Hot spots, island chains, and intraplate volcanism

Hot spots are small, isolated regions of volcanism that are stationary compared to the drifting lithospheric plates that make up the surface of the solid earth. The most common volcanic site for a hot spot is within the interior regions of oceanic plates, away from all convergent plate boundaries. Hot spots can exist at a given location for tens of millions of years. After an oceanic plate moves over a hot spot, a line of extinct volcanic islands, seamounts, and undersea guyots trail behind the hot spot for a distance of several thousand kilometers. Age dating of the volcanic materials at various distances along the island chain allows the calculation of the absolute velocity of the plate's motion over the hot spot.

Characteristics of Hot Spots

A hot spot is a circular region on the earth's surface that has a radius of about 1,000 kilometers. The region, especially if it occurs in an ocean basin, is usually elevated above its surroundings by about 1,000 meters. An active volcano or a tight cluster of volcanoes caps the uplifted area. Measurements of the amount of heat being radiated from the earth show that hot spots are regions of high heat flow from the interior of the earth. Most hot spots display a stronger gravitational attraction than their surroundings, indicating that dense mantle material may have risen into the lithosphere under the area. The region appears to be relatively stationary in reference to the earth's rotational poles; thus, hot spots do not move with the lithospheric plates that move across the earth's surface. Instead, the plate is said to move “over” the hot spot.

Volcanic chains associated with oceanic hot spots may have as much as 80 percent of their volcanoes submerged below sea level. Submerged volcanoes are called seamounts. Many seamounts are characterized by steep sides, having a 22-degree slope, and a flat top, which can be more than 20 kilometers across. This flat-topped, submerged volcanic structure is called a “guyot.” It is believed that guyots form from normal conical-shaped volcanoes. Oceanic lithosphere that moves over a hot spot becomes elevated, allowing the hot spot to form an island. Continued movement of the lithosphere would bring the now-extinct volcanic island off the apex of the hot spot, where the summit could be truncated by wave action and marine erosion. When the lithosphere has moved completely off the elevated region of the hot spot, the eroded volcano becomes fully submerged below the level of wave action to be preserved as a guyot in the volcanic chain.

Hot spots are known to have a long volcanic life, with the products of their volcanism forming a chain of igneous landforms that can extend hundreds of kilometers down the plate in the same direction that the plate is moving. Most of the hot spots in the Pacific Ocean basin have produced deposits of volcanic rocks for 25 million years or longer. The hot spot occurring at Hawaii has existed for more than 70 million years and has produced a 4,000-kilometer chain of extinct volcanoes on the Pacific Ocean floor that includes the Emperor and Necker string of seamounts, as well as the series of islands found at Midway and Hawaii. The ages of the extinct volcanoes within the chain are older in the direction that the plate moves over the hot spot.

Number and Location of Hot Spots

There are nearly fifty well-documented hot spots currently producing active volcanoes, and nearly three times that number of volcanoes have at one time or another been suggested as hot spots. Of these fifty, about twenty are located in the Pacific Ocean basin, fifteen are in the Atlantic Ocean, about five exist in the Indian Ocean, and fewer than ten occur on all of the continents. The Canadian geophysicist J. Tuzo Wilson developed the theory of hot spots in 1963, and by 1967 he had compiled a list of 122 hot spots that had been active within the last 10 million years. When the historic record is included, about 50 percent of all hot spots developed in continental regions, and 50 percent have been in ocean basins.

Hot spots provide considerable information about plate motions, making it important to locate hot spots in a plate tectonics framework. The earth's surface is covered with more than thirty lithospheric plates. The majority of the world's earthquakes occur along rather narrow belts, and these belts mark the boundaries between adjacent plates. Plates have three types of boundaries: divergent boundaries (where the two plates move directly away from each other); convergent boundaries (where the plates move toward each other); and transform fault boundaries (where plates slide past each other, with each moving parallel to the boundary between them). About one-fourth of all hot spots occur on or near divergent boundaries at either mid-oceanic ridges (Iceland, for example) or in continental rift valleys (the Afar hot spot in the Red Sea region of northern Africa). About one-tenth of the currently active hot spots occur on transform fault boundaries (the Azores are a good example). The remaining two-thirds of the hot spots occur in the interior regions of a plate away from all boundaries (for example, the island of Hawaii in the Pacific Ocean basin). Hot spots do not occur at convergent boundaries.

Distinguishing a Hot Spot

Most of the world's volcanoes do not develop at hot spots. A few key characteristics allow hot spot volcanism to be distinguished from other types of volcanoes. A long, linear belt of active volcanism commonly develops at both convergent and divergent plate boundaries. Within these bands of active volcanoes occurring at plate margins are intermixed extinct volcanoes, whereas the volcanic chain extending from a hot spot has active volcanoes only at the head of the chain, with extinct volcanoes making up the chain itself. Volcanism at plate margins develops in a line that parallels the plate boundary, whereas the volcanic chain of a hot spot trends at an angle of nearly 90 degrees to both convergent and divergent boundaries.

Volcanism at plate boundaries is associated with active earthquakes, but there are no earthquakes along the hot spot chain. This lack of earthquakes in the volcanic chain of a hot spot is such a distinctive feature that a hot spot trace is often called an “aseismic” ridge in geological literature.

Subtle differences in terminology are often used to distinguish the tectonic setting for volcanism. The more abundant, active belt of volcanoes associated with an oceanic convergent boundary is called a volcanic “arc” (such as Japan). A volcanic chain occurring at a divergent boundary in the ocean basins is called either a “mid-oceanic ridge” or a “rise” (such as the East Pacific Rise). The term “volcanic island chain” usually refers to the volcanic trace of an oceanic hot spot.

Mantle Plumes

The causes for the formation of hot spots remained unknown until Jason Morgan hypothesized the existence of mantle plumes in 1971. A mantle plume is a thin, cylindrical column of mantle material that originates near the boundary of the core and mantle. The lower mantle is thought to be fairly stagnant. This nonconvecting, lower mantle is called the mesosphere. Thermal irregularities in the liquid outer core cause stalk-like columns of hot rock material to rise and penetrate vertically through the mesosphere, the asthenosphere, and possibly into the lower portions of the lithosphere before the plumes begin to melt. The more buoyant magma continues to rise through fractures in the upper lithosphere to emerge as volcanoes. A hot spot is the direct surface manifestation of a mantle plume.

Researchers have been able to map mantle plumes passing through the upper mantle under various hot spots using waves that are generated by earthquakes. Mantle plumes that would be about 100 kilometers wide after 150 million years have been simulated in laboratory experiments. Studies have also shown that there is a very strong correlation between the distribution of hot spots and the occurrence of topographic highs on the top of the outer core.

Absolute Plate Motions

The stationary nature of mantle plumes and the long life of the associated hot spots allow several aspects of plate motion to be determined. The velocity that a plate has as it passes over a hot spot can be calculated by dividing the distance the plate has traveled by the length of time that has passed while traveling that distance. Plate velocities are usually expressed as centimeters per year. The concentration of various radioactive elements in the lava of a volcano allows for the dating of how long ago the volcanic rock formed. In a volcanic chain, the lava found on an extinct volcano was formed when the volcano was at the hot spot, and the rock's age will indicate how much time has elapsed since the volcano was over the hot spot. The distance the volcanic rock has traveled since it was formed is merely the distance from the hot spot to the current position of the volcano within the chain. Velocity analyses for the Hawaiian Islands yield a consistent velocity for the Pacific plate of 10 centimeters per year for the past 30 million years. Similar values have been calculated for the Cook-Austral island chain, which leads away from the Macdonald hot spot located in the South Pacific.

The volcanic island chain generated by a hot spot is a record of the direction in which the plate has moved. The line formed by the volcanic trace (pointing away from the hot spot) provides the compass bearing of the direction that the plate has moved. When multiple hot spots occur on the same plate, their volcanic traces typically form a series of mutually parallel lines. All five island chains caused by hot spots in the South Pacific show the same west-northwest bearing, which corresponds to the direction of movement of the Pacific plate.

When a hot spot occurs on a divergent boundary, it can form two simultaneous volcanic chains, one on each of the plates that are forming. The Iceland and Tristan da Cunha hot spots, which occur on the Mid-Atlantic Ridge, each have two aseismic ridges leading away from the active volcanic islands. One of the two ridges trends in an easterly direction across the eastern Atlantic basin in the direction of movement of Europe and Africa, while the other ridge has a westerly bearing, showing the movement direction of North America and South America.

The volcanic trace can record changes in the direction a plate has moved. The volcanic trace associated with the hot spot located under Yellowstone National Park in Wyoming has recorded a change in direction of motion for the North American plate. During the past 15 million years, this hot spot has caused the development of the Snake River volcanic plain, which trends to the southwest from Yellowstone. Magnetic studies have shown that the North American plate, which drifted westward for more than 200 million years, took a sharp turn to the southwest about 14 million years ago (verifying the evidence from the hot spot trace of Yellowstone).

The classic example of a hot spot recording a change in the direction of plate motion is Hawaii. The Hawaiian-Emperor chain has a distinct bend at Midway Island. The younger Hawaiian chain has trended west-northwest for nearly 40 million years. The Emperor chain was formed from 70 million to about 40 million years ago and has a trend of due north.

American geophysicist Jason Morgan used the twenty-five hot spots (and their volcanic traces) that are associated with the Atlantic Ocean and its neighboring continents to reconstruct the plate movements of the last 200 million years. His hot spot reconstruction of how North America and South America split apart from Europe and Africa to form the Atlantic Ocean agrees well with magnetic studies of plate motions for these continents.

Tectonic Evolution

As an oceanic plate is formed at a divergent boundary of a mid-oceanic ridge, it moves across an ocean basin and is destroyed as it is subducted into a trench at a convergent plate boundary. When a hot spot forms under this moving plate, the volcanic island chain that forms will eventually be carried into the trench where the plate is being consumed. Often the volume of volcanic material in the chain is so large that it disrupts the normal subduction process.

There are three effects that hot spot traces can have when they reach a convergent boundary. The most common effect is to cause a kink, cusp, or sharp bend in the normally smooth shape of a volcanic arc. This is well displayed by the Emperor seamounts, which are the trace of Hawaiian hot spot. The seamounts are being subducted along the Kurile and Aleutian trenches southwest of the Alaska Peninsula. At their point of entry into the trench, the volcanic arc changes its orientation from northwest to northeast, a bend of almost 90 degrees.

As the volcanic island chain is subducted, it can disrupt the mechanism that generates arc volcanism, causing a gap in the active volcanoes associated with an arc. In the southeastern Pacific basin, the Nazca plate is being subducted beneath the South American plate along the Peru-Chile Trench. The “arc” of active volcanoes corresponds to the AndesMountains, which run the entire length of western South America. The volcanic chain called the Nazca Ridge trends in an easterly direction from the Easter Island hot spot. The Andes Mountains have an 800-kilometer gap without an active volcano where the Nazca Ridge enters the trench.

In numerous cases the lavas associated with the volcanic chain are so abundant that they cannot be subducted beneath the oncoming plate. The hot spot material can stop the subduction process, in which case the volcanoes of the island chain will “dock” or weld themselves to the leading edge of the opposing plate. These pieces of old hot spot traces that have been added to the leading edge of a continental plate are given a variety of terms: accreted terranes, displaced terranes, or exotic terranes. The western edge of the North American continent has picked up more than twenty displaced terranes during its 200-million-year western march, and about one-half of these represent old hot spot materials from ancient ocean basins.

Hot Spots Within Continental Plates

Plates move over time, while hot spots are stationary. Thus, a hot spot that forms under a continent may have the continent drift away from the plume, while the hot spot becomes an oceanic spot with an aseismic ridge leading back to the continent. An example is the Trinidad hot spot in the southwestern Atlantic Ocean, which has an easterly trending aseismic ridge (Columbia seamounts) that goes under Brazil. The hot spot trace over the continent corresponds to the locations of most of the explosive, deep-seated, kimberlite diamond pipes that represent outgassing from the mantle. From 120 million years ago to about 60 million years ago, the hot spot was under Brazil, whereas for the last 60 million years, the hot spot has created the Columbia aseismic ridge in the Atlantic basin. North America moved northwest over the Great Meteor hot spot 100 million years ago, causing kimberlites and volcanoes in New England. Today the hot spot is located well out in the Atlantic Ocean, and the New England seamount chain shows the trend it has followed since the continent drifted west off the hot spot.

Hot spots appear to be important in the breakup of continents. The rising mantle plumes weaken the continental plate and provide breakage points. When a continent is rifted apart, the break begins as a zigzag rift valley that switches direction at each continental hot spot. A modern example is the Ethiopian hot spot, which occurs at the angle between the Red Sea and the Gulf of Aden, where the Arabian platform is being rifted away from the African continent.

As two continents continue to split apart and the pieces diverge from each other, the rift valley evolves into an ocean basin, and the hot spots change from a continental environment to being mid-oceanic ridge hot spots. When the supercontinent Pangaea was rifted apart 200 million years ago, the hot spot now located at Tristan da Cunha went through this evolution. The hot spot began under Pangaea and caused a large lava plateau. As the rift valley formed, the plateau was split in half: One half is the Paraná plateau on the east coast of South America, and the other half is the Etendeka plateau on the west coast of Africa. As the continents drifted away from each other, the hot spot became situated atop a mid-oceanic ridge with two traces back to both of the diverging continents. Tristan da Cunha is on the Mid-Atlantic Ridge, and it has produced both the aseismic Rio Grande Ridge (trending off to the west and connecting to the Paraná on the South American plate) and the Walvis Ridge (trending off to the east and connecting to the Etendeka).

Intraplate Volcanism

The lavas produced at hot spots are the most common form of intraplate volcanism. Although the actual region of the hot spots makes up an insignificant percentage of the earth's surface, the island chains, submarine aseismic ridges, lava plateaus, and rift lavas generated by hot spots are estimated to cover nearly 20 percent of the planet's surface.

Over millions of years, the movements of plates can take old volcanic rocks from an oceanic hot spot and add them to the leading edge of a continental plate, where later they may be caught in a collision between two converging continents. Thus, volcanic material generated at a hot spot in the interior of an oceanic plate can, over time, find itself situated in a convergent zone between two continental plates. Examples of this occur in the Himalaya Mountains. Researchers have attempted to develop techniques that will identify the tectonic setting where the volcanic rock was originally formed and that will apply no matter where the rock currently is located.

The most promising of these techniques is called the trace-element discrimination diagram. A chemical analysis is made of the rock, and the relative amounts of three rare elements—titanium, yttrium, and zirconium—are compared. The chemistry of the rock is plotted on a triangular diagram in which three regions are identified: mid-oceanic ridges, volcanic arcs, and within-plate lavas. The magmatic processes that occur in mantle plumes are such that the yttrium content is very low compared to titanium and zirconium in the lavas that are generated at hot spots. These elements were selected to identify rocks formed by intraplate volcanism because their relative abundance does not shift when the rocks are subjected to the many tectonic and chemical processes that occur on the surface of the earth.

Discrimination-type diagrams have been used to explain the origin of intraplate volcanism when current plate configurations do not allow tectonic origins to be deduced. For example, discrimination diagrams indicate that the 30-million-year-old San Juan Volcanic Field, located in the state of Colorado, is the result of magmatic processes associated with a convergent plate boundary. The nearest convergent boundary at the time that these Colorado volcanoes were forming was more than 1,000 kilometers to the west, off the coast of California.

Another cause of intraplate volcanism is the failure of early-formed rifts related to the breakup of a continental plate to fully develop. Such “failed” rifts are commonly associated with old hot spots that occurred within a continental plate. The rift valleys produce an abundance of fissure eruptions, leaving extensive lava plateaus in the interior of the continent. Some examples of major failed rifts with associated intraplate volcanism are the Keweenaw Peninsula of upper Michigan, the Rio Grande Valley in New Mexico and Colorado, the Connecticut and Hudson River Valleys of New England, the Rhine River Valley in Europe, the Benue Trough of western Africa, and the East African Rift Valley in Kenya and Tanzania.

Principal Terms

aseismic: lacking earthquake activity

asthenosphere: the layer within the earth's mantle that displays convective motion while in the solid state; the convective motion, which results from the cooling of the earth's interior, causes the overlying lithosphere to move on the earth's surface

lithosphere: the brittle, outer layer of the earth that is broken into individual pieces called “plates”; each plate is composed of the crust and the rigid portion of the upper mantle

mid-oceanic ridge: a volcanic ridge on the floor of the ocean basins where a divergent boundary between two oceanic plates occurs; often called a “rise” when it is not centered in the ocean basin

subduction zone: a lithospheric plate margin where a converging oceanic plate is forced to slide down into the asthenosphere under the opposing converging plate; a subduction zone forms a deep trench on the ocean floor

Bibliography

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