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Stratovolcano Formation & Examples

Margaret Stone, Rachel Tustin
  • Author
    Margaret Stone

    Margaret has taught both college and high school English and has a master's degree in English from Mississippi State University. She holds a Mississippi AA Educator License.

  • 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.

Understand the definition of a stratovolcano, or composite volcano, and study its characteristics. Examine how they form, and explore composite volcano examples. Updated: 03/23/2022

What is a Composite Volcano?

Volcanoes are vents found in the crust of planets and satellites from which molten rocks, gas, and lava erupt. Although erupting volcanoes are known for their spectacular displays, some types of volcanoes are more dangerous than others. The deadliest type of volcano is the stratovolcano, also known as a composite volcano.

What is a stratovolcano? Stratovolcanoes are made of layers of lava and ash. According to the composite volcano definition, composite volcanoes typically feature steep sides that are deceptively attractive since the stratovolcano is actually the most deadly type of volcano. The cinder cone volcano is another type of volcano. Cinder cone volcanoes are typically much smaller than stratovolcanoes, which can sometimes reach heights of 10,000 feet. Shield volcanoes are the third type of volcano. These differ from stratovolcanoes both in structure and material, but the primary difference lies in the eruptions. While shield volcanoes typically produce non-eruptive lava flows, stratovolcanoes produce explosive eruptions.

Composite Volcano Characteristics

What are composite volcanoes made of? The material found in composite volcanoes is half lava and half pyroclastic material. Stratovolcanoes or composite volcanoes are comprised of layers that include thin layers of ash, lava flows, and lava domes that spill lava but do not flow a great distance. In fact, some composite volcanoes consist of several lava domes stacked on top of each other. Perhaps the most noticeable of composite volcano characteristics is the volcano's appearance. Its slopes are quite steep near the summit and typically topped by a small summit crater.


Mount Hood, one of several stratovolcanoes in the United States, is located in Oregon.

Photograph of Mount Hood


Layers of the Earth

The Earth itself is also composed of layers. One of the layers is located 3000 to 4000 miles beneath the surface of the planet. The inner core is the deepest layer and is comprised of hot iron and nickel. Because of its location at the center of the Earth, the inner core spins slightly faster than Earth. Some scientists speculate that there may be a deeper core inside the inner core. If so, it is likely completely composed of iron.

The Earth's outer core is made of liquid iron and nickel. The heat generated from the radioactive decay of uranium and thorium creates huge waves of liquefied metal. The motion created in the outer core generates electricity which produces the planet's magnetic field.

Found about 19 miles below the surface of the Earth, the mantle is made of slowly circulating iron, magnesium, and silicon. However, the temperatures in the mantle's deepest layer are much hotter. These high temperatures create a layer of melted rock called the asthenosphere. This hot, molten section of the mantel is thought to support the tectonic plates found on the lithosphere.

The lithosphere is the Earth's crust. This thin and brittle layer is composed of silica, aluminum, and oxygen. The lithosphere's thickness varies. It is only about 3 miles thick beneath the oceans but as much as 43 miles thick beneath the ground.

The Earth's tectonic plates are huge slabs of broken pieces of the lithosphere. The tectonic plates move less than five inches a year, but this slight movement results in seismic activity that causes earthquakes and volcanoes. When two tectonic plates converge or push together, mountains are formed. The Earth's continents are embedded in the tectonic plates and drift along as the plates move. This slow drift accounts for the changes in the Earth's geography over long periods of time.

Tectonic Plates & Composite Volcanoes

A stratovolcano usually forms in an area called a subduction zone. Subduction zones, or convergent plate margins, are created when a tectonic plate below the ocean slides under another plate beneath a continent. Subduction zones help push a hot liquid called magma to the Earth's surface.

How Are Composite Volcanoes Formed?

How are composite volcanoes formed? They are created when volcanic eruptions cause volcanic ash and lava to form layers around the sides of a volcano. These layers eventually pile up to create the cone and steep slopes that typify composite volcanoes. It takes thousands of years for stratovolcanoes to form, but these impressive volcanoes can stand for millions of years and erupt thousands of times during their existence. The summit of a composite volcano usually contains a crater. Stratovolcanoes are made from basalt, rhyolite, and andesite volcanic rock.

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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Additional Info

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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Frequently Asked Questions

How does a stratovolcano form?

Tectonic plate movement causes the seismic activity that creates volcanoes and earthquakes. A stratovolcano then forms when layers of ash and lava pile up to create the cone and slopes of the volcano. Stratovolcanoes take thousands of years to form, and they typically contain a crater at the summit.

What is a composite volcano also known as?

Composite volcanoes are also known as stratovolcanoes. Most composite volcanoes are found in the so-called Ring of Fire, an area in the Pacific basin.

What are two examples of composite volcanoes?

Mount Tambora is a composite volcano located in Indonesia. In 1815, Mount Tambora experienced the largest volcanic eruption ever recorded. Mount St. Helens, which is located in the U.S, in Washington state, is another example of a composite volcano.

What is the difference between a stratovolcano and volcano?

Volcanoes are vents found in the crust of planets and satellites from which molten rocks, gas, and lava erupt. There are three types of volcanoes: shield volcanoes, cinder cone volcanoes, and stratovolcanoes. Stratovolcanoes are formed from layers of ash and lava.

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