Types of volcano

Measured from the sea floor, Mauna Kea and Mauna Loa in Hawaii rise more than 9 km, yet their slopes are gentler than 10 degrees.

Shield volcanoes

Shield volcanoes are built almost entirely of fluid basalt lava flows. Lava pours out from the summit or from two or three rift zones that radiate from it like the spokes of a bicycle wheel, and thousands of thin flows overlap into a broad, gently sloping dome that looks from far away like a warrior's shield lying on the ground. BGS gives their slopes as less than 10°. They are the largest volcanoes on Earth. The Hawaiian Islands are a chain of shield volcanoes, including Kilauea and Mauna Loa, the world's largest active volcano.

Stratovolcanoes

Some of the best known mountains in the world are composite volcanoes, also called stratovolcanoes: Mount Fuji, Cotopaxi in Ecuador, Mount Shasta, Mount Hood and Mount St. Helens. They are tall, with steep, even sides of about 30 to 35°, and are made of repeating layers of lava flows, ash, cinders, blocks and volcanic bombs. Their magma is sticky, so it does not flow far, and gas builds up in it, which is why stratovolcanoes are more likely to erupt explosively. The tallest on Earth is Ojos del Salado in Chile, with a summit 6,893 m above sea level; the tallest in the United States is Mount Rainier at 4,392 m.

Cinder cones and lava domes

Cinder cones are the simplest volcanoes. A blast throws molten rock into the air; it cools quickly, falls and breaks into small pieces of bubbly cinder that pile up around the vent into a cone that can be up to about 300 m high. If the wind blows during the eruption, the cone becomes oval. Thousands of cinder cones dot western North America. Lava domes are not really a type of volcano but an eruption feature. The lava is too thick to flow away, so it squeezes out and piles up over the vent, sometimes as spines and sometimes like a giant muffin. Domes grow from the inside, and their cooled outer shell can shatter and spill hot rock and gas down the slope, which makes them dangerous.

A famous example is the round Novarupta Dome, which formed during the 1912 eruption at Katmai in Alaska. It measures about 244 m across and 61 m high. USGS describes it as one of the last squirts of lava from a much larger and longer eruption, the largest and most violent ever to occur in the United States. Domes like it often grow inside the craters or on the flanks of large stratovolcanoes.

Calderas

Magma is stored beneath a volcano in a chamber. When a very large eruption empties it, the roof can collapse and leave a steep-walled bowl, a caldera, which can be tens of kilometres across. A caldera can also form when an eruption removes the summit of a single stratovolcano. BGS uses Tambora in Indonesia as an example. On 10 April 1815 it produced the largest eruption in recorded history, removed a peak estimated at 4,000 m and left a summit caldera 6 km across and 1.1 km deep. Shape tells you a lot: a gentle shield suggests runny basalt and lava flows, a steep cone suggests sticky magma and a history of explosions.