Volcano Watch — Volcanoes of the Big Island

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The Island of Hawai'i consists of five coalesced volcanoes, a submarine volcano that has already subsided below sea level, and another, Lō‘ihi Seamount, that has yet to grow to sea level.
 

The Island of Hawai'i consists of five coalesced volcanoes, a submarine volcano that has already subsided below sea level, and another, Lō‘ihi Seamount, that has yet to grow to sea level.

The following is a brief history of each of these volcanoes, starting with the youngest and progressing to the oldest.

Lō‘ihi, which is in transition between preshield and shield stages, has infrequent small eruptions, and has had small earthquake swarms nearly every year since 1980. The flat-topped summit, about 3,180 feet below sea level, is connected to a well-developed south rift zone and a less pronounced north rift zone. There are two deep collapse pits, similar in size to Halema'uma'u at Kīlauea's summit, located in the southern part of the summit region. Hot water escapes from vents near its summit and along its upper south rift zone.

These observations, and the recovery of fresh, glassy lava fragments, indicate that Lō‘ihi is an active volcano destined to emerge as an island within the next 200,000 years, grow, and coalesce with Hawaii.

Kīlauea Volcano, in the explosive substage of the shield stage, and the most active volcano on Earth, has erupted 60 times since 1840. Its lava flows are interbedded with ash deposits produced by infrequent explosive eruptions, most recently in 1790. Eruptions can occur anywhere at the summit or along the east or southwest rift zones.

The south flank of the volcano, bounded by the two rift zones, slips towards the ocean at rates of a few inches per year on a flat-lying fault about 6 miles deep. Large earthquakes, such as those that occurred in 1975 and 1989, are associated with large-scale movement along this flat-lying fault.

Mauna Loa Volcano, nearing the end of the shield stage, is declining in its eruption rate. Only three of its 36 eruptions since 1843 have occurred since 1950. In addition to the two prominent rift zones, repeated fissureeruptions have occurred randomly on the northern and northwestward flank of the volcano. In order to grow to its present estimated volume of roughly 10,000 cubic miles, Mauna Loa must have erupted, for much of its lifespan, at four or five times its historic rates.

Like Kīlauea, the southeastern flank of Mauna Loa slips slowly towards the ocean on a flat-lying fault that generates large earthquakes. The west flank also slips during large earthquakes. The flanks of Mauna Loa have spawned at least six catastrophic landslides that can be recognized as blocky debris on the sea floor adjacent to the island. The Alika slide off the west flank of Mauna Loa Volcano is inferred to have formed about 105 ka (thousand years before present); it appears to be the youngest of the large landslides around the Hawaiian Islands.

Hualālai Volcano, an active volcano in the postshield stage, last erupted in A.D. 1800-1801, once about 700 years ago, and three times between 900 and 1,200 years ago. It erupts every few hundred years, on average; the flows of 'a'a and pahoehoe advance quickly due to its steep slopes. The final summit caldera is buried, and spatter and cinder cones align along well-developed northwest and south rift zones.

At about 105 ka, the postshield-stage pumice cone Pu'u Wa'awa'a, and its associated 900-foot-thick flow at Pu'u Anahulu are the oldest exposed lavas on Hualālai. The youngest shield lavas are about 128 ka. Hualālai is a potentially dangerous volcano that is likely to erupt in the next 100 years.

Mauna Kea, a dormant volcano in its postshield stage, last erupted about 4,500 years ago. Lava flows and cinder cones have buried the final summit caldera. Although a few flows have funneled down stream beds and reached the coast, its youngest lavas are thick and pasty and formed large cinder cones and short flows. Its oldest exposed lavas are about 250,000 years old.

Many Mauna Kea eruptions were explosive and produced widespread ash deposits. These ash-producing eruptions may have been triggered by water/rock interaction as lava encountered glaciers at the summit of Mauna Kea. Three periods when mountain glaciers covered the summit region have been identified. The thick ash cover on its flanks is derived mostly from Mauna Kea eruptions, but perhaps three feet of it may be from explosive eruptions on Kīlauea about 39,000 years ago.

Mauna Kea could erupt again, although it is unlikely, because postshield-stage eruptions become less and less frequent before they cease altogether.

Kohala Volcano is extinct; it consists of shield lavas, including the oldest lavas from the island of Hawaii, dated at about 460,000 years old, and postshield-stage lavas as young as 60,000 years old. Its northwest rift zone extends through an elongate summit depression, apparently an extensional basin at the headwall of the Pololu landslide that moved towards the northeast. Waipi'o and Pololu valleys have formed along faults that bounded this landslide, which occurred before eruption of postshield-stage lavas began about 260,000 years ago. The thick ash cover on Kohala Volcano is derived mostly from Mauna Kea, although some is probably of local origin.

Mahukona, the first volcano to form part of the island of Hawaii, is located off the northwest shore of the island; it is now completely submerged. Mahukona has a prominent west rift zone, but the location of its summit and any other rift zones are now buried beneath flows from the adjacent Kohala and Hualālai Volcanoes. It apparently became extinct at the transition between the shield and postshield stages about 450,000 years ago.

A stairstep series of drowned coral reefs occur on the northwest- and southwest-facing slopes of the volcano. These reefs mark former shorelines and record the subsidence of the island from about 465,000 years ago until the present. At its zenith, Mahukona probably rose no more than 820 feet above sea level; it was completely submerged shortly after 435,000 years ago.

The Island of Hawaii will change shape in the future as Kīlauea extends its southern and eastern shorelines and as Lō‘ihi grows to sea level and eventually coalesces with the island. At the same time, however, continued subsidence will reduce the size of Kohala, Mauna Kea, Hualālai, and Mauna Loa Volcanoes.

At the present rate of subsidence of about 1/8 inch per year, Kohala will become a separate island about 350,000 years from now. Between 600,000 and 850,000 years from now, Hualālai and Mauna Kea will become separate islands; exactly when that time will come cannot be estimated without knowing when Mauna Loa will stop erupting.