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Chapter 7, section 1
What is a volcano?
Volcanoes and Plate Tectonics
 A volcano is a structure on the surface of our planet where molten
rock,fragments, and gas erupt through the crust
Magma- liquid melted rock located inside the Earth
Volcanism- the process that involves the movement of magma to or
towards earth’s surface
Large areas of magma form where rock has melted. The magma pushes
toward the crust because it is less dense than the surrounding rock
As it moves upwards rock that the magma contacts melts as well
Sometimes the magma breaks through the surface of Earth
Lava- magma that erupts onto Earth’s surface
Vent- the opening through which the magma flows onto Earth’s surface
Volcanoes around the
world →
There are over 600 volcanoes that have erupted in the last 50 years
Just like earthquakes, volcanoes mainly occur at plate boundaries
Major volcanic zones →
1) Subduction zone- occur at convergent plate boundaries where a
oceanic plate moves underneath another plate (oceanic or continental)
o When the plate moves underneath into the hot asthenosphere
water from the subducting plate mixes with the rock and it
Volcanism →
melts, forming magma
o A major volcanic zone is around is around the Pacific Ring of Fire
o Oceanic-oceanic → island arcs form in the ocean
o Oceanic-continental → volcanoes form on land
2) Mid-ocean ridges- occur at divergent plate boundaries as plates pull
apart and moves upward. As the magma reaches the crust it cools, and
forms new lithospheric crust (creating new ocean floor)
o These volcanoes mostly go unnoticed because they are under
water. The exception is Iceland, a country located directly on a
mid-ocean ridge
3) Hot spots- an area where a plume of magma within the asthenosphere
touches the lithospheric plate and melts the rock above it
o Does NOT occur at a plate boundary! They occur in the middle
of a plate
o The hot spot remains stationary, it is the plate that moves
slowly over the hot spot. A new volcano is made where the plate
is melted by the hot spot.
o Most hot spots occur in the ocean, creating islands, although
this is not always true
o Examples:
1) Hawaiian islands- there are over 80 volcanoes
created from a hot spot
The first island was created over 5.5 million years ago
2) Yellowstone- located in the U.S., hydrothermal features
have been created by a hot spot.
Yellowstone may turn into a supervolcano one day