Some mountains are made of solid rock, like the Rocky Mountains or the Swiss Alps. But, some mountains are actually volcanoes.
Volcanoes are vents, or openings in the Earth's crust, that release ash, gases and steam, and hot liquid rock called lava. When the lava cools and hardens, it forms into the cone‐shaped mountain we think of as a volcano. Most of the world's volcanoes are found around the edges of tectonic plates, both on land and in the oceans.
On land, volcanoes form when one tectonic plate moves under another. Usually a thin, heavy oceanic plate subducts, or moves under, a thicker continental plate. When this happens, the ocean plate sinks into the mantle.
Water trapped in the rocks in this plate gets squeezed out. This causes some of the rocks to melt. The melted rock, or magma, is lighter than the surrounding rock and rises up. This magma collects in magma chambers, but it is still miles below the surface.
When enough magma builds up in the magma chamber, it forces its way up to the surface and erupts, often causing volcanic eruptions.
In the ocean, volcanoes erupt along cracks that are opened in the ocean floor by the spreading of two plates called a mid‐ocean ridge. Magma from the Earth's upper mantle rises up to fill these cracks. As the lava cools, it forms new crust on the edges of the cracks. These mid‐ocean ridges are actually long chains of underwater volcanoes that circle the Earth like the seams on a baseball.
About 80 to 90 percent of all volcanic eruptions occur where the plates spread apart.
The Ring of Fire is a large circle of explosive volcanoes around the Pacific Ocean. The circle is formed by the subduction of the Pacific Plate and some smaller plates under surrounding plates. © AMNH
The Mount Rainier volcano in Washington is considered “active.” Volcanoes are classified as active (erupted recently), dormant (expected to erupt in the future), and extinct (not expected to erupt again). © USGS

Hot Spots

Some volcanoes pop up in random places, often far from the edge of a tectonic plate. These volcanoes are found over "hot spots."
A hot spot is an intensely hot area in the mantle below the Earth's crust. The heat that fuels the hot spot comes from very deep in the Earth. This heat causes the mantle in that region to melt. The molten magma rises up and breaks through the crust to form a volcano.
While the hot spot stays in one place, rooted to its deep source of heat, the tectonic plate is slowly moving above it. As the plate moves, so does the volcano, and another one forms in its place. The volcano that moved is no longer active. This is why a chain of extinct volcanoes is often found extending from a hot spot.
Hot spots are found around the globe, on land and in the ocean. The Hawaiian Islands are the youngest volcanic mountains in a long chain of volcanoes that formed over a hotspot. They are still forming today. Another hot spot is under Yellowstone National Park, where the heat causes boiling mud pools and geysers like Old Faithful.
The Hawaiian Islands are still forming above a hotspot (left). © USGS
Old Faithful geyser in Yellowstone National Park (right). © INEEL
This article comes from OLogy, the American Museum of Natural History’s website for kids.