18-2 Volcanic 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 ->>>>>

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

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

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

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

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

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

Video Summary