Volcano Watch — Inactive benches quickly become stable

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Many streams of lava entering the ocean are shattered to bits in the surf zone. These loose fragments gradually build layers of rubble on the steep submarine slope. The resulting nearshore submarine deposits are complexly interbedded lava flows and fragments ranging in size from boulders to tiny shards of glass and minerals.

As the deposits build above sea level, they are capped by lava flows, at least at the surface. Superficially solid, this unstable edge, which we call a delta or, more often, a bench, commonly slips beneath the waves by slope collapse and landsliding.

Because all of the Big Island's dry land was built on bench deposits, benches must become stable, such that they don't continue sliding into the sea. Our surveys of several inactive benches indicate they become stable soon after lava stops flowing onto them.

When does a bench become stable? Surveys in 1993 showed that new land seaward of the pre-eruption coastline subsided gradually at the rate of a few centimeters (an inch or two) per month in the year after its emplacement. Another bench that recently became inactive, the East Kupapa`u bench near the end of Hawai`i County's access road, has been subsiding about 1.8 cm (0.7 inches) per month, according to surveys made since January 2002.

Benches cease to subside sometime between two and five years after they stop growing. We have surveyed the 1995 benches that lie near the current ocean entry periodically since summer 2000. These surveys show that subsidence is trifling, less than 1-2 millimeters (0.05 inches) per year, a range at about the limit of our ability to measure subsidence along the coast.

How do the benches become stable? The three most likely processes are cooling, compaction, and cementation. Increased stability for many benches probably is initiated as soon as the supply of active lava ends. When a bench is still active, molten lava can invade existing cracks and exert pressure on them, thereby decreasing the shear strength of the bench. This destabilizing force ends when the lava supply is severed. Liquid lava has very low shear strength, and its mere presence in the bench creates zones of instability. Deprived of its lava supply, the bench material is able to cool to earth-surface temperatures and increase its strength. An important benefit is the end of explosions that occur intermittently when hot lava interacts with the sea. Explosions, unsettling as they are for visitors, can really give a bench a bad day. They jostle the fragmental debris, which may then start the process of bench collapse. More often, though, explosions accompany collapse, rather than precede them.

Compaction occurs as the weight of the lava fragments and lava flows within the bench causes the material to settle more closely together. Seawater that permeates the deposits is forced out by compaction. These changes increase the area of grain-to-grain contact, which increases friction and adds strength to the deposits. Compaction is an ongoing process that begins even while the bench is being fed with new lava flows and may be a major cause of the subsidence found by our surveys on inactive benches.

Another process that helps to make benches more stable is cementing, which results when naturally occurring compounds are deposited between the grains of a bench, making it relatively solid. These cements are commonly iron sulfates and calcium carbonate minerals. Dewatering aids cementation as the newly deposited minerals close up the remaining space between grains.

Clays also form cementing agents. Volcanic glass is abundant in the fragments that form a bench. The glass forms because the lava chills quickly in the ocean setting. Glass breaks down to form more stable compounds, including clay minerals, that reduce pore space and further stabilize the bench.

The life history of a bench thus proceeds from unstable to stable. Relative stability probably begins soon after a bench becomes inactive. Sometime between age two and five, a bench ceases subsiding almost entirely. The process of cementation finishes the conversion from grit to rock as all the fragments become bonded together. Thereafter the bench will probably become a long-lived part of the Big Island's south coastline.

Volcano Activity Update

Eruptive activity of Kilauea Volcano continued unabated at the Pu`u `O`o vent during the past week. Molten lava is flowing near the end of the Chain of Craters road, and the National Park Service is allowing visitors to get up close to the action. Two lava deltas, Wilipe`a and West Highcastle, are growing with multiple ocean entries from each. The instability of the new lands is evident from the frequent collapses and explosions observed along their seaward edge.

Three earthquakes were reported felt during the week ending on October 3. A resident of Pahala was shaken by an earthquake at 4:37 p.m. on Friday, September 27. The magnitude-2.4 earthquake was located 7 km (4.2 mi) west of Pahala at a depth of 4.9 km (2.9 mi). An earthquake was felt in Hawai`i Volcanoes National Park at 5:37 p.m. on September 28. The magnitude-2.9 temblor was located 6 km (3.6 mi) north of Ka`ena Point at a depth of 8.9 km (5.3 mi). Two residents of Hilo felt an earthquake at 7:38 p.m. on September 29. The magnitude-2.7 earthquake was located 3 km (1.8 mi) north of Volcano at a depth of 30.6 km (18.4 mi).

As a follow-up to last week's Volcano Watch article, we will now include a weekly update on the inflation and the earthquake activity of Mauna Loa Volcano. Mauna Loa is not erupting. GPS measurements show that the distance across the summit caldera, Moku`aweoweo, is lengthening, and the ground is uplifting. We interpret the lengthening and uplifting to indicate swelling of the magma reservoir within the volcano.

Based on the present detection capabilities of the seismic network, scaling of the weekly earthquake activity in the summit region will be: 0-20 earthquakes, low; 21-40 earthquakes, moderate; 41-60 earthquakes, moderately high; more than 60 earthquakes, high. During the past week, seismicity was low. Five earthquakes were catalogued with the largest having a magnitude of 2.2.