Volcanoes are geological structures that form when molten rock, ash, and gases erupt from beneath the Earth’s surface. These vents in the Earth’s crust can release superheated materials. The type of molten rock and gases involved influence the resulting shape and size of the volcanic landform. This geological diversity leads to various types of volcanoes, each with distinct features and eruptive behaviors.
Shield Volcanoes
Shield volcanoes are characterized by their broad, gently sloping shape, which resembles a warrior’s shield lying on the ground. Their form results from the highly fluid, low-viscosity basaltic lava that erupts from them. Basaltic lava flows easily and spreads out over vast distances before solidifying, building up the volcano through successive layers of thin flows.
Eruptions from shield volcanoes are typically non-explosive and effusive. These effusive eruptions can produce lava fountains, but the low viscosity of the basaltic magma allows gases to escape without significant pressure buildup. Notable examples of these immense structures include Mauna Loa and Kīlauea in Hawaii, which are among the most active volcanoes on Earth and are composed almost entirely of basalt.
Stratovolcanoes
Stratovolcanoes, also known as composite volcanoes, possess a classic steep, conical shape with a prominent summit crater. Their structure is built up over thousands of years by alternating layers of viscous lava flows, ash, and rock fragments. The cooler, more viscous lavas, often andesitic or rhyolitic in composition, do not flow far and pile up around the vent, contributing to the steep profile.
Eruptions from stratovolcanoes are typically explosive and can be very dangerous due to the high viscosity and gas content of their magma. The thick, gassy magma traps pressure, leading to powerful blasts of ash, pyroclastic flows, and volcanic gases. Pyroclastic flows, which are fast-moving avalanches of hot debris and gases, and lahars, volcanic mudflows, are associated with these eruptions. Well-known stratovolcanoes include Mount Fuji in Japan, Mount St. Helens in the United States, and Mount Vesuvius in Italy, known for their potential for catastrophic eruptions.
Cinder Cone Volcanoes
Cinder cone volcanoes are the smallest and most common type, characterized by a simple, symmetrical cone shape with a bowl-shaped crater at the summit. These volcanoes are formed primarily from the accumulation of ejected volcanic fragments known as cinders or scoria. These fragments are pieces of gas-rich basaltic lava that are blown into the air, cool rapidly, and solidify as they fall around a single vent.
Cinder cone eruptions are typically short-lived and explosive, expelling these fragments in “fire fountains”. The loose, unconsolidated nature of the cinders allows for steep slopes, typically between 30 to 40 degrees, which is close to their angle of repose. Smaller, ranging from tens to hundreds of meters tall, cinder cones can occur individually or in clusters on the flanks of larger volcanoes or within volcanic fields. Parícutin in Mexico, which famously grew from a cornfield in 1943, is a classic example.