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Chapter 18
Volcanic Activity
Magma
• A mixture of molten rock, suspended mineral
grains, and dissolved gases deep beneath
Earth’s surface.
• Forms when temperatures are high enough to
melt rock
– 800°C-1200°C
Factors That Affect Magma Formation
• Temperature and
Pressure
– As pressure increases
the temperature at
which a substance melts
increases.
• Presence of water
– Lowers the melting point
temperature
Three Major Types of Magma
1. Basaltic magma
– Least explosive
2. Andesitic magma
3. Rhyolitic magma
– Most explosive
• Magmas differ in the source rock from which
they form, viscosity, silica content, gas content
and explosiveness
Intrusive Activity Affects Earth’s Crust
• Magma can force rock
apart and enter newly
formed fissures (cracks)
• Magma can cause rocks
to break off and sink
into the magma
chamber
• Magma can melt rock
into which it intrudes
Types of Intrusions
• Batholiths, stocks, sill,
dikes, and laccoliths are
plutons that are
classified according to
their size shape and
relationship to
surrounding rock.
• Batholiths are the
largest plutons and
often form the cores of
many of Earth’s major
mountain chains.
Volcanoes
• Lava flows onto Earth’s
surface through a vent
• Multiple lava flows can
accumulate to form a
volcano
• A crater is a depression
that forms around a vent at
the summit of a volcano
• A caldera is a large crater
that forms when a volcano
collapses during or after an
eruption.
Three Types of Volcanoes
• Shield volcanoes
• Cinder cone volcanoes
• Composite cone volcanoes
Tephra
• Rock Fragments ejected from a volcano
Where Volcanoes Form
• Convergent and divergent
plate boundaries
• Hotspots
– Usually hot areas in the
mantle that are stationary
for a long period of time
• Flood basalts
– when lava flows from
fissures to form flat
plains or plateaus