Plumes and megaplumes

A plume is a pipe that extends into the mass of hot rocks that exist in the mantle of the earth and brings them to the surface, forming a “hot spot.” A megaplume is a supermass of extremely hot rocks that moves very slowly under the surface of the earth and influences the breakup of tectonic plates.

Hot Spots

There are more than one hundred regions of the world known as “hot spots,” which are fed by plumes of hot rock rising from deep in the earth's mantle. These hot spots are responsible for a particular type of volcanic activity, which, unlike other active volcanoes, has its origins deep in the interior of the earth. Plumes are found far away from the most active centers of volcanic activity and are usually most active in flat landscapes or at the bottom of oceans rather than in mountainous regions, as is true of more-typical volcanoes.

The hot spots come from material found deep within the earth's mantle, the solid layer of rocks that extends to more than 3,000 kilometers below the earth's surface, just above the core. Plumes apparently arise in regions of the mantle that are stirred about as the large continental plates that cover part of the earth move slowly across the surface. This movement of plates has been going on since early in the earth's history, beginning at least 4.6 billion years ago, when the earth's crust was just forming. As the huge plates of rock that make up the continents formed, they were originally one giant mass, but they began breaking up and moving apart at a fraction of an inch per year. “Plate tectonics,” as the study of these movements is called, describes how the continents reached their present locations; they are still moving apart, and still only by fractions of an inch every year. The plate movement helps explain the building of mountain ranges, for as the plates crash into one another, they push their margins up into mountains such as the Himalayas and the Andes. Plate movement also helps scientists to understand the activities of volcanoes. Most volcanic activity occurs in those areas where the major plates that make up the earth's surface (including the Eurasian, American, African, Pacific, Indian, and Antarctic plates) come together. At these margins of plate contact, the pressure of the plates pressing against one another creates fissures and breaks in the earth's surface, through which magma—the hot, molten material coming from deep in the mantle—can flow. The plates that form the continents are called “continental plates.” Other plates are found at the bottom of the oceans; these are called “oceanic plates.”

Hot Spot Volcanoes

As the plumes of mantle material move upward toward the surface, they feed and create what are called “hot spot” volcanoes. These range in size according to how deep the plume has reached into the depths of the earth. The deepest plumes create the largest volcanoes. The material coming up through the plumes (magma) consists of gigantic blobs of melted rock. Plumes and the hot spots connected to them move much more slowly than the continents above them. When one of the continental plates crosses over a plume, the magma flowing upward creates a large structure that looks like a dome. Such domes are usually about 125 miles wide and can be hundreds of miles long. Approximately 10 percent of the earth's surface is covered with these domelike structures. As the magma continues to burst upward, the dome increases in size; as it does, cracks and small openings appear, and the hot magma flows through these openings onto the surface. The most well-known domes created by plumes are the Hawaiian Islands. Geologists believe that all the islands in the Hawaiian chain were created from a single plume. As the Pacific plate passed over the hot spot, the islands popped up and out from the ocean floor, and the plume pumped huge quantities of magma into and out of the resulting dome.

In the Atlantic, volcanic islands formed from plumes are found along the mid-ocean ridge. The Azores and Ascension Islands appear on this ridge. The location of these island chains and hot spots can help scientists understand the movement of tectonic plates. The plumes appear to be fixed in relationship to one another and move at velocities of only a few millionths of an inch each year. Sophisticated measuring instruments can account for this movement, however. They also move in what appear to be well-established tracks, and as they move, their heat weakens the rocks above. Over time, these weakened surfaces begin to crack, causing rifts, or giant cracks in the earth. Some of these rifts become huge valleys, such as that found in the East African nation of Ethiopia.

Hot Spot Distribution

Of the hundred or so hot spots, more than half are found on the continental plates, with about twenty-five found in Africa. The African plate has remained over these hot spots for millions of years. The shape of the continent, which is covered by hundreds of basins, domes, and ridges, was greatly influenced by the slow movement of the continental plate over these plume-fed hot spots. Hot spots are also found in great numbers under the Antarctic and Eurasian plates. Hot spots are more likely to be found under slow-moving plates, since those continents that are moving more rapidly, such as North and South America, have only a very few areas of volcanic activity caused by hot spots.

One area in North America located over a hot spot is Yellowstone National Park. The hot spot below the park creates the many geysers, including Old Faithful, that are found in the region. Geysers are created when surface water seeps into the ground. When it comes in contact with boiling magma, the water is heated rapidly; it then boils upward until it explodes through cracks in the earth's crust.

Hot spots do have a limited life span. Typically, a plume feeding a hot spot cools off and disappears after about 100 million years. Their positions also change. The plume feeding the Yellowstone geysers originated farther to the north, around the Snake River in Idaho, 400 miles away, around 15 million years ago. Over time, the North American plate has slipped across it, putting the hot spot in its present location. Yet Yellowstone, too, is only a temporary home for this hot spot, and its slow movement to the southwest continues. The slow drift accounts for the volcanic activity in the area. Scientists believe that at least three major volcanic eruptions have taken place in this region during the last two million years. They predict that another massive explosion, hundreds of times greater than the huge Mount St. Helens eruption in 1981, will hit the area sometime in the next few thousand years.

Iceland

The hot-spot theory is an important contribution to the science of plate tectonics. Few geologists doubt the existence of these hot spots and the plumes that create them. One of the most intensely studied hot spots is the dome that makes up the North Atlantic island of Iceland. This megaplume, which was raised above the ocean floor more than 16 million years ago, lies across a formation known as the Mid-Atlantic Ridge. The dome is actually about 900 miles long, but only about 350 miles of it—Iceland—lies above sea level. To the south of the island, the dome tapers off gradually and dips below the sea. In 1918, a volcanic eruption under a glacier to the north of Iceland melted enough ice to create an oceanic flood of water that was twenty times greater than the yearly flow of the Amazon River, the world's largest river. Had the floodwater hit land many islands and the European coastline would have been devastated.

Within the next few million years, a very short period in geologic time, the Mid-Atlantic Ridge will have moved away from the hot spot, carrying Iceland with it. This will dry up the source of magma supplying the volcanoes on Iceland, and they will no longer erupt.

Seamounts and Guyots

Most volcanic hot spots never rise above sea level and remain underwater volcanoes. Magma erupting from these plumes forms structures called “seamounts.” These are isolated, though they form into long chains along the surface of the oceanic plate. A few seamounts are found with extensive fissures and cracks. Within these cracks, magma has cooled over hundreds of thousands of years, piling up thousands of magma flows, one on top of the other. A few of these are high enough to break through the ocean's surface. These seamounts become volcanic islands and dot various ridges in the crust, forming island chains such as the Galapagos Islands off the west coast of Ecuador. The tallest seamounts rise more than two and one-half miles above the sea floor and are found to the east of the Philippine Islands, where the crust is over 100 million years old. Generally, the older the crust, the larger the number of undersea volcanoes. The majority of seamounts are found in the Pacific Ocean, where there are between five and ten volcanic hot spots in every 5,000 miles of ocean floor. The Hawaiian Islands were created as the oceanic plate passed over a plume and formed the Emperor Seamounts, the name used by students of plate tectonics to refer to the Hawaiian Island chain.

Other plumes, in ancient geological times, formed undersea volcanoes called guyots (pronounced GHEE-ohs). Dozens of these once rose high above the Pacific Ocean. Over millions of years, however, constant wave action eroded the tops of these guyots below the sea's surface. Guyots, over time, moved away from their sources of magma, the hot spots, and this also helps to account for their disappearance beneath the ocean.

Mid-Cretaceous Megaplume Activity

The plumes described here are moderate in size and are considered to be a normal part of the earth's mantle. Some geologists are convinced that millions of years ago the earth went through an extremely intense period of volcanic eruptions. During this period, giant megaplumes exploded from deep within the earth, expelling huge quantities of molten material. These structures spread across the earth's surface, becoming ten times larger than average plumes. These “superplume” explosions were responsible for the volcanic activity that affected the ocean floor during the mid-Cretaceous period about 90 million to 100 million years ago. One result of this unusually violent activity was the creation of hundreds of seamounts in the western Pacific. Another area affected by these megaplumes was the Parana River basin in Brazil, where hundreds of rift valleys were created. It was also during this time that the Andes Mountains in South America and the Sierra Nevada in the western United States were formed.

Megaplume activity during the mid-Cretaceous led to a 100-foot rise in sea level and a 10-degree increase in the temperature of the earth's air. This increase was caused by the release of huge amounts of carbon dioxide into the air during volcanic explosions. A key result of this activity was an enormous increase in plankton, the microscopic organisms that drift in the oceans and are the first link in the ocean's food chain. As the plankton died, they devolved into huge deposits of oil. Perhaps 50 percent of the world's known oil supply dates to this period of megaplume activity. The volcanic activity of the giant plumes also brought large quantities of diamonds from the earth's interior closer to the surface, from which they are mined.

Role in Earth's History

Plumes and megaplumes have had a dramatic influence on the history of the earth. Plume activity has created islands, volcanoes, valleys, and mountains. The superheated rocks brought from far within the interior of the earth have helped to form geysers, oil deposits, and diamonds. Tracking the slow movement of hot spots can help scientists to predict future events, such as the possibility of volcanic eruptions or the creation of new rift valleys. The study of past volcanic explosions can help scientists to keep the public informed about the potential harm that can be expected from future eruptions of plumes.

Volcanic eruptions have occurred periodically throughout the earth's history. From a geological point of view, periods of intense volcanic activity last for relatively brief spans of time, perhaps from 2 million to 3 million years. There are particularly intense periods of major activity about every 30 million years due to a combination of causes, including volcanic activity, impact events, and sea-level falls. These latter periods—and the mid-Cretaceous period might have been one of them—coincide with mass extinctions of life, as volcanic gases flow into the atmosphere, releasing thousands of tons of sulfur and other dangerous chemicals. Some of the released gases are converted into acids that also have a devastating impact on living things. Hot spots and megaplumes expel huge amounts of ash, dust, and molten rock from their cracks and fissures. These materials absorb the sun's radiation and can cause intense heating or cooling of the atmosphere. The dust can also shade out the sun's light for long periods of time. The reduced sunlight can cause mass extinctions of plants and animals because of the extreme cold produced. Intense volcanic activity can also produce acid rain, which could kill the leaves of plants and make the oceans and lakes unlivable. Scientists believe that the earth has been victimized by such violent activity at least three times in the past, the last time being about 65 million years ago. Hot spots and megaplumes were responsible for much of this violent volcanic activity.

Principal Terms

crust: the rock and other material that make up the earth's outer surface

guyot: a formation made by plume activity in the ocean that has a flat top wholly under water

hot spot: a heat source fed by a plume that reaches deep into the earth and produces molten rock

magma: molten rock generated deep within the earth that is brought to the surface by volcanoes and plumes

mantle: the part of the earth below the crust and above the core composed of dense, iron-rich rocks

plate tectonics: the theory that accounts for the major features of the earth's surface in terms of the interactions of the continental plates that make up the surface

seamount: an isolated dome formed under the sea by plumes reaching a height of at least 2,300 feet

tectonics: the history of the larger features of the earth—rocks and mountains, islands and continents—and the forces and movements that produce them

Bibliography

Ballard, Robert D. Exploring Our Living Planet. Washington, D.C.: National Geographic Society, 1983. A well-illustrated guide to modern theories of continental drift, plate tectonics, and the activities of volcanoes. A good place to begin an investigation of the history of the earth's formation and the various forces that have created the earth's features. Includes pictures, maps, and an index.

Condie, Kent C. Plate Tectonics and Crustal Evolution. 4th ed. Oxford: Butterworth Heinemann, 1997. An excellent overview of modern plate tectonics theory that synthesizes data from geology, geochemistry, geophysics, and oceanography. A very helpful tectonic map of the world is enclosed. The book is nontechnical and suitable for a college-level reader. Useful “suggestions for further reading” follow each chapter.

Davies, Geoffrey F. Mantle Convection for Geologists. New York: Cambridge University Press, 2011. Begins with strong foundational material upon which to build convection concepts. Chapter 7 covers the plumes and hot spots. Although the title implies technical writing, the author's intended the text be for anyone studying geological processes or university level students.

Eicher, Don L., A. Lee McAlester, and Marcia L. Rottman. The History of the Earth's Crust. Englewood Cliffs, N.J.: Prentice-Hall, 1984. A brief introduction to plate tectonics and geological history. A good beginning for those unfamiliar with the topic. Useful illustrations, charts, and an index.

Erickson, Jon. Plate Tectonics: Unraveling the Mysteries of the Earth. New York: Facts on File, 2001. An excellent, well-written, easily understandable description of the forces shaping the earth's geology, including a detailed and illustrated discussion of plumes and hot spots. Megaplumes, however, are not described. A very good introduction to the subject. Illustrations, bibliography, index.

Foulger, G. R. Plates vs. Plumes: A Geological Controversy. New York: Wiley-Blackwell, 2010. Discusses volcanism, seismology, and other topics related to plumes. Constantly returns to plate and plume hypotheses as each new topic is presented. Provides a website for discussion of the plume model and non-plume models in volcanism.

Kearey, Philip, Keith A. Klepeis, and Frederick J. Vine. Global Tectonics. 3rd ed. Cambridge, Mass.: Wiley-Blackwell, 2009. A textbook written in somewhat technical language; nevertheless contains some good illustrations and a detailed discussion of megaplumes. Designed for college courses in geology. Index and bibliography.

Olsen, Kenneth H., ed. Continental Rifts: Evolution, Structure, Tectonics. Amsterdam: Elsevier, 1995. The various essays provide good explanations of plate tectonics and continental rifts. Slightly technical but suitable for the careful reader. Illustrated.

Reynolds, John M. An Introduction to Applied and Environmental Geophysics. 2d ed. New York: John Wiley, 2011. An excellent introduction to seismology, geophysics, tectonics, and the lithosphere. Appropriate for those with minimal scientific background. Includes maps, illustrations, and bibliography.

Seyfert, Charles K., and L. A. Sirkin. Earth History and Plate Tectonics: An Introduction to Historical Geology. New York: Harper & Row, 1973. A textbook for geology students, but easy to understand and well illustrated. Read this book after consulting some of the briefer descriptions of the formation of the earth's mantle and crust.

Sullivan, Walter. Continents in Motion: The New Earth Debate. 2d ed. American Institute of Physics, 1993. A somewhat dated but still useful summary of the differing points of view of various theorists of the earth's formation and how such views have changed over time. Popularly written; easily understood without a technical background in geology.