Composite Volcanoes – Structure, Example, Formation and Life Cycle

Composite Volcanoes: Composite volcanoes, also referred to as stratovolcano, are a type of volcano that has a distinctive shape and also eruptive characteristic. These volcanoes are tall, steep-sided cones and are formed through a combination of both explosive as well as effusive volcanic activity. Let us understand more about the structure, eruptive behavior, formation, and characteristics of composite volcanoes.

Composite Volcanoes- Structure

Composite Volcano- Overview

Aspect	Description
Alternate Names	Stratovolcanoes, Conical Volcanoes
Shape	Tall, steep-sided cone
Eruption Type	Both explosive and effusive eruptions
Lava Composition	Andesitic or dacitic lava
Volcanic Materials	Alternating layers of lava flows, ash, and rocks
Hazardous Eruptions	Commonly associated with explosive eruptions
Hazards	Pyroclastic flows, ashfall, lava flows
Volcanic Explosivity	Can range from moderate to highly explosive
Examples	Mount St. Helens (USA), Mount Fuji (Japan), Mount Vesuvius (Italy)
Tectonic Setting	Often found near convergent plate boundaries
Monitoring and Mitigation	Monitored for early warning and risk mitigation
Notable Historic Eruptions	Vesuvius in AD 79, Krakatoa in 1883

Composite Volcano

Composite volcanoes, or stratovolcanoes, are one of the most common categories of volcanoes. Multiple eruptions spanning hundreds of thousands of years create a composite volcano. The eruptions gradually build up the composite volcano, layer by layer, until it reaches tens of thousands of meters in height. Some strata may be made of lava, while others may be made of ash, rock, or pyroclastic flows. Large amounts of thick magma can build up inside a composite volcano, causing it to detonate in a volcanic explosion.

Structure of a Composite Volcano

Composite volcanoes possess a central conduit that links the magma from the solid layers of the mantle to the outer surface. During the initial phases of composite volcano formation, this central conduit is a typical geothermal vent, either at ground level or slightly elevated.

As eruptions occur and additional layers accumulate, the vertical length of the conduit also increases. The central conduit may branch into secondary ducts, and these may or may not extend all the way to the surface.

When the branching duct extends to the surface, it gives rise to a parasitic cone, leading to the release of small quantities of lava. If the branching duct doesn’t reach the surface, it is referred to as a dyke. The upper part of the central conduit is termed the summit, and its opening is called the crater.

The area beneath the summit, within the volcano’s layers, is identified as a conduit. These layers consist of alternating ash and solidified lava. The steepness of the volcano’s slope is determined by the strength and scale of the eruption.

Examples of Composite Volcano

Most composite volcanoes are predominantly located in the Pacific’s “Ring of Fire.” Some famous examples of composite volcanoes are:

• Mount Fuji in Japan

• Mount Cotopaxi in Ecuador

• Mount Shasta in California

• Mount Hood in Oregon

• Mount St. Helens and Mount Rainier in Washington

Formation of Composite Volcano

Composite volcano are formed at subduction zones, where one tectonic plate is forced beneath another. This process occurs either when oceanic crust descends beneath another oceanic plate (as seen near Japan and the Aleutian Islands) or when oceanic crust is subducted beneath continental crust (as observed beneath the Andes and Cascades mountain ranges).

Water is trapped in porous basalt and minerals at subduction zones. As the plate descends deeper, temperature and pressure increase, initiating a phenomenon known as “dewatering”. The release of water from hydrates lowers the melting point of rock in the mantle. The resulting melted rock, or magma, is less dense than solid rock, causing it to ascend and become lava.

During this ascent, decreasing pressure allows volatile chemicals like water, carbon dioxide, sulphur dioxide, and chlorine gas to escape from the solution. Ultimately, the pressure buildup leads to the rupture of the stony plug covering the vent, resulting in a powerful explosion.

Characteristics of Composite Volcano

Some of the major geologic features found on composite volcanoes include:

• Central Crater: Found at the summit of a composite volcano, there is usually a sizable crater serving as the central vent for eruptions. This crater may exhibit continuous activity, releasing gases and intermittent ash, or it might remain inactive during periods between eruptions.

• Lava Dome: A rounded mass of dense lava can accumulate within the summit crater, forming a lava dome around the volcanic vent. Lava domes frequently occur in the concluding phases of a volcanic eruption and may pose a risk of explosive eruptions should the dome lose stability and collapse.

• Pyroclastic Flows: These fast-moving currents of hot ash, pumice and volcanic gas rush down the slopes of a composite volcano during an eruption. They can travel at speeds over 100 mph.

• Lava Flows: Lava flows, while less common in composite volcanoes, can develop on the sides and contribute to the overall conical shape during less explosive eruptions.

Life Cycle of Composite Volcano

The life cycle of a composite volcano can be categorized into following stages:

• The Emergence of Magma: As the pressure from the gases within the magma increases and ascends through the layers of the mantle, any susceptible rock in a vent fractures. Consequently, the magma erupts onto the Earth’s surface.

• Period of Active Volcanic Activity: During this phase, the volcano is highly active, experiencing eruptions approximately once every hundred years. With each successive eruption, the lava and ash discharged from the volcano solidify, contributing to the growth of a cone. Each layer adds to the overall height of the volcanic cone.

• Erosion of Cones: Similar to other natural formations, the cone formed through various eruptions undergoes gradual erosion due to natural elements. Consequently, the cone degrades, and the duct becomes sealed.

• Final Remnants: Over an extended period, erosion eliminates all traces of the volcanic cone or its eroded structure. Only the lava plateau formed persists.

Conclusion

A volcano is an opening in the earth’s crust through which gases, molten rock materials (lava), ash, steam etc. are emitted outward in the course of an eruption. There are various types of volcanoes: cinder cones, composite volcanoes, shield volcanoes, and lava volcanoes. Composite volcanoes also known as stratovolcanoes are tall, steep cones that produce explosive eruptions. They are also called stratovolcanoes because of their stratified layers of deposits that form the flanks of the volcano.

FAQs on Composite Volcano

Define a composite volcano?

Composite volcano, also known as stratovolcano is a cone-shaped volcano built from several layers of lava, pumice, ash, & tephra.

How is a composite volcano structured as?

Composite volcanoes are structured as cones. They are created with alternate layers of ash and lava.

Are composite volcanoes explosive or nonexplosive?

Composite volcanoes are explosive in nature.

What is a composite volcano also known as & why?

Composite volcanoes are also called stratovolcanoes because of their stratified layers of deposits that form the flanks of the volcano.

What is the difference between composite volcano and a cinder cone?

Composite volcanoes are tall, steep cones that produce explosive eruptions. Whereas, the Cinder cones are the smallest volcanoes and result from accumulation of many small fragments of ejected material.