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A Comprehensive Guide to Composite Volcanoes

A Comprehensive Guide to Composite Volcanoes

Composite or Stratovolcanoes are the most common type of volcanic structures on Earth. Read this article, which covers some important facts about their structure, formation, and characteristics with famous examples.
Gaurav Athavale
Last Updated: Feb 21, 2018
Did You Know?
The highest volcano on Earth (in terms of land elevation) is the Nevado Ojos del Salado mountain, located on the Argentina-Chile border. It is also the highest composite/stratovolcano on the planet. The height of this geological structure has been measured to be almost 6900 meters.
Composite volcanoes are generally formed by the accumulation of different layers of lava, ash, tephra, pumice, etc., which are deposited over a number of eruptions. These volcanoes are mainly cone-shaped in appearance with a single crater at the top. The slopes vary from intermediate to steep, though the latter are more common. The lava that erupts from such volcanoes consists of medium to high content of silica, making it more felsic in nature. Thus, the erupted lava is characterized by high viscosity and mostly does not flow over longer distances.

The composite layered structure may also consist of alternate layers of hardened lava and ash deposited during consequent eruptions. The cone itself takes thousands of years to form, and in some cases, the volcanic crater might collapse leading to the formation of a caldera. Mount Krakatoa and Mount Vesuvius are the most well-known composite or stratovolcanoes, as they cause a lot of damage during their deadliest eruptions.
The following sections shall explain to you the various characteristics of composite volcanoes.
How Is a Composite Volcano Formed?
These volcanoes mostly form at subduction zones, where the oceanic crust gets consumed beneath the continental crust, and even when island arcs arise from the collision of two oceanic plates. The examples of the former category are present in the Andes Mountain Range in South America, and the Cascade Range in North America. Composite volcanoes in the latter category are present in the Pacific Ring of Fire, and also on the Japanese Islands.

The formation processes start with the addition of water from the basaltic igneous rocks and the minerals that are carried into the mantle during subduction. This released water reaches the upper mantle, i.e., asthenosphere. This process takes place only at specific temperature and pressure ranges depending on the minerals and the depth to which the subducted plate is consumed at the asthenosphere. The mantle rock undergoes partial melting as its melting point is lowered due to the release of water. Thus, the magma formed slowly rises upward and accumulates at the base of the lithosphere. With further increase in partial melting, the magma rises further and gets 'stored' in a magma chamber at the volcano base.

Gases like carbon dioxide and sulfur dioxide are also dissolved in this mixture along with several volatiles. At a particular point when the critical limit is reached regarding pressure and temperature buildup, the magma bursts out or erupts through the Earth's surface in the form of lava along with steam, and starts cooling down. With successive eruptions, a composite or stratovolcano is formed with a typical cone-shaped structure.
Examples of Famous Composite Volcanoes with Pictures
The images provided below showcase spectacular photographs of some of the most famous composite volcanoes on Earth.
Mount Merapi
Mount Merapi
Tungurahua Volcano
Tungurahua Volcano
Mount Cotopaxi
Mount Cotopaxi
Mount Hood
Mount Hood
Mount Mayon Volcano
Mount Mayon Volcano
Mount Shasta
Mount Shasta
Mount Etna
Mount Etna
Eyjafjallajokull Volcano
Eyjafjallajokull Volcano
Mount Baker
Mount Baker
Mount Fuji
Mount Fuji
Mount Krakatoa
Mount Krakatoa
Mount Ontake
Mount Ontake
Mount Pinatubo
Mount Pinatubo
Mount Rainier
Mount Rainier
Mount St. Helens
Mount St. Helens
Mount Vesuvius
Mount Vesuvius
Popocatepetl Volcano
Popocatepetl Volcano
Tambora Volcano
Tambora Volcano
Structure of Composite Volcanoes
composite volcano structure
The above diagram illustrates the basic structure of a stratovolcano with labeled parts. The interior of the volcano base consists of the magma chamber, where magma from the asthenosphere starts to accumulate. Above the magma chamber, the conduit system is present, through which the magma rises upwards. Several vents might be present on the volcano slopes, which have separate outlets that are linked to the main central column. Through these vents, steam and small amounts of lava might get released intermittently. At the volcano top, a crater is present, which mainly forms after repeated eruptions.

If the lava that erupts from the side vents cools down, then small dikes might be formed that increase the strength of the volcanic cone. Also, at the crater, the central conduit channel may break up into small vents, which can simultaneously release magma. The exterior of the conical flank portion might consist of alternate ash and lava flow layers that are deposited during numerous eruptions. This appearance is due to the periodic deposition of lava and ash material.

If the eruption of a stratovolcano is massive, then the volcano crater might collapse along with the cone, leading to the formation of a caldera. When it becomes extinct, a lake may form inside the crater if water gets accumulated due to precipitation or due to melting of ice. The most common magma types that erupt from such volcanoes are andesite, rhyolite, and dacite. They have a larger percentage of feldspar and quartz containing minerals.
Eruption Characteristics and Hazards
In addition to being the most common kind of volcano, composite volcanoes are also one of the most hazardous ones as the force of eruption is greater than in any other type. As the magma is quite viscous, the dissolved gases do not escape easily, and hence are released with an explosive force. Pyroclastic and mudflows are a common characteristic of composite volcanoes, due to which several individuals have been killed and injured during eruptions. Pyroclastic material is a mixture of hot mud, volcanic debris, ash particles, and volatiles that cascade down the slopes at high speeds. Mount Etna and Mount St. Helens are two of the most explosivestratovolcanoes ever known. The four main components of composite volcano eruptions are mentioned below:
Volcanic Ash
During eruption, ash clouds are one of the first materials to be released in the atmosphere. Ash is classified into various kinds depending on the thickness, density, and shape of the particles. Many times, an eruption might occur without lava flows, and might just consist of ash release along with some pyroclastic material. It is a hazard to flights as well to those living in the regions nearby. Often, airplanes have encountered engine and system problems and failures because of coming in contact with volcanic ash. Large amounts of such ash can cause respiratory problems and asphyxiation, and may also lead to death.
Lava Flows
As the lava that erupts from these volcanoes has a high silica content, it is quite thick and viscous. Hence mostly, it does not flow over long distances. The speed of flow is also quite less and hence, people are mostly not killed by its contact. The lava flows are known to destroy property more than causing human casualties. When an eruption occurs in colder regions, where ice sheets or glaciers are present, the ice might melt due to coming in contact with lava flows. In some cases, if the erupted lava is thick, it may close off or block the vent wherein the next eruption could be very explosive.
Mud Flows
Also called lahars or debris flows, they are mainly composed of water, ash, and mud, with the presence or absence of pyroclastic debris. These flows are mostly formed due to mixing of water with all the other volcanic products. Precipitation and runoff due to ice melting are the main sources of water for the formation of mud flows. The flows might vary in thickness depending on the amount of water and material mixed with it. They travel at high speeds and can quickly destroy entire cities or towns situated near a volcano. Human casualties are more in this case as compared to the ones due to lava flows.

Apart from these aspects of eruption, large fragments and pieces of igneous rocks may also be thrown off, which are potentially hazardous if they strike any building or civil structure. Such fragments are called lava bombs, and after eruption, they can travel for several kilometers at high speeds. Though such rocks do not explode on impact, the damage caused is often equivalent to that of an explosion.

Effects of Composite Volcano Eruptions
A violent and long eruption can have extreme effects on the climate of that region, and in rare cases, may also affect the global climate. Huge amounts of harmful gases like sulfur dioxide, carbon monoxide, methane, nitrogen dioxide, etc., are released in the atmosphere, sometimes till the altitude of the stratosphere. Massive volcanic clouds may even block the sun for months together, which can induce a drastic change in climate with the prevalence of cooler temperatures. This happened when Mount Tambora erupted in Indonesia in 1815, and its ash clouds blocked sunlight for almost a year. The surrounding regions experienced a sudden drop in temperature as well as cooler climatic conditions, along with a small famine.

Another aftereffect of a composite volcano eruption includes the possibility of occurrence of an earthquake. If the eruption force is massive, it may travel through the ground surface in the form of seismic waves and cause devastating earthquakes in the same region. Hence, it is better if such regions are not much inhabited. One famous example is that of the Mount Pinatubo volcanic eruption in Philippines in 1991.

A volcanic eruption, especially in case of stratovolcanoes, can change the geomorphology of the area, causing upheaval or even subsidence of the lithology. This mainly occurs in combination with an earthquake induced by the eruption. A few areas might suffer destabilization.

Scores of lives are lost every few decades due to eruptions from such volcanoes. Added to this is the loss of property and habitable areas due to lava flows, mud flows, pyroclastic material, etc., which can also claim lives. Entire cities and towns may get buried under the ash and pyroclastic material (for example, the destruction of Pompeii during the Mount Vesuvius eruption).
In spite of being dangerous in active or dormant state, composite or stratovolcanoes have formed some of the most beautiful mountains that attract a lot of tourists every year.