Volcanoes by Michael J Branney & Jan Zalasiewicz

Author:Michael J Branney & Jan Zalasiewicz [Branney, Michael J & Zalasiewicz, Jan]
Language: eng
Format: epub
ISBN: 9780191024733
Publisher: Oxford University Press
Published: 2020-10-26T00:00:00+00:00

Cracks, explosions, and block-and-ash flows

Volcanologists can listen to the sound of rising viscous magma. Sound waves through the Earth’s crust arise from the breaking of the magma. Recorded on seismographs, they can be used to forecast what a volcano may do next. For example, a growth spurt accompanied by elevated noise-levels as the magma repeatedly breaks may herald a phase of violent vulcanian explosions. Rapid growth can destabilize the dome. A dome interior contains pressurized gases, and when confining pressure is abruptly released, say by a rock-slide or avalanche, it can explode with considerable violence.

Tap a block from a collapsed dome lightly with a hammer, and it may miraculously spring apart into a little cloud of lava fragments and ash. This is immensely satisfying and it takes quite a long time before the average geologist can be persuaded to resist tapping every block within eyesight. What is happening here is that the blocks are thermally stressed through cooling-contraction while gas held in microscopic pores is pressurized. Fracturing abruptly releases the stress. The same phenomenon accompanies block collisions within hot rock-avalanches from lava domes, and so the amount of fine rock powder increases dramatically until the avalanches transform into highly mobile ‘block-and-ash flows’. These hot flows travel surprisingly rapidly down valleys, carrying hot, bouncing lava blocks the size of a fridge or garage enveloped in dusty gas, burying and burning houses, bridges, and roads. Block-and-ash flows are a particular hazard downslope of growing lava-dome volcanoes, such as during the recent eruptions of Unzen volcano in Kyushu, Japan, and the Soufrière Hills volcano on the Caribbean island of Montserrat. Local residents were displaced for months, some not able to return. Fortunately block-and-ash flows rarely travel for more than a few kilometres. But the lava domes that produce them can continue growing for several years, prolonging the hazard.

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