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Composite Volcano: Definition, Facts & Examples

Instructor: Rachel Tustin

Dr. Rachel Tustin has a PhD in Education focusing on Educational Technology, a Masters in English, and a BS in Marine Science. She has taught in K-12 for more than 15 years, and higher education for ten years.

The earth is not a quiet place, covered with plates in constant motion. Volcanoes come in all shapes and sizes, depending on where and how they have formed. In this lesson, we will look at composite volcanoes, including how and where they form, and where on earth they can be located.

The Allure of Volcanoes

Volcanoes have fascinated humans for thousands of years. Before the age of technology, they would erupt seemingly out of nowhere. Sometimes those eruptions would be as if bombs of gas and sediment had exploded. In other cases, such as the Hawaiian islands, those eruptions are slower and accompanied by the creep of molten rock along the earth's surface. Some volcanoes regularly erupt, where others may only erupt every few thousand years. In any case, their unpredictability has become a subject of human fascination.

Earth is like a Cake - of Rock

Earth is like a cake with many layers, each layer having different properties, including temperature and density. We live on the crust, which is the topmost layer. Also known as the lithosphere, the crust is made up of tectonic plates that are always in motion. Tectonic plates are irregular slabs of solid rock that can be a few hundred to several thousand kilometers wide, depending where on the earth they are located.

There are two types of lithosphere on Earth: oceanic and continental. Oceanic plates are thinner, around 8 KM, and occur beneath the large oceans and seas of the world. Continental plates are much thicker, around 25 km, and form the continents of the world.

Layers of the Earth
Layers of the Earth

Beneath the lithosphere is a layer of molten rock known as the mantle. Because the rock here is molten, and under less pressure than the core beneath of it, it can flow and behave like a liquid. Imagine asphalt being laid out on a road, and that is how magma, or molten rock, in the mantle, flows.

An Explosive Masterpiece

There are several types of volcanoes in the world, but the most explosive and active tend to be composite volcanoes. Also known as stratovolcanoes, these form along the boundaries of tectonic plates in subduction zones. These plates do not stand still, but rather move due to the moving magma, or molten rock, inside the earth.

In subduction zones, one of these plates is sliding underneath the other. The plate that is being forced back into the mantle, where the rock melts back into magma. Anything that happens to be on the plate, including any trapped seawater and other minerals, melts as well. This molten mix, in turn, melts the plate above it, forming a new volcano.

Composite volcanoes tend to form over thousands of years and are made up of layers. Some layers of volcanic ash, pumice, mixed with layers of lava. This layering structure makes them stronger, and so they tend to form steep-sided volcanoes.

Subduction Zones
Subduction Zones

Where in the World are Composite Volcanoes?

The vast majority of composite volcanoes occur in an area of the world known as the 'Ring of Fire.' Around the boundaries of the Pacific Ocean, the Pacific Plate is being subducted beneath the continents. As a result, the Pacific Ocean is slowly shrinking and along its boundaries, you find numerous volcanoes. Nearly sixty percent of the volcanoes in the world are composite, otherwise known as stratovolcanoes.

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