Earth Science: Plate Tectonics: The Unifying Theory of Geology

Introduction

Plate tectonics is the unifying theory of geology, the framework into which are fitted all other explanations of large-scale geological phenomena, such as earthquakes, volcanoes, and the existence of ocean basins and continents. Plate tectonics describes and explains the movements of lithospheric plates, which are large areas of rocky crust, like fragments of eggshell thousands of miles across, that float and drift on the asthenosphere (the molten or malleable upper layer of Earth's mantle). Some of these moving plates bear the continents, while others underlie the ocean basins. Given the great expanse of geologic time—Earth is over 4.5 billion years old-even small plate velocities of inches or centimeters per year can radically rearranged the continents and oceans of Earth over many millions of years. Many lines of evidence show that Earth's continents have clustered together and then drifted apart again repeatedly during its history.

“Tectonics” is a geology term referring to all large-scale processes that shape planetary crusts, including that of Earth; plate tectonics is thus not the only possible kind of tectonics. As of 2008, Earth was the only body in the solar system definitely known to have plate tectonics.

Historical Background and Scientific Foundations

The interior bulk of Earth, which has an average radius (distance from center to surface) of 3,960 miles (6,370 km), is divided into core, mantle, and crust. The core is a ball at the center of the planet consisting mostly of iron, nickel, and sulfur: It has a radius of about 1,860 miles (3,000 km). It is surrounded by the mantle which is rocky, rather than metallic, and about 1,800 miles (2,900 km) thick. The mantle, in turn, is topped by the crust, whose outer face is the surface of Earth. The crust is divided into the areas of oceanic and continental crusts: Oceans overlie most oceanic crust, while most of the surface overlying continental crust is above sea level. The oceanic crust is thin (3–4.3 mi [5–7 km]), basaltic (<50% SiO²), dense, and young (<250 million years old). In contrast, the continental crust is thick (18.6–40 mi [30–65 km]), granitic (>60% SiO²), light, and old (250–3,700 million years old). SiO₂ Silicon dioxide (SiO²) is known as the mineral quartz when it is in a crystalline form or glass when it is an amorphous form; it is a relatively light mineral, and the continents float on the mantle because they contain so much SiO². Oceanic crust eventually sinks into the mantle as it ages and thickens (accumulating solidified mantle material on its lower surface), since it does not contain enough SiO². Oceanic crust that sinks or is forced down into the mantle is continually replaced by fresh oceanic crust along certain ridges that zig-zag across the ocean floors, traversing Earth like the seams on a baseball.

The outer crust is subdivided into seven major (e.g., North American, South American, Pacific) and about a dozen minor lithospheric plates. These lithospheric plates, composed of crust and the outer layer of the mantle, contain varying proportions of oceanic and continental crust.

The visible continents and the oceanic floors, both a part of the lithospheric plates upon which they ride, shift position slowly over time in response to the geothermal forces generated by radioactive decay and residual heat left over from the formation of Earth. This heat causes the soft and liquid material in the mantle to turn over like the contents of a boiling pot, only more slowly; the lithospheric or tectonic plates shift about on the surface of this rolling mantle material like islands of scum jostling about on the surface of boiling soup. The resulting plate velocities are usually measured in centi-meters per year.

Containing both crust and the upper region of the mantle, lithospheric plates are approximately 60 miles (approximately 100 km) thick: They are thicker under continental crust than under oceanic crust. Miles below the surface, a sudden change in composition from crust material to mantle pyriditite is termed the Mohorovicčič discontinuity (or simply the Moho for easier pronunciation) by geologists in honor of the man who discovered it in 1910, Croation seismologist Andrija Mohorovičič (1857–1936).

Plate tectonic boundaries are regions where the edges of lithospheric plates meet. There are three types of plate tectonic boundary, namely, divergent, convergent, and transform. Divergent boundaries describe areas under tension where plates are pushed apart by magma upwelling from the mantle. New oceanic crust is scrolled out from both sides of these ridgelike boundaries, aging and thickening as it slowly moves away from its origin; such a boundary runs all the way down the middle of the Atlantic from the Arctic to the Antarctic. As the twin sheets of oceanic crust expand from the mid-ocean ridge, the Atlantic sea-floor spreads and the Americas get farther away from Africa and Europe.

Convergent boundaries are sites of collision between plates. This results either in crustal uplift (e.g., when continental crust collides with continental crust, squeezing up mountains) or in subduction, where the edge of an oceanic plate is driven downward into, and ultimately dissolved in, the molten mantle. Because Earth must remain the same size—no significant amount of matter is being added to or removed from it—its surface area is constant, so destruction of crust at subduction zones is exactly balanced, over time, by creation of new crust at divergent boundaries. Most creation and destruction of crust affects only oceanic crust, which is why continental crust is, on average, far older than oceanic crust—much of it almost as old as Earth itself.

Transform boundaries are areas such as the San Andreas fault in California, where the edges of tectonic plates slide along each other. The jerky stick-and-slip motion of the rubbing plate edges causes earthquakes. At triple points where three plates converge (e.g., where the Philippine sea plate merges into the North American and Pacific plate subduction zone), the situation be-comes more complex. Also, mid-plate stresses can exist due to forces acting on plate boundaries and may result in bowing, fracturing, or mid-plate earthquakes such as the 1812 New Madrid earthquake, which devastated part of what is now the state of Missouri.

When oceanic crust collides with oceanic crust, both plates may subduct, V-ing downward to form an oceanic trench up to 36,000 feet (10,973 m) deep (as in the Marianas trench in the western Pacific, the deepest point in the world ocean). As the oceanic crusts subduct, material may be scraped off and clump up to form an accretion prism and oceanic island arcs.

When oceanic crust collides with the less-dense, more-buoyant continental crust, the oceanic crust subducts under the lighter continental crust and the continental crust may be wrinkled under compression to form mountain chains (e.g., the Andes). The subducting oceanic crust melts as it penetrates deeper into the asthenosphere, and rising molten material and gases from the melted crust may contribute to the formation of volcanic arcs such as those found along the Pacific Rim.

Although lithospheric plates move very slowly, the plates have tremendous mass. Accordingly, at collision, each lithospheric plate carries tremendous momentum (the mathematical product of velocity and mass) and the kinetic energy to drive subduction. Ultimately, all

energy for plate-tectonic processes begins as heat in Earth's interior. As this heat radiates into space through Earth's surface at a rate of about 0.07 watts per square meter, cooled upper mantle material slowly sinks and hotter material rises. The resulting rolling motion imparts motion to the plates.

Subduction zones are usually active earthquake zones; they are the only sites of deep earthquakes, whose sources range down to a depth of about 430 miles (700 km) in areas termed Benioff zones. Plate friction (drag) and mineral phase transitions—sudden collapses of molecular structure in large mineral bodies under heat and pressure—create the explosive forces observed in deep earthquakes. The release of forces due to sudden slippage of plates during subduction can be sudden and violent. Subduction zones also usually experience frequent shallow and intermediate-depth earthquakes. Undersea earthquakes can result in large waves known as tsunamis. A subduction earthquake under the Indian Ocean caused the Boxing Day tsunami of December 26, 2004, which killed almost a quarter of a million people, mostly along the coasts of India, Indonesia, Sri Lanka, and Thailand.

Because continental crusts do not subduct, a collision between continental crusts results in an uplift of both crusts with resultant mountain-building (orogeny). The formation and continued upward growth of the Himalayas at a rate of approximately one centimeter per year is a result of the collision of India with Asia.

Modern Cultural Connections

In the nineteenth and early twentieth centuries, several theories of crustal change and continental drift were current, including proposals that Earth as a whole was expanding or contracting. Several forms of the idea of continental drift were proposed early in the twentieth century, but the idea that continents might move was considered a crackpot notion by most geologists. This changed when technological advances made during World War II and throughout the 1950s allowed the

discovery of prominent undersea ridges and rift valleys. The formation, structure, and dynamics of these ridges were ultimately explainable only by continental drift. In 1960, geologist and U.S. Navy Admiral Harry Hess (1906–1969) provided the missing explanatory mechanism for plate tectonics by suggesting that thermal convection currents in the athenosphere drive plate movements. Subsequently, geologists Drummond Matthews (1931–1997) and Fred Vine (1939–1988) confirmed Hess's assertions regarding seafloor spreading. Measurements of the magnetism of rocks in the Atlantic sea floor showed alternating north-south striping of magnetic field direction, with mirror-image striping on either side of the mid-Atlantic ridge. These stripes are a clear record of the growth of the sea floor in sheets spreading from the ridge; as rock cooled in the crust forming along the line of origin, it recorded the direction Earth's magnetic field happened to be pointing at the time. When Earth's magnetic field flipped, as it does occasionally, fresh crust on both sides of the ridge would record an opposite magnetic direction for awhile. As the crust grew and Earth's field flipped many times over tens of millions of years, two sets of stripes were formed, one to the east of the ridge and another to the west. No plausible explanation for the existence of these stripes exists, other than sea-floor spreading and movement of the continents.

The principal proofs of plate-tectonic theory lie in:(1) the geometric fit of the displaced continents (matching distant coastlines); (2) the similarity of rock ages and Paleozoic fossils in corresponding bands or zones in areas that were once adjacent, such as South America and West Africa; (3) the existence of ophiolite suites (slivers of oceanic floor containing fossils) found uplifted into the upper levels of mountain chains; (4) radiometric dating evidence, which shows that rock ages are similar

in equidistant bands symmetrically centered on the Mid-Atlantic ridge divergent boundary; and (5) paleomagnetic studies that show bands of magnetic orientation symmetrical to divergent boundaries (as described earlier for the Atlantic). These convergent and independent sources of evidence convinced working geologists in the 1960s and afterward that the motions described by plate tectonics are a reality.

Additional evidence is the fact that the age of the rocks making up the oceanic crust increases as their distance from the divergent boundary (i.e., mid-ocean ridge) increases. Rock-age studies also show that oceanic crust is young, containing no rocks formed more than 250 million years ago.

Understanding continental drift has indirect benefits for human societies. In particular, volcanoes, earthquakes, and tsunamis can cause large numbers of deaths and vast property loss: When such events can be predicted, their effects can be greatly reduced. Today, one reason that scientists study the nature of oceanic subduction is to be better able to forecast major eruptions and earthquakes. In California, for example, a network of ground-motion detectors tracks the slippage of the Pacific plate along the edge of the North American plate, which creates the San Andreas fault system.

Pseudoscientific beliefs about continental drift are held by a few people, such as that cultural similarities between South America and Africa (pyramid-building cultures arose in both) are explained by the fact that the two continents were once in contact. Such beliefs have no basis in fact: Human beings only evolved in the last million years or so, while it has been well over 100 million years since Africa was in contact with South America.

Primary Source Connection

Before comprehensive theories develop and scientists know how natural mechanisms work, there is a tendency to shape explanations and analyze data in terms of prevailing theory. For example, prior to the now well-tested and accepted theory of plate tectonics and evidence of glacial movements, there were various attempts to explain erratic boulders (rock not native to a particular area). In the example that follows, in 1839, English naturalist Charles Darwin (1809–1882), best known for his advance of evolutionary theory, compiled the following “Note on a rock seen on an iceberg in 61° south latitude” that was then published by the Journal of the Royal Geographical Society of London in

which Darwin documented anecdotal observations that he thought might be relevant to developing an explanation of erratics.

NOTE ON A ROCK SEEN ON AN ICEBERG IN 61° SOUTH LATITUDE

HAVING been informed by Mr. Enderby, that a block of rock, embedded in ice, had been seen during the voyage of the schooner Eliza Scott in the Antarctic Seas, I procured through his means an interview with Mr. Macnab, one of the mates of the vessel, and I learnt from him the following facts: On the 13th of March, when in lat. 61 S., and long. 103–40 E., a black spot was seen on a distant iceberg, which, when the vessel had run within a quarter mile of it, was clearly perceived to be an irregularly-shaped but angular fragment of dark-coloured rock. It was embedded in a perpendicular face of ice, at least 20 feet above the level of the sea. That part which was visible, Mr. Macnab estimated at about 12 feet in height, and from 5 to 6 in width; the remainder (and from the dark colour of the surrounding ice, probably the greater part) of the stone was concealed. He made a rough sketch of it at the time, as represented at p. 524. The iceberg which carried this fragment was between 250 and 300 feet high.

Mr. Macnab informs me, that on one other occasion (about a week afterwards) he saw on the summit of a low, flat iceberg, a black mass, which he thinks, but will not positively assert, was a fragment of rock. He has repeatedly seen, at considerable heights on the bergs, both reddish-brown and blackish-brown ice. Mr. Macnab attributes this discolouration to the continued washing of the sea; and it seems probable that decayed ice, owing to its porous texture, would filter every impurity from the waves which broke over it.

Every fact on the transportation of fragments of rock by ice is of importance, as throwing light on the problem of ‘erratic boulders,’ which has so long perplexed geologists; and the case first described possesses in some respects peculiar interest. The part of the ocean, where the iceberg was seen, is 450 miles distant from Sabrina land (if such land exists), and 1400 miles from any certainly known land. The tract of sea, however, due S., has not been explored; but assuming that land, if it existed there, would have been seen at some leagues distance from a vessel, and considering the southerly course which the schooner Eliza Scott pursued immediately prior to meeting with the iceberg, and that of Cook in the year 1773, it is exceedingly improbable that any land will hereafter be discovered within 100 miles of this spot. The fragment of rock must, therefore, have travelled at least thus far from its parent source; and, from being deeply embedded, it probably sailed many miles farther on before it was dropped from the iceberg in the depths of the sea, or was stranded on some distant shore. In my Journal, during the voyage of H.M.S. Beagle, I have stated (p. 282), on the authority of Captain Biscoe, that, during his several cruises in the Antarctic Seas, he never once saw a piece of rock in the ice. An iceberg, however, with a considerable block lying on it, was met with to the E. of South Shetland, by Mr. Sorrell (the former boatswain of the Beagle), when in a sealing vessel. The case, therefore, here recorded is the second; but it is in many respects much the most remarkable one. Almost every voyager in the Southern Ocean has described the extraordinary number of icebergs, their vast dimensions, and the low latitudes to which they are drifted: Horsburgh* has reported the case of several, which were seen by a ship in her passage from India, in lat. 35°–55 S. If then but one iceberg in a thousand, or in ten thousand, transports its fragment,the bottom of the Antarctic Sea, and the shores of its islands, must already be scattered with masses of foreign rock, the counterpart of the “erratic boulders” of the northern hemisphere.

C.R. Darwin

darwin, c.r., “note on a rock seen on an iceberg in 61° south latitude.” journal of the royal geographical society of london 9 (march 1839): 528–529.

See Also Earth Science: Gradualism and Catastrophism.

bibliography

Books

McPhee, John. Basin and Range. New York: Farrar, Straus, & Giroux, 1980.

Periodicals

Darwin, C.R., “Note on a Rock Seen on an Iceberg in 61° South Latitude.” Journal of the Royal Geographical Society of London 9 (March 1839):528–529.

Matthews, Drummond H., and Simon L. Klemperer. “Deep Sea Seismic Reflection Profiling.” Geology 15 (1987): 195–198.

Vine, F.J. “Spreading of the Ocean Floor: New Evidence.” Science 154 (1966): 1405–1515.

Web Sites

United States Geological Survey. “This Dynamic Earth: The Story of Plate Tectonics.” March 27, 2007. http://pubs.usgs.gov/gip/dynamic/dynamic.html (accessed February 6, 2008).

K. Lee Lerner