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Lecture 4

ESS 101 Lecture Notes - Lecture 4: Subduction, Vestmannaeyjar, Heimaey

Earth and Space Sciences
Course Code
ESS 101
Swanson Terry

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The nature of volcanic hazards is strongly tied to viscosity and gas content of the magma
Basaltic composition eruptions tend to be non-explosive (effusive)
o Basalt has low viscosity - very fluid
Gas escapes easily
o Basalt has low gas content
o Loss of life = rare in effusive explosions ; more damage to structural things that can't
move because there will be greater lava flow
Basaltic eruptions happen at different tectonic settings around the world
o Four different locations
Iceland sits on a basaltic spreading ridge - hotspot
East Africa
Columbia Plateau
Iceland is situated over the mid-Atlantic seafloor spreading margin
o Also a hotspot located beneath the island
o Basaltic volcanism = prevalent on island this nation
o Because Iceland is located at polar latitudes, Arctic Circle glacial ice presents more
problems when Icelandic volcanoes erupt
o 1973, basaltic eruption almost destroyed important fishing port of Vestmannaeyjar on
island of Heimaey
Pyroclastic eruptions (explosive) - most devastating volcanic eruptions
o More human casualties
Ex: Pompeii
Mt. Vesuvius had cataclysmic pyroclastic eruption in 79 AD
o Two phases:
Gas pressure builds up as magma rises
Catastrophically erupts
Hot material is then ejected into the air
Because of its composition and high density, it falls
o Dense clouds of super-heated gas and ash fall down the flanks at 100-160 km/hr
Anything in the path are lost
o Most often associated w/ subduction zone volcanism
o These eruptions are known as nuee ' ardentes (glowing avalanche)
Lahar flows happen when snow or glacial ice is melted by hot pyroclastics
o Mud and debris tend to flow down slope of the volcano within preexisting drainages
Lahar flows are very dangerous
o More difficult to predict their magnitude as they can be generated by small eruptions
o Number one potential volcanic hazard from Mt. Rainier
Volcanic Hazards Risk Assessment (Ash/Pyroclastic eruptions, Lahars) things to look out for:
o Magnitude of event
o Frequency of event
Why does magma movement produce seismic activity (minor earthquakes)?
o Crust is not elastic. As magma moves up, crust must rupture
Volcanic eruptions on cooling climate:
o You generally want the eruption plume into the stratosphere (above weather zone)
If the ash is low, it quickly condenses and falls
o Volcanoes near equator have bigger effects on climate
Ash is put into both hemispheres
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