June 26, 2009
On June 12, as the International Space Station passed over the Kuril Islands northeast of Japan, an astronaut looked out the window and saw Sarychev Peak, on Matua Island, in the early stage of an eruption, and he took some photos with a digital camera.
This detailed astronaut photograph is exciting to volcanologists because it captures several phenomena that occur during the earliest stages of an explosive volcanic eruption. The main column is one of a series of plumes that rose above Matua Island on June 12. The plume appears to be a combination of brown ash and white steam. The vigorously rising plume gives the steam a bubble-like appearance. The eruption cleared a circle in the cloud deck. The clearing may result from the shockwave from the eruption or from sinking air around the eruption plume: as the plume rises, air flows down around the sides like water flowing off the back of a surfacing dolphin. As air sinks, it tends to warm and expand; clouds in the air evaporate.
In contrast, the smooth white cloud on top may be water condensation that resulted from rapid rising and cooling of the air mass above the ash column. This cloud, which meteorologists call a pileus cloud, is probably a transient feature: the eruption plume is starting to punch through. The structure also indicates that little to no shearing wind was present at the time to disrupt the plume….
By contrast, a cloud of denser, gray ash—probably a pyroclastic flow—appears to be hugging the ground, descending from the volcano summit. The rising eruption plume casts a shadow to the northwest of the island (image top). Brown ash at a lower altitude of the atmosphere spreads out above the ground at image lower left. Low-level stratus clouds approach Matua Island from the east, wrapping around the lower slopes of the volcano. Only about 1.5 kilometers of the coastline of Matua Island (image lower center) are visible beneath the clouds and ash.
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