Once again Kilauea volcano’s lava has intruded into human territory in Hawaii. Scientists are trying to understand how and where magma will next appear on Earth’s surface, to better tell people how to avoid this recurring natural hazard. Mapping these hidden pathways is an ongoing project at the Hawaiian Volcano Observatory, where I once worked as a staff volcanologist.
Kilauea consists of a summit caldera, Halema‘uma‘u, from which regions of fractured rock and erupted lava, called radial rift zones, extend to the southwest and east. The volcano also has an ocean-facing south flank that periodically slumps seaward during landslides whose lofty headwalls are called “palis.” The entire Kilauea assembly grew on the south flank of its much larger neighboring volcano, Mauna Loa. If Mauna Loa was not already present, Kilauea would likely have developed a third radial rift zone aimed toward this huge neighbor. The mountain erupts because magma rises buoyantly from Earth’s mantle, like a cork rising through water. Molten rock is less dense than its solid counterpart. Such rock also contains dissolved gases that increase this density contrast, most dramatically near Earth’s surface where they begin to appear as gas bubbles. Think of the frothy top of a glass of your favorite carbonated beverage.
The subsurface plumbing of this region is evolving and often complex. Typically magma rises straight up, more or less, through a container of the volcano’s solid rock. When the magma reaches shallow depths, however, it may find a path of lesser resistance along one of the rift zones, where it may erupt or simply cool to a subsurface solid mass called a dike. A dike may be finger thin or meters thick and is generally oriented as a tall vertical blade. Many of the older, highly eroded volcanoes elsewhere in the Hawaiian chain expose many examples of such dikes.
A few days ago, shortly before ground-breaking fissures began to appear within the Leilani Estates subdivision of homes on the east rift zone of Kilauea, a restless lake of molten lava partly filled the Halema’uma’u pit crater. At the same time, at a place called Pu‘u ‘Ō‘ ō on the east rift zone between Halema‘uma‘u and Leilani, another pool of restless lava was churning slowly. Volcanologist do not know for certain the Halema‘uma‘u and Pu‘u ‘Ō‘ ō lava pools were hydraulically connected, but it seems like a reasonable assumption. Possibly these two pools had been in a stable relationship via this deep connection. But then changing stress within the rift zone east of Pu‘u ‘Ō‘ ō—perhaps a result of many eruptions during the preceding three-plus decades—permitted magma to feed into a new subsurface dike or family of dikes that then quickly propagated toward Leilani. Both magma pools disappeared as shared melt fed eruptions at the new dikes. And of course, when magma wedges its way into new dikes the ground shakes in earthquakes. Two of the quakes to date at Leilani are of magnitudes large enough to hint at a possibility of ground-breaking that is greater than that seen so far in the area.
It is difficult to predict what may happen going forward. If a sufficient volume of eruptible magma is now available within Kilauea, extensive lava flows could bury much of the lower east rift zone and adjacent downslope areas during the coming months. This happened in 1959 when a small village was overrun by new lava—many papaya fields were buried, a graveyard was partly covered with lava and a lighthouse at the shoreline where the east rift zone enters the Pacific Ocean was nearly surrounded by the flow. (The lighthouse itself was spared.)
A rather different scenario: eruptions at Leilani and nearby areas will cease in the near term. If this happens, the new lava will cool and become part of the landscape within weeks or months. A growing array of instruments on the ground, which can make critical measurements of temperature, chemistry and the amount of land bending and flexing caused by intrusive magma, will help scientists form reasonable ideas about which scenario—or something in between—people can expect.