Volcanoes are openings in the Earth’s crust that allow molten rock, known as magma, to escape from beneath the surface. This material, along with volcanic ash and gases, is expelled during an eruption. While the common image of a volcano is a towering, conical mountain, these geological features exhibit diverse forms and behaviors. A volcano’s shape and eruption style are influenced by magma composition and the forces bringing it to the surface.
Shield Volcanoes
Shield volcanoes are distinguished by their broad, gently sloping profiles, resembling a warrior’s shield laid flat on the ground. This distinctive shape results from the eruption of highly fluid, low-viscosity lava, primarily basaltic in composition. This runny lava flows easily and spreads out over vast areas before solidifying, building up wide, low-angle flanks.
Eruptions from shield volcanoes are typically non-explosive and effusive, meaning the lava flows out rather than erupting violently. While these eruptions can cover extensive landscapes, they generally pose less immediate danger to life compared to more explosive types. Mauna Loa in Hawaii is a prominent example, recognized as one of Earth’s largest active volcanoes by both mass and volume.
Stratovolcanoes
Stratovolcanoes, also known as composite volcanoes, have the iconic steep-sided, conical shape often associated with volcanoes. They are constructed over many eruptions from alternating layers of hardened lava flows, volcanic ash, cinders, and other fragmented rock. Their lava is typically more viscous, or thicker, preventing it from flowing far before solidifying and contributing to their steep profile.
Eruptions from stratovolcanoes are often explosive and can be quite dangerous, driven by the buildup of gas pressure within the viscous magma. Mount Fuji in Japan, with its nearly perfect symmetrical cone, is a well-known stratovolcano. Another notable example is Mount St. Helens in Washington, which is recognized for its significant explosive eruption in 1980.
Cinder Cone Volcanoes
Cinder cone volcanoes are generally the smallest and simplest type, characterized by a steep conical hill with a bowl-shaped crater at its summit. They form from the accumulation of ejected volcanic fragments, such as cinders and scoria, which are bits of gas-charged lava blown into the air that solidify and fall around the vent. These fragments build up the cone’s characteristic steep slopes.
Their eruptions are typically explosive but tend to be relatively short-lived. The process often involves “fire fountaining,” where gas-rich lava is propelled high into the air, creating the cinders. Parícutin in Mexico is a famous example, which emerged from a cornfield in 1943 and grew into a substantial cinder cone over nine years.