Hotspot (geology)
Introduction
A geological hotspot is a region in the Earth's mantle where a column of hot plume from the core-mantle boundary rises through the mantle and crust, resulting in volcanic activity on the surface. Unlike most volcanism, which occurs at tectonic plate boundaries, hotspots are typically found in the interior of tectonic plates. This phenomenon is thought to be the result of a narrow stream of hot mantle convecting up from the Earth's core–mantle boundary called a mantle plume.
Formation and Characteristics
Hotspots are formed when a mantle plume rises towards the Earth's crust. The rising plume is thought to begin as a thermal anomaly at the core-mantle boundary, with temperatures that are hundreds of degrees hotter than the surrounding mantle. As the plume rises, it begins to melt due to the decrease in pressure, forming magma. This magma then rises through the crust and erupts on the surface, forming a volcano.
Hotspots are characterized by high heat flow and seismic activity, and they often form chains of volcanoes known as hotspot tracks. These tracks form as the tectonic plate moves over the stationary hotspot, creating a line of volcanoes that grows older the further it is from the hotspot. The most famous example of a hotspot track is the Hawaiian-Emperor seamount chain.
Hotspot Volcanism
Hotspot volcanism is the process by which magma from a hotspot reaches the Earth's surface and erupts to form a volcano. This type of volcanism is unique in that it can occur in the middle of tectonic plates, far from plate boundaries. The magma produced by hotspots is typically basaltic, due to the composition of the mantle.
The process begins when a mantle plume rises towards the crust, melting to form magma. This magma then rises through the crust due to its lower density, eventually reaching the surface and erupting to form a volcano. Over time, as the tectonic plate moves over the stationary hotspot, a chain of volcanoes can form.
Evidence for Hotspots
There are several pieces of evidence that support the existence of hotspots. One of the most compelling is the existence of hotspot tracks, chains of volcanoes that grow older the further they are from the hotspot. These tracks provide a record of the movement of tectonic plates over stationary hotspots.
Another piece of evidence is the unique geochemical signature of hotspot volcanoes. The magma produced by hotspots has a different composition than that produced at plate boundaries, reflecting the composition of the deep mantle. This unique geochemical signature can be used to trace the source of the magma back to a hotspot.
Seismic evidence also supports the existence of hotspots. Seismic waves travel more slowly through hot material, and seismologists have detected low-velocity zones in the mantle beneath known hotspots. These low-velocity zones are thought to represent the hot, partially molten mantle plume.
Notable Hotspots
There are about 50 known hotspots around the world, with some of the most notable being the Hawaii, Yellowstone, and Iceland hotspots.
The Hawaiian hotspot is located in the middle of the Pacific Plate and is responsible for the formation of the Hawaiian Islands. The hotspot is currently located beneath the Big Island of Hawaii, which is the youngest and most volcanically active island in the chain.
The Yellowstone hotspot is located beneath the North American Plate and is responsible for the formation of the Yellowstone Caldera, one of the largest volcanic systems in the world. The hotspot is currently located beneath Yellowstone National Park, where it causes frequent geothermal activity.
The Iceland hotspot is unique in that it is located on the Mid-Atlantic Ridge, a divergent plate boundary. This location results in a high level of volcanic activity, with eruptions occurring on average once every five years.