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September 24, 1998

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

Ongoing Subsidence at Kilauea Volcano

Kilauea Volcano's summit has been slowly subsiding since 1983, the year that the ongoing Pu`u O`o-Kupaianaha eruption started. This broad and gentle cone-shaped downwarping of the summit is centered about 1,500 m (one mile) south of Halema`uma`u Crater, where the ground's surface is now 1.3 m (4.3 ft) lower than it was in 1983.

The summit's subterranean magma chamber is the source of the lava currently being erupted from the volcano. Magma rises up from a hot spot deep in the Earth's mantle below Hawai`i and fills the shallow magma chamber, causing the ground's surface to bulge upward and outward. When magma leaves the reservoir to feed an eruption or dike intrusion, the ground above it moves downward and inward.

At present, magma is being supplied to the chamber from the hot spot, and it is also being withdrawn to feed the eruption. The observed subsidence of the summit implies that more magma is being withdrawn than is being supplied. The amount of subsidence can be converted to a volume change to estimate the difference between the eruption and supply rates.

In order for the summit to regain the same shape it had in 1983, it would take about 35 million cubic meters (46 million cubic yards) of material to fill the downwarping. We approximate this subsidence volume by finding the volume of a cone with a radius of 5 km (3 miles) and a height of 1.3 m (4.3 ft).

It turns out, in fact, that the subsidence volume overestimates the volume loss needed in a buried magma chamber by about 50 per cent. This is because the Earth's crust is compressible. Some of the subsidence is from shrinking of the magma chamber and some from the stretching of the surrounding rock. Magma withdrawal needed to produce the observed subsidence is, then, about 23 million cubic meters (30 million cubic yards). This is only about one percent of the lava erupted since 1983.

In addition to subsiding, Kilauea's summit has also stretched. Its southeastern side has moved seaward about 0.5 m (1.6 ft) since 1983. The stretching can account for part of the subsidence, which can be likened to the thinning of taffy when it is stretched. This process will further reduce the estimated imbalance between magma supply and eruption rates.

On average, the summit's magma supply during the current eruption has been balanced by the eruption of lava from vents in the east rift zone. The observed summit subsidence may have resulted from an eruption rate that is only one per cent greater than the magma supply rate. This balance is an important factor in making the current eruption the longest rift zone eruption in the recorded history of Kilauea Volcano.

Eruption and Earthquake Update

There was no change in the eruptive activity at the Pu`u `O`o vent during the past week. Lava continues to flow through a network of tubes from the vent to the sea coast and enters the ocean in two locations near Kamokuna. The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying frequent collapses of the lava delta. The steam clouds are highly acidic and laced with glass particles.

Two earthquakes originating from a shallow depth near Pu`ulenaCrater in lower Puna were reported felt by a resident of Leilani Estates subdivision. Both earthquakes were less than 2 in magnitude. The earthquakes occurred at 4:37 a.m. and 4:54 a.m. on the morning of September 19.

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Updated: 2 Oct 1998