Historic Dispute : Are Earth's continents stationary throughout geologic time

views updated

Historic Dispute : Are Earth's continents stationary throughout geologic time?

Viewpoint: Yes, the best available and most widely accepted models of Earth's structure once precluded large-scale horizontal motion of continents.

Viewpoint: No, geologic evidence shows that the continents had once been in very different positions, and thus must have moved great distances across Earth's surface over time.

Scientific ideas and theories change through gradual evolution and sudden revolution. Sometimes the systematic exploration of shared principles leads to an expansion and accumulation of knowledge about a subject; other times, competing concepts about the most fundamental hypotheses of a field clash for acceptance. For several decades during the twentieth century, earth scientists wrestled with an idea at the very foundation of their field: had the continents we observe on Earth today been stationary throughout history, or had subterranean forces reshaped the pattern of land and sea throughout time? The reluctance of many scientists to accept the concept of continental drift was rooted not only in reservations they had about the nature of the evidence cited to support the novel theory, but also in concerns about the very kind of theory it was.

To understand why the theory of continental drift seemed so unacceptable to earth scientists during the twentieth century, it helps to understand the guiding methods of geology that had been developed during the previous two hundred years. One important principle that geology shared with other sciences was a commitment to simplicity—scientists sought to find the simplest theory to explain their observations. Extraneous assumptions, elaborate constructions, and obscure interpretations would all be challenged if simpler, more obvious explanations could account for the same data. This more or less philosophical preference for simplicity has been an important element of the history of science and has often played a role in scientific controversies.

A methodological or philosophical concept that is special to the earth sciences is called uniformitarianism. It is related to the general commitment to simplicity mentioned above, but arises more directly from the subject matter of Earth's history. Uniformitarianism suggests that scientists may invoke no forces to explain geologic events of the past that are not observed on Earth today. Earth has changed through history, but always and only as a result of observable transformations—not through the work of invisible forces or mysterious conditions. Uniformitarianism as a methodology itself changed somewhat during the nineteenth and twentieth centuries, but it was still a valuable concept to geologists at the time they were confronted with the unorthodox concept of continental drift.

The suggestion that Earth's continents had shifted over time, breaking off from an original supercontinent into drifting configurations that gradually came to the array of continents observed today, was extraordinarily novel. Even the theory's proponents were initially unable to explain how the continents moved, and to many scientists it seemed that for every observation that could be explained by continental drift, there were several—from geology, paleontology, and physics—that could not. Over time, new discoveries and new interpretations of controversial observations accumulated in support of the idea that the continents are not stationary. Today, we know that the brittle outer portion of Earth, called the lithosphere, is in motion. We have sophisticated ways of directly measuring this motion using lasers and satellites. The modern theory of plate tectonics accommodates the idea that the continents are in constant motion. This theory states that Earth is divided into a few large, thick slabs of rock called lithospheric plates that are slowly moving and changing in size. Continental crust comprises the upper portion of some of these plates, ocean crust comprises the upper portion of other plates, and still others have both continental and oceanic crust comprising their upper portions. Plate tectonics is a unifying theory, or paradigm, in the earth sciences. It holds such an important place in our study of Earth because it explains the formation of mountain belts, the occurrence and distribution of volcanoes and earthquakes, and the age and distribution of material in Earth's crust. The saga of the long dispute over accepting the idea that continents move is a paradigmatic story of a revolutionary transformation in scientific ideas.

—LOREN BUTLER FEFFER

Viewpoint: Yes, the best available and most widely accepted models of Earth's structure once precluded large-scale horizontal motion of continents.

Existing Theories and Their Problems

One of the major geological problems in the second half of the nineteenth century was the question of how mountains originate. Although it was well-known that the rocks which formed mountains had originally been deposited in shallow coastal seas, opinions varied as to the nature of the forces and processes that caused these thick sediments to be thrust upward, tens of thousands of feet above sea level. In Europe, where the mountains best known to geologists were the inland Alps, the most influential theorist was Austria's Eduard Suess. He postulated that Earth's crust experienced irregular periods of collapse, causing a global redistribution of water and land, and changing the patterns of erosion and sediment deposition. This explained how the Alps could contain rocks that were once on the ocean floor. Suess also coined the term "Gondwanaland" to describe a now-sunken continent that he estimated had once existed in the southern hemisphere. In his theory, oceans and continents were not permanent: they could move up and down, continents becoming oceans and vice versa. However, they certainly could not drift sideways.

In the United States, geologists' experience came primarily from the Appalachians, a coastal mountain chain. American scientists, led by Yale professor James Dwight Dana, therefore, favored a different theory: continental permanence. Dana and his followers believed that the oceans and continents had been in roughly the same position throughout Earth history. They thought that mountains were raised up as a result of sedimentary accumulation along the coast in a trough called a "geosyncline." The American theory also recognized that continents and ocean floors are made of two fundamentally different materials, with distinct chemical compositions and densities. In contrast to Suess's theory, the two were not interchangeable. As the saying went, "once a continent, always a continent; once an ocean, always an ocean."

What theories like these had in common was the assumption that Earth was gradually cooling, and therefore contracting. This ongoing shrinkage was imagined to be the ultimate driving force behind subsidence, mountain building, and other geological processes. By the end of the first decade of the twentieth century, however, many realized that no theory relying on contraction could possibly work, for two reasons. First, the discovery of radioactivity meant that there was a new source of heat to account for within Earth. The Irish geologist John Joly calculated that Earth was not cooling, and thus not shrinking. However, he argued, the heat released by radioactive decay deep inside Earth could be the energy source that powered geological forces.

The second problem with the idea of a contracting Earth was equally fundamental. Surveyors in India and the United States had made very detailed measurements of the force of gravity in many different places, to correct for instrumental error. They believed that enormous mountain ranges like the Himalayas would exert a gravitational pull on the plumb lines used to level astronomical instruments. To the geophysicists' surprise, the effective mass of mountains was much less than predicted. The only possible solution was "isostasy," the idea that Earth's crust is flexible and that its pieces are in balance with each other. This made it impossible that continents (which are made of lighter material than the ocean floors) could sink to become oceans, as Suess had argued. Dana's theory that continents were permanently fixed in place seemed closer to the truth, but it would soon be challenged by Alfred Wegener.

The Initial Response to Wegener's Challenge

Although the American theory of continental permanence agreed with geophysical evidence about Earth's structure and composition, Wegener pointed out that it failed to account for a wide range of geological evidence about the actual history of Earth. In the 1920s, scientists who rejected his hypothesis had to provide other explanations for the observed correlation in fossils, stratigraphy, and glaciation, especially between South America, Africa, and India. European geologists had formerly relied on rising and sinking continents to account for these similarities, and indeed Wegener obtained his concept of Gondwanaland from Suess. But if Suess's theory was physically impossible, what response could American geologists, who insisted that these southern continents had always been widely separated, offer?

Ever since Charles Darwin showed that species are related by descent, it had been highly implausible that the same groups of plants and animals could have arisen twice in two different places. "Land bridges" were one of the most popular solutions to the problem of how species could cross oceans. Even if isostasy made it impossible for whole continents to rise and fall, it was still possible for small areas of the ocean floor to be lifted up temporarily, creating a thin pathway for migration. Charles Schuchert, the most influential American paleontologist of his time, favored this hypothesis. He believed that land bridges had connected the continents of the southern hemisphere at certain points in the past, much as the isthmus of Panama connects North and South America today. This model had the added advantage of explaining the evidence for widespread simultaneous glaciation in parts of the southern continents, because it implied that the Antarctic Ocean had been completely cut off from external circulation, leading to much colder than normal temperatures.

Schuchert was very aware that his theory of land bridges had to be in agreement with geophysical principles. He enlisted Bailey Willis, another eminent American geologist, to help describe the processes that could have formed these inter-continental links. By the early 1930s, they had successfully proposed an alternative to Wegener's theory that was accepted by many American paleontologists and subsequently appeared as a standard model in textbooks. Schuchert took Wegener's arguments very seriously and was even willing to accept that continents had moved horizontally over relatively short distances, providing the forces to build mountains. However, he adamantly rejected drift on a global scale because it seemed to threaten the very science of paleontology, which depended on continents remaining at the same latitude so that their past climate could be inferred. If the continents had wandered, and been at different latitudes in the past, Schuchert's interpretation of the principle of uniformitarianism would be jeopardized. Schuchert was very concerned drift denied geologists' essential ability to understand the past in terms of present conditions. Although Wegener offered a powerful solution to certain problems in paleontology and historical geology, the idea of drift also required major sacrifices. Since there was more than one way to explain the homologies between the southern continents, most American geologists followed Schuchert and stuck to the established theory of continental permanence.

If geologists were resistant to Wegener's suggestions, geophysicists had no trouble rejecting drift outright. Although Wegener had a good understanding of the problems facing existing models of Earth's structure and of the implications of isostasy for new models, his proposal that continents underwent horizontal displacement (drifting) met widespread disbelief. Willis demanded to know how the ocean floor could be soft enough to allow continents to pass through it, while simultaneously strong enough to force mountains to rise up on land. Moreover, the prevailing mathematical treatment of isostasy, the "Pratt model," implied that pieces of Earth's crust could not move horizontally at all. The leader of American geophysical surveying, William Bowie, believed that the Pratt model's practical success in dealing with field data meant that it accurately represented reality. Like other scientists, he refused to consider changing this model since it was clearly adequate and reliable.

The Pratt model required a very weak crust, one that would be incapable of lateral motion. (An equivalent version of isostasy, the "Airy model," would have let continents move horizontally, but was mathematically more complex and thus less popular.) Bowie repeatedly argued that continental drift was simply an impossibility. As a geophysicist, he was unconcerned with the masses of historical evidence that suggested the existence of Gondwanaland in the past. Another physicist, British mathematician Harold Jeffreys, argued that measurements of earthquake vibrations proved the solidity of Earth and thus provided another reason why drift was impossible. Because Wegener's theory violated the laws of physics, geologists would have to provide some other explanation for their own data. Paleontologists and historical geologists could not refute physical calculations (hence Schuchert's need to rely on Willis), while geophysicists had no interest in accounting for the details of Earth history. Because of these divisions between the various professional communities, the greatest virtue of Wegener's approach—that it drew together data from many different fields—was ineffective.

It is important to realize that geologists and geophysicists alike did not reject Wegener's idea simply because they refused to acknowledge the data. Wegener's hypothesis was one of many new theories competing in the early twentieth century to explain a wide range of confusing geological data. His theory of continental drift was disadvantaged from the start by his choice of methodology. American Earth scientists were firmly committed to the method of "multiple working hypotheses," as expounded by the senior geologist T. C. Chamberlin. According to this philosophy, a proper geological report should begin with objective data, interpretations should be left until the conclusion, and more than one hypothesis should be considered as a possible explanation. By these standards, Wegener's writings were completely unacceptable. He began with his all-embracing theory, and then presented selected facts as demonstrations of its unquestionable validity. His enthusiasm made it very difficult for many scientists to take him seriously. Because it did not meet American standards of scientific logic, Wegener's theory of drift did not receive a full hearing. At a famous symposium held by the American Association of Petroleum Geologists in 1926 (two years after Wegener's book was translated into English), nearly all the speakers rejected continental drift decisively, criticizing Wegener's dogmatic writing style as much as his ideas.

A Continuing Rejection

Once they had rejected continental drift, Earth scientists considered the matter closed and were unreceptive to new evidence in its favor. The highly detailed comparison of South American and African rock formations made by South African geologist Alexander du Toit, and his strong advocacy of continental drift through the 1930s and 40s, had little impact. His writing was unfortunately just as zealous as Wegener's and his carefully gathered data in support of drift were dismissed by Willis as a "fairy tale." Geophysicists, and a younger generation of geologists who were increasingly turning to quantification and laboratory experimentation, saw no need to revise their models of Earth's crustal structure in light of old-fashioned, field-based observations.

The most striking demonstration that the permanence of continents was a scientific non-issue in the 1930s-50s comes from the work of British geologist Arthur Holmes. One often-repeated criticism of Wegener's model was that there was no adequate force to move the continents. Yet while Wegener did not live long enough to solve this problem, Holmes developed a sophisticated model of continental motion based on convection (heat-driven) currents below the crust. This model, similar in many ways to our present understanding, finally demonstrated that continental movement was physically possible. Nevertheless, Holmes's work, which was well-known from the 1930s onward in Europe and the southern hemisphere, did not change American minds. Holmes did have a persistent critic in the influential Cambridge physicist Jeffreys, who continued to argue for a solid, contracting Earth even after the general acceptance of plate tectonics. But the rapid change in most scientists' views during the 1960s indicates that the problem up until that time was not a lack of evidence or of a plausible mechanism:

[Image not available for copyright reasons]

it was a lack of interest. Even in light of Holmes's work, most Earth scientists were unwilling to reconsider the assumption of continental permanence because they saw no pressing need to do so.

—BRIAN C. SHIPLEY

Viewpoint: No, geologic evidence shows that the continents had once been in very different positions, and thus must have moved great distances across Earth's surface over time.

Continental Drift and the Theory of Plate Tectonics

Continental drift, in the context of the modern theory of plate tectonics, is explained by the movement of lithospheric plates over the asthenosphere (the molten, ductile, upper portion of the Earth's mantle). Precisely used, the term "continental drift" is actually rooted in antiquated concepts regarding the structure of the Earth. Today, geophysicists and geologists explain the movement or drift of the continents within the context of plate tectonic theory. The visible continents, a part of the lithospheric plates upon which they ride, shift slowly over time as a result of the forces driving plate tectonics. Moreover, plate tectonic theory is so robust in its ability to explain and predict geological processes that it is equivalent in many regards to the fundamental and unifying principles of evolution in biology, and nucleosynthesis in physics and chemistry.

The theory of plate tectonics gained general acceptance between 1940 and 1980. Although first advanced in the early part of the twentieth century, less than one out of 10 professional geologists accepted arguments of continental drift prior to the Second World War. The resistance on the part of geologists was well founded because the original theory of continental drift asserted that the continents moved through and across an underlying oceanic crust much as ice floats and drifts through water. In contrast, according to a survey of professional geologists conducted in the 1970s, four out of five geologists accepted the arguments of plate tectonics enough to characterize the theory as well established by fact. Although the ultimate validity of any theory is not ultimately dependent upon authority (i.e., the popularity of the theory with experts), the acceptance of a hypothesis—enough to regard it as a scientific theory—within the scientific community is usually a result of the theory's ability to better explain and make predictions based upon existing data. During the twentieth century, the weight of evidence, collected from multiple lines of inquiry, made it clear that only the theory of plate tectonics could explain all the existing data related to the apparent drift of the continents.

Based upon centuries of cartographic depictions that allowed a good fit between the Western coast of Africa and the Eastern coast of South America, in 1858, French geographer Antonio Snider-Pellegrini, published a work asserting that the two continents had once been part of larger single continent ruptured by the creation and intervention of the Atlantic Ocean. In the 1920s, German geophysicist Alfred Wegener's writings advanced the hypothesis of continental drift depicting the movement of continents through an underlying oceanic crust. Wegener's hypothesis met with wide skepticism but found support and development in the work and writings of South African geologist Alexander Du Toit who discovered a similarity in the fossils found on the coasts of Africa and South Americas that derived from a common source. Other scientists also attempted to explain orogeny (mountain building) as resulting from Wegener's continental drift. The technological advances necessitated by the Second World War made possible the accumulation of significant evidence regarding Wegener's hypothesis, eventually refining and supplanting Wegener's theory of continental drift with modern plate tectonic theory.

Plate tectonic theory asserts that Earth is divided into core, mantle, and crust. The crust is subdivided into oceanic and continental crust. The oceanic crust is thin (3-4.3 mi [5-7 km]), basaltic (<50% SiO 2), dense, and young (< 250 million years old). In contrast, the continental crust is thick (18.6-40 mi [30-65 km]), granitic (>60% SiO 2), light, and old (250-3,700 million years old). The outer crust is further subdivided by the subdivision of the lithospheric plates, of which it is a part, into 13 major plates. These lithospheric plates, composed of crust and the outer layer of the mantle, contain a varying combination of oceanic and continental crust. There is a compositional change from crust material to mantle pyriditite called the Mohovisic discontinuity, and the lithospheric plates move on top of mantle's asthenosphere.

Boundaries are adjacent areas where plates meet. Divergent boundaries are areas under tension where plates are pushed apart by magma upwelling from the mantle. Collision boundaries are sites of compression either resulting in subduction (where lithospheric plates are driven down and destroyed in the molten mantle) or in crustal uplifting that results in orogeny. At transform boundaries, exemplified by the San Andreas fault, the continents create a shearing force as they move laterally past one another.

New oceanic crust is created at divergent boundaries that are sites of sea-floor spreading. Because Earth remains roughly the same size, there must be a concurrent destruction or uplifting of crust so that the net area of crust remains the same. Accordingly, as crust is created at divergent boundaries, oceanic crust must be destroyed in areas of subduction underneath the lighter continental crust. The net area is also preserved by continental crust uplift that occurs when less dense continental crust collides with continental crust. Because both continental crusts resist subduction, the momentum of collision causes an uplift of crust, forming mountain chains. A vivid example of this type of collision is found in the ongoing collision of India with Asia that has resulted in the Himalayan mountains that continue to increase in height each year. This dynamic theory of plate tectonics also explained the formation of island arcs formed by rising material at sites where oceanic crust subducts under oceanic crust, the formation of mountain chains where oceanic crust subducts under continental crust (e.g., Andes mountains), and volcanic arcs in the Pacific. The evidence for deep hot convection currents combined with plate movement (and concurrent continental drift) also explained the mid-plate "hot spot" formation of volcanic island chains (e.g., Hawaiian islands) and the formation of rift valleys (e.g., Rift Valley of Africa). Mid-plate earthquakes, such as the powerful New Madrid earthquake in the United States in 1811, are explained by interplate pressures that bend plates much like a piece of sheet metal pressed from opposite sides.

Evidence in Support of the Theory of Plate Tectonics

Wegener's initial continental drift assertions were based upon the geometric fit of the displaced continents and the similarity of rock ages and Paleozoic fossils in corresponding bands or zones in adjacent or corresponding geographic areas. Wegener also argued that the evidence of Paleozoic glaciation in South Africa, South America, India and Australia—sites far removed from estimates of the geographical extent of glaciation—argued strongly for continental drift. Although Wegener's theory accounted for much of the then existing geological evidence, Wegener was unable to advance a verifiable or satisfying mechanism by which continents—with all of their bulk and drag—could move over an underlying mantle that was solid enough in composition to be able to reflect seismic S-waves.

Modern understanding of the structure of Earth is derived in large part from the interpretation of seismic studies that measure the reflection of seismic waves off features in Earth's interior. Different materials transmit and reflect seismic shock waves in different ways, and of particular importance to the scientific debate regarding continental drift is the fact that liquid does not transmit a particular form of seismic wave known as an S wave. Because the mantle transmits S-waves, it was long thought to be a cooling solid mass. Geologists later discovered that radioactive decay provided a heat source with Earth's interior that made the asthenosphere plasticine (semi-solid). Although solid-like with regard to transmission of seismic S-waves, the asthenosphere contains very low velocity (inches per year) currents of mafic (magma-like) molten materials.

Although also explained by theories of crustal upthrusting and collapse, another line of evidence in support of plate tectonics came from the long-known existence of ophiolte suites (slivers of oceanic floor with fossils) found in upper levels of mountain chains. The existence of ophiolte suites are consistent with the uplift of crust in collision zones predicted and explained by continental drift and plate tectonic movements.

As methods of dating improved, an important line of evidence in support of plate tectonics derived from the dating of rock samples. Highly supportive of the theory of sea floor spreading (the creation of oceanic crust at a divergent plate boundary such as the Mid-Atlantic Ridge) was evidence that rock ages are similar in equidistant bands symmetrically centered on the divergent boundary. More importantly, dating studies show that the age of the rocks increases as their distance from the divergent boundary increases. Accordingly, rocks of similar ages are found at similar distances from divergent boundaries, and the rocks near the divergent boundary where crust is being created are younger than the rocks more distant from the boundary. Eventually, radioisotope studies offering improved accuracy and precision in rock dating also showed that rock specimen taken from geographically corresponding areas of South America and Africa showed a very high degree of correspondence, providing strong evidence that at one time these rock formations had once coexisted in an area subsequently separated by movement of lithospheric plates.

Similar to the age of rocks, studies of fossils found in once adjacent geological formations showed a high degree of correspondence. Identical fossils are found in bands and zones equidistant from divergent boundaries. Accordingly, the fossil record provides evidence that a particular band of crust shared a similar history as its corresponding band of crust located on the other side of the divergent boundary.

The line of evidence that swayed most geologists to accept the arguments in support of plate tectonics derived from studies of the magnetic signatures or magnetic orientations of rocks found on either side of divergent boundaries. Just as similar age and fossil bands exist on either side of a divergent boundary, studies of the magnetic orientations of rocks reveal bands of similar magnetic orientation that were equidistant and on both sides of divergent boundaries (e.g., the Mid-Atlantic Ridge). Tremendously persuasive evidence of plate tectonics is also derived from correlation of studies of the magnetic orientation of the rocks to known changes in the Earth's magnetic field as predicted by electromagnetic theory. Paleomagnetic studies and discovery of polar wandering, a magnetic orientation of rocks to the historical location and polarity of the magnetic poles as opposed to the present location and polarity, provided a coherent map of continental movement that fit well with the present distribution of the continents.

Paleomagnetic studies are based upon the fact that some hot igneous rocks (formed from volcanic magma) contain varying amounts of ferromagnetic minerals (e.g., Fe 3 O 4) that magnetically orient to the prevailing magnetic field of Earth at the time they cool. Geophysical and electromagnetic theory provides clear and convincing evidence of multiple polar reversals or polar flips throughout the course of Earth's history. Where rock formations are uniform or not grossly disrupted by other geological processes, the magnetic orientation of magnetite-bearing rocks can also be used to determine the approximate latitude the rocks were at when they cooled and took on their particular magnetic orientation. Rocks with a different orientation to the current orientation of the Earth's magnetic field also produce disturbances or unexpected readings (anomalies) when scientists attempt to measure the magnetic field over a particular area.

Additional Support for Plate Tectonics

Overwhelming support for plate tectonics came in the 1960s in the wake of the demonstration of the existence of symmetrical, equidistant magnetic anomalies centered on the Mid-Atlantic Ridge. Geologists were comfortable in accepting these magnetic anomalies located on the sea floor as evidence of sea floor spreading because they were able to correlate these anomalies with equidistant radially distributed magnetic anomalies associated with outflows of lava from land-based volcanoes.

Additional evidence continued to support a growing acceptance of tectonic theory. In addition to increased energy demands requiring enhanced exploration, during the 1950s there was an extensive effort, partly for military reasons related to what was to become an increasing reliance on submarines as a nuclear deterrent force, to map the ocean floor. These studies revealed the prominent undersea ridges with undersea rift valleys that ultimately were understood to be divergent plate boundaries. An ever-growing network of seismic reporting stations, also spurred by the Cold War need to monitor atomic testing, provided substantial data that these areas of divergence were tectonically active sites highly prone to earthquakes. Maps of the global distribution of earthquakes readily identified stressed plate boundaries. Improved mapping also made it possible to view the retrofit of continents in terms of the fit between the true extent of the continental crust instead of the current coastlines that are much variable to influences of weather and ocean levels.

Also damaging to older theories of an undulating crust theory were compositional chemical studies that showed that the oceanic crust was substantially younger than the continental crust. Not only were there symmetrical bands of rock on either sides of divergent boundaries with similar dating, but no rocks older than 250 million years old were ever discovered in oceanic crust. Compositional studies also allowed plate tectonic theory to explain isostacy (buoyant characteristics) and provided gravitational data showing less dense material in mountainous areas.

In his important 1960 publication, "History of Ocean Basins," geologist and U.S. Navy Admiral Harry Hess (1906-1969) provided the missing explanatory mechanism by suggesting that the thermal convection currents in the asthenosphere provided the driving force behind plate tectonics. Subsequent to Hess's book, geologists Drummond Matthews (1931-1997) and Fred Vine (1939-1988) at Cambridge University used magnetometer readings previously collected to correlate the paired bands of varying magnetism and anomalies located on either side of divergent boundaries. Vine and Matthews realized that magnetic data reveling strips of polar reversals symmetrically displaced about a divergent boundary confirmed Hess's assertions regarding seafloor spreading.

The degree with which the geological community resisted acceptance of Wegener's theory of continental drift is clearly demonstrated by the fact that Hess's assertion of thermal currents was drawn from work done by Arthor Holmes in the 1930s. The fact that there was also initial resistance to plate tectonic theory is evidenced by the fact that Vine and Matthews's publication of their findings in Nature came on the heels of the rejection a few months prior by the same editors of a paper by Canadian geologist L.W. Morley proposing a similar plate tectonic hypothesis. Eventually, however, the evidence for plate tectonics became overwhelming and, although murkier as to insights into earlier continental configurations, plate tectonics powerfully explains the forces that caused the continents to break apart form and drift from a common Pangaea supercontinent to their present configuration on the ever-evolving face of Earth.

—K. LEE LERNER

KEY TERMS

CONTINENTAL CRUST:

The outer most layer of the lithosphere comprising the existing continents and some subsea features near the continents. Continental crust is composed of lower density rocks such as granite and andesite.

CRUST:

The outermost layer of Earth, 3-43 mi (5-70 km) thick and representing less than 1% of Earth's total volume.

GONDWANALAND:

An inferred supercontinent in the southern hemisphere, used to explain why the fossils and rock formations of today's southern continents are so similar. According to Suess, it existed between Africa and India before sinking to become part of the ocean floor; according to Wegener it broke apart into the continents we now see.

HOMOLOGY:

A structural similarity between two objects from different places (e.g., rock formations, fossils), which suggests, but does not prove, that they have a common origin.

ISOSTASY:

The "floating" behavior of components of Earth's crust. Just as icebergs float because ice is less dense than water, mountains are buoyed up by deep "roots" that are of lower density than the crustal material they displace.

LITHOSPHERE:

The near-surface region of the crust and upper mantle that is fractured in plates that move across a plasticine mantle.

OCEANIC CRUST:

The outer most layer of the lithosphere lying beneath the oceans. Oceanic crust is composed of high-density rocks, such as basalt.

POLAR WANDERING:

Earth's magnetic field is like a dynamo driven by electrical currents in the Earth's liquid outer core. Igneous (volcanic) rocks contain small amounts of magnetic minerals that align themselves with Earth's magnetic field as they cool. These rocks provide a paleomagnetic record. Earth's magnetic poles undergo regular changes (reversals or flips) exemplified by the Jaramillo Event, a reversal of Earth's magnetic field approximately 900,000 years ago. Apparent polar wandering also is the result of restricted magnetic pole movement and continental drift.

UNIFORMITARIANISM:

Traditionally the most important principle of geology, it states that "the present is the key to the past." The history of Earth must be explained entirely according to processes actually occurring and observable today; references to speculative or unknown external forces are not allowed.

Science in Dispute