Hualālai

Hualālai (pronounced [huwəˈlaːlɐi] in Hawaiian) is an active volcano on the island of Hawaiʻi in the Hawaiian Islands. It is the westernmost, third-youngest and the third-most active of the five volcanoes that form the island of Hawaiʻi, following Kīlauea and the much larger Mauna Loa. Its peak stands 8,271 feet (2,521 m) above sea level. Hualālai is estimated to have risen above sea level about 300,000 years ago. Despite maintaining a very low level of activity since its last eruption in 1801, and being unusually inactive for the last 2,000 years, Hualālai is still considered active, and is expected to erupt again sometime in the next 100 years. The relative unpreparedness of the residents in the area caused by the lull in activity would worsen an eruption's consequences.

The area near Hualālai has been inhabited for centuries by Hawaiian natives, dating back to before recorded history. The coast to its west in particular had several royal complexes. The volcano is also important ecologically, is home to many rare species and several nature reserves near the summit, and is a popular hiking attraction. Today the coast near Hualālai is dotted by vacation resorts, some built on historic flows, and a National Historical Park.

Hualālai stands at 8,271 ft (2,521 m) with a prominence of 3,071 ft (936 m). It is the westernmost of the five major volcanoes that form the island of Hawaiʻi. Being in the post-shield stage of development, Hualālai is overall much rougher in shape and structure than the more youthful Mauna Loa and Kīlauea. Hualālai's structure is denoted by three rift zones: a well-developed one approximately 50° to the northwest, a moderately developed one to the southeast, and a poorly developed one trending northward about 3 mi (5 km) east of the summit. Over 100 cinder and spatter cones are arranged along these rift zones. Hualālai has no summit caldera, although there is a collapse crater about 0.3 mi (0.48 km) across atop a small lava shield. Much of the southern slope (above the modern town of Kailua-Kona) consists of lava flows covered by a layer of volcanic ash from 10 to 100 cm (4 to 39 in) thick. Of the volcanoes on the island, it is the third-tallest, third-youngest, third-most active, and second-smallest, making up just 7% of the island.

A major subfeature of Hualālai is Puʻu Waʻawaʻa, Hawaiian for "many-furrowed hill", a volcanic cone standing 372 m (1,220 ft) tall and measuring over 1.6 km (1 mi) in diameter. It extends for 9 km (6 mi), and has a prominence of 275 m (902 ft), north of the summit at 19°46′15″N 155°49′56″W / 19.77083°N 155.83222°W / 19.77083; -155.83222 (Puu Waa Waa). The cone is constructed of trachyte, a type of volcanic lava not found on other volcanoes on the island. Trachyte flows move more slowly than the typically "runny" Hawaiian lavas due to its high (over 62%) silica composition (typical basalt is only 50% silica). Geologists hypothesize that Puʻu Waʻawaʻa originally formed during a pumice eruption a little over 100,000 years ago, and has continued to build since then, with at least three distinct trachyte flows recognized. The eruptions, although partially covered by flows from Hualālai and Mauna Loa, have built a distinctive structure known as the Puʻu Anahulu ridge.

Hualālai's westward-facing flank forms a large underwater slump known as the North Kona slump. An area of about 1,000 km² (390 sq mi), the slump consists of an intricate formation of beaches and scarps 2,000 to 4,500 m (6,600 to 14,800 ft) below the waterline. This area was explored more closely in a 2001 joint Japan-United States project to explore the volcano's flanks, utilizing the remotely operated vehicle ROV Kaikō. Data collected showed that the lava flows there originated in shallow water 500 to 1,000 m (1,600 to 3,300 ft) deep, and that unlike similar slumps at other volcanoes, the slump at Hualālai formed gradually.

Hualālai is a known source for xenoliths, rock from the Earth's mantle that have been brought up in lava flows. Many prehistoric deposits, as well as those from the 1801 event, contain xenoliths of large size and abundant quantity.

Lava attributed to a shield-stage Hualālai has been found just offshore of its northwest rift zone. Tholeiitic basalt, indicative of the submarine subphase of the volcano's construction, has been found in wells driven into the volcano at a depth of 75 ft (23 m). These lavas persisted until an estimated 130,000 years ago. Hualālai entered the post-shield stage, the stage it is now in, about 100,000 years ago. Pumice and trachyte eruptions at Puʻu Waʻawaʻa may be a sign of this change.

Geological mapping of Hualālai indicates that as much as 80% of its surface has been topped by lava flows during the last 5,000 years, entirely composed of shield alkalic basalt. More than half of this is under 3,000 years old, and about 12% is less than 1,000 years old. Between 1700 and 2016, eruptions originated from six vents; four of these lava flows poured into the sea to the west coast.