Volcanic region hotter than the surrounding mantle
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.[1] 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.[2] 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.[3][4]
Origin
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.[5] 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.[6] Whether or not such mantle plumes exist has been the subject of a major controversy in Earth science,[4][7] but seismic images consistent with evolving theory now exist.[8]
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.[9] 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.[8]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.[8] Only one, (Yellowstone) has as yet been consistently modelled and imaged from deep mantle to surface.[8]
Composition
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,
basalticmagma rises through the continental crust, which melts to form
rhyolites. These
rhyolites can form violent eruptions.[10][11] 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.[12]
The hotspot hypothesis is now closely linked to the
mantle plume hypothesis.[13][8] The detailed compositional studies now possible on hotspot basalts have allowed linkage of samples over the wider areas often implicate in the later hypothesis,[14] and it's seismic imaging developments.[8]
Contrast with subduction zone island arcs
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 chains
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 are
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).[15] 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.[8]
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.[16]
^Donald Hyndman; David Hyndman (1 January 2016). Natural Hazards and Disasters. Cengage Learning. pp. 44–.
ISBN978-1-305-88818-0.
^Wolfgang Frisch; Martin Meschede; Ronald C. Blakey (2 November 2010). Plate Tectonics: Continental Drift and Mountain Building. Springer Science & Business Media. pp. 87–.
ISBN978-3-540-76504-2.
^O'Connor, J. M.; le Roex, A. P. (1992). "South Atlantic hot spot-plume systems. 1: Distribution of volcanism in time and space". Earth and Planetary Science Letters. 113 (3): 343–364.
Bibcode:
1992E&PSL.113..343O.
doi:
10.1016/0012-821X(92)90138-L.