• Despite the high temperature in the
mantle, most of the layer remains solid
due to extreme pressure
– Magma: liquid rock produced under
Earth’s surface
• Melting Point: the
temperature at which a
substance will change from a
solid to a liquid
• Magma can form in three
ways
– Temperature rises above the
melting point of rock
– Pressure decreases lowering
the melting point of rock
– Addition of fluids, like water,
can decrease the melting point
• Volcanism: any activity that includes
the movement of magma and gases
toward or onto Earth’s surface
– Magma rises because it is less dense
than the surrounding rock
• magma that flows onto Earth’s surface is
called lava
• Flows from an opening, or vent
• When magma erupts from a volcano and
then cools & hardens, it forms an igneous
rock
• Mafic: magma that
• Felsic: magma that
is rich in
is rich in feldspar
magnesium and
and silica
iron
–
Light
in
color
– Dark in color
– Makes up
– Makes up oceanic
continental crust
and continental
crust
only
• Viscosity:
resistance to
flow
–Affects the
force with
which a
particular
volcano will
erupt
• Quiet Eruptions
–Form from mafic magma
–Low viscosity
–Occur with oceanic volcanoes
–Magma “oozes” out of the vent
• When mafic lava cools
rapidly, a crust forms on
the surface of the flow
• If the lava continues to
flow after the crust forms,
the crust wrinkles
– Pahoehoe: a volcanic rock
that forms from hot fluid
lava
• As it cools it forms a
smooth, ropy texture
• Means “ropy” in
Hawaiian
• If the crust deforms rapidly, or grows too
thick to form wrinkles, the surface breaks
into jagged chucks
– Aa: a jagged volcanic rock that forms from lava
that
• has the same composition as pahoehoe
– Blocky Lava: a volcanic lava that has a high
silica content, and is highly viscous
• Explosive Eruptions
–The felsic lavas of continental
volcanoes tend to be cooler and
stickier than the fluid lavas
produced by oceanic volcanoes
• Pyroclastic material: fragments of rock
that form during a volcanic eruption
• What are the differences
between an explosive eruption
and a quiet eruption?
• Volcanoes are usually
located at the edge of
plate boundaries
– Ex: A major zone of
active volcanoes
encircles the Pacific
Ocean
• Known as the Pacific Ring
of fire
– Subduction zones
• Continental-oceanic
convergent boundaries
• Oceanic-Oceanic
convergent boundaries
• The largest amount of magma is found
at divergent boundaries along midocean ridges
– The magma erupts to form underwater
volcanoes
• These volcanoes produce pillow lava
• Shield Volcanoes:
volcanic cones that are
broad at the base and
have gently sloping
sides
– Covers a wide area
– Generally forms from
quiet eruptions
– Layers of hot, mafic lava
flow out around the vent,
harden, and slowly build
up to form the cone
– Ex: Hawaiian Islands
• Cinder Cones: a
volcano that has a
very steep slope
– Not usually tall- max
height is not more than
a few hundred meters
– Form from explosive
eruptions and are
made of pyroclastic
material
• Composite Volcanoes:
made of alternating
layers of hardened lava
and pyroclastic material
– Quiet eruptions deposits
lava and an explosive
eruption follows depositing
ash
– Also known as
stratovolcanoes
– Develop to form large
volcanic mountains
• Caldera: a large, circular
depression that forms
when the magma
chamber below a volcano
partially empties and
causes the ground above
to sink
– Calderas may fill with
water to form lakes
• Ex: Mount Mazama in
Oregon (known as
Crater Lake)
• Hot Spot: an area of
volcanic activity within the
interior of a tectonic plate
– Magma rises from the mantle
in the middle of a plate, and
not at a plate boundary
– The rising magma creates a
volcano, producing an island
– As the plate shifts over the
magma, a new volcano and
new island is formed
– Continues creating a chain of
islands
• Ex: Hawaiian Islands