Hotspot (geology)

In geology, a hotspot is a volcanic location fed by underlying mantle that is much hotter than the surrounding area. While most volcanoes form at the boundaries where tectonic plates meet, hotspots are unique because their position is independent of those edges. This means they can appear in the middle of a plate. They are essential for understanding how heat moves from deep inside the Earth to the surface.

Scientists use two main hypotheses to explain how these hotspots function. The first is the mantle plume theory. This suggests that hotspots are caused by mantle plumes, which are streams of hot mantle rising from the core–mantle boundary. These plumes act like thermal diapirs, or rising blobs of heat, moving upward through the Earth.

The second theory suggests the mantle beneath a hotspot is not actually hotter than its surroundings. Instead, the crust above is unusually thin or weak. This allows melt to rise passively from shallower depths through lithospheric extension, which is the stretching of the Earth's outer layer.

Researchers often categorize hotspots into two distinct types based on their depth. Primary hotspots originate from the deep core/mantle boundary. These create large volcanic provinces and long, linear tracks. Examples include Iceland, Hawaii, Easter Island, Afar, Louisville, Reunion, and Tristan. Secondary hotspots originate at the upper/lower mantle boundary. These do not create massive provinces but instead form island chains. Confirmed examples of secondary hotspots include Samoa, Tahiti, Cook, Pitcairn, Caroline, and MacDonald.

The concept of hotspots was significantly advanced by J. Tuzo Wilson in 1963. He postulated that the Hawaiian Islands formed because a tectonic plate moved slowly across a hot region beneath the surface. This idea helped explain why volcanoes appeared in a line away from plate boundaries. For a long time, scientists debated whether mantle plumes actually existed. However, modern seismic imaging has provided evidence consistent with the mantle plume theory.

Hotspot volcanism varies greatly depending on the local geology. Most hotspot volcanoes are basaltic, such as those in Hawaii or Tahiti. Basaltic magma is less explosive than the magma found in subduction zones. Subduction zones occur when a dense oceanic plate is forced downward into a trench, releasing water that melts the overlying plate. In contrast, when hotspots occur in continental regions, the magma must rise through thick continental crust. This process can melt the crust to form rhyolites. These rhyolites can lead to violent eruptions, such as those seen at the Yellowstone Caldera.

One of the most visible results of a hotspot is a volcanic chain. As a tectonic plate moves over a stationary hotspot, it creates a time-progressive trail of volcanoes. In the Hawaiian archipelago, the islands become progressively older and more eroded toward the northwest.

Similarly, Yellowstone sits at the end of a chain of extinct calderas that get older toward the west. Geologists use these trails to track the movement of tectonic plates. However, this work is complex because some chains, like the Galápagos, are not time-progressive. Additionally, hotspots do not always stay fixed relative to one another.

Studying hotspots connects many different fields of Earth science. By looking at the composition of basaltic samples, scientists can link different volcanic areas together. They can also use seismic tomography to image deep structures. For example, in 2020, researchers detected an oceanic plateau formed 100 million years ago. This was thought to be the plume head of the Hawaii-Emperor chain, now located 800 km deep under eastern Siberia. This deep connection shows how surface features are tied to the massive processes occurring deep within the Earth's interior.