Roughly 1,350 volcanoes around the world are considered potentially active, and about 500 of them have erupted in just the last century, according to the U.S. Geological Survey. That is a lot of mountains built from the inside out. Every one of them, from a snow-capped stratovolcano in the Cascades to a lava-spitting island in Hawaii, exists because of the same basic ingredient: magma finding a way to the surface. How that magma forms, and why it erupts where it does, comes down to a small number of very specific geological settings.

Magma Starts Deep and Fights Its Way Up

The USGS explains that magma originates many tens of miles beneath the ground, where extreme heat partially melts solid rock. That molten rock is lighter than the solid material around it, so it rises, pushed along by buoyancy and by the pressure of gases dissolved inside it. When it finally forces its way through a weak point in the crust, it erupts, sometimes as a slow-moving river of lava, sometimes as a violent blast of rock fragments, ash, and gas that can throw fine particles miles into the sky.

Where Plates Collide

Most of the world’s volcanoes form at convergent plate boundaries, where one tectonic plate is forced beneath another in a process called subduction, according to National Geographic Education. As the descending plate sinks deeper, rising heat and pressure melt the mantle rock above it, generating fresh magma that rises to the surface and builds chains of volcanoes called volcanic arcs. Roughly 425 of these volcanoes ring the Pacific Ocean in the horseshoe-shaped belt known as the Ring of Fire, including the Aleutian Islands and the Andes.

Where Plates Pull Apart

Volcanoes also form at the opposite kind of boundary, where plates are moving away from each other. Along divergent boundaries like the Mid-Atlantic Ridge, magma continuously rises from the mantle to fill the gap left behind, building new crust on both sides as it cools. That underwater mountain range stretches nearly 60,000 kilometers, or about 37,000 miles, making it the longest chain of volcanoes on Earth, even though almost all of it is hidden beneath the ocean.

The Hotspots That Ignore Plate Boundaries

Not every volcano needs a plate boundary at all. Hawaii and Yellowstone sit atop what geologists call hotspots, unusually hot, long-lived regions deep in the mantle that melt through the plate above them regardless of where a boundary happens to be, per USGS research. Because the hotspot itself stays roughly fixed while the Pacific Plate slowly slides over it, Hawaii’s volcanic activity has left behind a trail of islands that get progressively older to the northwest. Kauai’s oldest rocks are about 5.5 million years old, while the Big Island, still sitting over the hotspot, has rock less than 700,000 years old and an active volcano to show for it.

Not All Eruptions Look the Same

The setting that builds a volcano also shapes what it looks like. Thin, runny lava at hotspots and some divergent boundaries tends to build broad, gently sloped shield volcanoes, like those in Hawaii. Thicker, gas-rich magma at subduction zones tends to erupt more explosively, building the classic steep-sided stratovolcanoes like Mount Fuji or Mount St. Helens. Same basic process, wildly different results, depending on exactly where on the planet that magma finally breaks through.