Earth Science
Chapter 6
Section 1
A. Volcanoes:
1. What is a Volcano?
- A weak spot in the Earth’s crust where molten material,
or magma comes to the surface.
- Magma – A molten mixture of rock-forming
substances, gases and water vapor from the mantle.
- Lava – Magma that has reached the Earth’s surface.
- Solidified lava forms Extrusive Igneous Rock.
- Volcanic activity is a constructive force, building up the
Earth’s surface.
2. Location of Volcanoes:
- Most volcanoes occur along divergent plate boundaries,
such as a mid-ocean ridge, or in subduction zones around
the edges of oceans. Others form at “Hot Spots” far
from the boundaries of continental or ocean plates.
- Ring of Fire – One major volcanic belt, formed by the
many volcanoes that rim the Pacific Ocean.
a. Volcanoes at Divergent Plate Boundaries:
- At a mid-ocean ridge, lava pours out of cracks in the
ocean floor. Only in a few places do the volcanoes of a
mid-ocean ridge rise above the ocean’s surface.
Examples: Iceland and the Azores islands in the Atlantic.
b. Volcanoes at Convergent Plate Boundaries:
- Ocean to Ocean – One ocean plate subducts under
another. A trench is formed. The formation of a string of
volcanic islands is formed along the opposite side of the
trench.
Examples: Japan, New Zealand, Indonesia, the Caribbean
Islands, and the Aleutians.
- Ocean to Continent – The ocean plate is subducted
beneath the continental plate. A trench is formed.
Volcanic mountains form inland as magma is pushed to
the surface from subduction. Some Examples: The
Andes of South America, and the volcanic mountains of
the Pacific Northwest in the U.S.
- Hot Spots – magma from deep within the mantle melts
through the thin crust. Hot spots lie in the middle of
ocean or continental plates, far from plate boundaries.
Examples: Hawaiian Islands – Ocean plate hotspot.
Yellowstone National Park – Continental plate hotspot.