Shield volcanoes are known for their broad and gently sloping profiles. Unlike the towering, conical shapes of other volcano types, they are characterized by wide bases and non-explosive eruptions. Their morphology and eruptive behavior set them apart.
What Makes a Shield Volcano Unique
Shield volcanoes derive their name from their resemblance to a warrior’s shield, characterized by their low profile and gently convex slopes. These volcanoes typically have very wide bases and slopes that are usually only a few degrees, gradually steepening to about 10 degrees near the summit. Their distinctive shape results directly from the type of lava they erupt.
They are primarily built from highly fluid, low-viscosity basaltic lava. This runny lava travels great distances before solidifying, leading to their broad, spread-out form rather than a steep cone. Shield volcanoes can reach immense sizes. For example, Mauna Loa in Hawaii, one of Earth’s largest active volcanoes, measures about 100 miles (160 km) wide at its base.
How They Form and Erupt
Shield volcanoes form through the gradual accumulation of numerous thin, widespread lava flows. Successive layers of this fluid basaltic lava cool and harden, slowly building the volcano’s low, broad profile over hundreds of thousands to millions of years, with intermittent eruptions.
The eruptions from shield volcanoes are typically effusive, meaning they involve gentle outpourings of lava rather than violent explosions. This non-explosive behavior is attributed to the low viscosity and low gas content of the basaltic magma. Gases escape easily from the fluid magma, preventing explosive pressure buildup. While generally gentle, eruptions can become more explosive if water enters the volcanic vent.
Shield volcanoes are commonly associated with geological hotspots, where magma rises from deep within the Earth’s mantle. They can also form in continental rift zones, where tectonic plates pull apart, allowing magma to surface. Lava often erupts from a central summit vent or from fissures along the volcano’s flanks, sometimes traveling through lava tubes that extend flows over large areas.
Where Shield Volcanoes Are Found
Shield volcanoes are distributed globally, primarily in oceanic settings where tectonic plates move over stationary hotspots. The Hawaiian Islands are a prominent example, composed entirely of shield volcanoes, such as Mauna Loa and Kīlauea. Mauna Loa is considered the largest active volcano on Earth by mass and volume, rising over 13,600 feet (4,169 meters) above sea level, with its base extending deep below the ocean surface.
Kīlauea, also in Hawaii, is one of the world’s most active volcanoes, known for frequent effusive eruptions. Other notable locations include the Galápagos Islands, featuring active shield volcanoes like Wolf volcano, and Iceland, situated over both a hotspot and a mid-ocean ridge. Shield volcanoes can also be found in continental rift zones, such as in East Africa, with examples like Nyamuragira in the Democratic Republic of Congo.