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Volcano Watch: Spatter erupted from the heart of the Koa‘e fault system at Kilauea

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Time-lapse movie of Pu‘u ‘O‘o crater.

(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

Seldom does one find evidence of an eruption where none was known before. We generally assume that Kilauea erupts only at the summit or along its two rift zones. That view has to change, if only slightly, because of a discovery made on June 22.

In early June, a small earthquake swarm shook the west-central part of the Koa`e fault system, which connects the east and southwest rift zones south of Kilauea’s summit. The Hilina Pali Road runs along the fault system from the Chain of Craters Road to Kulanaokuaiki Campground, where the road bends across a pali, escapes the fault system, and heads for Kipukanene.

The earthquakes, centered 1-2 km west of the campground, were small, and no one examined the ground for new cracks until a marvelous InSAR image became available on June 21. This radar image, obtained from space, showed that significant ground deformation took place during the earthquakes.

On June 22, an HVO geologist examined the area on the ground and found many small new cracks. This was exciting, but the big news came when, at 11:15 a.m. and totally unexpectedly, he found tiny bits of spatter lying next to an older, pre-2012 crack. On July 5, similar material was found 700 m farther east along the same crack system.

Spatter is ejected during eruption, but no eruption was known in the fault system—until now. There’s not much spatter but enough to tell that it was spit from a wide crack and mostly blown a few tens of meters southward by the trades.

The eruption was tiny, to judge from the two very limited areas in which spatter fell. But an eruption is an eruption, and this one tells us that a dike of magma must have been injected underground into the fault system between Kilauea’s rift zones.

How old is the newly discovered spatter? It did not erupt during the early June earthquake swarm; it is too discolored and otherwise modified to be so young. It was erupted within the past 220 years, because it fell onto volcanic ash formed in 1790. Chemical tests are underway to compare it with lava erupted at known times since 1790. We hope to find some resemblance that will help date the eruption.

Magma intrusion into the Koa`e had been previously suggested. Earthquakes in 1973, and probably also in 1965, propagated westward into the fault system from an eruption site on the east rift zone, as if magma were moving underground. Also in 1973, surveying at three places across the fault system showed that the ground was arched up along what was thought to be a dike. Two geophysical surveys likewise suggested that magma intruded a short distance into the Koa`e in 1973.

These interpretations were all reasonable, but no smoking gun—volcanic ejecta—had been found. It still hasn’t been found for the 1973 and 1965 events, but now we know that magma did intrude and erupt in the fault system at least once since 1790. This makes it likely that the interpretations of magma intrusion in 1973 (and probably in 1965) are correct.

The recognition of dikes in the fault system helps explain two puzzling observations. Given all the open cracks in the Koa`e, why isn’t the earth’s gravitational field warped? With less rock mass, gravity should be weaker than elsewhere. But if many of the cracks are filled with solidified magma at shallow depth, there would be little mass deficit and no gravity anomaly.

The fault system is a barrier to seaward groundwater flow. With open cracks, one would think the reverse would be true. Dikes in the Koa`e, filling cracks underground, might form natural barriers to water flow, as happens elsewhere in Hawai`i.

The emerging picture is that magma can intrude and occasionally erupt in the Koa`e fault system during eruptions at adjacent parts of the east rift zone, between Hi`iaka and Mauna Ulu. Can magma in the southwest rift zone also intrude the fault system? Does there even have to be an accompanying eruption in a rift zone?

The two spatter discoveries have triggered new questions about the Koa`e fault system, an important but often overlooked part of Kilauea.

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