Lecture Notes 17 Eruptive Styles and Landforms I.doc

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Lecture 17
Eruptive Styles and Landforms I
How do volcanoes erupt?
Internal heat
• Volcanoes are a mechanism by which
Earth expels internal heat quickly
• Internal heat increases with depth
–Geothermal gradient
– Anomalously hot zones:
• Hot spots
Spreading centers
From rock to magma
• What causes solid rock to melt?
– Decrease P
(most important mechanism)
• Decompression melting
– Increase Tº
– Increase water content
Phase change from solid to liquid
accompanied by volume expansion
– Fractures developing in overlying
rocks
Volcanic eruption
• Sudden occurrence of a violent
discharge of volcanic materials
Volcanic eruption
Step by step
1. Hot, solid rock from asthenosphere
rises closer to the surface
– Pressure decrease causes rocks to melt partially
Decompression melting
2. Volume increase induces fractures
– More hot material rises
– More rocks liquefy
3. Volatiles gradually come out of the melt
– Gas bubbles push magma upward
4. Magma fragments
– When bubbles ≥ 75% volume
– Powerful gas jet expels magma in the atmosphere
Eruptive style
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• Style: peaceful vs explosive
Relation between:tectonic environment, magma composition, eruptive style
6 types of eruptions
– Caution: a volcano can change eruption type through time
Relation between: eruptive style, eruption type, magma composition, volcanic explo-
sivity index
3 Vs
• Factors controlling volcanism and
volcanic landforms
1. Viscosity
2. Volatiles
3. Volume of magma
Peaceful eruptive style
1. Viscosity
• Low/medium magma viscosity
2. Volatiles
• Low/medium volatile content
3. Volume of magma
Small to large volume
Icelandic-type eruptions
• Tectonic environments:
– Spreading center
– Hot spot
• Magma composition: basaltic
Low viscosity: lava flows like water
• Low volatile content: volatiles
escape easily
• Small volume
Landform: lava plateau
Lava plateau
• Small area covered by nearly
horizontal layers of solidified lava
• Surface rupture:
Fissures: linear fractures (≤ 25 km)
Flood basalts:
A special type of very large
"icelandic" eruptions from the
geological past
Largest volcanic events on Earth Very large volume of low-viscosity,
lava with low-volatile content
• Millions km3
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