MAGMA

All volcanoes are fueled by
magma deep beneath
Earth’s surface
 Magma is a mixture of molten
rock, suspended mineral
grains, and dissolved gases
 Depending on the composition
rocks begin to melt at
temperatures between 800°C
and 1200°C
Factors affecting magma
formation

Temperature
 If rocks melt at temperatures found in the
asthenosphere, and temperature increases
with depth, then why isn’t the entire mantle
liquid?
 What other factors affect the formation?
Pressure
Pressure increases with depth because of
overlying rocks.
 As pressure increases, the temperature at
which a substance melts also increases
 Most rocks in Earth’s lower crust and upper
mantle do not melt to form magma even
though the temperatures may be hot
enough

 Greater Pressure = Greater Melting
Temperature
Water Content
Water can be found in the pore spaces
of some rocks and can be bound into
the crystal structure of some minerals
 A wet mineral or rock will melt at a lower
temperature than the same mineral or
rock under dry conditions
 Greater water content = lower melting
temperature

Magma Composition

Many different factors determine the
composition of magma
 Viscosity
○ The internal resistance to flow
 Gas Content
 Silica Content
 Source Material
Magma Types

3 Different types of magma
 Basaltic
○ Forms when rocks in the upper mantle melt
○ Low viscosity
○ Contain small amounts of dissolved gases
and silica
 Eruptions are relatively quiet

Andesitic
 Found along subduction zones
 Relatively high silica content
 Has an intermediate viscosity
○ Volcanoes said to have intermediate eruptions

Rhyolitic
 Forms when molten material rises and mixes
with the overlying silica and water rich
continental crust
 Highest viscosity
○ Resistant to flow
 Large volume of trapped gas
○ Very explosive eruptions
Viscosity

Viscosity of magma depends upon
temperature and composition
 Hotter the magma or lava, the lower the
viscosity
 Amount of silica in magma or lava also
increases viscosity
○ High in silica = high in viscosity
Intrusive Activity

Magma is less dense than surrounding
rocks.
 Moves upward and comes into contact with
or intrudes the overlying crust
 What happens deep in the magma chamber
as the magma slowly cools?
○ As they cool they begin to form intrusive
igneous rock bodies ranging in size
○ These intrusive igneous rock bodies are called
plutons
Batholiths & Stocks

Batholiths = largest plutons
 Cover at least 100 km2
 Common in the interiors of major mountain
chains
 Composed primarily of granite

Stocks are irregular shaped and smaller
in size than batholiths
Laccoliths
A mushroom-shaped pluton with a round
top and flat bottom
 Relatively small in comparison to
batholiths and stocks

Sills

Sill = a pluton that forms when magma
intrudes parallel to layers of rock
 Range from only a few centimeters to
hundreds of meters in thickness
Dikes
Dike = a pluton that cuts across
preexisting rocks
 Form when magma invades cracks in
surrounding rock bodies

Anatomy of a Volcano

Volcanism produces various features
that alter Earth’s landscape

Vent
 Opening in the crust through which lava erupts
 Over time lava flowing from the vent can
accumulate forming a mountain known as a
volcano

Crater
 A bowl shaped depression, around the vent, at
the top of a volcano
 Connected to the magma chamber by the vent
Calderas
Craters are usually less than 1km in
diameter
 Larger depressions = caldera

 Up to 50 km in diameter
 Can form when volcano summit or side
collapses into an empty magma chamber
Types of Volcanoes

Shield
 Broad, gently sloping sides
 Nearly circular base
 Formed when layers of lava accumulate
 Non-explosive eruptions

Hawaiian Islands
 Ex. Mauna Loa, Kilauea

Cinder-Cone
 Formed when material ejected into the air
falls back to Earth and piles up around the
vent
 Steep sides
 Small
 Magma contains more water and silica =
more viscous

Explosive eruptions

Composite
 Layers of volcanic fragments alternate with
lava
 Magma contains large amounts of silica,
water, & gas
 Much larger than cinder-cones
 Violently explosive

Ex. Mount St. Helens
Volcanic Material

Tephra = rock fragments thrown into the
air during an eruption
 Classified by size
○ Dust, ash, lapilli, volcanic blocks
○ Volcanic bombs
Pyroclastic Flow
Clouds of gas, ash, and other tephra
 Move at incredible speeds

 200km/h
Temperature can exceed 700°C
 1902 Mount Pelée

Where Do Volcanoes Occur?

80% occur along convergent boundaries
 Subduction zones
 Pacific Ring of Fire

15% occur along divergent boundaries
 Rift zones along ocean ridges

5% occur away from boundaries
Hot Spots

Hot Spots = unusually hot regions of
Earth’s mantle
 Plumes of material rise toward the surface
 Magma melts through the crust to form
volcanoes

Creates chains of volcanoes
 Provide important information about plate
motions
○ Rate and direction