Some mountains are made of solid rock, like the
Rocky Mountains or the Swiss Alps. But, some mountains
are actually volcanoes.
are vents, or openings in the Earth's
crust, that release ash, gases and steam, and hot
liquid rock called lava. When the lava cools and
hardens, it forms into the cone-shaped mountain
we think of as a volcano. Most of the world's
volcanoes are found around the edges of tectonic
plates, both on land and in the oceans.
On land, volcanoes form when one tectonic plate
moves under another. Usually a thin, heavy
oceanic plate subducts, or moves under, a thicker
continental plate. When this happens, the ocean
plate sinks into the mantle.
Water trapped in the rocks in this plate gets
squeezed out. This causes some of the rocks to melt. The melted rock, or
magma, is lighter than the surrounding rock and
rises up. This magma collects in magma
chambers, but it is still miles below the surface.
When enough magma builds up in the
magma chamber, it forces its way up to the
surface and erupts, often causing volcanic
In the ocean, volcanoes erupt along cracks that are
opened in the ocean floor by the spreading of two
plates called a mid-ocean ridge.
Magma from the Earth's upper mantle rises
up to fill these cracks. As the lava cools, it forms
new crust on the edges of the cracks. These mid-ocean ridges
are actually long chains of underwater volcanoes
that circle the Earth like the seams on a baseball.
About 80 to 90 percent of all volcanic eruptions
occur where the plates spread apart.