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Boring Lava Field
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The Boring Lava Field (also known as the Boring Volcanic Field)[3] is a Plio-Pleistocene volcanic field of cinder cones, small shield volcanoes, and lava flows in the northern Willamette Valley of the U.S. state of Oregon and adjacent southwest Washington. The volcanic field is named for the town of Boring, Oregon, located 12 miles (20 km) southeast of downtown Portland and on the edge of the densest cluster of volcanic vents. The zone became volcanically active about 2.7 million years ago, with long periods of eruptive activity interspersed with quiescence. Its last eruptions took place about 57,000 years ago at the Beacon Rock cinder cone volcano. The individual volcanic vents of the field are considered extinct, but the field itself is not.

Key Information

The Boring Lava Field covers an area of about 1,500 square miles (3,900 km2) and has a total volume of 2.4 cubic miles (10 km3). This region sustains diverse flora and fauna within its habitat areas, which are subject to Portland's moderate climate. The highest point of the field is at Larch Mountain, which reaches an elevation of 4,055 feet (1,236 m).

The Portland metropolitan area, including suburbs, is one of the few places in the continental United States to have extinct volcanoes within a city's limits. The Boring Lava Field has played an important role in local affairs, including the development of the Robertson Tunnel, recreation, and nature parks. Because of the field's proximity to densely populated areas, eruptive activity would be a threat to human life and property, but the probability for future eruptions affecting the region is very low. The field may also influence future earthquakes in the area, as Extrusive rock from its historic eruptions may affect ground movement.

Geography

[edit]
A hiker stands on a rock mound with snow surrounding him; forest can be seen below the mountain
A hiker atop Larch Mountain, the highest elevation in the field

The Boring Lava deposits received their name based on their proximity to the town of Boring,[5][6] which lies 12 miles (20 km) southeast of downtown Portland.[3] The term "Boring Lava" is often used to refer to the local deposits erupted by vents in the field.[7] They are located in the western portion of Oregon.[8] The deposits were given this name by R. Treasher in 1942.[9][10][11] In 2002, as geochemical and geochronological information on the Boring deposits accumulated, they were designated part of the larger Boring Lava Field.[12] This grouping is somewhat informal and is based on similarities in age and lithology.[13]

The Boring Lava deposits lie west of the town of Boring.[10] The Global Volcanism Program lists the volcanic field's highest elevation as 4,055 feet (1,236 m),[14] at Larch Mountain;[15] most vents reach an elevation of 660 to 980 feet (200 to 300 m).[15] Located in the Portland Basin, the field consists of monogenetic volcanic cones that appear as hills throughout the area, reaching heights of 650 feet (200 m) above their surroundings. The collection includes more than 80 small volcanic edifices and lava flows in the Portland–Vancouver metropolitan area, with the possibility of more volcanic deposits buried under sedimentary rock layers.[3] The borders for the Boring Lava Field group are clear, except on the eastern side, where distinctions between Boring deposits and those from the major Cascade Volcanoes are less clear; many geologists have arbitrarily placed the eastern border at a longitude of 122 degrees west.[16] In total, the Boring Lava Field covers an area of about 1,500 square miles (4,000 km2), and it has a total volume of 2.4 cubic miles (10 km3).[16]

Physical geography

[edit]

With a variable topography, the Portland area ranges from river valley floors to terraces reaching elevations of 400 feet (120 m).[17] The Willamette Valley is marked by hills reaching heights of more than 1,000 feet (300 m),[18] and it is also physically separated from the lower Columbia River valley.[17] The Columbia River flows west from the eastern Portland region, merging with the Willamette near Portland before moving north. Tributaries for the Willamette include the Pudding, Molalla, Tualatin, Abernethy, and Clackamas rivers; tributaries for the Columbia River include the Washougal and Sandy rivers.[19] The Columbia River has significantly shaped the geology of the area.[20]

Multnomah Creek drains from Larch Mountain, one of the volcanic cones in Boring Lava Field.[21] Local streams near the community of Boring receive seepage from the local aquifer. This aquifer, part of the greater Troutdale sandstone aquifer, is made of sandstone and conglomerate[22] and supplies water to domestic wells in the Mount Norway area.[23] Boring Lava is known to have formed intrusions into local sedimentary rock, and thus it may guide flow of groundwater locally.[23]

Climate

[edit]

Portland's climate is moderate, with long growing seasons, moderate rainfail, mild winters, and warm, dry summer seasons. The area has more than 200 frost-free days annually. Temperature can vary widely, reaching a historic maximum of 116 °F (47 °C), though the usual July maximum is below 80 °F (27 °C), and the average minimum for January is above 32 °F (0 °C).[19] Yearly, precipitation averages between 35 and 45 inches (890 and 1,140 mm) in most river valleys, with a mean of 42.04 inches (1,068 mm) from 1871 through 1952. It shows variability, with a historic low of 26.11 inches (663 mm) at Portland in 1929 and a maximum of 67.24 inches (1,708 mm) in 1882.[19] More than 75 percent of this precipitation occurs between October and March; July and August mark the driest months with means below 1 inch (25 mm), while November, December, and January represent the wettest with averages greater than 6 inches (150 mm).[24] Prevailing winds originate from the south during winter and from the northwest during the summer season, with the exception of prevailing winds at the mouth of the Columbia River Gorge, where winds predominantly move to the east. The southern winds have the highest velocities of the three, only rarely occurring with potentially destructive force.[25]

Ecology

[edit]
A foot path runs through forest with shrubs and many tall trees.
Old-growth forest at Larch Mountain

The Portland area has a moderate climate, and precipitation is not typically very heavy, allowing for significant growth of vegetation, which can hamper fieldwork in the area.[25] Many forests that covered the area were partly cleared for agriculture, timber, or cemetery applications in the early 20th century.[26] These cleared and burned land plots sustain rich stands of secondary forest, featuring gorse, huckleberry, nettles, poison oak, salal, and blackberry. Myriad species of fern, as well as rapid-growth deciduous trees like alder and vine maple are also frequent. Forests support stands of Douglas fir, western hemlock, western redcedar, Pacific dogwood, bigleaf maple, Oregon ash, red alder, cascara buckthorn, Pacific madrone, and Oregon white oak; within swamps and moist areas in creeks, the shrub Devil's club can be observed.[25] Other trees that sometimes dominate forest areas include black cottonwood and red alder.[27] Forest communities have many additional shrubs including Indian plum, western hazel, and snowberry. Ground layer plants include the herbaceous sword fern and stinging nettle.[27]

In contemporary times, clearing of forests for housing development has left about half of the Boring Lava region un-forested. As a result, water quality has decreased due to higher sedimentation and turbidity, and flooding has worsened over time.[28] Streams within the area are of either first or second order, with moderate to low flows and average gradients between 10 and 12 percent. Cool and clear, many sustain macroinvertebrates, and a smaller number support amphibians and fish.[26] The riparian zones in the Lava Field area host diverse species, and they are influenced by uplands that serve as migration connections for birds, mammals, reptiles, and some amphibians.[27]

The United States Fish and Wildlife Service provided a list of potentially threatened or endangered species in the Boring Lava area, labeling them "sensitive" species.[29] Among plant species, they determined the following species to be sensitive: whitetop aster, golden Indian paintbrush, tall bugbane, pale larkspur, peacock larkspur, Willamette daisy, water howellia, Bradshaw's lomatium, Kincaid's lupine, Howell's montia, Nelson's checkermallow, and Oregon sullivantia.[30] For animal and marine life, northwestern pond turtles, willow flycatchers, long-eared myotises, fringed myotises, long-legged myotises, Yuma myotises, Pacific western big-eared bats, and northern red-legged frogs have been identified as species of concern; pileated woodpeckers, bald eagles, cutthroat trout, and coho salmon are also considered sensitive.[30]

History

[edit]

The nearby Portland area was inhabited by the Chinook people,[31] though much of the local indigenous culture is poorly understood as a result of disturbance of archeological sites and artifacts by erosion and human development.[32] Oral history, limited archeological evidence, and ethnographic research inform current knowledge about local Native American communities.[32] The area surrounding Portland constituted one of the most densely populated communities in the Pacific Northwest, made up predominantly of Chinook people including the Multnomah and the Clackamas.[31] In 1805 and 1806, Meriwether Lewis and William Clark documented villages and encampments near what is now East Portland, trading with members of the community and describing their plank houses, language, customs, and material culture.[31] Chinookian villages married amongst themselves and with members of the Tualatin tribe as well as the coastal Tillamook people. They imported slaves, traded among villages, and acted as intermediaries for the English and American fur trade. They also developed skill as craftspeople in making clothing, baskets, tools, and architecture.[33] Local indigenous populations were greatly reduced after forced displacement and disease,[34] but a small Native American community persists in Multnomah County.[35]

The Portland area has historically been a center for trade since the city was founded in 1845. With time, commerce has diversified. Iron mining and smelting was common between 1867 and 1894, with paper mills becoming established as an industry in 1885. Plants manufacturing cement and conducting aluminum reduction, and shipyards can be found in the region. Industrial chemical production represents an important industry in Portland. Most of these industries rely on resources outsourced from other areas, except for the paper industry; business is driven by low power costs and the local industrial mineral market. Other important manufacturing industries in the nearby region include food processing and logging.[36]

A glass tube containing geological samples including some from Boring Lava Field is seen running along the wall of the Washington Park station platform.
A core sample showing Portland's geological history, which includes Boring Lava Field deposits, is exhibited on the Washington Park station platform.

In 1893, the Kelly Butte Natural Area was designated upon a petition from the Portland City Council. Named after a pioneer family, the park covers an area of 22.63 acres (9.16 ha), including part of the Boring Lava Field, on public land 6 miles (9.7 km) to the southeast of downtown Portland, Historically, it sustained a quarry, prompting the creation of the Kelly Butte Jail, which used prisoner labor (under guard supervision) to gather crushed rocks for building roads in Portland until the 1950s.[37] In general, rocks from the Boring Lava Field have been used for masonry projects including retaining walls, garden walls, and rock gardens, especially oxidized and scoriaceous rocks.[38] Despite the prevalence of quarrying activity in historical times, there is no ongoing mineral or aggregate resource mining near the Boring Lava Field.[39]

In 1952, after a local vote, the Kelly Butte Civil Defense Center was built between 1955 and 1956, costing about $670,000. The center was constructed to host local government agents should a nuclear attack on Portland occur; it had an area of 18,820 square feet (1,748 m2), intended to host 250 people in case an emergency government became necessary. It was known throughout the United States as a model facility for local governments, and in 1957, the docudrama A Day Called X included footage of the Defense Center. The center was left obsolete after a 1963 Portland City Council vote to abolish it passed; in 1968, just one permanent employee remained. Eventually the building was converted into an emergency services dispatch center from 1974 through 1994, when it was abandoned due to rising costs for renovation and space limitations. That same year the building was vacated, and then it was sealed off in 2006. A sixty-bed isolation hospital operated at Kelly Butte from September 1920 until 1960, supporting patients with communicable disease. A 10 million-US-gallon (38 millionL) water tank stood in the area from 1968 through 2010, when it was replaced with a 25 million-US-gallon (95 millionL) underground reservoir that cost $100 million, despite opposition from local environmental groups like the Friends of the Reservoirs. Historically, the park has also housed a police firing range, and Kelly Butte remains a recreational space today, administered jointly by Portland Parks and Recreation and the Portland Water Board.[37]

In 1981, the Portland city government built a reservoir at the north end of Powell Butte (part of the Boring Lava Field), which still serves the city. In 1987, Portland government created Powell Butte Nature Park, covering 600 acres (240 ha) of meadows and forest within the city. Planning started in 1995 for a second water reservoir in the area, which was built between 2011 and 2014. The new reservoir is underground, buried under topsoil and native plants, and it has a volume of 50 millionUS gallons (190 million L). With the new reservoir came improvements to the Powell Butte park, including resurfaced and realigned trails, reduced environmental impacts, better accessibility measures, and reduction of steep grades. The government also built a visitor center, caretaker's house, public restrooms, maintenance yard, and a permeable parking area that permitted filtration of rainwater through asphalt to an underground stone bed, where it could be absorbed by the soil and then into the nearest aquifer.[40]

Built between 1993 and 1998, the Robertson Tunnel runs for 2.9 miles (4.7 km) through the Tualatin Mountains. It contains Washington Park station, the deepest train station in North America, located 260 feet (80 m) underground. The station displays a core that exhibits Boring Lava deposits. For the first 3,900 feet (1,200 m) of the tunnel, the core shows Boring lava flows with cinder, breccia, and loess dated from 1.47 million to 120,000 years ago, which have been deformed by the Sylvan fault. With the Oatfield fault, the Sylvan fault trends to the northwest, extending 9.3 miles (15 km) northwest and 16 miles (25 km) southeast of the tunnel. It is of Quaternary age and lacks surface expression, possibly as a result of its extensive burial by loess along its length.[41]

In 2000, the nonprofit Friends of Mt. Tabor Park was formed to help maintain the Mount Tabor Park area,[42] located 3.5 miles (5.6 km) east of downtown Portland.[43] They have an organizational website and publish a bi-annual newsletter called the Tabor Times. Membership requires dues, and they also rely on donations and a gift shop for financial support.[42]

In September 2017, the Hogan Butte Nature Park opened in the city of Gresham, encompassing an area of 46 acres (19 ha) that includes the extinct Boring Lava Field volcano Hogan Butte. This park opened after more than 25 years of processing, supported by a 1990 bond from the city and two regional Metro bonds. Collaborators for opening the park include the U.S. Forest Service, local citizens, Metro, The Trust for Public Land, and the Buttes Conservancy organization.[44] Gresham is one of just a few places in the United States with volcanoes contained in its city limits.[44] Mount Sylvania and Mount Scott lie within the limits of Portland, in the southwestern and southeastern parts of the city, respectively.[45]

Geology

[edit]
Graphic showing the oceanic Juan de Fuca tectonic plate advancing under the continental North American tectonic plate, with rising magma showing where volcanoes have formed as with the Boring Lava Field
Volcanism in the Boring Lava Field results from subduction of the oceanic Juan de Fuca tectonic plate under the North American tectonic plate.

There are 90 volcanic centers[a][10] within a 20-mile (32 km) radius of Troutdale and more than 32 vents within a 13-mile (21 km) radius of Kelly Butte. Mostly small cinder cone vents, these volcanoes also include some larger lava domes from shield volcanoes at Mount Sylvania, Highland Butte, and Larch Mountain. The Boring Lava Field marks the densest volcanic vicinity in this group, encompassing an area of 36 square miles (93 km2).[10] It includes more than 80 known small vents and associated lava flows, with more volcanic deposits likely present under sedimentary rock deposits from the Missoula Floods[3] (also known as the Bretz or Ice Age Floods),[46] which took place between 21,000 and 15,000 years ago and probably destroyed small cinder cones (including those made from tuff) and maar craters, burying them under up to 98 feet (30 m) of silt from slack water.[3] The Global Volcanism Program reports that the field includes somewhere between 32 and 50 shield volcanoes and cinder cones, with many vents concentrated northwest of the town of Boring.[15]

Regional setting

[edit]

Considered an outlier of the Cascade Range,[7] the Boring Lava Field lies about 62 miles (100 km)[15] to the west of the major Cascade crest.[7] It marks one of five volcanic fields along the Quaternary Cascade arc, along with Indian Heaven, Tumalo in Oregon, the Mount Bachelor chain, and Caribou in California.[47] Like the Cascade Range, the Boring field was also generated by subduction of the oceanic Juan de Fuca tectonic plate under the North American tectonic plate, but it has a different tectonic position, with its eruptive activity more likely related to tectonic rifting throughout the region.[3] The Boring Lava Field has erupted material derived from hot mantle magma, and the subducting Juan de Fuca plate may be as shallow as 50 miles (80 km) in depth at their location.[48]

The High Cascades, a segment of the Cascade volcanic arc that includes the Boring Lava Field,[49] is characterized by basaltic lava flows with andesite, tuff breccia, and volcanic ash.[49] The High Cascades may lie over a graben (a depressed block of the Earth's crust bordered by parallel faults), and activity at the Boring field and throughout the Portland area may be associated with deformation of the block.[50] Portland lies within the Portland Basin, part of the forearc (the region between an oceanic trench and the associated volcanic arc) between the Cascades major arc and the Pacific Coast Ranges, which consist of Eocene to Miocene marine sedimentary rock deposits and Eocene intrusions and extrusions of basalt that were emplaced on the Siletz terrane.[20] The eastern boundary for the Portland Basin is the Cascades, while the Tualatin Mountains lie to the west, along an anticline formation that has been changing since the Miocene.[20] The Boring Lava Field sits on the floor of the Portland Basin,[51] residing in the forearc setting between tectonic extension to the south and compression to the north.[52] The uneven distribution of vents within this forearc suggests a local zone of crustal expansion. Over the last 2.7 million years, the volcanic field has irregularly rotated clockwise and migrated to the northwest at an average rate of 0.37 inches (9.3 mm) ± 0.063 inches (1.6 mm) per year relative to the surrounding crust. This northwest trending is consistent with other faults in the nearby area.[53] The Boring Lava Field represents the youngest episode of volcanism within the Cascade forearc,[54] and while there is no evidence that they were associated with a slab window (a gap that forms in a subducted oceanic plate when a mid-ocean ridge meets with a subduction zone and plate divergence at the ridge and convergence at the subduction zone continue, causing the ridge to be subducted), they likely interacted with the regional mantle wedge.[55]

Composition and local setting

[edit]

The Boring Lava Field shows a similar composition to the High Cascades that run through Oregon and southern Washington state,[7] with Pliocene to Pleistocene[56] basalt lava flows and breccias.[7] It was active during the late Tertiary into the early Quaternary.[21] Within the field, lava shows a diverse composition overall,[54] varying from low-K,[b] tholeiitic to high-K, calc-alkaline eruptive products.[52] Some of the low-K tholeiite deposits likely originated from vents closer to the High Cascades, and they are overlain by Boring Lava materials.[8] J. M. Shempert, a geologist at Portland State University, proposed that mantle sources for the two different lava types may be different and that the calc-alkaline sources are more refractory.[c][57]

Like the surrounding High Cascades, Boring Lava Field erupted lava made of olivine basalt and basaltic andesite;[58] these sub-alkaline basalts and basaltic andesite predominate among Boring Lava deposits.[4] The olivine basalt deposits have fine to medium textures, and the basaltic andesite lava flow deposits have relatively little pyroclastic rock in them,[58] suggesting that explosive eruptions were uncommon within the field.[59] Dark gray to light gray in color, Boring Lava produces columnar and platy joints, which can be seen in Oregon east of Portland and in Clark County in Washington state.[23] It is usually phyric, though one sample from Rocky Butte consists of labradorite with olivine phenocrysts that have been transformed to iddingsite.[60] The Boring Lava reaches thicknesses of more than 400 feet (120 m).[7] Boring Lava has a more mafic (rich in magnesium in iron) composition than the nearby volcano Mount Hood, but they are similar in age.[21] There is a small amount of andesite in the lavas from the field, mostly erupted from monogenetic vents or Larch Mountain.[61] Sometimes, Boring Lava overlaps with volcaniclastic conglomerate from other Cascade eruptions in Multnomah County and the northern part of Clackamas County.[23][62] The Boring Lava also contains tuff, cinder, and scoria; it is characterized by plagioclase laths that show a pilotaxitic texture with spaces between them that show a diktytaxitic texture.[63] The Boring Lava exposures show aeromagnetic anomalies with short wavelengths and high amplitudes suggestive of their relatively young geological ages.[64]

At points where the Boring Lava sits over Troutdale Formation deposits, landslides are frequent, producing steep head scarps with heights of up to 66 feet (20 m). These scarps tend to have grabens at their bases and Boring Lava blocks at their tops, and they show variable slide surfaces from hummocky to flat. A number of these exposures show dips up to 35 degrees, as well as minor faults. The landslides range in thickness from 20 to 79 feet (6 to 24 m).[65] Portland's wet climate leads to weathering,[66] which at the Boring Lava Field has reached depths of up to 25 feet (7.6 m), altering the upper 5 to 15 feet (1.5 to 4.6 m) of soil to a red, clay-like material. At the cinder cone in Mount Tabor Park, an outcrop of quartzite-pebble xenoliths (rock fragments enveloped in a larger rock during the latter's development and solidification) can be observed among local cinder specimens, dating from Miocene to Pliocene Troutdale deposits.[60] While the volcanic rock of Boring Lava was being emplaced over rock from the Troutdale formation,[67] there was deformation that uplifted and dropped fault blocks to the southeast of Portland.[68] Along the Washougal River, a large landslide occurred as a result of failure due to the Boring Lava pushing down on rock from the Troutdale formation.[69] Intrusions of Boring Lava formed outcrops at Highland Butte, La Butte, and potentially in the subsurface regions near Aurora and Curtis, and these intrusions have been associated with normal faulting at Parrett and Petes Mountain, Aurora, Curtis, and Swan Island (along the Molalla River).[70][71] Faults together with igneous intrusions are usually accompanied by stretching and doming as a result of magma influxes or collapses from the evacuation of the magma flows.[70] Similarly, faults north of Oregon City might have resulted from subsidence after magma chambers emptied or lava was extruded as a result of Boring Lava eruptions.[72] Some of the Boring Lava vents cut off hydrogeologic units in the surrounding area.[73]

Eruptive vents on the western edge of the field formed along a fault line that trended to the northeast, located north of present-day Carver.[68] Boring Lava was erupted by vents in the volcanic field,[6][74] and it has been exposed at elevated topographic levels in intact volcanic cones and dissected lava plains. There is likely more lava deposited under Quaternary sedimentary mantle throughout the region,[6] though activity was confined to a relatively concentrated area.[75]

Subfeatures

[edit]
Map of the Boring Lava Field with a total of 95 vents including 74 named cones, with major landmarks marked including Portland, Vancouver, and other cities as well as the Columbia River

D. E. Trimble (1963) argued that the Boring Lava Field was produced by eruptive activity at 30 volcanic centers;[76] these include shield and cinder cone volcanoes.[76] J. E. Allen reported 95 vents in 1975, dividing them into four clusters: 17 vents north of the Columbia River, 14 vents west of the Willamette River, 19 vents east of the Willamette River and north of Powell Valley Road, and 45 vents east of the Willamette River and south of Powell Valley Road (Highway 26).[77] Of these, 42 were unnamed, and several volcanoes contained multiple vents.[77] Generally, all lava flows in the field can be traced to specific vents in the field,[78] but documented source vents have mostly been confirmed through chemical analysis or petrographic comparisons,[79] with a few exceptions.[80]

In the eastern part of the Boring cluster, volcanic vents have average diameters less than 1.6 miles (2.6 km), with average heights less than 1,090 feet (330 m) from base to summit. The lava flows from Highland Butte and Larch Mountain, both shield volcanoes, encompass a wide area, the Boring Lava deposits averaging thicknesses of 100 to 200 feet (30 to 61 m) not considering areas next to volcanic cones in the field. Most of the summit craters have been destroyed, though there are partial craters at Bobs Hill (located 20.5 miles (33.0 km) northeast of Portland) and Battle Ground Lake (located 20.5 miles (33.0 km) north of Portland);[60][81] Mount Scott also has an intact summit crater.[82] Many of its vents retain the shape of a volcanic cone, with loess extending above an elevation of 400 feet (120 m).[60] The Rocky Butte plug, which reaches a height of 330 feet (100 m) above its surroundings, was dated to 125,000 ± 40,000 years old by R. Evarts and B. Fleck from the United States Geological Survey (USGS). Mount Tabor is also prominent in the area, dated by the USGS to 203,000 ± 5,000 years old, as are Kelly Butte, Powell Butte, and Mount Scott. Scott has been dated to 1.6 million years ago.[82]

A series of lava tubes were documented near the Catlin Gabel School along the western slope of the Portland Hills. These formations, created by lava flows cooling at the surface while their hot interior kept draining, were first identified by R. J. Deacon in 1968, then analyzed by L. R. Squier in 1970;[83] they were studied in greater detail by J. E. Allen and his team in 1974.[83] The Catlin Gabel tubes lie among cinder cones and lava flows from the Pliocene to Pleistocene, and they are the oldest known lava tubes in Oregon, older than the Holocene.[83] The tubes were produced by a small vent at the southern end of the northern segment of the field,[84] extending 2.5 miles (4.0 km) from its base to the south and then the west.[85] They originated from the uppermost lava flow from a series of eruptions that ran into a valley on the western slope of the Portland Hills.[86] The Catlin Gabel tubes have a width of 2,500 feet (760 m), with slopes averaging 150 feet (46 m) per mile for an average grade of 3 percent. On average, these tubes have a thickness of 235 feet (72 m) near their center, with an upper lava unit thickness of 90 feet (27 m) that has since been modified by erosion and the deposition of up to 30 feet (9.1 m) of Portland Hills silt. The Catlin Gabel tubes also sit atop 434 feet (132 m) of silt from the Troutdale Formation.[85] Running along the tube's arc are five depressions, which were created through the collapsing roofs of the lava tubes within a subsegment that is 6,000 feet (1,800 m) in length. The characteristics of the tube system are not well documented, since only the collapsed segments are accessible. Some of the channels have been reduced to rubble, and analysis has revealed that they trended northwest, had widths up to 40 feet (12 m) and depths no more than 60 feet (18 m), and required special engineering procedures to permit the construction of a 15-story building above them.[86]

Oregon vents

[edit]

The following vents are in Oregon:

Boring Lava Field Vents in Oregon
Name Elevation Notes
Chamberlain Hill 890 feet (271 m)[87]
Cook's Butte 718 feet (219 m)[87]
Highland Butte 1,594 feet (486 m)[88]
Kelly Butte 400 feet (122 m)[87]
Larch Mountain 4,061 feet (1,238 m)[2]
Powell Butte 614 feet (187 m)[89]
Rocky Butte 612 feet (187 m)[90]
Ross Mountain 1,380 feet (421 m)[87]
Swede Hill 995 feet (303 m)[87]
Mount Scott 1,093 feet (333 m)[88][91] Named for Harvey W. Scott[92]
Mount Sylvania 978 feet (298 m)[93]
Mount Tabor 630 feet (192 m)[94]
Mount Talbert 715 feet (218 m)[95]
TV Hill 1,275 feet (389 m)[87]
Walker Peak 2,450 feet (747 m)[88]

Washington vents

[edit]

The following vents are in Washington:

Boring Lava Field Vents in Washington
Name Elevation Notes
Battle Ground Lake 509 feet (155 m)[96]
Bob's Mountain 2,110 feet (643 m)[87]
Bob's Mountain (N) 1,775 feet (541 m)[87]
Bob's Mountain (S) 1,690 feet (515 m)[87]
Brunner Hill 680 feet (207 m)[87] 2 vents
Green Mountain 804 feet (245 m)[87]
Mount Norway 1,111 feet (339 m)[87]
Mount Pleasant 1,010 feet (308 m)[87]
Mount Zion 1,465 feet (447 m)[87]
Nichol's Hill 1,113 feet (339 m)[87]
Pohl's Hill 1,395 feet (425 m)[87]
Prune Hill (E) 610 feet (186 m)[87]
Prune Hill (W) 555 feet (169 m)[87]
Tum-Tum Mountain 1,400 feet (427 m)[87]
Panorama view of Portland with a central area revealing hills corresponding to buttes in the Boring Lava Field
The buttes of the Boring Lava Field are visible toward the center of this panorama of Portland, Oregon.

Eruptive history

[edit]
Houses cover a forested area that sits atop Mount Scott, one of the Boring Lava Field volcanic vents.
Eruptions took place in areas that are now populated, such as this community on Mount Scott.

Eruptions at Boring Lava Field occur in a concentrated manner, often in clusters of three to six vents, as at Bobs Mountain and Portland Hills.[80] These types of vents typically produced similar types of magma in relatively short periods of time, and they also frequently show alignment. Vents in the field have generally produced basalt and basaltic andesite, with some andesitic eruptions, including those that produced the Larch Mountain shield volcano.[3]

Prior to the 1990s, there was little potassium-argon dating data available for the lava field,[97] and despite the field's proximity to an urban area, little was known about its composition until recent years.[16] Weathering, fine grain size, and glassy content further limit argon–argon dating for the field.[80] Recent argon–argon dating suggests that eruptive activity at the Boring Lava Field began between 2.6 and 2.4 million years ago, yielding far-reaching basalt lava flows, the Highland Butte shield volcano, a number of monogenetic vents, and one andesitic lava flow. These took place near the southern Portland Basin, and were followed by about 750,000 years of quiescence. About 1.6 million years ago, eruptive activity resumed to the north of the previously active area, with alkalic basalt lava flows[80] generating the Mount Scott shield volcano.[4] As eruptions shifted to the east over time, the Larch Mountain volcano was produced by eruptions in the foothills of the Cascade Range. Activity spread out over the area, extending to its current expansive state about one million years ago.[3] In addition to spreading out geographically, the lava composition in the field's vents became more diverse.[4] This period continued until about 500,000 years ago, with no activity until about 350,000 years ago,[98] after which activity continued through roughly 60,000[3][99] to 50,000 years ago according to several sources,[48][98] or about 120,000 years ago according to I. P. Madin (2009).[46] R. Evarts and Fleck originally reported that lava flows at the Barnes Road deposit of the field represented the youngest eruptive products in the Boring area, with a radiometric dating age of 105,000 ± 6,000 years.[66] These eruptions followed a relatively even age distribution over time;[46] younger vents and associated deposits lie in the northern portion of the field, while older deposits are confined to the south.[4]

The products of the Boring Lava Field were erupted discontinuously over an erosion surface.[100] Activity took place during the late Tertiary and early Quaternary, in what is now the Portland area as well as the surrounding area, with a particularly concentrated pocket of activity to the east.[6] Nearly all of these eruptions were confined to single vents or small vent complexes, with the exception of a lava plain southeast of present-day Oregon City.[74] Boring Lava generally consists of flowing lava; only one eruptive deposit contains tuff, ash, and tuff breccia, and one vent to the northeast of the Carver area displayed evidence of explosive eruptions that later became effusive.[101]

Recent activity and current threats

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Sometime less than 100,000 years ago, magma at Battle Ground Lake in Washington state interacted with water to form the eponymous maar volcano, destroying a lava flow dated to 100,000 years ago. The last volcanic center to form in the field was Beacon Rock, a cinder cone produced by eruptions about 57,000 years ago, which was eroded by the Missoula Floods to leave only its central volcanic plug.[3][4] While the known volcanic vents in the Boring Lava Field are extinct, the field itself is not considered extinct. The probability of future eruptions in the Portland–Vancouver metropolitan area is very low.[3] It is rare that more than 50,000 years pass without an eruption in the region; given the past eruptive history of the field, an eruption is predicted to occur once every 15,000 years on average.[102]

About half of the Boring Lava Field eruptions took place in what are today densely populated areas of the Portland–Vancouver metropolitan area. Though the formation of a small cinder cone vent might not extend far beyond its surroundings, depending on location, similar eruptions could lead to deposition of volcanic ash that could lead to serious infrastructural consequences, covering large areas. A larger eruption, like the ones that built Larch Mountain or Mount Sylvania, could span years to decades. It is unclear where exactly a future eruption might take place, but it would probably occur in the northern portion of the field.[102]

Many seismic faults in the northeastern section of the northern Willamette Valley formed as a result of intrusions of Boring Lava, as supported by their orientation, lengths, displacements, age, and proximity to Boring Lava intrusions. Though intrusions from any future eruptions at the Boring field are "probably minimal",[71] Boring Lava might play a role in determining the intensity of ground shaking during future earthquakes in the area.[71]

Recreation

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Trails in the city of Gresham travel over parts of the Boring Lava Field and its cones. Mount Tabor and Powell Butte are better known for their recreational uses than other cones;[103] Powell Butte Nature Park has 9 miles (14 km) of trails.[40] The Mt. Tabor Park is open to bicyclists and pedestrians from 5 a.m. through midnight and to motorized vehicles from 5 a.m. through 10 p.m. each day, except for Wednesdays when the park roads are not open to automobiles.[43] The Hogan Butte Nature Park offers views of Mount Adams, Mount Hood, Mount Rainier, and Mount St. Helens,[104] as well as running trails and sites for picnicking.[44]

In addition to the nature park on Hogan Butte, a number of smaller cinder cones are also publicly accessible. The Gresham Saddle Trail traverses Gresham Butte and Gabbert Butte, running for 3.3 to 3.7 miles (5.3 to 6.0 km). The trail is considered to be of moderate difficulty, and it offers no amenities. It includes the Gabbert Loop Trail, which extends for 1 mile (1.6 km) through forests of maples, alders, ferns, and firs.[103]

See also

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Notes

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References

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Sources

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Boring Lava Field

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The Boring Lava Field, also known as the Boring Volcanic Field, is a comprising more than 80 small volcanoes, including cinder cones, maars, and shield volcanoes, located in the Portland-Vancouver metropolitan area across northwestern and southwestern Washington, . This field covers an area of approximately 4,000 km², with exposed volcanic rocks spanning about 500 km² and a total eruptive volume of roughly 10 km³, primarily consisting of lavas ranging from to and low-silica . The field is centered in the Portland Basin and named after the community of , near several of its southeastern vents. Geologically, the Boring Lava Field represents tectonically anomalous volcanism within the , positioned trenchward of the main Cascade volcanic arc and unrelated to the nearby High Cascades. Volcanic activity initiated around 2.6 million years ago (Ma) with low-potassium (low-K) tholeiitic eruptions, followed by a hiatus of about 0.8 million years until 1.6 Ma, after which dispersed eruptions of variable composition—ranging from low-K tholeiite to high-potassium (high-K) calc-alkaline—occurred at an average recurrence interval of roughly 15,000 years. The youngest dated vents within Portland city limits, such as Powell Butte (~100-125 ka), are up to approximately 200 ka old, with the field's overall youngest at ~57 ka; no recorded activity. The field's eruptive products show geochemical evidence of limited influence from subducted slab fluids, primarily from mantle sources, and possible late Neogene intra-arc rifting, distinguishing it from typical back-arc or arc volcanism. Notable features include the highest summit, Larch Mountain at 1,236 m (4,055 ft), and several urban-integrated cones such as Rocky Butte and Kelly Butte, which now form public parks and reservoirs in Portland. Despite its proximity to a major population center of over 2.5 million people (as of 2024), the field's dormancy and monogenetic nature indicate low future eruption risk, though it serves as a key site for studying subduction-related volcanism in urban settings.

Geography

Location and extent

The Boring Lava Field is a situated in the northern of the , centered near the city of , and extending northward across the into state. It lies in the region of the Cascade Volcanic Arc, positioned westward of the main arc axis. The field encompasses an area of approximately 4,000 square kilometers (1,500 square miles), primarily within the Portland Basin. Over 80 volcanic centers have been identified across the field, with the majority concentrated in Multnomah and Clackamas counties in , and scattered outliers in Clark and Skamania counties in Washington. The approximate central coordinates are 45°18′N 122°30′W. The field underlies significant portions of the Portland metropolitan area, including urban neighborhoods such as Sellwood-Moreland and the district, where volcanic features like cinder cones and lava flows are integrated into the cityscape.

Physical features

The Boring Lava Field is dominated by low-relief shield volcanoes and monogenetic cinder cones, accompanied by extensive basaltic lava flows that form the primary surface expressions of its . Cinder cones, reaching heights of up to 120 meters, are scattered throughout the field, often with associated deposits and agglutinate, as seen in remnants like . Shield volcanoes exhibit broad, gently sloping profiles, such as Powell Butte, a low forested hill capped by volcanic material rising to about 187 meters (614 feet) above . These landforms contrast with the surrounding alluvial plains of the Portland Basin, creating isolated buttes and hill clusters that rise up to 200 meters above the adjacent lowlands. Lava flows, averaging 10-30 meters in thickness, cover large areas and contribute to flat-topped plateaus, with examples including the Canemah flow exhibiting . Fissure vents have produced linear flows, while explosive activity has formed and rare tuff rings, such as the Battle Ground Lake . Erosion has exposed layered sequences of these flows in some localities. The overall topography consists of gently rolling hills with elevations ranging from near to 1,236 meters (4,055 feet) above at Larch Mountain, shaped by post-eruptive weathering and burial under younger sediments. Hydrological features, including lava tubes and collapse sinks, significantly influence local drainage patterns by channeling water and creating impermeable barriers. For instance, at , a largely buried high-silica center, such structures have altered stream courses and formed small depressions. These elements, combined with the field's low-relief morphology, result in a landscape where volcanic remnants protrude as prominent, rounded hills amid urban development.

Climate and hydrology

The Boring Lava Field lies within the warm-summer zone (Köppen Csb) characteristic of the , featuring mild temperatures and distinct seasonal precipitation patterns. Annual precipitation averages approximately 914 mm (36 inches), with the majority—about 737 mm (29 inches)—falling during the wet winter months from to , while summers from to remain relatively dry with less than 150 mm (6 inches) total. Temperatures typically range from a winter low of 2°C (36°F) to a summer high of 29°C (84°F), with an annual average around 11°C (52°F), supporting a long of over 200 frost-free days. This climatic regime significantly influences the field's landscape, as the abundant winter rainfall accelerates chemical of the basaltic lavas and fosters dense cover on the nutrient-rich volcanic soils, particularly in undeveloped hill areas. In low-lying portions of the field, seasonal river overflows can lead to temporary flooding, which exposes and erodes older lava flows, enhancing surface . The permeable nature of the basaltic rocks further modulates local water retention, preventing widespread except in topographic depressions. Hydrologically, the Boring Lava Field occupies the northern within the Portland Basin, where it is bordered by the to the south and west and the to the north, with several tributaries intersecting the volcanic deposits. The region's is primarily stored in the permeable basaltic aquifers of the Willamette Lowland aquifer system, including interbedded Boring lavas and underlying flows, which exhibit high due to vesicular and fractured zones. Springs often emerge along fault lines and from the margins of lava flows, including occasional outflows associated with preserved lava tubes in the Portland Hills; however, the field lacks major lakes, though small crater ponds, such as Battle Ground Lake formed in a volcanic , provide localized features. Projections from recent assessments indicate that will exacerbate risks in the region through warmer temperatures and reduced summer , potentially altering rates and increasing variability in levels across the , though no direct links to volcanic activity have been observed.

Geology

Regional tectonic setting

The Boring Lava Field is situated in the region of the , approximately 100 km west of the main Cascade volcanic arc axis near . This positioning renders the field tectonically anomalous, as forearc volcanism is uncommon in subduction systems and typically occurs farther landward in back-arc settings. The field overlies the Portland Basin, a structural depression formed amid the ongoing convergence of the oceanic Juan de Fuca Plate with the continental North American Plate. Tectonic activity in the region is driven by the oblique of the young, warm Juan de Fuca Plate beneath the North American Plate at a rate of approximately 4 cm per year. This process dips the slab eastward at angles of 10–45°, releasing hydrous fluids that promote in the mantle wedge, though the location of the Boring field suggests additional mechanisms such as intra-arc rifting or edge-driven mantle upwelling to explain the volcanism. Possible influences include clockwise rotation and northward translation of the Siletzia , a accreted to the margin in the Eocene, which may have facilitated localized extension. The field developed atop the Miocene Columbia River Basalt Group, a vast continental flood basalt province, and elements of the underlying Siletzia terrane, with volcanism spanning the Plio-Pleistocene from about 2.7 million years ago to roughly 50,000 years ago. Although no major active faults traverse the field directly, the broader area experiences diffuse seismicity tied to the locked Cascadia megathrust, which poses risks of great earthquakes (magnitude 8–9) every few centuries. The Portland Basin itself remains relatively aseismic compared to the arc, highlighting the anomalous nature of the volcanic activity without prominent crustal faulting.

Magma composition and volcanism

The magmas erupted in the Boring Lava Field are predominantly basaltic to basaltic andesite in composition, with SiO₂ contents typically ranging from 48 to 58 wt%, reflecting derivation from mantle sources with limited crustal contamination. Minor occurrences of more alkalic rocks, such as hawaiite and mugearite, appear in some vents, alongside low-K tholeiitic basalts (LKT), calc-alkaline basalts (CAB), and high-K calc-alkaline basalts (HKCA), with overall alkali contents (Na₂O + K₂O) lower than those typical of the adjacent Cascade Arc volcanoes. These compositions indicate a spectrum from subduction-modified mantle melts to ocean-island basalt (OIB)-like magmas, with K₂O varying from 0.13 to 1.71 wt%. Volcanism in the field is characterized by effusive eruptions producing extensive lava flows from fissures and small shield volcanoes, alongside mildly explosive activity that forms cinder cones and scoria deposits, but lacks evidence of large Plinian eruptions. Magma sources are primarily asthenospheric, at depths of approximately 30–80 km, varying by magma type, with contributions from slab-derived fluids enriching incompatible elements such as Ba, Sr, and K₂O, as evidenced by elevated Ba/Nb and Pb/Ce ratios relative to mid-ocean ridge basalt (MORB). No caldera formation has occurred, consistent with the monogenetic nature of the vents and absence of large-volume silicic magmatism. Petrological studies reveal anomalies including enrichment in incompatible trace elements, suggestive of fluid influx from the subducting Juan de Fuca plate, while some LKT and OIB-like s show minimal high field strength element (HFSE) depletion. Shallow chambers at depths less than 10 km are inferred from the presence of crustal and mantle xenoliths in some flows, indicating rapid ascent with limited differentiation. using K-Ar and ⁴⁰Ar/³⁹Ar methods confirms the field's activity spans the to , with pulses of basaltic .

Vents and subfeatures

The Boring Lava Field encompasses over 80 volcanic vents, with approximately 70 concentrated in and roughly 10 in Washington. These vents consist primarily of monogenetic features such as , shields, and plugs, formed by localized eruptions of basaltic to andesitic magmas. In , prominent vents include the , which stands about 130 m (430 ft) high (relief above surrounding terrain) and is situated within Portland . Another key feature is Rocky Butte, a (eroded to a ) dated between 285,000 and 500,000 years old via potassium-argon methods. The complex, located in the , comprises multiple aligned vents that generated lava flows extending into river deltas. Washington's vents are sparser and generally smaller, exemplified by Beacon Rock, an erosion-resistant plug and the field's youngest vent. Additional outliers near , such as those associated with , produced modest lava flows confined to local topography. Subfeatures within the field include lava tubes developed beneath Portland in the thicker flows, such as the Catlin Gabel system in the west hills, providing evidence of subsurface channeling during eruptions. Maars and possible rings occur near , indicating phreatomagmatic activity where magma interacted with groundwater. Erosion-resistant buttes, such as Powell Butte and Kelly Butte, formed from consolidated and lavas, were historically quarried for construction materials like aggregate. Mapping of these vents and subfeatures relies on efforts by the U.S. Geological Survey (USGS) and state geological surveys, which employ geomorphic analysis of landforms, soil stratigraphy, and geochemical fingerprinting of rocks to delineate vent sources and flow extents.

Eruptive history

Timeline and episodes

The Boring Volcanic Field initiated its eruptive activity approximately 2.6 million years ago during the epoch, with the earliest documented eruptions occurring in the southern Portland Basin through the extrusion of low-potassium tholeiitic basalts. This initial phase, spanning roughly 2.6 to 2.2 million years ago, involved shield-building volcanism that produced widespread lava flows and established the foundational structure of the field. Following a hiatus of about 500,000 years, activity resumed around 1.7 million years ago and persisted as the dominant pulse until approximately 0.5 million years ago, characterized by more dispersed eruptions of moderately alkaline basalts that migrated northward at a rate of about 9 mm per year. Within this main phase, episodic clusters of heightened activity are evident, including a notable burst around 1.2 million years ago that contributed to mid-field development, and another peak near 0.5 million years ago involving scattered vents and flows across the central and northern extents of the field. then entered a prolonged repose period exceeding 100,000 years before a final cluster of eruptions between 350,000 and 50,000 years ago, which produced isolated and associated lavas in the northern and western portions. The youngest confirmed episode occurred at Beacon Rock around 58,000 years ago, marking the end of documented activity with no verified eruptions, though the field's intermittent nature over 2.5 million years underscores long repose intervals typically greater than 100,000 years. Ages for these episodes have been determined primarily through techniques, with potassium-argon (K-Ar) methods applied to older units exceeding 1 million years and more precise ⁴⁰Ar/³⁹Ar dating used for younger Pleistocene materials to achieve higher resolution. These are supplemented by stratigraphic correlations and paleomagnetic analysis, which confirm the sequence and polarity reversals aligning with global chronologies, such as the Jaramillo subchron for some mid-phase lavas.

Recent activity and hazards

The Boring Volcanic Field has experienced no confirmed eruptive activity since approximately 58,000 years ago, when the produced its last eruption. Individual volcanic centers within the field are considered extinct by the U.S. Geological Survey (USGS), though the overall field retains a very low potential for future monogenetic eruptions due to its tectonic setting in the of the . Such events, if they were to occur, would likely be small-scale, with (VEI) values of 0 to 2, involving basaltic lava flows and minor emissions typical of the field's composition. The USGS Cascades Volcano Observatory (CVO) monitors the region encompassing the Boring Volcanic Field through a network of seismometers, GPS stations for ground deformation, and gas emission sensors, primarily focused on higher-threat but extending to detect any anomalous activity in the Portland Basin. As of November 2025, background remains low, with no detected precursors such as increased earthquakes, ground uplift, or elevated gas emissions indicative of magmatic unrest. No eruptive or significant precursory activity has been recorded in the , aligning with the field's long dormancy. Potential hazards from a hypothetical future eruption include slow-moving basaltic lava flows that could inundate urban areas, localized ash fall disrupting transportation and air quality, and lahars triggered by interaction with waterways or rainfall in the densely populated Portland-Vancouver metropolitan area. Urban expansion has heightened exposure, placing over 2 million residents within the field's ~4,000 km² extent. The USGS assesses the annual probability of an eruption as very low—considerably less than risks from nearby Cascade stratovolcanoes or the Cascadia subduction zone earthquake—prompting emphasis on ongoing monitoring rather than imminent threats in 2025 forecasts. Contingency planning is integrated into regional frameworks by the Oregon Office of Emergency Management, including public awareness, evacuation route development, and coordination with USGS for early warnings in Multnomah County and surrounding jurisdictions.

Human history

Indigenous and early settlement

The Portland Basin, encompassing the Boring Lava Field, has been occupied by for over 10,000 years, with evidence of human presence dating back to approximately 11,000 years ago through small, mobile groups that established winter villages and seasonal camps for resource gathering. Chinookan-speaking groups, including the Chinook, Multnomah, and Clackamas, inhabited the region for millennia, relying on the area's rivers and fertile soils for subsistence economies centered on , , and gathering. These communities utilized the volcanic landscape of the Boring Lava Field, employing local rocks to craft tools such as grinding stones and projectile points, while the elevated buttes provided strategic vantage points for defense and observation. Archaeological evidence includes seasonal campsites on volcanic soils near waterways, supporting at sites like and in surrounding uplands, though specific sites on field vents are limited; petroglyphs documented along the nearby reflect broader cultural practices in the region. No direct accounts of Boring Lava Field eruptions exist due to their prehistoric timing (50,000–130,000 years ago). European exploration began with the in November 1805, as the descended the and noted the "bold and rockey" shores flanked by pine-covered hills, corresponding to the basaltic terrain of the Boring Lava Field area near modern Portland. Initial permanent settlements emerged in the 1840s amid the migrations, with communities like Milwaukie establishing agricultural outposts that incorporated local from the lava field for building foundations and structures, leveraging the durable stone amid abundant timber resources. The volcanic landscape influenced settlement patterns by providing natural barriers and elevated sites for farms and mills, while avoiding low-lying flood-prone areas shaped by ancient lava flows. No historical records indicate direct impacts from eruptions, as all activity predated human occupation, allowing settlers to adapt to the stable but rugged terrain without volcanic hazards. By the early , the community's identity solidified with the naming of , in 1903 after settler William H. Boring, a Civil War veteran who donated land for the local school and , marking the transition from scattered pioneer homesteads to formalized within the lava field's eastern extent.

Modern urbanization and impacts

Since the early 20th century, the Portland metropolitan area has undergone substantial urban expansion, overlaying significant portions of the Boring Lava Field with residential, commercial, and infrastructural developments. This growth has integrated volcanic features into the urban fabric, with the field spanning approximately 3,900 square kilometers underlying parts of the Portland-Vancouver region, where over half of its volcanic centers now lie within densely populated zones. The (PDX), situated on volcanic soils of the field along the floodplain, exemplifies this overlay, as do major highways like Interstate 205, which traverse lava flows and contribute to increased impervious surfaces in the area. By 2025, the metro area's population exceeds 2.5 million residents, exerting pressure on the field's geological landscape through ongoing suburbanization. Quarrying activities have further altered the field's morphology, particularly through basalt extraction for construction materials. At Rocky Butte, a prominent within the field, quarries on the east side operated historically, supplying rock for local infrastructure such as the Multnomah County Jail in the 1940s; these operations created exposed geological sections while forming artificial depressions in the landscape. Such extraction, prominent through the mid-20th century, supported Portland's building boom but diminished natural landforms. Urbanization has introduced environmental impacts, including groundwater contamination risks from stormwater runoff carrying pollutants across the permeable basaltic layers of the field. In the Portland Basin, overpumping and urban infiltration have affected aquifers associated with Boring lavas, leading to lowered water levels and potential pollutant migration in areas like east Multnomah County. Land use conflicts have arisen, with proposed developments on volcanic vents and flows often halted by Oregon's Goal 5 preservation laws, which prioritize protection of significant geological resources; for instance, inventories and protection plans for sites like the Boring Lava Domes have refined regulations to balance growth with conservation since the 1990s. The Boring Lava Field also holds cultural significance, contributing to Portland's identity as the "City of Roses" through its fertile volcanic soils that enhance horticultural growth in the mild climate. Recent urban planning efforts incorporate the field's geology into sustainable strategies, such as hazard mitigation and greenspace preservation, based on the 2021 mitigation action plan with ongoing implementation as of 2025.

Ecology

Flora and vegetation

The Boring Lava Field supports diverse vegetation adapted to its volcanic substrates, primarily fertile soils derived from weathered basaltic lava flows. Dominant plant communities include mixed coniferous-deciduous forests featuring ( menziesii), western redcedar (), and bigleaf (), which thrive on well-drained, nutrient-rich soils in forested uplands and slopes. Open oak savannas, characterized by Oregon white oak () and associated grasses, occur on exposed lava flows and drier buttes, reflecting historical prairie-forest ecotones now largely remnant. Vegetation exhibits adaptations to the field's challenging substrates, with colonizing disturbed or rocky areas through mechanisms like serotinous cones or nitrogen-fixing capabilities. Red alder () serves as an early successional species on young or disturbed lavas, rapidly establishing in open, nutrient-poor sites before yielding to shade-tolerant . Rare Willamette Valley prairie plants, such as Nelson's checker-mallow (Sidalcea nelsoniana), may occur in remnant meadows near the field, favored by moist microhabitats. like Scotch broom () proliferate post-disturbance in open areas, outcompeting natives through rapid growth and . Vegetation zonation varies with and , influenced by the region's moderate maritime . Wetter lowlands along streams and wetlands host sedge-dominated communities, including slender sedge (Carex obnupta) and cattail (Typha spp.), providing riparian buffers. Drier buttes and uplands feature grasslands with fescues and bunchgrasses, transitioning to fragmented habitats at urban edges where development disrupts continuity. Recent studies post-2010 identify biodiversity hotspots in undeveloped vents like Powell Butte and , where intact lava remnants preserve high plant diversity amid urbanization. Climate shifts, including prolonged droughts, are favoring drought-tolerant species such as Oregon white oak and vine maple (Acer circinatum) over moisture-dependent by 2025, as documented in regional assessments. As of 2025, regional assessments note that events like the 2021 have stressed forests, potentially accelerating shifts toward drought-tolerant species in exposed volcanic areas. Invasive species continue to impact remnant habitats, with efforts focused on control in parks like Powell Butte.

Fauna and habitats

The Boring Lava Field supports a variety of mammals adapted to its mosaic of urban, forested, and open landscapes, including (Odocoileus hemionus columbianus) that forage in meadows and woodlands, coyotes (Canis latrans) that roam buttes and edges of developed areas, and bats such as the (Myotis lucifugus) and (Eptesicus fuscus) that utilize rock crevices and lava tubes like the Catlin Gabel system for roosting. Urban-adapted species thrive near human settlements, with raccoons (Procyon lotor) commonly scavenging in parks and along streams. Ground squirrels (Urocitellus spp.) also inhabit the field's buttes and open spaces, contributing to the area's mammalian diversity amid fragmentation. Birdlife in the Boring Lava Field is rich, with approximately 200 species recorded across its parks and natural areas, including raptors like the (Buteo jamaicensis) that nest on elevated buttes such as Powell Butte and . Migratory waterfowl, including Canada geese (Branta canadensis), utilize riparian zones near lava-influenced streams like Johnson Creek, while songbirds such as warblers, , flycatchers, and the occasional (Sturnella neglecta) frequent woodlands and meadows during breeding and migration seasons. Other notable residents include band-tailed pigeons (Patagioenas fasciata), lazuli buntings (Passerina amoena), and pileated woodpeckers (Dryocopus pileatus), with spring migrations bringing diverse flocks to sites like , where over 190 species have been documented as of 2025. Habitats within the Boring Lava Field are characterized by fragmented woodlands of Douglas-fir and bigleaf maple interspersed with open meadows and riparian corridors along streams altered by ancient lava flows, providing essential refugia for wildlife despite urban encroachment. These riparian zones host diverse communities, serving as migration pathways for birds and mammals, while buttes offer elevated nesting and foraging sites that enhance connectivity in the Portland basin. Urbanization has reduced habitat corridors, limiting movement for species reliant on contiguous forests and wetlands. Recent surveys in the 2020s highlight ecological pressures, with declining populations of amphibians such as the northwestern salamander (Ambystoma gracile) attributed to habitat loss from development and in the Portland area, including sites influenced by the Boring Lava Field. No species endemic to the volcanic features have been identified, but protected areas like Powell Butte and act as refugia, supporting stable populations of deer, raptors, and songbirds amid broader regional declines. Northern red-legged frogs (Rana aurora) and northwestern pond turtles (Actinemys marmorata) persist in wetlands but face ongoing threats from fragmentation.

Recreation and conservation

Parks and recreational sites

The Boring Lava Field encompasses several urban parks and natural areas in the Portland metropolitan region, offering public access to its volcanic features through preserved greenspaces. These sites highlight the field's cinder cones, lava flows, and forested buttes, providing opportunities for outdoor engagement amid suburban settings. Park, a 176-acre site on an extinct , features reservoirs from the early and serves as a central recreational hub with paved and unpaved paths circling the summit. Visitors can explore three looped trails—the 1-mile Red Trail, 1.7-mile Green Trail, and 3-mile Blue Trail—offering views of the city's skyline and forested slopes. The park includes picnic areas, a off-leash zone, sports courts, and an outdoor stage, making it suitable for family outings and community gatherings. Powell Butte Nature Park spans 611 acres of meadow and forest on another , with over 9 miles of multi-use trails accommodating hikers, mountain bikers, and equestrians. Trails like the Orchard and Cedar Grove Loop provide access to wildflower meadows and wildlife viewing spots, including for such as red-tailed hawks. The park's expansive size allows for longer loops, such as the 5.5-mile Powell Butte Full Trail System, emphasizing the area's natural isolation within city limits. Tryon Creek State Natural Area covers 658 acres of second-growth forest overlying Boring Lava Field flows, with 8 miles of hiking trails, 3.5 miles of equestrian paths, and a 3-mile paved bike route along the creek. Popular routes like the 3.4-mile Triple Bridge Loop traverse outcrops and riparian habitats, ideal for and nature immersion. The area includes a barrier-free Trillium Trail for inclusive access. Rocky Butte Natural Area, at 19.64 acres, preserves a steep with unpaved paths leading to panoramic vistas of and the . The site's forested trails allow for short geological explorations of exposed volcanic rock, connecting visitors to the field's monogenetic vents. Common activities across these sites include , , and , with informal geological tours possible at Rocky Butte due to its prominent cone features. Collectively, the parks offer dozens of miles of trails, many ADA-compliant with paved sections and accessible parking. Entry is free at city-managed sites like and Powell Butte, drawing urban residents for daily recreation. Recreation in the Boring Lava Field dates to the early 1900s, when buttes like hosted picnics and outings for Portland families, with the park formally acquired in 1909 and landscaped for public use. By the mid-20th century, former quarry sites such as —mined for jail construction in the —were repurposed into protected greenspaces, shifting from industrial extraction to conservation-focused parks after acquisitions in the .

Protection and management

The Boring Lava Field lacks designation as a national monument but receives protection through federal, regional, and local frameworks. Portions of the field, particularly in its eastern extent, fall within the boundaries of , managed by the U.S. Forest Service to preserve natural and geological features. Locally, the City of Portland designates key sites, such as the Boring Lava Domes in the Johnson Creek watershed, as protected environmental resources under its Inventory and Protection Program, originally established in the 1980s and updated through environmental overlay zones that restrict development around volcanic vents to maintain ecological and geological integrity. Management responsibilities are shared among federal and regional entities, with the U.S. Geological Survey's Cascade Volcano Observatory conducting ongoing monitoring of potential volcanic hazards and geological stability across the field. Metro, the regional government for the Portland area, oversees ecological management through its Parks and Nature System Plan, which includes efforts to control invasive species and restore trails in areas like the East Buttes, funded by voter-approved measures in the 2020s that support habitat preservation and biodiversity corridors. These initiatives emphasize geoheritage value, integrating the field's volcanic features into urban planning to enhance educational and scientific access. Challenges in protection arise from the field's location amid rapid urban expansion in the Portland-Vancouver metropolitan area, requiring a balance between development pressures and preservation of volcanic landforms and habitats. Recent post-2020 studies and planning, including Metro's projects and Portland's 2022 Environmental Overlay Zone updates with GIS-based mapping, recommend expanded vent buffer zones to address climate adaptation needs, such as increased risks, while promoting connectivity for .

References

  1. https://www.usgs.gov/publications/boring-volcanic-field-portland-vancouver-area-oregon-and-washington-tectonically
  2. https://pubs.geoscienceworld.org/books/book/1732/chapter/107173614/The-Boring-Volcanic-Field-of-the-Portland-Vancouver
  3. https://www.usgs.gov/observatories/cvo/science/boring-volcanic-field-hills-portland-basin
  4. https://volcano.si.edu/volcano.cfm?vn=322800
  5. https://www.volcanodiscovery.com/boringlava.html
  6. https://fred.stlouisfed.org/series/PORPOP
  7. https://pubs.geoscienceworld.org/gsa/geosphere/article/10/6/1283/132214/40Ar-39Ar-geochronology-paleomagnetism-and
  8. https://www.usgs.gov/faqs/does-portland-oregon-have-a-volcano-within-its-metropolitan-area
  9. https://pubs.geoscienceworld.org/gsa/books/edited-volume/885/chapter/3929513/The-Boring-Volcanic-Field-of-the-Portland
  10. https://www.weather.gov/pqr/elevations
  11. https://pubs.usgs.gov/of/1980/1049/plate-1.pdf
  12. https://weatherspark.com/y/757/Average-Weather-in-Portland-Oregon-United-States-Year-Round
  13. https://www.weather.gov/media/pqr/climate/ClimateBookPortland/pg1.pdf
  14. https://pubs.usgs.gov/pp/1424b/report.pdf
  15. https://www.oregon.gov/owrd/wrdreports/gw_report_3_e_portland.pdf
  16. https://parks.wa.gov/about/news-center/field-guide-blog/battle-ground-lake-state-park-history
  17. https://news.oregonstate.edu/news/new-oregon-climate-assessment-documents-continued-warming-and-signs-adaptation
  18. https://www.researchgate.net/publication/283971314_The_Boring_Volcanic_Field_of_the_Portland-Vancouver_area_Oregon_and_Washington_Tectonically_anomalous_forearc_volcanism_in_an_urban_setting
  19. https://www.geosociety.org/gsatoday/archive/19/9/article/i1052-5173-19-9-4.htm
  20. https://www.researchgate.net/publication/253665749_The_Boring_Volcanic_Field_of_the_Portland_Basin_Diverse_primitive_mafic_magmas_erupted_in_a_frontal_arc_setting
  21. https://ui.adsabs.harvard.edu/abs/2021AGUFM.T11E..06Z/abstract
  22. https://www.frontiersin.org/journals/earth-science/articles/10.3389/feart.2020.535879/full
  23. https://pubs.usgs.gov/pp/1443/report.pdf
  24. https://geoinfo.nmt.edu/publications/periodicals/isochronwest/63/iw_v63_p03.pdf
  25. https://www.vulcanospeleology.org/sym03/ISV3x06.pdf
  26. https://www.usgs.gov/media/images/boring-volcanic-field-map-and-portland-basin-a-hillshade-image
  27. https://pubs.usgs.gov/publication/70157239
  28. https://www.usgs.gov/cascades-volcano-observatory
  29. https://www.usgs.gov/programs/VHP/volcano-updates
  30. https://www.oregon.gov/oem/hazardsprep/pages/volcanoes.aspx
  31. https://multco.us/info/natural-hazards-mitigation-planning
  32. https://www.oregonhistoryproject.org/narratives/commerce-climate-and-community-a-history-of-portland-and-its-people/introduction-3/first-peoples-in-the-portland-basin/
  33. https://www.ci.woodland.wa.us/media/12356
  34. https://www.oregonencyclopedia.org/articles/wappato_valley_villages/
  35. https://lentshistory.com/first-people-of-the-clackamas-and-willamette/
  36. http://www.oregon-archaeology.com/archaeology/
  37. https://www.nps.gov/articles/columbia-river-area-petroglyphs.htm
  38. https://lewisandclarkjournals.unl.edu/item/lc.jrn.1805-11-05
  39. https://www.oregonencyclopedia.org/articles/milwaukie/
  40. https://restoreoregon.org/wp-content/uploads/2013/12/Pioneer-Houses-and-Homesteads_web.pdf
  41. https://www.kgw.com/article/travel/whats-in-a-name/boring-oregon-name-history-boxing/283-65a93b56-4998-4dd7-ac30-e3a2db905744
  42. https://www.opb.org/article/2025/10/01/all-science-no-fiction-northwest-airports-are-sinking-satellite-analysis-shows/
  43. https://trimet.org/publicart/pdf/greenline-i205-cultural-history.pdf
  44. https://www.portland.gov/parks/rocky-butte-natural-area
  45. https://www.oregonencyclopedia.org/articles/rocky-butte/
  46. https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=9100TMA2.TXT
  47. https://www.portland.gov/bps/planning/environ-planning/documents/boring-lava-domes-supplement-johnson-creek-basin-plan-1997/download
  48. https://andrsnflowers.com/guide-to-the-best-flower-growing-regions-for-garden-roses/
  49. https://www.portland.gov/sites/default/files/council-documents/2022/2021-mitigation-action-plan_0.pdf
  50. https://www.portland.gov/sites/default/files/2018-12/Portland_Plant_List_2016_Update_Final2.pdf
  51. https://www.nwcb.wa.gov/weeds/scotch-broom
  52. https://portlandtribune.com/2025/11/13/2021-heat-dome-left-rhode-island-sized-damage-in-oregons-washingtons-western-forests/
  53. https://www.frontiersin.org/journals/forests-and-global-change/articles/10.3389/ffgc.2025.1637821/full
  54. https://www.portland.gov/parks/powell-butte-nature-park
  55. https://www.travelportland.com/attractions/extinct-volcanoes/
  56. http://rubble.blogspot.com/2012/09/more-fun-with-boring-lava-catlin-gabel.html
  57. https://ebird.org/region/L252795
  58. https://ebird.org/region/L447381
  59. https://www.traveloregon.com/things-to-do/destinations/parks-forests-wildlife-areas/mt-tabor-park/
  60. https://www.oregonlive.com/environment/2009/06/oregons_native_amphibians_on_t.html
  61. https://www.jcwc.org/powell-butte-amphibian-survey-discoveries/
  62. https://www.birdallianceoregon.org/blog/northern-spotted-owl-found-on-mt-tabor-died-due-to-rodent-poison/
  63. https://www.portland.gov/parks/mt-tabor-park
  64. https://www.travelportland.com/attractions/mount-tabor-park/
  65. https://www.alltrails.com/parks/us/oregon/powell-butte-nature-park
  66. https://stateparks.oregon.gov/index.cfm?do=park.profile&parkId=103
  67. https://stateparks.oregon.gov/index.cfm?do=main.loadFile&load=_siteFiles%252Fpublications%252F%252FTryon_Creek_brochure_7.17.LOW_RES021523.pdf
  68. https://www.accesstrails.org/overview/powell-butte/
  69. https://www.oregonlive.com/terryrichard/2009/05/go_play_on_a_volcano_mount_tab.html
  70. https://www.portland.gov/council/documents/ordinance/passed/190834
  71. https://www.oregonmetro.gov/sites/default/files/2018/03/23/Parks-and-Nature-System-Plan.pdf
  72. https://www.oregonmetro.gov/news/wildlife-corridors-protect-animals-big-and-small
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