Stratovolcano

Some mountains are big volcanoes.

They are built of many layers. These layers are made of old lava. They are also made of rock.

These volcanoes can pop and blow. This happens because gas gets trapped inside. The blast can send ash into the air.

The ash can look like gray snow. It can even change how warm the Earth is. These mountains are very powerful.

A stratovolcano is a tall, cone-shaped mountain.

These volcanoes are built by many layers. These layers are made of hardened lava and tephra, which is rock from an eruption.

Because the layers are mixed, we also call them composite volcanoes.

Most stratovolcanoes form at subduction zones. This is a place where one crust plate slides under another.

As the plate goes down, it lets out water. This water helps the mantle melt to make magma. The magma then rises to a magma chamber.

These volcanoes often have explosive eruptions. This happens because their lava is viscous. Viscous means the lava is thick and sticky. It does not flow easily. This traps gas inside. When the pressure gets too high, the gas blasts out. This can create pyroclastic flows. These are fast, hot clouds of ash and gas.

They can move very fast and cause great danger. Some eruptions even send ash so high it changes the temperature of the Earth.

A stratovolcano is a tall, cone-shaped mountain.

These mountains are also called composite volcanoes. They are built by many alternating layers, or strata. These layers consist of hardened lava and tephra. Tephra is rock that comes from an eruption. Because these layers are often mixed and uneven, the name composite is used. These volcanoes are very common around our world. More than 700 stratovolcanoes have erupted in the last 11,700 years.

Most stratovolcanoes form at subduction zones.

This happens when one oceanic crust plate slides under another plate. As the plate sinks, it carries minerals like chlorite and serpentine. These minerals release water into the mantle. This process is called dewatering. The water lowers the melting point of the mantle. This causes flux melting, which creates magma. The magma then rises and gathers in a magma chamber.

Many stratovolcanoes are famous for their explosive eruptions. This happens because the lava is viscous. Viscous means the lava is thick and sticky. It does not flow easily like water. This thickness traps volcanic gases inside the magma. When the pressure becomes too high, the gases blast out. This can create pyroclastic flows. These are fast, hot mixtures of ash and gas. They can move at speeds over 50 miles per hour.

History shows us how powerful these eruptions can be. In 79 A.D., Mount Vesuvius erupted in Italy. It killed about 2,000 people and buried cities like Pompeii. In 1883, Krakatoa in Indonesia erupted and claimed 36,000 lives. More recently, Mount St. Helens erupted in 1980 in Washington State. In 1991, Mount Pinatubo in the Philippines had a huge eruption. This event sent an ash cloud 40 kilometers into the sky.

These volcanoes can even change how the Earth feels. The 1991 eruption of Mount Pinatubo released 22 million tons of sulfur dioxide. This created a cloud that lowered global temperatures by 0.5 °C. Volcanic ash can also be a problem for planes. The ash is made of tiny pieces of rock and glass. It can damage airplane engines during flight. We also see these mountains in other places. Some scientists think Zephyria Tholus on Mars might be a 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.