Mount Fuji is an example of a composite volcano produced by the collision of tectonic plates.
Some of history’s most explosive volcanic eruptions have involved volcanoes created by the collision of tectonic plates. These convergent boundaries are home to a dangerous type of volcano, known as the composite volcano. When these large, classically shaped volcanoes erupt, they frequently produce powerful explosions, large pyroclastic flows and deadly mudslides, effecting both the surface and the atmosphere.
The Earth’s crust is comprised of around a dozen fractured pieces, called plates. As they move on top of the planet’s fluid mantle, the plates collide with each other in certain areas. The areas where these plate collisions occur are called convergent zones. When collisions involve oceanic and continental plates, the heavier oceanic plates are pushed beneath the more buoyant continental plates. This process creates a subduction zone, leading to the formation of volcanoes.
Composite volcanoes, or stratovolcanoes, are typical in subduction zones, and account for around 60 percent of all volcanoes. These conically shaped volcanoes are formed by alternating layers of hardened lava and pyroclastic material, creating a strong, composite structure. They have steep sides, rising as much as 10,000 feet, with a crater at the summit. A high concentration of stratovolcanoes is found in subduction zones around the Pacific Rim, referred to as the “Ring of Fire.”
Characteristics of Composite Eruptions
Composite volcanoes typically eject andesite lava, which is cooler and thicker than the oceanic basalt lava. Andesite lava contains large amounts of trapped gases, leading to explosive eruptions. The steep sides of stratovolcanoes leave them prone to landslides, avalanches and mudflows. Composite volcano eruptions are unpredictable due to their propensity to suffer sector collapses, where entire sides, or flanks, of the volcano fail during an eruption. The 1980 eruption of Mount St. Helens is an example of sector collapse in a composite volcano. Because they are fueled by a slow magma supply rate, composite volcanoes typically erupt infrequently.
The Cascade Range
The Cascade Range, located in the Pacific Northwest, provides an example of composite volcanoes formed by plate subduction. In this case, the Juan de Fuca plate is colliding with and subducting under the North American plate. This has led to the formation of some of the world’s largest and most iconic stratovolcanoes, including Mount Rainier, Mount St. Helens and Mount Shasta.
Mount Fuji, located in Japan, is another example of a large composite volcano formed by a subduction zone. Here, the Philippine Sea plate is being subducted under Japan. However, Mount Fuji is also unique in that geologists have identified a tear in the Philippine Sea plate underneath the volcano. This tear, or hotspot, allows mantle magma to fuel the volcano. While the original eruptions were andesite in nature, eruptions have been basaltic in nature since the tear developed. This is highly unusual for a stratovolcano.
The 1991 explosive eruption of Pinatubo, located in the Philippines, was the second largest eruption of the 20th century. This composite volcano is part of a complex subduction zone involving the Eurasian plate and the Philippine plate. This eruption also included enormous lahars, or mudslides.