Continental growth

Continents are believed to have increased in size during the Earth's history by accretion of additional crustal material along their margins. This process has played a significant role in the formation of valuable mineral deposits such as gold, silver, copper, gas, and oil.

Geosynclines

Geologists believe that the continents have increased in size during geologic time by the accretion of additional material along their margins. This additional material usually consists of younger rocks deposited in a deeply subsiding belt known as a geosyncline, which is then welded to the continent by compressive forces. In some cases, the additional material may represent portions of a preexisting continent—or even an entire continent itself—that has been “drifted in” by the mechanism known as plate tectonics.

The idea of geosynclines dates back to the work of two nineteenth-century American geologists. In the 1850s, James Hall pointed out that the crumpled strata of mountain ranges along the continental margins were thicker than the equivalent strata in the continental interiors. In 1873, J. D. Dana suggested the term “geosyncline” (literally, “great Earth downfold”) for elongated belts of thick sedimentary rocks deposited along the continental margins. Further geologic fieldwork, primarily in the Alps and in the Appalachians, showed that the sedimentary rocks of the geosynclines had been deformed by compressive forces emanating from the ocean basins. By the 1950s, the generally accepted picture of continental growth was that of a stable continental interior, called the shield or craton, surrounded by increasingly younger belts of deformed rock. Each belt was believed to represent a geosynclinal sequence that was deposited in a bordering trough and then welded to the shield by lateral accretion. Yet, no satisfactory mechanism for the source of the compressive forces from the ocean could be discovered.

In North America, the central shield is called the Canadian Shield. Its exposed portion occupies the eastern two-thirds of Canada, the U.S. margins of Lake Superior, and most of Greenland. The Canadian Shield is the largest exposure of Precambrian rocks in the world, consisting predominantly of igneous and metamorphic rocks. There is also a buried portion of the Canadian Shield extending westward to the Rocky Mountains and southward to the Appalachian Mountains, the Arbuckle Mountains in Oklahoma, and into Mexico. This buried portion of the shield has a thin cover of largely Paleozoic sedimentary rocks deposited in shallow transgressing seas. These Paleozoic rocks are still flat-lying except where they have been gently warped into broad domes and basins. Along the margins of the Canadian Shield, four belts of deformed sedimentary rocks represent the former geosynclines. Geologists named the deformed belt on the eastern side of the shield the Appalachian geosyncline, the belt on the south side the Ouachita geosyncline, the belt on the west side the Cordilleran geosyncline, and the belt on the north side the Franklin geosyncline. Because of compressive forces emanating from the ocean basins, overturned folds and thrust faults are present in all four geosynclinal belts.

Similar patterns of continental growth are found elsewhere in the world. Each continent has at least one shield. These include shields in South America, Africa, northern Europe, Siberia, eastern Asia, India, Australia, and Antarctica. The remnants of geosynclinal belts are located adjacent to these shields. The most famous of these geosynclinal belts is the Tethyan geosyncline, found along the southern margin of Europe and Asia. The present-day Alps and Himalayas have risen out of this geosynclinal belt.

Plate Tectonics

During the 1960s, the concept of plate tectonics gradually emerged as the result of the work of oceanographers trying to explain the origin of the planet's major seafloor features. These features include mid-ocean ridges rivaling the largest mountain ranges on the Earth and volcanic island chains with associated deep oceanic trenches that rim the Pacific. The plate tectonics theory has revolutionized not only the field of oceanography but also the field of geology. According to the plate tectonics theory, the surface of the Earth is covered by a series of rigid slabs or plates that are capable of moving slowly over the Earth's interior. Geologists recognize seven major plates, each one usually containing a continent. The plates are presumed to behave as separate units, and where plates jostle each other, intense geologic activity occurs along their boundaries.

Three types of activity are believed to occur at plate boundaries. Divergence occurs where two plates are moving apart. The result of the divergence of two continental plates is believed to be the formation of a new ocean basin, a process referred to by geologists as seafloor spreading.

When two plates are moving toward each other, the result is convergence. In this case, three possibilities arise, depending on the nature of the plate boundary. If two continental plates converge, the intervening oceanic sediments are believed to be compressed into a new mountain range, such as the Himalaya. If a continental plate runs into an oceanic plate, the oceanic plate is believed to slide beneath the overriding continental plate, producing a deep oceanic trench. Geologists refer to this process as subduction. Finally, one oceanic plate may override another oceanic plate, producing a trench and adjoining volcanic island arc.

The third type of movement found along plate boundaries occurs when two plates slide past each other horizontally, just as trains pass each other in opposite directions on adjacent tracks. Such movement may proceed continuously or in a series of abrupt jerks, depending on the amount of friction encountered along the plate boundary. The abrupt jerks result in the type of Earth movement known as earthquakes, and these are particularly associated with the famous San Andreas fault in California.

Geologists speculate that convection cells of rising and sinking material in the mantle (the Earth's intermediate layer) may carry the plates slowly along. Geologists are now using 3-D seismic tomography, which shows that larger forces may be at play than convection cells. In the late 1990s, the idea of plume tectonics was suggested. The mechanism by which rigid plates can slide across the Earth's interior is unconfirmed. Presumably, a “plastic” layer in the upper mantle provides the necessary lubrication for the crustal plates to move.

The significance of plate tectonics for the concept of continental growth has been recognized by scientists. Because most of the jostling takes place at plate boundaries, that is where they find downwarped geosynclinal belts, earthquakes, volcanic activity, and recently formed mountains. By comparison, the stable plate interiors are places where little jostling takes place and thus where the quiescent continental shields are located.

Continuing Continental Growth

An example of a present-day geosynclinal belt being squeezed between two converging shields is the Tethyan geosyncline. This geosyncline is believed to have originated between the Eurasian and African Shields during the Mesozoic era. It must have resembled a broad tropical seaway extending from the Caribbean eastward through the Mediterranean and the Himalayas to Indonesia on the borders of the Pacific. Thick marine sediments accumulated on the floor of this seaway, and they are preserved today as richly fossiliferous limestone sedimentary rocks.

During the Cenozoic time, geologists believe, the convergence of several continental plates initiated the destruction of this seaway. The Indian subcontinent, for example, is believed to have drifted northward until it collided with Asia, producing the Himalayas, the highest mountain chain on the Earth. A second collision occurred as the Arabian plate (a minor subplate) drifted north to collide with Asia Minor, forming the Zagros and other mountains. Finally, Africa is believed to have drifted northward, resulting in the compressive forces that have produced the Alps. The result of these collisions is the near obliteration of the old Tethyan seaway. The only relics of it that survive are the Caspian Sea, the Black Sea, and the Mediterranean Sea. In addition, the sedimentary rocks that accumulated in the geosyncline have been folded and thrust northward against the Eurasian continental platform. Scientists have a clear picture, therefore, of continental growth occurring because of crustal plates moving toward each other, with the intervening sediments being welded to the shields by lateral accretion.

As indicated earlier, continental growth can also result when a portion of a preexisting continent, or even an entire continent, collides with another continent. An example of such a collision is believed to be provided by the formation of the Ural Mountains at the end of the Paleozoic era. Geologists now believe that these north-south trending mountains, eroded down to their roots, were formed by the compression of sediments deposited in a seaway lying between Europe and Asia. In other words, the present-day continent of Eurasia, which is twice as large as any other continent, was once two separate continents.

In the cases of the destruction of the Tethyan geosyncline and the formation of the Ural Mountains, the role of plate tectonics seems clear because the plates that did the moving can be identified. Sometimes, however, relationships are not so apparent—for example, in the case of the deformation of North America's Appalachian geosyncline and the thrusting of its Paleozoic sediments against the shield. No continental plate lies along the East Coast of North America that might account for the compression. As a result, geophysicists have postulated an elaborate scenario that involves two stages. They assume, first, that the Atlantic Ocean closed at the end of the Paleozoic era as a result of the collision of Europe with North America and then that Europe moved eastward again, resulting in the reopening of the Atlantic because of seafloor spreading.

The deformation of the Cordilleran geosyncline along the West Coast of North America offers no such problems. It can be explained by the collision of the North American plate with the Pacific Ocean plate. According to plate tectonics, the Pacific Ocean is a separate plate even though it lacks a continent. Thus, the deformation of the Cordilleran geosyncline has resulted from the sliding of the Pacific Ocean floor beneath the North American continent in the process known as subduction.

Study of Continental Growth

Scientists have studied continental growth in many ways. Foremost among them has been field investigations in the rock strata found along the margins of the shields. Using the fossils contained within these rocks, as well as radioactive dating, geologists have pieced together a detailed history of geosynclinal accretion. By analyzing the geometry of the folds and faults, scientists have also been able to infer the direction from which the compressive forces came.

An example of such geologic field investigations is seen in the deciphering of the rocks of the Canadian Shield. These rocks constitute the largest outcrop of Precambrian strata exposed anywhere in the world today. To early workers, they appeared to be a hopeless tangle of similar-looking igneous and metamorphic rocks. After years of painstaking research, however, scientists have been able to identify an orderly sequence of mappable rock units within the Canadian Shield, so that at least four distinct cycles of deposition and mountain making during Precambrian time are recognized.

Another way in which scientists have approached the subject of continental growth is by examining the rock types that compose the shields and ocean floors. They have found that the continental rocks are largely granitic and are rich in silica, aluminum, and potassium. These rocks also have a slightly lower average density than do the rocks underlying the ocean basins, and they stand higher, as if both were floating on interior layers of the Earth. By contrast, the rocks of the ocean floors consist of slightly heavier basalt lava and related volcanic rocks.

To everyone's surprise, the granitic rocks of the continents have not proved to be the oldest rocks on the Earth. Radioactive dating indicates that slivers of seafloor rocks incorporated in the granites claim this distinction. Thus, scientists have concluded that the shields do not represent parts of the Earth's original crust but have been built up through time by lateral accretion. Their granites may have come from the reworking of seafloor rocks.

A third way scientists investigate continental growth is through the detailed study of the sea floor itself. This study began with the Deep Sea Drilling Project in 1968 using the Glomar Challenger, a drillship that was retired in 1984. The program continued through 2004 under the name Ocean Drilling Project, utilizing the JOIDES Resolution, a 143-meter drillship capable of recovering samples of rock and sediments from depths of 9,000 meters under the ocean surface. The Integrated Ocean Drilling Program (IODP) replaced the Ocean Drilling Project in 2004 and ran through 2013. The International Ocean Discovery Program (IODP) replaced this program in 2013, building on the four previous projects. The IODP successfully fosters global research, operating multiple drilling platforms and exploration vessels that research and monitor the Earth and climate change. The findings of these programs have been of great significance for plate tectonics.

Evidence indicates a very young age for the ocean basins—less than 200 million years old, which is less than one-twentieth the Earth's presumed age of 4.7 billion to 5 billion years. Further, the Atlantic and Indian Oceans are widening, while the Pacific is shrinking by a few centimeters each year, which means the crustal plates will eventually collide, thus providing the mechanism for further continental growth.

Some scientists suggest the rate of continental growth may have been faster at one time, but it slowed down about three billion years ago. Research suggests there have been six distinct periods of growth in Earth's history, which occur every five to seven hundred years. These periods are suggested to occur simultaneously with continents' major formation and deformation. However, scientists continually debate the details of these findings, and many studies focus only on zircon, a mineral used to estimate crust growth, failing to consider the impact of the lithospheric mantle root.

Mineral Deposits

The same processes that have thrust former geosynclinal belts against the shields have also produced rich mineral deposits in the resulting mountain chains. These mineral deposits fall into three categories: metals, such as gold, silver, and copper. Nonmetallic deposits, such as certain abrasives, gemstones, and the building stones granite, marble, and slate; and the important energy resources petroleum, natural gas, and anthracite coal.

A good example of a metal deposit found in the deformed rocks of a former geosyncline is California's famous Mother Lode. This zone of gold veins, which is more than 200 kilometers long but barely one kilometer wide, can be traced along the western slopes of the Sierra Nevada, a mountain range that has risen out of the former Cordilleran geosyncline. The gold discoveries—which attracted the “Forty-Niners” to California and led to the rapid growth of San Francisco and neighboring cities—were nuggets and flakes of gold derived from these veins and washed down into the sand and gravel deposits of rivers at the foot of the mountains. The early settlers realized the gold was from a source upstream, and they called this source the Mother Lode (literally, “parent vein”). Eventually, the settlers traced the streams up to their headwaters in the Sierra Nevada and discovered the Mother Lode itself.

The fabulously rich oil deposits of the Middle East are another example of an economic resource related to continental growth. The Tethyan seaway, which stretched from the Caribbean to the Pacific during Mesozoic time, was the site of extensive deposits of thick limestone sedimentary rocks. These limestones are now oil-bearing and have been caught in the closing vise between the northward-moving Arabian plate and the portion of the Eurasian continent known as Asia Minor. Because of this compression, the limestones have been shaped into a series of gently undulating folds. Migrating oil has been trapped in the crests of the upfolds (technically known as anticlines), where it is obtained by drilling wells down into the anticlinal structures.

In 1988, more than 40 percent of the world's oil imports came from the Middle Eastern oil fields, which enabled these nations to wield a political and economic influence far out of proportion to their geographic size or population. In the 1970s, when these nations paralyzed the free world's economic system with an oil embargo. Though the major consumers of Middle Eastern oil were Western Europe and Japan, the dislocation of world oil supplies had severe consequences in the United States as well. Americans were asked to turn down their thermostats, the nationwide speed limit was reduced to fifty-five miles per hour, and automobile companies were told to improve the gas mileage of their cars. As the ripple effects of the oil shortage spread through the United States economy, a recession was triggered that cost people their jobs and set off a major stock market decline. Despite these experiences and abortive attempts to market electrically powered automobiles, later generations of U.S. car drivers migrated back to low-mileage cars, trucks, and recreational vehicles. The appetite for Middle Eastern oil had allowed subsequent manipulation of world oil supply, as again occurred early in 2000 when oil production was limited, and prices rose dramatically.

Principal Terms

fault: a fracture in rock strata with relative displacement of the two sides

fold: an upward or downward bend in layered rock strata

geosyncline: an elongate subsiding trough in which great thicknesses of sedimentary and volcanic rocks accumulate

granite: a light-colored crustal rock produced by the underground cooling of molten rock

lateral accretion: the process by which crustal material is welded to a shield by horizontal compression

mantle: the Earth's 2,900-kilometer-thick intermediate layer, which is found beneath the crust

plate tectonics: a theory that describes the Earth's outer shell as consisting of individual moving plates

shield: a continental block of the Earth's crust that has been stable over a long period

subduction: the process by which one crustal plate slides beneath another as a result of horizontal compression

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