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VOLCANOES
CH 10 Volcanoes
4 Factors affecting eruptions
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1. Magma composition
 Basaltic – low silica and low viscosity
 Rhyolitic – high silica and high viscosity
 Andesitic – midway between the 2
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2. Magma temperature = hotter flows easier
3. Magma viscosity – determined by temp and
composition. High viscosity = violent eruptions
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4. Dissolved gases = more gasses means more violenc
3 Volcanic Materials
1. Lava flows – 3 textures
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1. Aa – rough and jagged
2. Pahoehoe - wrinkly and ropey texture
3. Pillow - forms underwater
2. Gases – water vapor, CO2, nitrogen, sulfur
and chlorine
3. Pyroclastic materials – ejected fragments
3 SIZES
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1. Ash – fine material
2. Cinders and lapilli – medium/pebble sized
3. Bombs and blocks – big stuff, block = hard,
bombs = lava
Aa LaVa on the left
PAHOEHOE
PILLOW LAVA
BLOCK
BOMB
Volcanic Terms
Crater – steep walled depression at top
Conduit – pipe connecting magma chamber to the
surface
Vent – top of the conduit
Caldera – large depression in a volcano due to collapse
of volcano
Volcanic neck – remnant of eroded volcano
Columnar jointing – 6 sided columns from cooling
magma
Lava plateau – raised flat area of lava/fissure=crack
Caldera = Crater Lake
Volcanic Neck = Ship Rock
Columnar Jointing
Columnar Jointing = Devil’s Tower
FISSURE ERUPTION
3 VOLCANO TYPES:
1. SHIELD CONE
--broad base & gently sloped sides
--huge size
--mild eruptions
--mostly lava flows
--basaltic lava/oceanic usually
--low in silica
--example = Hawaii
Shield Volcano = Hawaii
2. CINDER CONE
--narrow base & steep sides
--small size
--violent eruptions for size
--mostly tephra
--granitic lava/continental usually
--high in silica
--example = Paricutin & Sunset Crater
CINDER CONE
3. COMPOSITE CONE (Stratovolcano)
--medium base & moderately sloped
--medium size
--both violent & mild
--both lava flows & tephra
--andesitic lava / found “near” coasts
--medium amount of silica
--example = Mt. St. Helens & Andes
Composite/Strato--Volcano
Mount Saint Helens
Mount Saint Helens
Extra-Terrestrial Vulcanism (3)
1. IO = moon of jupiter
2. Olympus Mons = huge hot spot shield
volcano
3. Moon = has lava flows but NO
volcanoes
IO = moon of Jupiter
Olympus Mons = Mars Volcano
Marias on the Moon
3 Volcano Dangers
1. Pyroclastic flows – hot gases, glowing ash,
and larger rock fragments
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Very hot
Can attain 200 km/hr about 120 mph
2. Lahars – mudflow formed from volcanic ash
and water (melted snow or rain)
3. Eruptions – you’re there one minute and
gone the next…
Pyroclastic Flow
LAHAR
Igneous Activity
Plutons (intrusive igneous bodies)
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Classified by shape, size, and relationship to
surrounding rock layers (5 types)
 Sill – horizontal and between rock layers
 Dike – vertical and cut across rock layers
 Laccolith – dome/mushroom shaped plutons
 Batholith – more than 100 km2 of exposed rock
 Stock – less than 100 km2 of exposed rock
3 Factors for Origin of Magma
Read pages 291 – 292
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1. The role of heat
2. The role of pressure
3. The role of water
3 Volcano Locations
1. Convergent plates
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Subducting plate melts and magma rises
 Japan and Mt St Helens
2. Divergent plates
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Plate diverge and magma rises through the rift
 Mid-ocean ridge
 Iceland
 East African Rift
3. Intraplate (hot spots)
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Mantle plume rises towards the surface
 Hawaii and Yellowstone