Volcano Watch — Why are Mount St. Helens' eruptions generally more explosive than those of Kīlauea?

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Residents of the Big Island have been living with the nearly continuous eruption of Kīlauea for a bit over 20 years. They have become familiar with its eruptive style-quiet eruptions of lava that cover large areas with black lava flows, often miles away from the source vent--sometimes even reaching the ocean.

Close up view of the eruption column of Mount St. Helens

Close view of the eruption column of Mount St. Helens on May 18, 1980; the volcano is visible in the bottom of the photograph.

(Public domain.)

The flows consume the forest and occasionally engulf roads and houses, but they can be safely witnessed at close range, a feature that attracts thousands of visitors to the Island each year.

In contrast, many people remember the big eruption of Mount St. Helens in 1980 and know that a lot of people, most of whom were amateur volcano-watchers-were killed in an enormous explosion. Why was the 1980 Mount St. Helens eruption so violently explosive while the current Kīlauea eruption is so sedate?

The answer lies in the chemical compositions of the magmas produced by the two volcanoes. The basalt magma erupted by Kīlauea contains about 52% silica and about 0.5 % water while the dacite lava erupted by Mount St. Helens in 1980 contained more of both: about 64% silica and about 4% water. Silica is a general term for silicon dioxide (SiO2), which occurs in a number of different forms and is the most abundant chemical constituent of the earth's crust. Most people have heard of quartz-one of the crystalline forms of silica. You can find quartz at the jewelry store, as rock crystal-colorless quartz-or as amethyst, a purple or bluish-purple variety. Here we are concerned with the silica that is dissolved in the liquid part of magma.

Magma-the name for lava before it erupts--is strongly influenced by its viscosity. Viscosity is the resistance to flow in a liquid. A liquid is said to have a high viscosity if it flows slowly and a low viscosity if it flows freely. Everyday examples are water (low viscosity) and hot tar (high viscosity).

OK, so what does viscosity have to do with silica? The silica dissolved in the liquid part of magma tends to link together into molecular chains-the more silica, the more chains. These chains get tangled up with one another--like spaghetti--and increase the resistance to flow. So, increasing the silica content of the magma liquid increases the lava's viscosity.

As magma rises, the pressure on it progressively drops until eventually the water dissolved in the liquid begins to form bubbles of steam. The way these bubbles separate from the magma and escape to the atmosphere determines whether or not explosions will occur. In low-viscosity magma, gas bubbles float upward easily, like bubbles in beer, expanding as the surrounding pressure drops, arriving at the surface at atmospheric pressure, and escaping into the atmosphere with little fanfare. In high-viscosity magma, bubbles also form, but they can't float upward because the resistance to flow is too great. Furthermore, the resistance to flow is so great that it takes time for the bubbles to respond to the pressure decrease as the magma rises. The bubbles arrive at the surface trapped in the magma at a pressure greater than the atmospheric pressure. This pressure difference is a recipe for an explosion: as the high-pressure gasses in the bubbles expand in response to the lower atmospheric pressure, they rip the magma into countless tiny fragments of volcanic ash.

Mount St. Helens' magma is inherently more explosive than the Kīlauea magma: it has more water in it than Kīlauea magma, and is delivered to the surface at a higher pressure because of higher magma viscosity. So Mount St. Helens tends to have explosive eruptions and Kīlaueaa eruptions are generally non-explosive. Kīlauea does have explosions, but they require special circumstances, such as magma coming into contact with ground water or sudden release of pressurized gas deep with the volcano.

Volcano Activity Update

Non-explosive eruptive activity at the Pu`u `O`o vent of Kīlauea Volcano continued unabated during the past week. Scattered surface breakouts from the western "Kohola" lobe of the Mother's Day flow are seen throughout the inflating flow. A finger of lava from the eastern side of the Kohola flow made it over the sea cliff and trickled onto the Wilipe`a delta before stopping. Two surface flows from breakouts of the Mother's Day tube system are visible on Pulama pali down to Paliuli. From the diminishing size of the steam plume, the volume of lava entering the ocean at the West Highcastle delta seems to be decreasing.

One earthquake was reported felt during the past week. A resident of Leilani Estates subdivision felt an earthquake at 8:59 p.m. on March 17. The magnitude-2.0 event was located 1 km (0.6 mi) northeast of Pu`ulena Crater at a depth of 4 km (2.4 mi).

Mauna Loa is not erupting. The summit region continues to inflate, and the rate of inflation has increased slightly during the past month. The seismic activity is low, with no earthquakes located in the summit area during the last seven days.