PAGE2Volcanoes are not randomly distributed over the Earth's surface. Most are
concentrated on the edges of continents, along island chains, or beneath the sea forming
long mountain ranges. More than half of the world's active volcanoes above sea level
encircle the Pacific Ocean to form the circum-Pacific "Ring of Fire." In the past 25 years,
scientists have developed a theory--called plate tectonics--that explains the locations of
volcanoes and their relationship to other large-scale geologic features.

The most simple answer is that volcanoes are caused by the fact that magma is produced in the upper mantle of the Earth.
Magma is what geologists call molten rock before it makes it to the surface. Magma is produced in 3 main locations. One is
subduction zones where one of the Earth's plates dives under another. The sediments and the plate itself give off gases which
rise through the overlying mantle and help to cause it to partially melt, producing the magma that then erupts. These are often
called arc volcanoes because they lie along arcuate lines that parallel the subduction zones. Another place where significant
magma is produced is at spreading centers. Most of these are in the oceans, but some are on continents. Here, two tectonic
plates spread apart. Mantle material wells up to fill the space, and in the process it partially melts in the lower pressure it finds
itself in. This produces magma which can well up to the surface and erupt to produce new crust. Another place where lots of
magma is produced is at hotspots. We don't really understand hotspots very well. We know that they don't care that the plates
are moving around up on the surface so almost certainly they are deep structures. We also know that they can last a long time -
the Hawaiian hotspot has been active for at least 70 million years.

yyThis cross-section shows the parts of a volcano. A conduit feeds magma to the surface. Near the surface, the gas expands and
fragments the lava into ash. Some magma passes through dikes to feed vents. Some magma intrudes parallel to layers to make
sills.
More than 80 percent of the Earth's surface -- above and below sea level -- is of volcanic origin. Gaseous
emissions from volcanic vents over hundreds of millions of years formed the Earth's earliest oceans and
atmosphere, which supplied the ingredients vital to evolve and sustain life. Over geologic eons, countless volcanic
eruptions have produced mountains, plateaus, and plains, which subsequent erosion and weathering have sculpted
into majestic landscapes and formed fertile soils

About 500 active volcanoes are known on Earth, not counting those that lie beneath the sea.

The Earth's crust, on which we live and depend, is in large part the product of millions of once-active volcanoes
and tremendous volumes of magma that did not erupt but instead cooled below the surface. Such persistent and
widespread volcanism has resulted in many valuable natural resources throughout the world. For example,
volcanic ash blows over thousands of square kilometers of land increases soil fertility for forests and agriculture by
adding nutrients and acting as a mulch. Groundwater heated by large, still-hot magma bodies can be tapped for
geothermal energy. And over many thousands of years, heated groundwater has concentrated valuable minerals,
including copper, tin, gold, and silver, into deposits that are mined throughout the world

The largest eruption in the world this century occurred in 1912 at Novarupta on the Alaska Peninsula. An
estimated 15 cubic kilometers of magma was explosively erupted during 60 hours beginning on June 6 -- (which is
equivalent to 230 years of eruption at Kilauea (Hawaii) or, about 30 times the volume erupted by Mount St.
Helens (Washington) in 1980.)