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Volcanoes
volcano --a vent in the surface of Earth,
through which magma and gases are
expelled.
Volcanism: any
activity that includes
moving magma onto
Earth’s surface.
Mantle and crust melt to form liquid rock
under Earth’s surface (magma).
As magma rises, it enlarges because it
melts surrounding rock and intrudes into
other rocks, causing them to fall into the
magma “body.”
Magma that gets to the surface is
called lava.
When this magma/lava cools, it is
called igneous rock, if it
cools INSIDE the crust, it is
called intrusive igneous rock
or a pluton.
Volcanoes are likely to form in:
1. Tectonic plate boundaries
a. Convergent—plates move toward
each other
b. Divergent—plates move away
from each other
2. Hot Spots—areas
not on the edge
of plates
a. Mantle
plumes
rise/reach
lithosphere, forms volcano
b. Plate continues to move, volcano
goes with it and becomes inactive,
new one forms
c. Hawaii was formed this way---by
a hot spot.
3. Mid Ocean Ridges—forms from
divergent plates
a. Volcanoes on these ridges contain
pillow lava (lava rapidly cooled by
water)
i. Iceland is split in ½ between two
tectonic plates.
Fissures exist in the
middle of Iceland
where lava flows to
the surface.
4. Subduction zones—one plate moves
under another
a. Oceanic plate moves under
continental plate
b. Island arc—oceanic lithosphere
subducts under oceanic
lithosphere.
Most “major” zone is the Pacific Ring of Fire
Volcanic eruptions can be
signaled by a change in EQ
activity and are either:
a) quiet: produced by mafic
magma, runny with low
viscosity and low volume of trapped gases
(ex. Hawaiian volcanoes)
b) explosive: produced
by felsic magma, a sticky
lava with a high viscosity
and a high volume of
trapped gases (ex. Mt. St. Helens in
Washington)
Fragments of rock that form from a volcanic
eruption are called pyroclastics.
a) dust
b) ash
c) bombs-red hot lava
cools as it spins in the
air after ejection from
the dome.
d) lapilli: rocks less than
64 mm diameter.
e) blocks: can be as large as
a house
Types of volcanoes:
1. shield: wide
area…from quiet
eruptions.
2. cinder: small
area…from explosive
eruptions.
3. composite:
alternating layers from
explosive and quiet
eruptions.
A caldera is a depression that forms when
the top of the volcanic cone collapses
inward.
Newberry Caldera, Oregon