The Volcanoes Volcanoes have fascinated humans for

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.

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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
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.For example, Mt. Rainier in Washington state, is an example of a composite volcano.

Considered among the most dangerous volcanoes in the world, it poses a huge threat to the Puyallup River Valley where it is located. Covered in glacial ice, should it erupt again it would produce enormous mudflows called lahars that could reach as far as neighboring Seattle.Mount Fuji, in Japan, is another example, of a composite volcano. Forming the largest mountain in Japan, it last erupted in 1708. Located 90 miles from Tokyo, scientists are aware that its cone is under tremendous pressure.

These stresses are being monitored using seismic sensors all over Japan, and any earthquake in the vicinity could be an indication that an eruption of the volcano itself is not far behind.

Ring of Fire
Ring of Fire

Lesson Summary

The earth is covered with tectonic plates that form earth’s lithosphere.

Beneath the plates of the lithosphere, is a molten layer of rock, known as magma, called the mantle. The magma is constantly moving, which forces the plates to move across the earth. In subduction zones, the movement of magma forces one plate beneath another. It is along these subduction zones that composite volcanoes form. Composite volcanoes are made up of layers of magma and ash.

Therefore, composite volcanoes tend to have steeper sides and produce eruptions of gas and sediment.


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