Kimia's+Notes+Chapter+6+Section+1


 * __Volcanoes and Plate Tectonics__**

__ Key Terms __ -Volcanoes- a weak spot in the crust where molten material, or magma, comes to surface. -Magma- a molten mixture of rock-forming substances, gases, and water from the mantle. -Lava- when magma reaches its surface. -Ring of Fire- one major volcanic belt. -Island Arc- the resulting volcanoes create a string of islands. -Hot Spot- an area where materila from deep within the mantle rises and then melts, forming magma.

Introduction In 2002, Mount Etna erupted in glowing fountains and rivers of molten rock.Located on the island of Sicily in the Mediterranean Sea, Mount Etna is Europe's largest volcano.Over te last 2,500 years,it has erupted often.The ancient Greeks believed that Mount Etna was one home of Hephaestus,the Greek god of fire.Beneath the volcano was the forge where Hephaestus made beautiful metal objects for other Greek gods.

Volcanoes and Plate Boundaries There are about 600 active volcanoes on the land.Many more lie beneath the sea,where it is difficult for scientists to observe and map them.Volcanic belts form along the boundaries of Earth's plates.At plate boundries,huge pieces of the crust spread apart or collide.As a result,the crust often fractures,allowing magma to reach the surface.Most volcanoes form along spreading boundaries such as mid-ocean ridges and along colliding boundaries where subduction takes place.For example,Mount Etna formed near the boundary of the Eurasian and African plates.

Spreading Boundaries Volcanoes form along the mid-ocean ridges,which mark spreading boundaries.Along the rift valley,lava pours out of cracks in the ocean floor,gradually building new mountains.Volcanoes also form along spreading boundaries on the land.For example,there are several large volcanoes along the Great Rift Valley in East Africa.

Colliding Boundaries Many volcanoes form near colliding boundaries where oceaninc plates return to the mantle.Volcanoes may form where two oceanic plates collide or where an oceanic plate collides with the continetal plate.Many volcanoes occur near boudaries where two oceanic paltes collide.Through subduction,the older,denser the plate sinks beneath a deep-ocean trench into the mantle.Some of the rock above the subducting plate melts and forms magma.Because the magma is less dense than the surrounding rock,it rises toward the surface.Eventually,the magma breaks through the ocean floor,creating volcanoes.

Hot Spot Volcanoes A volcano forms above a hot spot when magma erupts through the crust and reaches the surface.Some hot spot volcanoes lie in the middle of plates far from any plate boundaries.A hot spot in the ocean floor can gradually form a series of volcanic mountains.For example,the Hawaiian Islands formed,one by one,over millions of years as the Pacific plate drifted over a hot spot.Hot spots can also form under the continents.Yellowstone Nationsl Park in Wyoming marks a hot spot under the North American plate.