Volcanic activity
PG. 89
Magma
 Temperature beneath the lithosphere causes rock to
melt
 As pressure increases, the melting point of rock
increases
 At any pressure, a wet rock will melt at a lower temp
Types of magma
 Magma formed from the 3 major types of igneous
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rock- basalt, andesite, and granite
Major types of magma: basaltic, andesitic, and
rhyolitic
Hawaiian volcanoes- basaltic
Mt. St. Helens in Washington are andesitic
Dormant volcanoes in Yellowstone are rhyolitic
Mt. St. Helens
Magma composition
 The viscosity of magma is based on silica content more
silica, higher viscosity
 Basaltic magma
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Volcanoes erupt quietly
About 50% silica
Made from rocks in the upper mantle
Oceanic and continental crust
 Andesitic magma
 found along subduction zones
 Made from oceanic crust and sediments
 60% silica, eruptions are in the medium range
 Rhyolitic magma
 Comes from water rich continental crust
 Very explosive
 70% silica content
 Basaltic volcano
 http://www.youtube.com/watch?v=5hE2DZdl0IA
 Andesitic volcano
 Rhyolitic volcano
Intrusive activity
 When magma cools, it forms structures called plutons
 Plutons are based on size and shape
 Largest plutons batholiths and stocks; form beneath
earths surface; coarse rock
 Laccoliths mushroom shaped pluton
 Sills and dikes smallest pluton
Anatomy of a volcano
 Magma erupts in an opening called a vent
 Around the vent is a crater
 Calderas can form when a volcano collapses into the
magma chamber
Types of volcanoes
 Shield volcano
 Mountain w/ broad gently sloping sides that forms when
basaltic rock is layered
 Cinder cone volcanoes
 Forms when material is ejected high in to the air and piles up
around a vent
 Has steep sides
 More explosive
 Composite volcanoes
 Much larger than cinder cone
 Dangerous to humans
 Lava alternates with volcanic fragments
Types
 Shield
 Cinder cone
 Composite
Volcanic Material
 Tephra- rock fragments thrown into the air
 Can be dust, ash, lapilli, volcanic blocks, or volcanic
bombs
 Rapidly moving volcanic material is called
pryoclastic flow
Volcano occurrences
 Convergent volcanism
 At a convergent boundary
 Usually form in the sea
 Divergent volcanism
 Magma forced upward through faults
 Causes seafloor spreading
Pg. 90
 Create a foldable of the 3 different types of
volcanoes. Include a diagram of each on your
foldable ( look on pgs. 482-483 for diagrams)
 Include the types of magma and how explosive it is