Some volcanoes explode with the force of an atomic bomb. Others spill rivers of gently flowing lava. What causes volcanoes to erupt? How do scientists study them? Let’s explore one of the most powerful, destructive volcanic eruptions in history.
The city of St. Pierre with Mt. Pelée in the distance.

More than a century ago, the city of Saint-Pierre was known as the Paris of the Caribbean. Located on the island of Martinique, Saint-Pierre was a center of trade in rum, sugar, cocoa, and coffee. Its boulevards were lined with beautiful homes and shops.

But in the spring of 1902, all of that changed…

On the morning of May 8, the nearby volcano Mt. Peléeexploded in a burst. A swirling cloud of hot gas, ash, and rocks, called a pyroclastic flow, rushed down the mountainside at 480 kilometers per hour (300 mph). It burned everything in its path, including the town of Saint-Pierre and nearly all the ships in the harbor. Within two minutes, close to 30,000 people were dead.

How does subduction cause volcanoes to form?

  • When an oceanic plate collides with a continental plate, it sinks into the mantle below.
  • As the oceanic plate sinks, fluid (shown in purple) is squeezed out of it.
  • The fluid flows up into the mantle rock above and changes its chemistry, causing it to melt. This forms magma (molten rock).
  • The magma rises and collects in chambers within the crust.
  • As magma fills the chamber, pressure grows. If the pressure gets high enough, the magma can break through the crust and spew out in a volcanic eruption. Most explosive volcanoes occur above subduction zones.

Why are some volcanoes so explosive?

It’s all a matter of chemistry. The way a volcano erupts depends on the amount of gas and silica (a molecule of silicon oxygen) in the magma below. Magma with lots of silica is thick and gooey, while magma low in silica is thin and runny. And in magma with lots of gas, bubbles form as it rises. The more bubbles that form, the more explosive the eruption!
Explore how different shapes of volcanoes have different kinds of eruptions.
A shield volcano, like Mauna Kea in Hawaii, has gentle slopes formed by oozing, runny lava. The magma is low in silica and low in gas, so it doesn’t erupt explosively. Courtesy of Nula666, Wiki Commons
The most explosive eruptions come from stratovolcanoes, like the Augustine Volcano in Alaska. When they erupt, stratovolcanoes blow huge columns of gas and ash into the air that can collapse in hot, fast-moving clouds called pyroclastic flows. © USGS
A cinder cone volcano, like Tavurvur in Papua New Guinea, forms when erupted fragments harden and fall to the ground, accumulating around the vent in a cone shape. The lava is low in silica, so the lava is runny. High gas levels make for the explosive eruptions that send it flying. Cinder cones typically form at the beginning of eruptions, and lava flow follows. © USGS
A lava dome, like the one of Chaitén Volcano in Chile, forms when thick lava oozes from a vent, piles up, and cools into a steep mound. The lava is thick because it’s high in silica, and it oozes instead of explodes because it’s low in gas. Sometimes lava domes form after explosive eruptions. © USGS

How do scientists study volcanoes?

Around the world, there are about 1,500 active volcanoes.Active volcanoes are those that have erupted in the past, and could erupt again. About twenty are probably erupting right now.
These days, eruptions rarely come as a surprise. Scientists are keeping a watchful eye on active volcanoes. They want to find out if magma is rising beneath a volcano — a sign that it could erupt. The goal? Reduce the risk to humans who live near them.
Check out some of the tools that scientists use to monitor volcanoes.
This portable monitoring station is called a “spider.” Spiders are dropped into an active volcano’s crater and transmit seismic data to geologists about the movement of magma inside. © USGS
The Global Positioning System (GPS) and satellite radar technology can signal if rising magma is causing a volcano’s shape to change. Satellites bounce radar signals off a volcano’s surface and compare images over time to detect small changes in the mountain’s shape. © NASA
Scientists observe a wall of digital monitors that constantly relay Mt. Etna’s vital signs. © AMNH
Thermal imaging reveals where Mt. Etna is warming under the surface. © AMNH
This article comes from OLogy, the American Museum of Natural History’s website for kids.