Science: Simple and complex volcanoes

Some volcanoes have a very simple cone shape. They are built up by just one eruption, or by very few subsequent eruptions. These cones, or domes, are built up over a few weeks or months.

Examples
--Sunset Crater in Arizona
--the small volcanoes of Craters of the Moon National Monument in Idaho
--Chaine des Puys volanic province in central France

At the other extreme, are "complicated" volcanoes - built up by multiple eruptions over long periods of time (thousands or even hundreds of thousands of years.) Phases of lava and pyroclastic accumulation are interspersed with phases of destruction caused by explosive activity, the collapse of a volcano's flanks, or subsidence of basement rock. During these eruptions, and during the intervals between, erosion due to runoff shapes these volcanoes.

Examples
the volcanoes of the San Francisco Peaks
near Flagstaff, Arizona
the volcanoes of the Cascades in the Pacific Northwest
Cantral Volcano in northern France



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Bibliography
Living Mountains, How and Why They Erupt, by Jacques Kornprobst and Christine Laverne

Science: Simple and Complex Volcanoes

A volcano is the point at which molten rock, which comes from deep within the earth, reaches the earth's surface. In some cases, volcanoes can simply be holes in the ground (vents), but generally they form conical mounds - built up progressively on a basement of pre-existing rock, and is composed of an accumulation of lavas and volcanic ejecta.

Molten rock reaches the surface by a central pipe, and by dikes, that bring this magma to the surface from a magma chamber situated anywhere from 1,000 feet to 2 miles beneath the vent.

The magma chamber for a particular volcano is generally very complex in shape, and is itself fed by magma from a deeper source, up to even 200 miles below the earth's surface.

The shape of the volcano above groundvaries depending on the chemical composition of the magma in question, as well as its temperature, as well as the amount of dissolved gas within it. All these factors combine to determine the frequency and violence of the eruptions, and thus what kind of cone, if any, is created.
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Bibliography
Living Mountains, How and Why They Erupt, by Jacques Kornprobst and Christine Laverne