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Ch. 7 Volcanoes
Ch. 7.1 Volcanoes and Plate
Tectonics
• Most rock in the very hot mantle /
asthenosphere remains solid due to very
high pressure.
• Sometimes conditions (lower pressure or
added water) allow part of the solid mantle
to melt and become flowing liquid magma.
Volcanism
• Volcanism is movement of magma toward
or onto the surface of the earth.
• Magma forms pockets, and the less dense
magma rises through the surrounding
rock.
• Most magma forms at plate boundaries,
especially at subduction zones, where
water enters with the subducting plate.
Some definitions….
• Lava—magma that erupts onto the earth’s
surface.
• Vent—the opening through which lava
flows onto the surface.
• Volcano—The structure formed by the
vent and the built-up volcanic material on
the surface around the vent.
Major Volcanic Zones
• Mostly at divergent and convergent plate
boundaries.
Subduction Zones
• With oceanic-continental plate
convergence, an ocean trench forms,
folded mountains form along the
continental edge, and magma forms at the
zone of subduction.
• Some magma rises to earth’s surface,
forming volcanic mountains near the
continent’s edge.
• With oceanic-oceanic plate convergence,
a trench and magma result, and a string of
volcanic islands (island arc) forms.
• As the islands grow larger, they may join
and form larger landmasses such as
Japan or the Philippines.
Mid-Ocean Ridges
• Divergent boundaries, where the greatest
amount of magma comes to the surface.
• New ocean floor created.
• Mostly unnoticed, except in Iceland, where
the Mid-Atlantic ridge is above sea level.
Hot Spots
• Areas of magma in the mantle that remain
stationary while the lithospheric plates
move above them.
• Creates a chain of volcanic islands.