Volcanoes: Facts about geology's fieriest features

Earth's top layer, the crust, is made of cool, hardened rock. But in some places, geological processes cause parts of the crust to melt. Or the crust can crack open enough to let melted rock from the next layer of Earth, the mantle, rise to the surface.

One place this happens is at the boundaries of tectonic plates, which are the huge pieces of crust that fit together like puzzle pieces and cover the surface of the planet. At places where two tectonic plates are pulling away from each other, magma — hot, molten rock — can rise from the mantle to the surface, forming volcanoes.

Volcanoes can also form where plates crash into each other. When one tectonic plate pushes beneath another, it's called subduction. The plate diving into Earth pulls down rocks and minerals full of water. When that water-rich rock gets put under pressure by the weight of the crust pressing down on top of it, it can melt. This melting forms volcanoes.

Volcanoes can also form at hotspots, which are places where a really hot plume of rock in the mantle bubbles up, weakening the crust.

The volcanoes that form where tectonic plates are pulling apart are now seen at midocean ridges, which are places under the ocean where two pieces of crust move away from each other. Two major midocean ridges are the Mid-Atlantic Ridge, which runs down the center of the Atlantic Ocean like a zipper, and the East Pacific Rise, which runs along the west coast of North America and then out to sea in the southern Pacific.

The volcanoes that form where tectonic plates are pushing together happen anywhere that ocean crust goes under ocean crust, or anywhere that ocean crust goes under the crust that makes up the continents. The main place to find these types of volcanoes is the "Ring of Fire" that circles the Pacific Ocean. There are more than 450 volcanoes in the Ring of Fire, according to the National Oceanic and Atmospheric Administration.

Volcanic hotspots, on the other hand, occur in the middle of continents. Hot mantle plumes sit in one place for long periods of time while the crust slowly creeps over them. Yellowstone National Park, the Hawaiian Islands, and American Samoa are all landscapes formed by hotspot volcanoes. Hotspots are also found in Iceland, in the Afar region of Africa, and under the ocean worldwide.

Cinder cones are simple volcanoes that form where vents, or openings, spit out ash and lava. They can be cone- or oval-shaped, usually with a bowl-like crater at their summits.

When a cinder cone erupts over and over, it can evolve into a stratovolcano. Stratovolcanoes are cone-shaped volcanoes that build up from layer after layer of lava and ash, one eruption at a time. They're also called composite volcanoes. Mount Rainier in Washington is a famous stratovolcano.

Shield volcanoes are big, broad volcanoes made up of oozy lava flows that erupt from multiple vents. Hawaii's Mauna Loa and Kilauea are both shield volcanoes. Though they don't form a classic cone shape, shield volcanoes can be massive: Mauna Loa has an estimated volume of 18,000 cubic miles (75,000 cubic kilometers). The biggest shield volcano is Tamu Massif, and it sits on a seamount east of Japan, its summit 6,500 feet (1,980 m) below the ocean surface.

One final type of volcanic system is the Large Igneous Province, a massive outpouring of lava made from rocks called basalts. These eruptions can flood millions of square miles with lava that's over a mile thick. Luckily, there are no large igneous provinces active today. But scientists think that in the past, these huge eruptions caused some of the planet's biggest mass extinctions.

The first hint that a volcano might erupt is usually a lot of tiny earthquakes. Some of these might be big enough that humans can feel them; others can be detected only by sensitive instruments. Either way, the quakes hint that magma is moving beneath the surface, pushing through breaks in the rock and causing the ground to tremble.

The ground may swell as unseen magma flows toward the surface. Often, the volcano starts belching out more volcanic gas. Sometimes, new vents open in the ground so this gas can escape. The ground may even get hot as magma warms it from below. On some snow-covered volcanoes, scientists watch for new lakes forming near volcanic vents. These snowmelt lakes can reveal hotspots of magma below.

In a non-explosive eruption, like those that happen at Hawaii's volcanoes, lava fountains or oozes from cracks in the ground. The lava in these types of volcanoes doesn't contain much dissolved gas, so it doesn't erupt with a lot of force. It's more like gooey syrup than a shaken-up soft drink.

Explosive eruptions, though, can blast huge amounts of rock and ash thousands of feet into the air. The May 1980 eruption of Mount St. Helens sent ash 12 miles (19 kilometers) into the sky in the first 10 minutes of the explosion. Volcanoes like Mount St. Helens also often release pyroclastic flows, which are avalanches of hot rock, gas and ash that spill down the slopes at more than 200 mph (320 km/h). Pyroclastic flows can be as hot as 1,300 F (700 C).

Explosive eruptions might also release molten lava flows and lava bombs, which are chunks of partially melted rock that can fly for miles. Some icy volcanoes produce huge mudslides called lahars when hot rock and hot gas melt ice and snow on their slopes.