Volcanoes are openings, or vents where lava, tephra (small rocks), and steam erupt on to the Earth's surface. Many mountains form by folding, faulting, uplift, and erosion of the Earth's crust. Volcanic terrain, however, is built by the slow accumulation of erupted lava. The vent may be visible as a small bowl shaped depression at the summit of a cone or shield-shaped mountain. Through a series of cracks within and beneath the volcano, the vent connects to one or more linked storage areas of molten or partially molten rock (magma). This connection to fresh magma allows the volcano to erupt over and over again in the same location. In this way, the volcano grows ever larger, until it is no longer stable. Pieces of the volcano collapse as rock falls or as landslides.
Molten rock below the surface of the Earth that rises in volcanic vents is known as magma, but after it erupts from a volcano it is called lava. Magma is made of molten rock, crystals, and dissolved gas—imagine an unopened bottle of soda with grains of sand inside. The molten rock is made of the chemicals oxygen, silicon, aluminum, iron, magnesium, calcium, sodium, potassium, titanium, and manganese. After cooling, liquid magma may form crystals of various minerals until it becomes completely solid and forms an igneous or magmatic rock.
Originating many tens of miles beneath the ground, magma is lighter than surrounding solid rock. It is driven towards Earth's surface by buoyancy, it is lighter than the surrounding rock, and by pressure from gas within it. Magma forces its way upward and may ultimately break though weak areas in the Earth's crust. If so, an eruption begins.
Magma can be erupted in a variety of ways. Sometimes molten rock simply pours from the vent as fluid lava flows. It can also shoot violently into the air as dense clouds of rock shards (tephra) and gas. Larger fragments fall back around the vent, and clouds of tephra may move down the slope of the volcano under the force of gravity. Ash, tiny pieces of tephra the thickness of a strand of hair, may be carried by the wind only to fall to the ground many miles away. The smallest ash particles may be erupted miles into the sky and carried many times around the world by winds high in the atmosphere before they fall to the ground.
Scientists have identified 169 volcanoes within the United States that most likely will erupt at some point in the future. Most of these volcanoes are located in Alaska, a state where eruptions occur almost every year. The rest of the volcanoes are located throughout the American West, and in Hawaii (see our volcano activity map for their locations). Kīlauea volcano on the Island of Hawai‘i is one of the most active volcanoes on Earth. It has been erupting almost nonstop since 1983!
There are about 1,500 potentially active volcanoes worldwide, not counting the volcanoes under the oceans. About 500 of these have erupted in the past 100 years. Many of these are located around the Pacific Ocean in what is known as the "Ring of Fire." In the U.S., volcanoes along the west coast and in Alaska (Aleutian volcanic chain) are part of the Ring of Fire, while Yellowstone and Hawaiian volcanoes form over a "hot spot."See the Eruption, Earthquakes, and Emissions around the world since 1960.
Cinder cones are the simplest type of volcano. They are made of small pieces of solid lava, called cinder, that are erupted from a vent. The ground shakes as magma rises from within the Earth. Then, a powerful blast throws molten rocks, ash, and gas into the air. The rocks cool quickly in the air and fall to the earth to break into small pieces of bubbly cinder that pile up around the vent. They accumulate as a small cinder cone that can be as high as a thousand feet above the surrounding ground. If the wind is blowing during the eruption, cinder is carried downwind before its deposited into an oval shape. Eruptions that form cinder cones also feed lava flows that spread outward from the eruptive vent. When you climb a cinder cone you can usually find the bowl-shaped crater marking the location of the vent. If eruptions of cinder and lava flows happen repeatedly from the same vent, the overlapping layers can form a composite volcano (stratovolcano). When looking at a map, you will find that thousands of cinder cones exist in western North America and in other volcanic areas of the world.
Some of the Earth's grandest mountains are composite volcanoes—sometimes called stratovolcanoes. They are usually tall with steep even sides and are made out of repeating layers of lava flows, volcanic ash, cinders, blocks, and volcanic bombs. Some composite volcanoes rise over 8,000 feet above their surroundings, but they reach much higher elevations when compared to the level of the sea (called above sea level). Ojos del Salado in Chile is the tallest composite volcano on Earth with a summit elevation (height above sea level) of 22,615 feet; the tallest in the U.S. is Mount Rainier in Washington State with a summit elevation of 14,410 feet. Some of the most famous and beautiful mountains in the world are composite volcanoes, including Mount Fuji in Japan, Mount Cotopaxi in Ecuador, Mount Shasta in California, Mount Hood in Oregon, and Mount St. Helens in Washington.
Shield volcanoes are built almost entirely of fluid lava flows. Lava pours out of vents in all directions, either from the summit (top) or along two to three rift zones (fractures) that radiate out from the summit like spokes on a bicycle wheel. As lava flows overlap one another, they construct a broad, gently sloping dome shape that from far away appears similar to a warrior's shield. Shield volcanoes build up slowly by the growth of thousands of lava flows that spread widely over great distances, and then cool as thin sheets. On Earth, some of the most massive volcanoes are shield volcanoes. In northern California and Oregon, many shield volcanoes are up to 3 or 4 miles wide and as tall as 1,500 to 2,000 feet. The Hawaiian Islands are made of a chain of shield volcanoes including K?lauea and the world's largest active volcano, Mauna Loa. Looking at pictures of volcanoes, you can usually identify them by shape as being a shield volcano or stratovolcano.
Lava domes are technically lava flows, but they contain lava that is too thick to flow away from the vent. Lava squeezes out of the vent and accumulates as a giant pile over and around the vent. Some domes form pointy spines, while others appear as a giant muffin, as opening flower petals, or as steep-sided stubby flows or tongues. Lava domes often grow within craters or upon the flanks of large steep-sided composite volcanoes. Lava domes can be dangerous. They grow largely by expansion from within. As fresh magma fills the inside, the cooler and harder outer surface shatters and spills hot rock and gases down the mountainside. The circle-shaped Novarupta Dome that formed during the 1912 eruption of Katmai Volcano, Alaska, measures 800 feet across and 200 feet high. This dome was one of the last squirts of lava to emerge during a much larger and long-term eruption. The eruption at Katmai was the largest and most violent eruption ever to occur within the United States.