Nonexplosive Calderas: How Kīlauea and Mauna Loa’s Summits Sink Without Blowing Up

Some calderas form without an explosion. At the summit of a large shield volcano, such as Kīlauea or Mauna Loa, a caldera forms when magma drains out of a shallow chamber, often to feed eruptions on the volcano's rift zones, and the summit, left unsupported, collapses. These nonexplosive calderas are steep-sided, often flat-floored where lava flows or lava lakes have filled them, and may hold smaller pit craters. They can also form at other long-lived basaltic centers.

Kīlauea’s caldera and the lava lake in Halemaʻumaʻu, 6 January 2021. U.S. Geological Survey.

Kīlauea

The caldera measures about 2.6 by 2.0 miles (4.2 by 3.2 km). The current one collapsed about 500 years ago; many came before it, the last about 2,200 years ago, later filled by lava.

In 2018, during Kīlauea's largest eruption in about 200 years, an eruption on the lower East Rift Zone partly drained the summit magma chamber and the caldera floor collapsed. The lava lake in Halemaʻumaʻu drained away, and the pit crater doubled in width and became nearly ten times deeper.

Kīlauea’s summit before the 2018 collapse… U.S. Geological Survey.

…and after. U.S. Geological Survey.

More: the 2018 eruption and summit collapse, the 2020–2021 summit eruption and three big changes in three years at Halemaʻumaʻu.

Mauna Loa

Mokuʻāweoweo, Mauna Loa's caldera, is about 4 miles (6.2 km) long and 1.5 miles (2.5 km) wide, three oval depressions above 12,000 ft. Earlier calderas were filled in by later eruptions; this one formed 1,000 to 1,500 years ago, during a large eruption on the northeast rift zone that produced the Panaʻewa lava flow, covering 66 square miles and more than 33 ft thick in places.

Mokuʻāweoweo in 2009. U.S. Geological Survey.

Elsewhere in the parks

• Bering Land Bridge (Alaska): the Twin Calderas, two small ones formed 800,000 to 900,000 years ago at a long-lived basaltic center, 1,700 and 2,500 ft across and 50 to 120 ft deep.
• National Park of American Samoa: the islands are eroded Pleistocene shield volcanoes, and Pago Pago Harbor, on the park's boundary, sits in the caldera of the Pago shield volcano on Tutuila, which erupted about 1.27 million years ago.
• Wrangell-St. Elias (Alaska): several andesite shield volcanoes have nonexplosive calderas, among them the ice-filled caldera of Mount Wrangell.

The Twin Calderas, Bering Land Bridge. National Park Service.

Pago Pago Harbor, in the caldera of the Pago shield volcano. National Park Service.

Mount Wrangell’s ice-filled caldera, 1987. U.S. Geological Survey.

What erupts, and what you find

Lava is typically basalt to andesite, as on the shield volcanoes themselves. Eruptions are mostly effusive, VEI 0–1, in the Hawaiian style: fluid lava flows and sometimes fire fountains.

• Pit craters: small collapse craters inside the caldera, such as Halemaʻumaʻu.
• Lava lakes: molten or solidified, where the vent stays open to the magma chamber below.
• Fumaroles: where heat and volcanic or rainwater-derived gases escape, and wherever molten lava is present.

Halemaʻumaʻu with an active lava lake, 13 December 2021. U.S. Geological Survey.

Thermal map of Kīlauea’s caldera, 8 April 2019, nearly a year after the collapse: fumaroles in Halemaʻumaʻu and along the collapse fractures. U.S. Geological Survey.

Hazards

Vog, volcanic smog of sulfur dioxide gas and aerosols, affects people nearby and downwind. Rockfalls, subsidence and fumaroles are hazards too (geohazard guidance).

Sources

• National Park Service, "Nonexplosive Calderas"; rewritten in hubnx's own words. Photographs and images: National Park Service and U.S. Geological Survey.