Hotspot volcanism is a distinct form of volcanic activity that takes place far from tectonic plate boundaries, within the interior of a plate. This phenomenon, known as intraplate volcanism, creates some of the planet’s most recognizable island chains and massive continental calderas. The mechanism accepted as the cause of this anomalous heat and magma production is the mantle plume.
Defining Hotspot Volcanism
A volcanic hotspot is a localized area of the Earth’s surface that experiences persistent volcanism over geologic timescales. Unlike volcanoes at plate boundaries, a hotspot is defined by its comparatively fixed position beneath the moving lithosphere. The overlying tectonic plate slowly drifts across this stationary heat source, resulting in a continuous trail of volcanism.
The Hawaiian Islands provide the classic example, with active volcanoes located directly above the heat source. Yellowstone National Park, a continental hotspot, is marked by vast caldera systems and geothermal features created by the same underlying process beneath the North American plate.
The Mechanism of Mantle Plumes
The mantle plume is a buoyant column of abnormally hot rock originating deep within the Earth’s mantle that provides the heat for hotspot volcanism. This structure has two main components: a narrow, deep conduit called the tail and a wider, bulbous region at the top known as the plume head. The plume material rises due to thermal buoyancy.
As the hot plume material ascends, the primary mechanism of magma generation is decompression melting, not an increase in temperature. The melting point of rock depends heavily on pressure; as the hot mantle rock rises closer to the surface, the confining pressure decreases substantially. This reduction in pressure causes the material to partially melt. The resulting basaltic melt rises through the crust to erupt on the surface, forming the volcanic hotspot.
The plume’s tail acts as a long-lived feeder channel, sustaining volcanism over millions of years as the plate moves over it. The initial arrival of a large plume head at the base of the lithosphere is often linked to the formation of massive outpourings of lava known as Large Igneous Provinces (LIPs). After this initial large-scale melting event, the narrower tail sustains the focused, long-term volcanic activity characteristic of an established hotspot.
Tracking Plate Movement with Seamount Chains
The movement of a tectonic plate over a stationary mantle plume creates a definitive, linear feature known as a seamount chain or hotspot track. The Hawaiian-Emperor Seamount Chain, stretching nearly 6,000 kilometers across the Pacific Ocean floor, serves as a precise record of the Pacific Plate’s past direction and speed.
The volcanoes in a hotspot track exhibit a clear age progression. The active volcano, such as the Island of Hawai’i, is the youngest and located directly over the plume source. Moving away from the active center, the volcanoes become progressively older, more eroded, and eventually subside beneath the ocean surface to form submerged seamounts. The oldest seamounts in the chain can be up to 85 million years old, providing a long-term record of plate motion.
A significant feature is the sharp Hawaiian-Emperor Bend, which occurred approximately 47 to 50 million years ago. This bend records a major change in the Pacific Plate’s movement, shifting from a northward path to its current northwesterly motion. While the plume was traditionally considered fixed, paleomagnetic data suggests the Hawaiian plume itself may have drifted slightly during the formation of the older Emperor seamounts.
The Deep Earth Origin of Plumes
Mantle plumes are believed to originate from the deepest parts of the planet, specifically at the Core-Mantle Boundary (CMB). This boundary is a significant temperature discontinuity, with the outer core being approximately 1,000 degrees Celsius hotter than the overlying mantle rock. This heat transfer creates thermal instabilities that initiate the rise of the buoyant plume material.
Seismic imaging has revealed two immense structures in the lower mantle, located just above the CMB, known as Large Low Shear Velocity Provinces (LLSVPs). These colossal structures, one beneath the Pacific and one beneath Africa, are thousands of kilometers wide. The LLSVPs are characterized by slower seismic wave speeds, indicating they are hotter and possibly chemically distinct.
These LLSVPs are hypothesized to act as thermal and chemical reservoirs, serving as the nucleation sites where mantle plumes are generated. The edges of these LLSVPs correlate closely with the locations of many present-day hotspots and ancient Large Igneous Provinces, suggesting a direct link between these deep structures and surface volcanism.