Studying volcanic ash flows

In response to the reawakening of Anatahan Volcano on May 10, seismologists installed a seismometer on the island on May 20. They returned on June 6 to replace a faulty battery. While servicing the electronic gear, they noticed something rather odd: some plastic one-gallon water bottles left there had been melted. The area around the bottles and the seismic gear was covered with a thin layer of fresh volcanic ash-clearly deposited there by the recent eruptions. Apparently the ash was hot enough to affect the plastic bottles but wasn’t hot enough to melt the more durable seismic gear.

How did the ash get there? The ash that melted the bottles couldn’t have fallen from the sky, because plants more than a meter (3 feet) high were unaffected even though vegetation close to the ground was scorched. The hot ash must have moved — flowed — along close to the ground surface. This behavior may sound improbable or even impossible, but actually it’s a common feature of explosive volcanism.

When magma rises from deep within the earth and approaches the surface, the reduced pressure causes dissolved gases to come out of solution and form bubbles. If the viscosity of the magma is sufficiently great — as is usually true of volcanoes that erupt silica-rich lava — the gases will escape violently, ripping the magma into countless tiny fragments of volcanic ash.

The mixture of hot gases and ash is propelled upward into the atmosphere as an eruption plume. If the proportion of gas to ash is relatively high, the density of the mixture is less than the surrounding air, so the plume rises like a hot-air balloon. Winds blow the ash cloud away from the volcano, and ash rains more-or-less vertically onto the countryside over a broad area.

Ash transported in this manner usually loses most of its heat before it reaches the ground and rarely causes death or injury directly. However, if the depth of the ash-fall deposit becomes great enough, its weight can cause buildings to collapse, especially if the ash is wet. There were many fatalities due to this process during the 1991 eruption of Mount Pinatubo in the Philippines.

If the proportion of gas to ash in the eruption plume is relatively low, the mixture is denser than air and acts somewhat like water in a vertical fountain: the mixture rises to some maximum height, collapses back onto the ground, and flows outward. The ground-hugging ash-gas mixture behaves as a high-temperature, high-velocity fluid.

Volcanologists have coined many terms for such flows, but the most common is “pyroclastic flow” if the gas content is low or “pyroclastic surge” if the gas content is high. Temperatures range upward to 600 degrees Celsius (1,100 degrees Fahrenheit) or more, and speeds in excess of 100 meters/sec (225 mile/hour) have been recorded. The areas affected range upward to more than 1,000 square kilometers (400 square miles).

Given these statistics, it’s not surprising that pyroclastic flows and surges are among the most hazardous volcanic phenomena. In 1902 an eruption of Mount Pelee on the island of Martinique in the Caribbean produced pyroclastic surges that killed virtually the entire population of St. Pierre — about 28,000 people — in a matter of minutes. The pyroclastic surge that melted the plastic bottles on Anatahan was at the extreme low end of the size scale, yet it probably would have caused severe injury or death if people had been present.

The most recent Hawaiian examples are pyroclastic surges erupted from Kilauea caldera in 1790.

Activity Update

Effusive eruptive activity at the Pu`u `O`o vent of Kilauea Volcano continued unabated during the past week. Lava is cascading down Paliuli and ponding at the base near the Kohola arm of the Mother’s Day flow. Scattered surface flows are visible on Pulama pali from the east and west lobes of the main Mother’s Day flow. Lava stopped entering the ocean at the Highcastle delta last week, and there is no ocean entry at this time.

One earthquake was reported felt on the island during the past week. Residents of Ka`u, Holualoa, and Ahualoa felt the earth shake at 8:08 p.m. on Sunday, July 13. The magnitude-3.3 earthquake was located 16 km (9.6 mi) southeast of Na`alehu at a depth of 35 km (21 mi).

Mauna Loa is not erupting. The summit region continues to inflate slowly. Seismic activity remains low, with only one earthquake located in the summit area during the last seven days.

This article was written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory and is republished by with permission.

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