Types of volcanoes

Some lava is runny, like warm syrup.

Some lava is thick and sticky, like toothpaste. Runny lava spreads out flat. Sticky lava piles up tall. So volcanoes come in different shapes.

The U.S. National Park Service (NPS) says volcanoes are diverse. Some are small cones only a few hundred feet tall, and some are the most massive mountains on Earth.

The British Geological Survey (BGS) explains that a volcano's shape depends mostly on its magma: how sticky it is, how much gas it holds, what it is made of and how it reaches the surface. Runny lava flows far before it cools, so it builds wide, gentle slopes. Sticky lava cannot flow far, so it piles up near the vent into a steep cone. The three best-known types are shield volcanoes, composite volcanoes (also called stratovolcanoes) and cinder cones.

The U.S. Geological Survey (USGS) groups volcanoes into four main kinds: cinder cones, composite volcanoes, shield volcanoes and lava domes; the NPS adds maars, tuff rings, monogenetic volcanic fields and calderas, and divides volcanoes into monogenetic ones that usually erupt once and polygenetic ones that erupt many times. BGS notes that, strictly, there are two broad types, stratovolcanoes and shields, with cones, domes and calderas as features and processes.

The controlling variable is magma. The NPS explains that magma chemistry sets its physical properties, which control eruption dynamics and how easily lava flows; gas content and eruption rate also affect explosivity. BGS describes the scale: iron- and magnesium-rich basalt is hottest and runniest, while silica-rich magma is the most viscous. Viscous material piles up in steep cones of about 30 to 35 degrees; fluid basalt spreads into shields with slopes under 10 degrees.

A shield volcano is wide and flat.

Its lava is runny. It flows a long way before it cools. The volcanoes of Hawaii are shield volcanoes.

Shield volcanoes are built almost entirely from runny lava called basalt. Flow after flow spreads out in all directions and cools in thin layers, building a broad, gently sloping volcano shaped like a warrior's shield lying on the ground.

Shield volcanoes can be gigantic. The USGS says Mauna Loa in Hawaii rises 13,677 feet above sea level, and because the ocean floor around Hawaii is so deep, its top is more than 28,000 feet above the sea floor. Kilauea and Mauna Loa are two of the world's most active volcanoes. BGS explains that runny basalt lava usually erupts more gently than sticky lava, pouring out in rivers and sometimes spraying up as lava fountains.

Shield volcanoes are built by the accretion of thousands of highly fluid basalt flows that pour from a central summit vent or from rift zones on the flanks, spreading widely and cooling as thin, gently dipping sheets (USGS). BGS gives their slopes as less than 10 degrees. Hotspot volcanoes such as those of Hawaii are typically shields.

The USGS describes Mauna Loa, 13,677 feet above sea level with its summit over 28,000 feet above the deep ocean floor, as the largest shield volcano and the world's largest active volcano; BGS gives Mauna Kea and Mauna Loa as rising over 9 km above the surrounding sea floor. Not every basalt eruption builds a shield: in fissure eruptions, lava floods out of long cracks and stacks into lava plateaus, such as those along the Columbia River in Washington and Oregon, where the flows total more than a mile thick (USGS).

Some volcanoes are tall and pointy.

Their lava is sticky. Layers of lava and ash pile up, one on top of another. Mount Fuji in Japan looks like this.

Composite volcanoes, also called stratovolcanoes, are tall, steep, cone-shaped mountains. They are built from layer after layer of sticky lava flows, ash and broken rock.

The USGS says some of the grandest mountains on Earth are composite volcanoes, including Mount Fuji in Japan, Mount Cotopaxi in Ecuador and Mount Rainier and Mount St. Helens in Washington. Their sticky magma traps gas, so they are more likely to erupt explosively, throwing out ash as well as lava.

Composite volcanoes are typically steep-sided, symmetrical cones of large dimension built of alternating layers of lava flows, ash, cinders, blocks and bombs, and may rise as much as 8,000 feet above their bases (USGS). Most have a summit crater with a central vent or cluster of vents; lava solidified in flank fissures forms dikes that strengthen the cone like ribs. Their essential feature is a conduit system that carries magma from a reservoir deep in the crust.

BGS explains that stratovolcanoes form from viscous lava that does not flow easily, giving slopes of about 30 to 35 degrees, and that they are more likely to erupt explosively because gas builds up in the viscous magma. Andesite, named after the Andes, is perhaps their most common rock. When a composite volcano becomes dormant, erosion strips the cone until only its hardened plug and dikes remain as a volcanic neck (USGS).

A cinder cone is a small volcano.

Blobs of lava fly into the air. They cool into little rocks called cinders. The cinders fall and pile up into a round hill.

Cinder cones are the simplest kind of volcano. Gas-filled lava is blasted into the air from a single vent. It breaks into small pieces that harden and fall back as cinders, piling up into a round or oval cone, usually with a bowl-shaped crater on top. Most are small: the USGS says they rarely rise more than about a thousand feet.

In 1943 a cinder cone began growing in a farm field near the village of Parícutin in Mexico. Scientists from around the world came to study it. During about 9 years of activity, it built a cone about 1,200 feet high.

Cinder cones are built from particles and blobs of congealed lava ejected from a single vent; the gas-charged lava fragments solidify in the air and fall as cinders around the vent, forming a circular or oval cone, usually with a bowl-shaped summit crater, rarely more than a thousand feet or so high (USGS). The NPS classes them as usually monogenetic: they tend to erupt only once.

Parícutin, in Mexico, is the classic example. It began growing on a farm in 1943; explosive eruptions built a cone 1,200 feet high, and once the excess gas had escaped, lava poured quietly from the base. That order, eruption, cone and crater, then lava flows, is a common sequence for cinder cones. In about 9 years of activity Parícutin covered about 100 square miles with ash, and geologists from many countries learned a great deal from watching a volcano's whole life (USGS).

Some volcanoes ooze lava gently.

Others blast out ash. It depends on gas. Gas trapped in sticky lava pushes hard, like the fizz in a shaken bottle.

Why do some volcanoes ooze while others explode? BGS says it depends mostly on the gas in the magma. Think of a bottle of fizzy drink. Shake it, open it, and the gas rushes out, carrying drink with it.

In runny magma, gas bubbles escape easily, so the lava flows out in rivers. These are effusive eruptions. In sticky magma, gas is trapped and pressure builds until the magma bursts out explosively, throwing ash high into the sky. Volcanic ash is not soft like ash from a fire: the USGS explains that it is made of tiny, hard pieces of rock, minerals and glass, 2 millimeters across or smaller.

BGS distinguishes explosive eruptions, which send ash, gas and fragments high into the atmosphere, from effusive eruptions that produce lava flows, and attributes the difference largely to gas: in low-viscosity magma, gas escapes easily, while in viscous magma it is trapped and pressure builds, like a shaken bottle of fizzy pop. If viscous magma rises slowly enough to lose its gas, it oozes out as a lava dome instead. Effusive eruptions can still be enormous: Iceland's 2014 Holuhraun eruption produced about 1.4 cubic kilometers of lava over six months, covering 84 km².

Temperature varies with composition. The USGS California Volcano Observatory gives about 800 to 1,000 °C for silica-rich rhyolite and up to 1,100 to 1,250 °C for basalt and basaltic andesite. The USGS defines volcanic ash as fragments of rock, minerals and volcanic glass from 2 mm down to less than 0.004 mm; it is hard and abrasive, and wind can carry it great distances.

Very sticky lava makes a round hill called a dome.

Sometimes a volcano's top falls in. It leaves a giant bowl. Crater Lake in Oregon fills a bowl like this.

Lava domes form when lava is so sticky that it hardly flows at all. It piles up over and around the vent in a rounded mound. The NPS says Lassen Peak, in Lassen Volcanic National Park, is a dome.

Sometimes a big eruption empties the magma chamber under a volcano, and the top of the volcano collapses into the empty space. This leaves a huge bowl called a caldera. Crater Lake in Oregon fills a caldera left when a tall volcano, Mount Mazama, erupted and collapsed. A small cinder cone called Wizard Island now pokes out of the lake.

Lava domes are bulbous masses of lava too viscous to flow far; they grow largely by expansion from within, their cooled skins shattering and shedding fragments, and commonly occur in the craters or on the flanks of composite volcanoes (USGS). The Novarupta Dome, formed during the 1912 eruption of Katmai Volcano in Alaska, measures 800 feet across and 200 feet high. The NPS lists Lassen Peak as a dome.

Calderas form when a large eruption drains the magma chamber and the roof collapses. The USGS gives sizes from roughly circular depressions 1 to 15 miles across to elongated ones as much as 60 miles long. Crater Lake occupies the caldera of Mount Mazama, which the NPS dates to an eruption and collapse 7,700 years ago (an older USGS booklet says about 6,800). BGS cites Tambora in Indonesia, whose eruption on April 10, 1815, it calls the largest in recorded history: it removed an estimated 4,000 m peak and left a summit caldera 6 km across and 1.1 km deep.