Volcanism is a geological process that shapes planetary surfaces by moving molten rock from the interior to the exterior. This activity creates various landforms, with the volcano serving as a vent for this material. The shield volcano is a major classification, distinguished by its massive size and uniquely broad, low-profile shape. This form results directly from the specific physical properties of the magma that builds it over geologic time.
Defining the Structure and Composition
A shield volcano is characterized by a wide base and gentle slopes, giving it a profile resembling a warrior’s shield lying on the ground. This shape results from highly fluid lava flowing great distances from the central vent. The overall profile is convex, with lower slopes typically beginning at two to three degrees and gradually steepening to about ten degrees near the summit.
These formations are constructed almost entirely from basaltic lava flows, which are rich in iron and magnesium but low in silica content. The individual flows that build the structure are relatively thin, often less than one meter thick, but they accumulate over millions of years. Shield volcanoes are considered the largest type of volcano on Earth. For example, Mauna Loa on Hawaiʻi is estimated to contain about 80,000 cubic kilometers of basalt, dwarfing most other volcanic types.
The Eruptive Mechanism of Formation
The creation of the shield shape is tied directly to the physical properties of the magma it erupts, primarily its low viscosity. The basaltic magma of shield volcanoes is notably runny. This low viscosity allows trapped gases to escape easily, preventing the buildup of pressure that causes violent, explosive eruptions.
Instead, the eruption style is effusive, meaning the lava flows out gently and continuously from the vent or fissure. The fluid lava travels great distances from the source before cooling and solidifying. This widespread flow, where thin sheets of lava are continually layered, slowly builds the characteristic dome over long periods.
The magma often originates deep within the Earth’s mantle, typically rising above hotspots or along divergent plate boundaries. At hotspots, a fixed plume of hot material creates a chain of shield volcanoes as the tectonic plate moves over it. Lava tubes, which form when the surface of a flow hardens while the interior remains molten, help lava maintain its temperature and travel farther across the flanks.
Famous Global Examples
The Hawaiian Islands provide the most recognized examples of shield volcanoes, as the entire chain was built over a mantle hotspot. Mauna Loa, on the Big Island of Hawaiʻi, is the largest single-mountain volcano on Earth, extending from the seafloor to over four kilometers above sea level. Neighboring Mauna Kea, when measured from its submerged base, stands as the tallest mountain in the world.
Kīlauea, also in Hawaiʻi, is one of the most active volcanoes on the planet. Beyond the Pacific, shield volcanoes are found globally in various tectonic settings. For instance, the Galápagos Islands feature several large shield volcanoes, such as Wolf volcano. Iceland, situated on a divergent plate boundary, also hosts shield volcanoes, though they show a slightly different morphology due to their smaller volume.