Shield volcanoes are the largest type of volcano on Earth, distinguished by their gently sloping structures. These landforms owe their unique profile to the specific composition of the magma that forms them. Understanding where these volcanoes are located requires examining the underlying geological processes that allow this particular type of magma to reach the surface. The distribution of shield volcanoes points to two primary tectonic settings where conditions align for their formation.
Defining the Shield Volcano
Shield volcanoes are named for their resemblance to a warrior’s shield lying on the ground. They are characterized by a low, broad profile and slopes that typically do not exceed 10 degrees near the summit. This distinctive shape results from the highly fluid, low-viscosity lava that builds the structure. The lava is primarily basaltic, meaning it has a low silica content, allowing it to flow easily and travel great distances.
Repeated, non-explosive eruptions of this fluid lava create broad, thin sheets that stack up over long periods. This effusive eruption style, often classified as Hawaiian-type, is generally less hazardous than the explosive eruptions of steeper, composite volcanoes. Mauna Loa in Hawaii, the largest active volcano on Earth by volume, demonstrates the scale achievable by this slow, steady accumulation of lava flows.
Formation Mechanism 1: Oceanic Hotspots
The most common location for shield volcanoes is in the middle of tectonic plates, far from plate boundaries, a setting known as an oceanic hotspot. These locations are fueled by a deep-seated column of superheated rock, called a mantle plume, which rises from the Earth’s lower mantle. The plume remains relatively fixed, creating a stationary source of heat and magma independent of the moving lithospheric plates above it.
As the plume reaches the base of the oceanic crust, the reduction in pressure causes the underlying mantle rock to undergo decompression melting. This process generates large volumes of low-viscosity basaltic magma. This magma then breaches the crust, forming a shield volcano on the seafloor that eventually emerges as a volcanic island. The Hawaiian Islands exemplify this process, with volcanoes like Mauna Loa and Kilauea sitting directly above the active Hawaiian hotspot.
The continuous movement of the Pacific Plate over the stationary hotspot results in a distinct chain of progressively older and dormant volcanic islands and seamounts. The active volcanoes remain anchored directly above the plume. Plate motion carries older volcanoes away, causing volcanism to cease as the magma source is cut off. The Galapagos Islands are another example of this hotspot volcanism, forming as the Nazca Plate moves over a mantle plume beneath the archipelago.
The oceanic crust allows the basaltic magma to ascend relatively unimpeded. The resulting volcanoes are built almost entirely of thousands of thin, fluid basalt flows. Some flows travel through insulating lava tubes to spread across large areas. This mechanism explains the creation of the planet’s largest volcanic structures, which are rooted deep on the ocean floor and rise for many kilometers.
Formation Mechanism 2: Continental Rifts and Divergent Margins
Shield volcanoes are also commonly found in areas where tectonic plates are actively pulling apart, including both mid-ocean ridges and continental rift zones. The separation of plates at these divergent boundaries leads to a thinning of the Earth’s crust. This allows hot mantle material to rise closer to the surface. This upward movement reduces the pressure on the mantle rock, triggering decompression melting similar to the hotspot mechanism.
The resulting basaltic magma rises through the fractures and fissures created by the plate separation, emerging to build shield volcanoes. Iceland provides a unique, active example, as it sits atop the Mid-Atlantic Ridge, a major divergent boundary. It is also influenced by an underlying mantle plume. This dual geological setting fuels volcanism, resulting in numerous shield structures that formed the island’s vast lava fields.
On continents, shield volcanoes develop along continental rift zones, where the landmass is being slowly stretched and pulled apart. The East African Rift Valley is a prominent example of this process, where the crust is thinning and faulting. As the continental crust stretches, the decrease in pressure allows mafic magma to ascend, forming shield volcanoes and broad lava flows.
In continental rift settings, the magma may interact with the thicker crust, sometimes leading to varied magma compositions. However, the fundamental structure remains the broad shield shape built by fluid basaltic eruptions. This highlights that the availability of low-viscosity, basaltic magma is the direct geological requirement for their formation.