A volcano is a geological structure formed where a rupture in the crust allows hot magma, volcanic ash, and gases to escape from a planet’s interior. The materials that erupt and the forces involved determine the final physical shape, or morphology, of the resulting landform. Volcanoes are classified based primarily on their structure and the composition of the materials from which they are built. The viscosity, or thickness, of the magma is a primary factor influencing the volcano’s shape, as it dictates how far the lava flows before it solidifies.
Shield Volcanoes
Shield volcanoes are characterized by their broad, gently sloping profiles, resembling a warrior’s shield resting on the ground. This distinctive shape results from highly fluid, low-viscosity basaltic lava that flows easily and travels great distances from the central vent before solidifying. The lava flows are relatively thin, spreading out in wide sheets that slowly build up the mountain over successive eruptions. The magma is low in silica, allowing gases to escape easily, which leads to an effusive eruption style where lava flows out rather than exploding violently. Eruptions are non-explosive and often produce fire fountains of molten rock, commonly seen in Hawaii, such as at Mauna Loa.
Composite Volcanoes
Composite volcanoes, also known as stratovolcanoes, are the classic, steep-sided, symmetrical cones. They are built from alternating layers (strata) of hardened lava flows, fragmented rock, and volcanic ash, which contributes to their tall, conical shape. The magma erupted is high in silica, making it highly viscous. This thick magma traps volcanic gases, causing immense pressure to build up beneath the surface, resulting in highly explosive and dangerous Plinian eruptions. Since the viscous lava cannot flow far before cooling, it piles up steeply around the vent, as seen in examples like Mount St. Helens and Mount Fuji.
Cinder Cone Volcanoes
Cinder cone volcanoes are the smallest and morphologically simplest type of volcano. They are characterized by steep slopes (typically 30 to 40 degrees) and a distinctive bowl-shaped crater at the summit. These volcanoes are constructed almost entirely from loose, fragmented pyroclastic material called cinders, or scoria, which are bits of gas-charged lava that cool rapidly in the air. The cinders accumulate steeply around a single central vent, creating the cone structure. Cinder cones are generally short, ranging from tens to a few hundred meters in height, and are frequently found on the flanks of larger volcanoes or in clusters, such as ParÃcutin in Mexico.
Volcanic Calderas
Volcanic calderas are not cones built up by eruptions but rather massive, basin-shaped depressions in the ground. A caldera forms when a large volume of magma is rapidly expelled from a shallow magma chamber beneath a volcano, leaving a void. Without the underlying support, the land surface and the overlying volcanic structure collapse inward, creating a large depression often over one kilometer in diameter. This collapse typically follows a violent eruption that empties the magma chamber, such as the event that formed Crater Lake in Oregon. The resulting depression can sometimes fill with water, creating vast caldera lakes, as seen at Yellowstone National Park.