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Britannica Concise Encyclopedia: hot spot
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Region of the Earth's upper mantle that upwells to melt through the crust to form a volcanic feature. Most volcanoes that cannot be ascribed either to a subduction zone or to seafloor spreading at midocean ridges are attributed to hot spots. The 5% of known world volcanoes not closely related to such plate margins (see plate tectonics) are regarded as hot-spot volcanoes. Hawaiian volcanoes are the best examples of this type, occurring near the centre of the northern portion of the Pacific Plate. A chain of extinct volcanoes or volcanic islands (and seamounts), such as the Hawaiian chain, can form over millions of years where a lithospheric plate moves over a hot spot. The active volcanoes all lie at one end of the chain or ridge, and the ages of the islands or the ridge increase with their distance from those sites of volcanic activity.

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Sci-Tech Encyclopedia: Hot spots
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The surface manifestations of plumes, that is, columns of hot material, that rise from deep in the Earth's mantle. Hot spots are widely distributed around the Earth. One of their characteristics is an abundance of volcanic activity which persists for long time periods (greater than 1 million years). When the lithosphere (the rigid outer layer of the Earth) moves over a plume, a chain of volcanoes is left behind that progressively increases in age along its length. Hot spots are believed to be fixed with respect to each other and the deep mantle so that the age and orientation of these chains provide information on the absolute motions of the tectonic plates. See also Lithosphere; Plate tectonics.

The Hawaiian-Emperor seamount chain in the central Pacific Ocean is a good example of a volcanic chain that was generated at a hot spot. The 3400-mi-long (5700-km) chain is made up mainly of tholeiitic lavas and ash tuff and pumice deposits. The lavas may have evolved from an initial submarine shield-building stage, through an explosive stage as they build up to sea level, and finally to a subaerial post-erosional stage. See also Lava; Seamount and guyot.

Not all hot-spot volcanism is expressed in terms of highly lineated, multistage, volcanic chains. Aseismic ridges that extend up to or close to the axes of mid-oceanic ridges are another example of hot-spot volcanism. When a hot spot (for example, Iceland) is centered on the axis, pairs of ridges such as the Iceland-Faeroes Rise and the Greenland Rise are formed. Sometimes the plate (for example, Africa) has migrated off the hot spot (such as Tristan da Cunha), leaving behind ridge systems that no longer extend to the ridge axis (such as Rio Grande Rise and Western Walvis). See also Mid-Oceanic Ridge; Volcano; Volcanology.

Another characteristic of hot spots is their association with broad swells in the Earth's topography. The Hawaiian hot-spot swell is believed to have been formed in response to either thermal or dynamic effects in an underlying mantle plume. The crustal and upper-mantle structure, which is constrained by seismic refraction data, shows that the oceanic crust is of uniform thickness beneath the swell. The long-wavelength correlation that is observed between the gravity anomaly and the topography (about 37 mGal mi−1 or 22 mGal km−1) indicates that the mass excess of the swell is compensated by a low-density, high-temperature region below the crust. The uplift of hot-spot swells is believed to result from thermal perturbations in the underlying plume. The excess heights of swells suggest, on isostatic grounds, that temperature differences of about 450°F (250°C) occur between the plume and the surrounding mantle. Hot ascending plumes may raise the temperature of the overlying lithosphere, thereby thinning it.

Two classes of models have been proposed to explain hot-spot swells. In the reheating model, uplift is produced by thermal expansion that is confined to the conducting portion of the lithosphere (the thermal boundary layer). In the dynamic model, however, there is a contribution to the uplift that is produced by vertical normal stresses exerted to the seismically defined base of the lithosphere (the mechanical boundary layer) by convection.

The main distinguishing feature between the uplift models is that the reheating model predicts a higher heat flow than the dynamic model. Discrimination between these models therefore depends on how the subsidence history, heat flow, and long-term strength (which is controlled mainly by the temperature) differ from those for unperturbed lithosphere of the same age.

Science Dictionary: hot spot
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A place deep within the Earth where hot magma rises to just underneath the surface, creating a bulge and volcanic activity (see volcano). The chain of Hawaiian Islands (see Hawaii) is thought to have been created by the movement of a tectonic plate over a hot spot.

Wikipedia: Hotspot (geology)
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Ambox content.png
 This article is in need of attention from an expert on the subject. WikiProject Geology or the Geology Portal may be able to help recruit one. (July 2009)
This article is about the geologic term. For other uses, see Hotspot.
Diagram showing a cross section though the Earth's lithosphere (in yellow) with magma rising from the mantle (in red)

In geology, a hotspot is a location on the Earth's surface that has experienced active volcanism for a long period of time.

Contents

Characteristics

J. Tuzo Wilson came up with the idea in 1963 that volcanic chains like the Hawaiian Islands result from the slow movement of a tectonic plate across a "fixed" hot spot deep beneath the surface of the planet. Hotspots are thought to be caused by a narrow stream of hot mantle convecting up from the Earth's core-mantle boundary called a mantle plume,[1] although some geologists prefer upper-mantle convection as a cause.[2][3] This in turn has re-raised the antipodal pair impact hypothesis, the idea that pairs of opposite hotspots may result from the impact of a large meteor.[4] Geologists have identified some 40–50 such hotspots around the globe, with Hawaii, Réunion, Yellowstone, Galápagos, and Iceland overlying the most currently active.

Most hotspot volcanoes are basaltic because they erupt through oceanic lithosphere (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 under continental crust, basaltic magma is trapped in the less dense continental crust, which is heated and melts to form rhyolites. These rhyolites can be quite hot and form violent eruptions, despite their low water content. For example, the Yellowstone Caldera was formed by some of the most powerful volcanic explosions in geologic history. However, when rhyolitic magma is completely erupted, it may eventually turn into basaltic magma because it is no longer trapped in the less dense continental crust. 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.[5]

Trail

Prominent hotspot trails on a bathymetric map of the Pacific Ocean

As the continents and seafloor drift across the mantle plume, "hotspot" volcanoes generally leave unmistakable evidence of their passage through seafloor or continental crust. In the case of the Hawaiian hotspot, the islands themselves are the remnant evidence of the movement of the seafloor over the hotspot in the Earth's mantle. The Yellowstone hotspot emerged in the Columbia Plateau of the US Pacific Northwest. The Deccan Traps of India are thought to be the result of the emergence of the hotspot currently under Réunion Island, off the coast of eastern Africa.

Geologists use hotspots to help track the movement of the Earth's plates. Such hotspots are so active that they often record step-by-step changes in the direction of the Earth's magnetic poles. Thanks to lava flows from a series of eruptions in the Columbia Plateau, scientists now know that the reversal of magnetic poles takes about 5,000 years, fading until there is no detectable magnetism, then reforming in near-opposite directions.

Notable hotspot trails

Possible hotspot trails

Comparison with island arc

Hotspot volcanoes should not be confused with island arc volcanoes. While each will appear as a string of volcanic islands, island arcs are formed by the subduction of converging tectonic plates. 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 causes some rock to melt. It is this that fuels a chain of volcanoes, such as the Aleutian Islands, near Alaska.

List of hotspots

[8]

Distribution of selected hotspots. The numbers in the figure are related to the listed hotspots on the left.
World map showing the locations of selected prominent hotspots. 1) Divergent plate boundaries, 2) Transform plate boundaries, 3) Convergent plate boundaries, 4) Plate boundary zones, 5) Selected prominent hotspots
Over millions of years, the Pacific Plate has moved over the Hawaii hotspot, creating a trail of underwater mountains that stretch across the Pacific
Over millions of years, the Pacific Plate has moved over the Bowie hotspot, creating the Kodiak-Bowie Seamount chain in the Gulf of Alaska

Former hotspots

See also

Earth sciences portal


References

  1. ^ ""Hotspots": Mantle thermal plumes". United States Geological Survey. 1999-05-05. http://pubs.usgs.gov/gip/dynamic/hotspots.html. Retrieved 2008-05-15. 
  2. ^ Wright, Laura (2000-11). "Earth's interior: Raising hot spots". Geotimes. American Geological Institute. http://www.geotimes.org/nov00/hotspot.html. Retrieved 2008-06-15. 
  3. ^ Christiansen, Robert L.; G.R. Foulger and John R. Evans (2002-10). "Upper-mantle origin of the Yellowstone hotspot". GSA Bulletin (Geological Society of America) 114 (10): 1245–1256. doi:10.1130/0016-7606(2002)114<1245:UMOOTY>2.0.CO;2. http://www.dur.ac.uk/g.r.foulger/Offprints/Yellowstone.pdf. Retrieved 2008-06-15. 
  4. ^ Hagstrum, Jonathan T. (2005-07-30). "Antipodal hotspots and bipolar catastrophes: Were oceanic large-body impacts the cause?". Earth and Planetary Science Letters (Elsevier B.V.) 236 (1-2): 13–27. doi:10.1016/j.epsl.2005.02.020. http://www.mantleplumes.org/WebDocuments/Antip_hot.pdf. Retrieved 2008-03-21. 
  5. ^ Holbek, Peter (1983-11) (PDF). Report on Preliminary Geology and Geochemistry of the Ilga Claim Group. http://www.em.gov.bc.ca/DL/ArisReports/12214.PDF. Retrieved 2008-06-15. 
  6. ^ a b E. V. Verzhbitsky. "Geothermal regime and genesis of the Ninety-East and Chagos-Laccadive ridges". Journal of Geodynamics 35/3 (2003). http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V9X-47MJ4CX-3&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=aa2e7686477bc607611955f2aba48558. 
  7. ^ a b W. J. Morgan. "Convection Plumes in the Lower Mantle". Nature 230(1971). http://www.mantleplumes.org/Morgan1971.html. 
  8. ^ Steinberger, Bernhard (2000). "Plumes in a convecting mantle: Models and observations for individual hotspots". Journal of Geophysical Research 105 (11): 11127–11152. doi:10.1029/1999JB900398. http://www.geodynamics.no/STEINBERGER/papers/jgr00.pdf. Retrieved 2008-06-15. 

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