A volcano is a rupture in the Earth’s crust that allows magma, ash, and gases to escape from a reservoir beneath the surface. Pressure from dissolved gases drives the material upward, leading to an eruption. Once the molten material reaches the surface, it is called lava, and its accumulation builds the volcanic structure.
The volcano’s final shape and eruption style are determined by the viscosity of the magma feeding it. Viscosity is the resistance of a fluid to flow, controlled primarily by silica content and temperature. Low-viscosity magma flows easily, resulting in gentle eruptions, while highly viscous, silica-rich magma traps gas, leading to explosive events. Geologists classify volcanic landforms into three main types based on these characteristics.
Shield Volcanoes
Shield volcanoes are characterized by their vast size and gentle, sloping flanks that resemble a warrior’s shield. This broad, dome-like profile results from the low-viscosity, basaltic magma that builds the structure. Basaltic magma is low in silica, allowing it to flow great distances from the central vent before it cools and solidifies.
The eruptions are typically effusive and non-explosive, meaning the lava flows out steadily. Over time, successive thin layers of fluid lava stack up, creating a mountain that is far wider than it is tall. Mauna Loa and Kīlauea in Hawaii are classic examples, with Mauna Loa being the largest active volcano on Earth. The ease with which the lava flows generally makes these eruptions less hazardous than other types.
Cinder Cone Volcanoes
Cinder cone volcanoes, also known as scoria cones, are the smallest and most common type of volcano. They feature steep sides, a circular base, and a bowl-shaped crater at the summit. These volcanoes are built not from flowing lava, but from ejected fragments of rock, known as cinders or scoria, which cool rapidly before piling up around the vent.
The eruption style is moderately explosive, often described as Strombolian, where gas-rich magma bursts forth in short-lived eruptions. These explosions throw lava fragments into the air, which fall back down as porous rock fragments. Due to the loose nature of this material, the sides of the cone maintain a steep slope. Cinder cones are often monogenetic, meaning they typically erupt only once, and frequently appear on the flanks of larger volcanoes.
Stratovolcanoes (Composite Volcanoes)
Stratovolcanoes, also known as composite volcanoes, are symmetrical cones with steep sides. Their conical shape is created by the alternation of thick, high-viscosity lava flows and layers of ash and pyroclastic debris. This layering, or stratification, is what gives them the name stratovolcano.
The magma feeding these volcanoes is typically silica-rich, such as andesite or dacite, which makes it very thick and sticky. This high viscosity prevents gas from easily escaping, causing immense pressure to build up in the magma chamber. When this pressure is finally released, it results in the most violent and dangerous eruption style, known as Plinian eruptions, which can blast ash and gas miles into the atmosphere. The sticky lava cannot flow far before solidifying, which contributes to the steep slopes of the cone. Stratovolcanoes are responsible for significant hazards, including fast-moving pyroclastic flows and destructive mudflows called lahars. Famous examples include Mount St. Helens and Mount Fuji.