Stratovolcano

A stratovolcano, often called a composite volcano, is a tall, cone-shaped mountain.

These volcanoes are built by many alternating layers, or strata, of hardened lava and tephra. Tephra refers to fragmented rock pieces ejected during an eruption. Because these layers are often mixed and uneven, the term composite is used. Stratovolcanoes are among the most common volcanic types on Earth. More than 700 stratovolcanoes have erupted during the Holocene Epoch, which covers the last 11,700 years.

Some older, extinct stratovolcanoes erupted as far back as Archean times.

Most stratovolcanoes form at subduction zones.

This occurs at plate tectonic boundaries where an oceanic crust plate is drawn under a continental plate or another oceanic plate. As the descending plate sinks, it carries hydrous minerals like chlorite and serpentine into the mantle. These minerals undergo a process called dewatering, where they release water at specific pressures and temperatures. This water lowers the melting point of the mantle, causing flux melting. This process generates magma, which then rises through the crust. As it rises, the magma incorporates silica-rich crustal rock, creating an intermediate composition.

Before an eruption, magma often pools in a magma chamber within the crust. Scientists are still researching the exact triggers that cause these chambers to erupt. One possibility is magma differentiation, where the lightest, most silica-rich magma and volatiles accumulate at the top. Volatiles are substances like water, halogens, and sulfur dioxide. Another mechanism is fractional crystallization. This happens when anhydrous minerals like feldspar crystallize out of the magma. This process concentrates volatiles in the remaining liquid, leading to a "second boiling" that increases pressure.

Other triggers involve the movement or changing state of the magma itself. Fresh magma may be injected into the chamber, heating the existing cooler magma. This can force volatiles out of solution and increase the pressure. There is evidence for this through magnesium-rich olivine crystals found in some lava. These crystals show no reaction rim, meaning they erupted immediately after mixing. Additionally, the progressive melting of surrounding country rock can contribute. External triggers, such as earthquakes or sector collapse, can also start an eruption. A sector collapse occurs when part of the volcano's flank falls away in a landslide.

Stratovolcanoes are known for their highly viscous, or thick and sticky, lava.

This high viscosity prevents volcanic gases from escaping easily. Instead, the gases remain trapped and build up tremendous internal pressure. When the vent is breached, the magma and gases degas explosively. This results in dangerous pyroclastic flows. These are swift, ground-sweeping mixtures of hot volcanic debris, ash, and superheated gases. They can travel at speeds exceeding 50 miles per hour.

Another hazard is the lahar, which is a fast-moving mudflow made of volcanic debris and water.

History records many catastrophic eruptions from these volcanoes. In 79 A.D., Mount Vesuvius in Italy erupted, killing an estimated 2,000 people. It smothered the cities of Pompeii and Herculaneum with ash and pumice. In 1883, the eruption of Krakatoa in Indonesia claimed 36,000 lives. More recently, the 1991 eruption of Mount Pinatubo in the Philippines was the second largest of the 20th century. It released 22 million tons of sulfur dioxide into the atmosphere. This created a cloud that lowered global temperatures by as much as 0.5 °C.

Other notable modern eruptions include Mount St. Helens in 1980 and Mount Unzen in 1991.

Volcanic hazards extend beyond the immediate eruption site. Volcanic ash consists of jagged pieces of rock, mineral, and volcanic glass. These grains are abrasive and do not dissolve in water. Ash clouds pose a serious risk to aviation, as they can cause engine failure in aircraft. For people on the ground, heavy ashfall can be dangerous. A four-inch thick layer of ash can weigh up to 200 pounds per square yard. When wet, it becomes even heavier and can collapse structures.

Even the composition of the magma changes the danger level. For example, Nyiragongo in Africa has low silica content, making its lava much less viscous than typical stratovolcanoes.