The Anatomy of a Volcano

Igneous extrusion builds a volcano. A mound or peak forms, built of melted rock and associated materials. Rock formed by volcanic heat is called igneous, from Latin, meaning “of fire”. The sides of a volcano can be steep, slope gently, or even form a volcanic plateau. Volcanoes are built around vents or fissures by material from deep within the earth.

Where the tectonic plates of the earth’s crust come together, where one plate subsides under another, or where a hotspot forms within a plate, volcanoes are found. The Hawaiian volcanoes lie over a hotspot. Mount Lassen in the western United States formed where rock subsided beneath the continental plate. Magma, molten rock, forms deep beneath the earth’s surface in areas like these. A pool of magma becomes the reservoir that powers a volcano. When magma comes out of the ground, it is called lava.

Volcanoes are divided into shield volcanoes, cinder cones, and composite or stratovolcanoes. Shape, size, and composition are different in each kind of volcano. Another volcanic feature is a caldera, a deep depression in a volcanic peak.

Shield volcanoes

Shield volcanoes are large, wide, and gently sloped. Lava emerges relatively slowly and quietly from fissures on a stratovolcano. They are usually built of basaltic lava, which tends to be runnier than other types. Because it is relatively fluid, it travels great distances before it cools. It may also carry less of the gasses that produce spectacular eruptions, though any eruption is spectacular enough. The Hawaiian Islands are built of immense shield volcanoes. The top of Mauna Loa is more than 28,000 feet above its base on the bottom of the ocean.

Cinder Cones

Cinder cones are the smallest volcanoes. They may be only a few hundred feet high and less than a mile in diameter at the base. They are steep. The crater is a relatively large part of the volcano. Cinder cones are formed entirely of pyroclastics, frothy bits of lava thrown out as blocks, bombs, cinders or ash. Ash may coat the ground for miles around a cinder cone, while lava may be thrown out as bombs up to 3 feet across. A steep sided cone is built of fragments too heavy to travel far.

Pyroclastics include pumice, which is full of cavities and will float on water. Scoria is a similar rock, but with coarser cavities. It does not float. Tephra is a general term for rock thrown through the air by volcanoes. It is classified by size as ash, cinders (lapilli), or volcanic blocks and bombs.

A cluster of cinder cones stands northeast of Mount Lassen, and they are found many other places as well. Climbing a cinder cone is like walking uphill through dry crunching sand, but more exhausting. A lava flow may emerge from the base or lower side of a cinder cone without destroying it. The loose construction of a cinder cone often will not carry a lava flow to the top.


Stratovolcanoes, also called composite cones, are built up in alternating layers. Layers of lava may flow over and solidify loose layers of pyroclastic rock. Flow types alternate, until a tall mountain stands. These sloped volcanoes grow steeper near the top, creating beautiful peaks like Mount Fuji and Mount Shasta.

However, the beautiful form of a stratovolcano can be broken. While such a volcano is dormant, has stopped erupting for many years, a plug of hardened lava forms in the mountain’s vent. The plug resists extrusions until great pressures build up. An eruption, if it comes, blows out great broken blocks of lava, and can shatter the volcano. It may send down a rain of glowing ash. It also produces a cloud of dust and condensed steam.

Magma is created by subsidence. When rock from under the ocean is drawn under the continental plate, it may be heated enough to give up the water bound within it. The water rises into the mantle rock above it. That rock has its melting point lowered by the addition of the water. It softens, becoming less dense, and rises.

Eventually, it may pool in a magma chamber beneath a volcano, still carrying superheated water, carbon dioxide, and other gasses it has picked up in its rise. However, the pressure is less here, and so the gasses begin to escape, the way gasses erupt from an opened soda bottle. They may blow out the plug in the vent of the volcano, the way shaken champagne can pop its cork. The latest eruption of Mount St. Helens, a stratovolcano, blew away the side of the mountain.

These volcanoes can produce pyroclastic flows, a combination of rock particles and gas that may travel as fast as 450 miles per hour. The deadly gases within the cloud can reach 1800 degrees Fahrenheit or more.

Calderas and other formations

A caldera is a volcanic feature that forms when a volcano’s crater collapses after magma is removed from beneath it. Subsidence calderas form when the lava beneath a volcano is drained away, leaving its top to sink. Explosive calderas form when the lava within a volcano is blasted away.

The Kilauea caldera formed when runny basaltic lava flows from the sides of the volcano drained away lava and left the top of the volcano to subside. It contains a beautiful pit crater, Halema’uma’u, said to be the home of Pele, goddess of fire.

Mount Mazama, in contrast, created an explosive caldera. When it exploded, around 5700 B.C.E., it lost 12 cubic miles of stone. More than 700 years were required for the caldera to cool and fill with water, to form serene Crater Lake in the Oregon Cascades. Another explosive caldera formed in the Mediterranean at Santorini. The explosion probably helped destroy the Minoan civilization.

Spatter cones are similar to cinder cones. They are small and messy looking. They form when viscous (sticky) lava is shredded by a gassy explosion and solidifies in clumps. Magma containing many gasses also produces ash and tuff cones, usually when it interacts with water. The mechanism of their formation is not clearly understood.


Volcanoes are built of molten rock from deep underground. If the lava is less sticky, it will flow a long distance and build a wide, gently sloped, shield volcano. If the lava holds a high proportion of gasses, it makes a steep, relatively small, cinder cone of pyroclastic materials. If a volcano is built of alternating layers of lava flows and pyroclastic materials, it is called a composite, or stratovolcano.