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Hotspot (geology)

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File:Hawaii hotspot cross-sectional diagram.jpg
Diagram showing a cross section through Earth at the Hawaii hotspot. Magma originating in the mantle rises into the asthenosphere and lithosphere. A chain of volcanoes is created as the lithosphere moves over the source of magma. ("Ma" means millions of years old/ago).

In geology, hotspots (or hot spots) are volcanic locales thought to be fed by underlying mantle that is anomalously hot compared with the surrounding mantle.<ref name="USGSYellow">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Examples include the Hawaii, Iceland, and Yellowstone hotspots. A hotspot's position on the Earth's surface is independent of tectonic plate boundaries, and so hotspots may create a chain of volcanoes as the plates move above them.

There are two hypotheses that attempt to explain their origins. One suggests that hotspots are due to mantle plumes that rise as thermal diapirs from the core–mantle boundary.<ref name="Morgan">Template:Cite journal</ref> The alternative plate theory is that the mantle source beneath a hotspot is not anomalously hot, rather the crust above is unusually weak or thin, so that lithospheric extension permits the passive rising of melt from shallow depths.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="Foulger">Template:Cite book</ref>

OriginEdit

File:Partial melting asthenosphere EN.svg
Schematic diagram showing the physical processes inside the Earth that lead to the generation of magma. Partial melting begins above the fusion point.
File:Tectonic plates boundaries physical World map Wt 180degE centered-en.svg
Map showing approximate location of many current hotspots and the relationship to current tectonic plates and their boundaries and movement vectors

The origins of the concept of hotspots lie in the work of J. Tuzo Wilson, who postulated in 1963 that the formation of the Hawaiian Islands resulted from the slow movement of a tectonic plate across a hot region beneath the surface.<ref>Template:Cite journal</ref> It was later postulated that hotspots are fed by streams of hot mantle rising from the Earth's core–mantle boundary in a structure called a mantle plume.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Whether or not such mantle plumes exist has been the subject of a major controversy in Earth science,<ref name="Foulger" /><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> but seismic images consistent with evolving theory now exist.<ref name=Koppers2021>Template:Cite journal</ref>

At any place where volcanism is not linked to a constructive or destructive plate margin, the concept of a hotspot has been used to explain its origin. A review article by Courtillot et al.<ref name="Courtillot 2003 295–308" /> listing possible hotspots makes a distinction between primary hotspots coming from deep within the mantle and secondary hotspots derived from mantle plumes. The primary hotspots originate from the core/mantle boundary and create large volcanic provinces with linear tracks (Easter Island, Iceland, Hawaii, Afar, Louisville, Reunion, and Tristan confirmed; Galapagos, Kerguelen and Marquersas likely). The secondary hotspots originate at the upper/lower mantle boundary, and do not form large volcanic provinces, but island chains (Samoa, Tahiti, Cook, Pitcairn, Caroline, MacDonald confirmed, with up to 20 or so more possible). Other potential hotspots are the result of shallow mantle material surfacing in areas of lithospheric break-up caused by tension and are thus a very different type of volcanism.

Estimates for the number of hotspots postulated to be fed by mantle plumes have ranged from about 20 to several thousand, with most geologists considering a few tens to exist.<ref name=Koppers2021/> Hawaii, Réunion, Yellowstone, Galápagos, and Iceland are some of the most active volcanic regions to which the hypothesis is applied. The plumes imaged to date vary widely in width and other characteristics, and are tilted, being not the simple, relatively narrow and purely thermal plumes many expected.<ref name=Koppers2021/> Only one, (Yellowstone) has as yet been consistently modelled and imaged from deep mantle to surface.<ref name=Koppers2021/>

CompositionEdit

Most hotspot volcanoes are basaltic (e.g., Hawaii, Tahiti). As a result, they are less explosive than subduction zone volcanoes, in which water is trapped under the overriding plate. Where hotspots occur in continental regions, basaltic magma rises through the continental crust, which melts to form rhyolites. These rhyolites can form violent eruptions.<ref>Donald Hyndman; David Hyndman (1 January 2016). Natural Hazards and Disasters. Cengage Learning. pp. 44–. Template:ISBN.</ref><ref>Wolfgang Frisch; Martin Meschede; Ronald C. Blakey (2 November 2010). Plate Tectonics: Continental Drift and Mountain Building. Springer Science & Business Media. pp. 87–. Template:ISBN.</ref> For example, the Yellowstone Caldera was formed by some of the most powerful volcanic explosions in geologic history. However, when the rhyolite is completely erupted, it may be followed by eruptions of basaltic magma rising through the same lithospheric fissures (cracks in the lithosphere). An example of this activity is the Ilgachuz Range in British Columbia, which was created by an early complex series of trachyte and rhyolite eruptions, and late extrusion of a sequence of basaltic lava flows.<ref>Template:Cite journal</ref>

The hotspot hypothesis is now closely linked to the mantle plume hypothesis.<ref>Mainak Choudhuri; Michal Nemčok (22 August 2016). Mantle Plumes and Their Effects. Springer. pp. 18–. Template:ISBN.</ref><ref name=Koppers2021/> The detailed compositional studies now possible on hotspot basalts have allowed linkage of samples over the wider areas often implicate in the later hypothesis,<ref name=Bredow2018>Template:Cite journal</ref> and its seismic imaging developments.<ref name=Koppers2021/>

Contrast with subduction zone island arcsEdit

Hotspot volcanoes are considered to have a fundamentally different origin from island arc volcanoes. The latter form over subduction zones, at converging plate boundaries. When one oceanic plate meets another, the denser plate is forced downward into a deep ocean trench. This plate, as it is subducted, releases water into the base of the over-riding plate, and this water mixes with the rock, thus changing its composition causing some rock to melt and rise. It is this that fuels a chain of volcanoes, such as the Aleutian Islands, near Alaska.

Hotspot volcanic chainsEdit

File:Hawaii hotspot.jpg
Over millions of years, the Pacific plate has moved over the Hawaii hotspot, creating a trail of underwater mountains that stretches across the Pacific.
File:Puu Oo cropped.jpg
Kilauea is the most active shield volcano in the world. The volcano erupted from 1983 to 2018 and is part of the Hawaiian–Emperor seamount chain.
File:Mauna Loa Volcano.jpg
Mauna Loa is a large shield volcano. Its last eruption was in 2022 and it is part of the Hawaiian–Emperor seamount chain.
File:Bowie Seamount1.jpg
Bowie Seamount is a dormant submarine volcano and part of the Kodiak-Bowie Seamount chain.
File:Axial Exaggerated Bathymetry.jpg
Axial Seamount is the youngest seamount of the Cobb–Eickelberg Seamount chain. Its last eruption was in 2015.
File:Mauna Kea from the ocean.jpg
Mauna Kea is the tallest volcano in the Hawaiian–Emperor seamount chain. Many cinder cones have been emplaced around its summit.
File:Hualālai 1996.jpg
Hualalai is a massive shield volcano in the Hawaiian–Emperor seamount chain. Its last eruption was in 1801.

The joint mantle plume/hotspot hypothesis originally envisaged the feeder structures to be fixed relative to one another, with the continents and seafloor drifting overhead. The hypothesis thus predicts that time-progressive chains of volcanoes are developed on the surface. Examples include Yellowstone, which lies at the end of a chain of extinct calderas, which become progressively older to the west. Another example is the Hawaiian archipelago, where islands become progressively older and more deeply eroded to the northwest.

Geologists have tried to use hotspot volcanic chains to track the movement of the Earth's tectonic plates. This effort has been vexed by the lack of very long chains, by the fact that many are not time-progressive (e.g. the Galápagos) and by the fact that hotspots do not appear to be fixed relative to one another (e.g. Hawaii and Iceland).<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> That mantle plumes are much more complex than originally hypothesised and move independently of each other and plates is now used to explain such observations.<ref name=Koppers2021/>

In 2020, Wei et al. used seismic tomography to detect the oceanic plateau, formed about 100 million years ago by the hypothesized mantle plume head of the Hawaii-Emperor seamount chain, now subducted to a depth of 800 km under eastern Siberia.<ref>Template:Cite journal</ref>

Postulated hotspot volcano chainsEdit

File:CourtHotspots.png
An example of mantle plume locations suggested by one recent group.<ref name="Courtillot 2003 295–308">Template:Cite journal</ref> Figure from Foulger (2010).<ref name="Foulger"/>

List of volcanic regions postulated to be hotspotsEdit

File:Hotspots-more.jpg
Distribution of hotspots in the list to the left, with the numbers corresponding to those in the list. The Afar hotspot (29) is misplaced.

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Eurasian plateEdit

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African plateEdit

Antarctic plateEdit

South American plateEdit

North American plateEdit

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Australian plateEdit

Nazca plateEdit

Pacific plateEdit

File:Kodiak-Bowie Seamounts.jpg
Over millions of years, the Pacific Plate has moved over the Bowie hotspot, creating the Kodiak–Bowie Seamount chain in the Gulf of Alaska.
File:HotspotHighway.jpg
The Hotspot highway in the south Pacific Ocean

Former hotspotsEdit

See alsoEdit

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ReferencesEdit

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Further readingEdit

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External linksEdit

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