A Closer Look: Volcanoes

What is a volcano?

A volcano is a landform that is formed through the eruption and accumulation of lava and other solid material. It starts as a vent, hole, or crack in the Earth's surface, through which hot molten rock (lava), gases, and tephra erupt. Tephra is a generic term for any fragments of volcanic rock that are blasted into the air, such as ash and chunks of rock, which, depending on their size, have a variety of names.

Why do volcanoes erupt?

Volcanoes erupt because of changes in density, buoyancy, temperature, and pressure. A volcanic eruption requires magma, or melted rock. Rock melts by either an increase in temperature, a decrease in pressure, or by the addition of water to the system (since water lowers the temperature at which a rock can melt). Melted rock is less dense than the solid rock surrounding it. Buoyancy causes less dense material to rise through more dense material. As the magma rises, pressure decreases, which causes additional melting and a continued decrease in density. Magma rises until it either erupts, or enters material with the same density, at which point it will form a magma chamber.

Throughout this ascent, bubbles can form from gas in the magma. This gas increases the pressure in the magma. If the pressure becomes great enough, the overlying rock can fracture, at which point an eruption occurs. Generally, volcanoes stop erupting because all the trapped volatile gasses have degassed and there is no longer sufficient pressure to drive the magma out of the Earth. Alternately, volcanoes stop erupting because enough heat is lost so that the magma cools and is no longer buoyant.

What are the different types of volcanoes?

The United States Geological Survey has identified four principal types of volcanoes:

Cinder cones are the simplest types of volcano. They are built from pieces of lava and tephra that have been ejected from a single volcanic vent. As the high gas-content lava is blown violently into the air, it breaks into small fragments that solidify and fall as cinders around the vent to form a cone. Most cinder cones have a bowl-shaped crater at the summit. Cinder cones are commonly found on the flanks of shield volcanoes and stratovolcanoes. One of the most famous cinder cones, Paricutin, grew in the middle of a cornfield in Mexico in 1943.

Cinder cone.

Shield volcanoes are large and gently sloping. The Hawaiian volcanoes typify them. Shield volcanoes generally erupt fluid basaltic lava. Shield volcanoes are built up slowly by the accumulation of many of these highly fluid lava flows that spread widely over great distances. They are characterized by low gas contents and therefore low-exclusivity, and they often produce fountain-like eruptions. Made almost entirely of lava, shield volcanoes are a common product of hotspot volcanism.

Shield volcano in Hawaii.

Stratovolcanoes are characterized by eruptions of lava that is more viscous (resistant to flow) and higher in gas content. They are often found at subduction-related arcs. Viscous lavas allow gas pressures to build up to high levels by chemically accommodating water well, as well as effectively "plugging" the volcano. As a result, stratovolcanoes often erupt explosively. Typically steep-sided, they are built of alternating layers of lava flows and tephra (volcanic ash and rocks that are blasted into the air upon eruption). The layering of these volcanic materials gives stratovolcanoes their other common name, Composite volcanoes. Well-known examples of stratovolcanoes are Mount St. Helens in the United States and Mount Fuji in Japan.

Stratovolcano.

Volcanic domes, also referred to as lava domes, commonly occur within the craters or on the sides of large stratovolcanoes. Volcanic domes are rounded, steep-sided mounds built by lava too viscous to flow any great distance. This viscous lava piles over and around its volcanic vent. Domes may consist of one or more individual lava flows. A dome grows largely by expansion from within. As a dome swells with hot magma inside, its outer surface cools and hardens, and then shatters, spilling loose fragments down its sides. Monte Pelee in Martinique is an example of a volcanic dome.

What determines the shape and size of a volcano?

The shape of a volcano depends primarily on how viscous the erupting lava is, which is determined by the lava’s chemical composition. Although magma is made of several different chemical compounds, the relationship between a volcano's shape and the chemical composition of the magma is largely determined by a single component: silica (SiO2). The more silica in magma, the more viscous or resistant to flow it is. Higher silica content also allows magma to trap more gas, which produces violent eruptions.

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