A volcano is a geological rupture in the crust of a planetary body that allows hot materials, such as magma, ash, and gases, to escape to the surface. The shape and size of a volcano are direct results of the magma’s chemical composition and the resulting eruption style. Magma viscosity, or resistance to flow, is the primary factor determining a volcano’s final structure, leading geologists to classify these features based on their morphology.
Shield Volcanoes
Shield volcanoes are characterized by their vast, gently sloping profiles, which resemble the shape of a warrior’s shield lying on the ground. This low-angle topography is a consequence of the magma’s extremely low viscosity. The lava erupted is almost exclusively basaltic, a mafic composition that is very fluid and contains a low percentage of silica.
Because the lava is so fluid, it flows long distances rapidly away from the central vent before it cools and solidifies. This effusive eruption style is generally non-explosive, allowing thin layers of lava to accumulate over broad areas. Hawaiian volcanoes, such as Mauna Loa and Kilauea, represent the largest examples of this type.
Cinder Cone Volcanoes
Cinder cone volcanoes are steep, symmetrical cones formed by an explosive eruptive mechanism, contrasting with the gentle slopes of the shield type. These are the smallest and simplest type of volcano, built almost entirely from loose, pyroclastic fragments called cinders or scoria. Eruptions are typically short-lived but involve explosive bursts of gas-rich magma.
As the gas-charged magma is violently ejected into the air, it fragments and cools quickly, falling back to Earth as solidified, porous rock pieces. These fragments accumulate around the vent, forming a conical hill with steep sides, often between 30 and 40 degrees, and a bowl-shaped crater at the summit. Cinder cones rarely exceed 300 meters (1,000 feet) in height and often form as isolated features or on the flanks of larger volcanoes, such as ParicutÃn in Mexico.
Stratovolcanoes
Stratovolcanoes, also known as composite volcanoes, exhibit the classic, symmetrical cone shape often associated with volcanoes. They feature steep summits and are much larger than cinder cones. Their structure is built from alternating layers, or strata, of solidified lava flows and pyroclastic material like ash and volcanic bombs. This layering is precisely why they are called composite.
The magma feeding these volcanoes is typically intermediate to felsic in composition, such as andesite or dacite, meaning it has a high silica content. This high silica makes the magma highly viscous, trapping volcanic gases and leading to significant pressure buildup beneath the surface.
Stratovolcanoes are known for their highly explosive and dangerous eruptions. They often produce devastating hazards like pyroclastic flows (fast-moving currents of hot gas and rock) and lahars (volcanic mudflows). Famous examples include Mount St. Helens and Mount Fuji, which are often found in subduction zones where one tectonic plate slides beneath another. The viscous lava solidifies quickly, creating the steep, conical profile that ranges between 10 and 35 degrees.
Lava Domes
Lava domes form from the slow extrusion of lava that is so highly viscous it piles up immediately around the vent instead of flowing away. This type of lava is often rhyolite or dacite, representing the most silica-rich magma found on Earth. The resulting structure is a bulbous, circular, or mound-shaped protrusion with a rough, blocky surface.
The growth of a lava dome is a slow process where the lava oozes out and solidifies in place. It sometimes grows endogenously (from the inside) or exogenously (by new flows over the surface). Since the magma is so thick, it effectively plugs the volcanic conduit, preventing gases from escaping easily.
This blockage can lead to a dangerous buildup of pressure. The eventual collapse or explosion of the dome can generate destructive pyroclastic flows. The dome that formed within the crater of Mount St. Helens after its 1980 eruption is a well-documented example of this high-viscosity landform.