18-2 Volcanic Eruptions

18-2 Notes
Eruptions

Making Magma

Temperature
▪ Most rocks melt between 800C -1200C

▪ These temps are found in crust and

upper mantle

▪ Temp. increases with depth

▪ Pressure

▪ Also increases with depth
▪ Pressure can keep rocks from

melting

▪ Melting Temperature increases with

depth

▪ Water can make a big difference in

melting temperature

▪ See chart ->>>>>

Multiple Choice

Melting temperatures of dry rocks at the surface

• Are higher than those at depth

• Are lower than those at depth

• Are the same as those at depth

Multiple Choice

A rock that is "wet" (meaning water is involved)

• melts at higher temps than the same rock when its dry

• melts at lower temps than the same rock when its dry

• melts at the same temperature as a dry rock

• tells you that it the guitar player was sweaty

Composition of Magma

Determines a volcanoes explosiveness
Factors that influence composition are:
1. Dissolved Gases
▪ Provides the “bang” like a shaken up soda.
▪ Major gases include

-Water vapor
-Carbon dioxide
-Sulfur dioxide
-Hydrogen sulfide

2. Viscosity

-Resistance to flow
- High = thick (like tar)
- Low = runny (like water)
- Higher the silica, the higher the

viscosity

Drag and Drop

The more dissolved ​

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found in a magma the more​

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it can be.

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gases

explosive

Multiple Select

High viscosity means a liquid that (choose all that apply)

• flows fast

• is like water

• flows slow

• resists flow

Multiple Choice

High viscosity magmas are high in

• iron

• magnesium

• aluminum

• silica

Types of Magma

Basaltic Magma
▪ Originates in the upper mantle
▪ Less than 50% silica
▪ Iron rich
▪ Low viscosity magma
▪ Quiet Eruptions
▪ Little or no interaction with

continental crust

▪ Kilauea, Mauna Loa, Hawaii
▪ Surtsey, Iceland

Multiple Choice

Two places to view basaltic eruptions would be

• Washington and Alaska

• Hawaii and Iceland

• Indonesia and Japan

• Montana and Colorado

Multiple Choice

Basalt eruptions tend to be

• explosive because the magma is high in silica

• not as explosive because the magma is high in silica

• explosive because the magma is low in silica

• quiet because the magma is low in silica

Andesitic Magma

▪ 50-60% Silica
▪ Same comp. as the rock with

same name

▪ Oceanic-Continental

Subduction Zones

▪ Higher Silica = Intermediate

Viscosity

▪ Volcanoes with intermediate

explosivity

▪ Massive Explosions –

Tambora

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Andesitic magmas create ​

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eruptions because they have a higher ​

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content than basalt magmas due to ​

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of water associated with ​

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plates.

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explosive

silica

subduction

oceanic

Rhyolitic Magma

▪ Continental crust mixes in with

magma from deep in Earth

▪ More than 60% silica
▪ Same comp. as granite
▪ Rich in both silica and water
▪ High viscosity slows its

movement

▪ Large volume (4-6%) trapped

gas

▪ Very explosive
▪ Yellowstone super volcano

Multiple Choice

The high silica content in rhyolitic magma is caused by

• huge amounts of continental crust being melting into the mix

• huge amounts of oceanic crust being melted into the mix

• a 50:50 blend of ocean and continental crust in the mix

Explosive Eruptions

▪ When lava is too viscous to flow
▪ Pressure builds up until explosion
▪ Tephra – anything erupted in air

▪Ash – less than 2mm – can be
carried 40km into atmosphere

▪Bombs – lava that hardens in air

▪Blocks – as big as a car or small
house

▪ Sulfur dioxide can stay in upper

atmosphere for years lowering
global temperatures

▪ Tambora and the year without

summer

Drag and Drop

Rank the following forms of tephra in size from largest to smallest ​

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​

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​

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​

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block

bomb

ash

Pyroclastic Flows

• Rapidly moving clouds of

tephra (or pryroclastics)

• Often deadliest part of

eruption

• Searing hot
• Fast moving – can be over 100

mph

• Suffocating poisonous gas and

ash

• Mt Peleé – 29,000 people

killed in 1902 on Martinique

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The only person to survive the Mt. Pele eruption was in a well built

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