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Ch. 6 Volcanoes
Volcanoes
A volcano is a weak spot in the
crust where magma comes to the
surface
Magma is molten rock, gases, and
water from the mantle
Magma becomes lava when it
comes to the surface
Lava cools to become igneous rock
Types of Rock
Igneous- molten material, such as
lava or magma, cools and hardens
Slow cooling- large crystals (granite)
Fast cooling- small crystals (basalt)
Metamorphic- great heat and
pressure changes minerals or
other rock
Heat and pressure increases inside
the Earth
Types of Rock
Sedimentary- sediment squeezed
and glued together
Sediment- small pieces of rock or
fossils that have been broken down
by erosion or weathering
The Rock Cycle
The rock cycle slowly builds,
destroys, and changes rock
Continuous and many pathways
The Rock Cycle
Volcanoes and Plate Tectonics
Volcanic belts are found at plate
boundaries, such as the Ring of
Fire around the Pacific Plate
Volcanoes and Plate Tectonics
Divergent boundaries- magma
comes out of rift valleys to build
mountains
Convergent boundaries
Oceanic plate sinks
Rock on top melts and becomes magma
Magma erupts and becomes lava
Volcanoes at Plate Boundaries
Hot Spots
Hot spots- volcanoes that form when
magma erupts through crust to get
to the surface
Hawaii
Yellowstone National Park
Parts of a Volcano
Magma
chamber-collects
magma
Pipe- tube that
connects magma
chamber to
surface
Vent- opening on
the Earth’s crust
Parts of a Volcano
Lava flow- area
covered by lava
Crater- bowlshaped area around
a volcano’s central
vent
Caldera- large
circular depression
formed when
magma chamber
empties and the
roof collapses
Eruptions
Pressure and gases force magma out
of a volcano
Silica makes magma thick and sticky
Types of eruptions
Quiet eruptions- lava oozes to surface
Thin, runny lava that is low in silica
Hawaiian Islands
Examples are pahoehoe and aa
Eruptions
Types of eruptions
Explosive eruptions- thick and sticky
lava which traps water
Water makes eruptions more explosive
Produces pyroclastic material
Magma that erupts as fragments of molten
material and solidifies in the air
Pieces of volcano that breaks off
Mt. St. Helens in 1980
Life Cycle
Geologists determine how likely a
volcano will erupt again
Stages
Active- erupting or will erupt in the
near future
Mt. Shasta and Lassen Peak
Dormant- “sleeping,” but can become
active again
Long Valley in eastern Sierras
Extinct- unlikely to erupt again
Diamond Head in Honolulu
Volcanic Landforms
Made of lava, ash, and other
materials
Types of Volcanoes
Shield Volcano- made from
nonexplosive eruptions
Low in silica
Wide with gentle slopes
Cinder Cone- made from explosive
eruptions
High in silica
Steep, cone-shaped, found in clusters
Volcanic Landforms
Types of Volcanoes
Composite- alternating layers of
pyroclastic material and lava flow
Also known as stratovolcano
Most common
Volcanic Landforms
Lava Plateaus- lava flows out of long
cracks called fissures
Thin, runny lava flows far before
cooling
Shield VolcanoMedicine Lake, CA
Cinder Cone VolcanoMt. Bromo, Indonesia
Composite VolcanoMt. Shasta, CA
Lava PlateauModoc Plateau, CA
Magma Landforms
Formed when magma goes into
cracks, but does not come to surface
Magma cools underground
Erosion exposes harden magma
Magma Landforms
Types
Volcanic Necks- magma hardens in pipe
Dikes and sills- magma cools between
rock layers
Igneous intrusions
Sill are horizontal
Dikes cross rock layers
Batholiths- massive rock forms when
large body of magma cools
Magma Landforms