A geological hotspot is an area of persistent, intense volcanic activity situated far from tectonic plate boundaries. This volcanism is anomalous because most magmatic activity occurs where plates interact. Hotspots represent stationary thermal anomalies within the Earth’s mantle that generate localized melting beneath the moving lithosphere. The resulting volcanic structures are a distinct form of intraplate volcanism, providing a window into deep-earth processes independent of surface plate dynamics.
How Deep Mantle Plumes Create Hotspots
Hotspots are linked to deep mantle plumes, columns of unusually hot rock rising buoyantly through the Earth’s mantle. These plumes originate at the core-mantle boundary, specifically in the lowermost mantle layer known as the D” layer, approximately 2,900 kilometers beneath the surface. This boundary is a region of temperature contrast where thermal instabilities generate rising masses of hot material.
As the column of superheated rock ascends, it forms a structure similar to a mushroom, featuring a narrow conduit connected to a large, bulbous head. When this hot plume head reaches the base of the lithosphere, pressure reduction causes the rock to undergo decompression melting, generating vast quantities of magma. Hotspot volcanism is driven by this deep, anomalous heat source, unlike melting triggered by water (subduction zones) or shallow melting (rift zones).
The resulting volcanic activity is characterized by its immobility compared to the tectonic plate above it. The plume is anchored deep within the mantle, while the rigid lithospheric plate slides over it at a slow, continuous rate. This dynamic relationship creates a time-progressive sequence of volcanic features on the surface, directly recording the speed and direction of the overriding plate’s movement. Seismic imaging provides evidence supporting the existence of these deep mantle structures and the plume hypothesis.
Unique Geological Features Created by Hotspots
The most recognizable product of a hotspot is the formation of a linear chain of volcanoes, islands, and seamounts known as a hotspot track. As the tectonic plate moves across the fixed plume, the active volcano is carried away from the heat source, becomes extinct, and a new volcano forms directly above the plume. This process results in a chain where structures become progressively older and more eroded the further they are from the active center.
In oceanic settings, this chain often begins as a volcanic island, which eventually subsides and is worn down by erosion, becoming a flat-topped, submerged volcano known as a guyot or seamount. The length and age distribution of these tracks, such as the Hawaiian-Emperor Seamount chain, provide a long-term record of plate motion over tens of millions of years.
Large Igneous Provinces (LIPs) are also produced by hotspots, accumulating enormous volumes of volcanic rock over a short geological timeframe. These massive flood basalt eruptions are associated with the initial arrival of a plume’s large head at the base of the lithosphere, releasing millions of cubic kilometers of lava.
Comparing Oceanic and Continental Hotspot Examples
The nature of the crust overlying the mantle plume influences the resulting volcanic landscape and magmatic composition. Oceanic hotspots, like the one beneath Hawaiʻi, occur where the lithosphere is thin, measuring about 10 kilometers. The magma generated by the plume, which is basaltic and low in silica, can ascend quickly through this thin crust. This results in frequent, effusive eruptions of fluid lava that build broad, gently sloped shield volcanoes.
Conversely, continental hotspots, exemplified by the Yellowstone system, are situated beneath a thicker continental crust, which can be around 45 kilometers deep. This thick barrier prevents the deep-sourced basaltic magma from easily reaching the surface, causing it to stall and pool within the crust. As the magma resides there, it interacts with and melts the silica-rich continental rock, transforming its composition into highly viscous, gas-trapping rhyolitic magma. The pressure build-up from this sticky magma leads to less frequent but more explosive eruptions, which often result in the formation of vast caldera systems rather than tall shield volcanoes.