Volcano Watch — New insights from old ash

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Many people remember that Mount St. Helens erupted with terrible ferocity on May 18, 1980, after a long dormant period. In fact, this is the date now used to mark the reawakening of the volcano. Yet it began to stir well before its activity reached a crescendo on May 18.

The first sign of life was a gradual increase in the number of earthquakes near the volcano in mid-March. On March 25, the earthquake rate increased to the point that nearby seismometers were saturated-individual earthquakes could not be distinguished. This continued for two days, at which time the first small explosion occurred, punching a hole through the summit glacier and spreading dark-gray ash on its surface. Numerous other eruptions of dark ash preceded the cataclysmic eruption of May 18. Many of these were larger than the first one on March 27, but even the largest was inconsequential in comparison to the May 18 eruption and many post-May 18 eruptions.

The small, early eruptions are important because, before May 18, volcanologists were struggling to understand exactly what was happening within the volcano and what the likely course of the eruption might be. Small, explosive eruptions commonly occur when long-dormant volcanoes reawaken. Sometimes they give way to large eruptions of new lava (called juvenile lava) and sometimes they simply fade away into another repose interval. The challenge is to figure out which of these two outcomes is more likely.

The small eruptions may be driven by steam formed when rising magma encounters ground water within the volcano, by steam from a long-established hydrothermal system, or by gases coming directly from rising magma. The chance of a large eruption is much greater if the small, early eruptions are being driven by magma rising within the volcano.

The color of the early Mount St. Helens eruption plumes-dark gray or black-suggested that they were driven by relatively low-temperature steam, because wet ash is usually dark. So the possibility of small eruptions driven directly from newly risen magma could be at least tentatively excluded. To test the possibility that the steam was generated by rising magma boiling off groundwater, volcanologists collected ash samples from the volcano's summit and flanks and examined it for the presence of new (juvenile) lava. They assumed that any juvenile material would be fresh, glassy pumice. They didn't find any pumice-only old dense rock fragments. So there was no direct evidence that magma was driving the volcano's restlessness. Yet, the May 18 eruption showed that magma had indeed risen within the volcano and was quite near the surface. Consequently the pre-May 18 eruptions were almost certainly driven by magma-groundwater interaction.

How could magma have risen so close to the surface and driven the small eruptions without incorporating some juvenile material into the resultant ash? This question has puzzled volcanologists for more than two decades. The answer came from reexamination of old ash samples that were collected before the great eruption of May 18 and have been languishing in a warehouse ever since. The reexamination showed that juvenile material IS actually present in the old samples, but not in the form of glassy pumice. Instead, the juvenile material is dense, dark-colored lava easily mistaken for the older lava fragments that compose the majority of the ash. It turns out that the first magma to fight its way upward lost its gases before it reached the surface. So instead of forming frothy pumice, it formed dense lava. This small insight will help volcanologists diagnose the next reawakening volcano.

Volcano Activity Update

Eruptive activity of Kilauea Volcano continued unabated at the Pu`u `O`o vent during the past week. Lava flows through a tube system from the vent to the sea. Two breakouts from the tube system above Pulama pali feed surface flows that are igniting forest fires in the area. Lava continues to enter the ocean at the Wilipe`a and West Highcastle lava deltas. The broad eastern arm of the Mother's Day flow entered the ocean in three locations: Highcastle, Lae`apuki, and midway between the two. The Highcastle entry is only intermittently active. The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying sudden collapses of the new land. The steam clouds are highly acidic and laced with glass particles. The National Park Service has erected a rope barricade to delineate the edge of the restricted area. Do not venture beyond this rope boundary and onto the lava deltas and benches.

One earthquake was felt in the week ending on November 21. A resident of Hilo was shaken at 9:58 p.m. on November 20. The magnitude-2.6 temblor was located 12 km (7.2 mi) east of Pohakuloa at a depth of 25 km (15 mi).

Mauna Loa is not erupting. The summit region continues to inflate. The earthquake activity is low with only 1 earthquake located in the summit area during the last seven days.