Continents and subcontinents

Continents are large landmasses with elevations that are considerably higher than that of the surrounding crust. Subcontinents are smaller landmasses that converged over time to form the large continents familiar today. Because of this movement, continents have a wide variety of terrains and landforms.

The word “continent” comes from the Latin continere, which means “to hold together.” Continents are large landmasses composed of lighter rocks that ride on top of the denser rocks in the mantle, somewhat like a cork in water. This results in areas on the Earth's surface that are higher than sea level, producing dry land. The Earth was not formed with these continents in place; a long, complicated process resulted in the formation of the landmasses familiar today.

When the earth was first formed approximately 4.6 billion years ago, it was a molten ball composed of all the elements. The planet's high temperature was the result of heat released from several sources: the process that formed the planet, decay of radioactive elements, and intense meteoritic bombardment. This molten state allowed the different elements and the compounds they formed to differentiate or separate.

This differentiation process is similar to mixing different kinds of oil with water in a bottle. Shake up the bottle, and the different liquids will be mixed together. Let it sit, and they will begin to form layers. The water is densest and will form a layer on the bottom. Each of the different oils will then form a separate layer above the water, with the layer of the densest oil being on top of the water and the least-dense oil forming the uppermost layer.

Similarly, when the Earth underwent differentiation, the densest material—mainly iron and nickel—sank toward the interior and formed the planetary core. Compounds and elements that were medium in density settled on top of the denser core and formed the layers of the planetary mantle. The least-dense compounds floated to the surface, eventually forming the crust, seawater, and atmosphere. These least-dense compounds consisted principally of the elements silicon, oxygen, aluminum, potassium, sodium, calcium, carbon, nitrogen, hydrogen, and helium, with lesser amounts of other elements.

Formation of the Earth's Crust

In the twenty-first century, Earth has two kinds of crust: the heavier, thinner crust under the oceans and the lighter, thicker continental crust. The oceanic crust was created between 4.2 billion and 4.5 billion years ago, has an average density of 2.7 grams per cubic centimeter, and consists mainly of mafic rocks. Mafic rocks are made of minerals that consist mainly of magnesium and iron. The most common mafic rock in the oceanic crust is basalt, a dark, hard stone. The dark maria on the face of the Moon (dark, featureless plains seen easily with the naked eye) are the result of basalt that reached the Moon's surface after large meteor impacts. Beneath the Earth's crust is the upper mantle, which has a density of approximately 3.4 grams per cubic centimeter and consists of mafic rocks that contain an even larger percentage of magnesium and iron; hence, they are called ultramafic rocks. This layer formed at about the same time as the oceanic crust.

Mixed with these two layers were even lighter materials, mainly compounds of silicon, oxygen, and aluminum, but the planet's high temperature did not allow these materials to start solidifying until about 4 billion years ago. The first continental rocks began to form when this occurred, although they were continuously broken up. The oceanic rocks were mainly basalt; the continental rocks were mainly granite with a density of 2.7 grams per cubic centimeter. The cooling process was slowed by the formation of crystal structures within the oceanic crust and upper mantle that forced out certain rare-earth elements, including the radioactive elements. These elements had to go somewhere and are thus found concentrated in continental rocks. Continental granites contain about ten times as much uranium as the oceanic basalts and about one thousand times as much as the upper mantle rocks. The heat released by the decay of this concentration of radioactive elements helped keep the continental rocks molten longer than the oceanic rocks.

A large amount of volcanic activity also characterized this period in the Earth's history; large chains of volcanoes formed archipelagoes of islands. As a result of plate tectonics, these islands moved around on the Earth's surface and were eventually reabsorbed back into the Earth's interior at subduction zones. Subduction zones occur when an oceanic crustal plate meets a continental plate. The denser, heavier oceanic plate is forced under the other plate and into the upper mantle, melting and returning to the surface through volcanic activity. Given enough time, this process will completely recycle the oceanic crust; today, none of the original oceanic crust remains.

About 4 billion years ago, the intense meteoritic bombardment suddenly ended; the planet's surface began to cool more quickly, and more island chains were formed. However, the still-molten, lighter continental rocks sometimes flowed into large cracks called fissures in the volcanic islands, providing them with additional buoyancy. When these islands, riding on the surface of a plate, reached the subduction zones, they were too light to be subducted and were instead scraped off by the other plate. As the other plate moved, it continued to scrape off more of the light islands; over time, a large amount of this lighter material accumulated in front of the plate. Eventually, that plate was subducted by another plate, and the light continental material it had collected was added to any collected by the new plate. In this way, the amount of continental crustal material grew until it was large enough to be a subcontinent.

Subcontinents and Accretion

A subcontinent is an area of land that is too large to be pushed simply by the movement of oceanic crustal plates. Instead, these large pieces of land ride on top of moving mantle rock deep beneath the surface. Yet, while subcontinents are extensive, they are still not large enough to be considered continents. Modern examples of subcontinents include the island of Greenland, the Arabian Peninsula, and the Indian subcontinent. The importance of subcontinents is that they will eventually collide with one another. They can stick together when they do, forming even larger land areas and, eventually, continents. This process is called accretion.

When subcontinents or continents collide, the event is like an automobile crash in slow motion. The two large bodies are moving and do not stop immediately; they continue to plow into each other, causing the rock to bend, fold, and lift, forming mountain ranges. Areas where this has occurred in the past are called orogenic (mountain) belts; the process of mountain building is known as orogeny. The Himalaya are an example of the result of this process. The Indian subcontinent took millions of years to move from southeast Africa to its current position on the southern side of Asia. When it collided with Asia, the force was enough to raise a giant plateau, with the towering Himalayan Mountain chain on top. Large areas between the orogenic belts are called cratons. The American Midwest between the Appalachian and Rocky Mountains is an example of a craton.

This movement of the landmasses continues even after the formation of continents. The continents continue to ride on top of currents of rock in the mantle, slowly making their way across the Earth's surface. In the twenty-first century, continents are moving at about 2 to 10 centimeters per year, but this rate was faster in the past. Over millions of years, the continents have been able to move great distances, and the shape and distribution of continents in the past did not resemble the global features of today. At times, the continents were together to form supercontinents that lasted for millions of years before breaking apart.

and Erosion

In addition to accretion, two other major forces at work on continents are volcanism and erosion. Volcanism results from plate tectonics, and most volcanism thus occurs along the edges of the landmasses where subduction occurs, or fault lines are found. Volcanism recycles material that has been subducted and adds to the mass of the continents. Erosion, meanwhile, wears down landforms. Rain, ice, heat, wind, and flowing water all work to break apart the rocks and slowly wash the surface material away. Some of these sediments are washed to sea, while others collect in low-lying areas on the land. These sedimentary deposits can be several kilometers thick and eventually become sedimentary rock.

Study of Continents

Continents are vast in size and complexity. Likewise, the study of these landmasses is also vast and complex. Much of the work in learning about continents is hampered by scientists' ability to sample only the thinnest top layer of the crust easily. Moreover, much of the evidence of past activity is destroyed through erosion. As a result, studying the continents is a slow process involving many scientists using various techniques and instruments.

The most basic method of studying the continents involves studying the layers of rocks. Sometimes, these layers can be seen from the surface, and other times, scientists must use drills to remove core samples. Sometimes, these layers lie flat, while others are at all angles. Studying these layers makes it possible to learn what they are made of, how they were made, and even when they were made. Eventually, it becomes possible to conclude that various layers are related and sometimes even constitute the same layer. For example, a layer of rock found in North Dakota may be identical to a layer of rock found in Nebraska, hundreds of kilometers away. In this way, geologists can build maps showing where these layers of rock can be found.

These kinds of maps reveal much about the past of the land. If a type of rock found in the desert is made of material found only on the bottom of swamps, geologists can deduce that the desert was once a swamp and that the climate in the area was once different. It might even be possible to track the change from swamp to desert by examining the different layers, although sometimes the layers are destroyed through erosion. If the ages of the different layers are known, it is possible to build a storyline showing how the swamp changed to desert over a period of time. Also, the angle of the layers tells what happened to the land. If the layer is horizontal, it has probably been undisturbed since it formed. If tilted or folded over on itself, then some force was applied to the rocks. Also, if a layer of rock is found on one continent and also found on another continent, then it can be concluded that the two continents were together when the layer was formed.

Rocks themselves also provide clues. What are the rocks made of? If they are sedimentary rocks, then their material existed in some other rocks. Where were those rocks? If the rocks are basalts, it is possible to conclude that there was volcanic activity in the area at one time. The chemical structure can reveal much about the temperatures and pressures to which rocks have been exposed over the years.

Instruments on spacecraft can make measurements over vast regions. This speeds up the process and provides new views and evidence not previously possible. Computer models can include this information to determine the forces at work forming continents. In this way, clues that were previously unsuspected can be found. Once a computer model suggests something, scientists can investigate it to find scientific evidence to support or refute it. Through these and other processes, including Geographic Information Systems (GIS), geologists gradually learn more and more about the Earth and its history.

Scientists continue to make discoveries regarding Earth’s continents and subcontinents in the twenty-first century. In 2017, geologists officially recognized Zealandia as Earth’s eighth continent. Zealandia is over 90 percent underwater, with only New Zealand and a few other small islands breaching the surface. It is over 1 billion years old. In 2019, scientists confirmed the presence of the lost continent of Greater Adria found in the waters of southern Europe. Additionally, scientists continue to refine their understanding of and definition of subcontinents worldwide. 

Principal Terms

craton: a stable, relatively immobile area of the Earth's crust that forms the nucleus of a continental landmass

crust: the thin layer of rock covering the surface of the Earth; solid and cool, the crust makes up the continents and floor of the ocean and may be covered with thick layers of sediments

mafic rocks: rocks that contain large amounts of magnesium and iron, found mainly in the oceanic crust and upper mantle

mantle: the region of the Earth between the dense core and the thin crust; the mantle makes up most of the volume of the earth

orogenic belt: an area where mountain-forming forces have been applied to the crust

plate tectonics: the process that causes the continents and large unbroken land areas within the oceanic crust, called “plates,” to move slowly along with currents of rock in the upper mantle

sediments: rocks and soil that have been eroded from their original positions by forces of weather

subcontinent: an area of land that is less extensive in size and has a smaller variety of terrains than a continent

Bibliography

Kusky, T. M., M.-G. Zhai, and W. Xiao, editors. The Evolving Continents: Understanding Processes of Continental Growth. Geological Society Special Publication, 2010.

Moores, Eldridge, editor. Shaping the Earth: Tectonics of Continents and Oceans. New York: W. H. Freeman, 1990.

Newcomb, Tim. “Earth's Hidden Eighth Continent Is No Longer Lost.” Popular Mechanics, 25 Sept. 2023, www.popularmechanics.com/science/environment/a45226285/eighth-continent-zealandia-mapped. Accessed 20 July 2024.

Plummer, Charles C., Diane H. Carlson, and Lisa Hammersley. Physical Geology. 13th ed., Boston: McGraw-Hill, 2009.

Prothero, D. R., and Robert H. Dott. Evolution of the Earth. 8th ed., New York: McGraw-Hill, 2009.

Rogers, John J. W., and M. Santosh. Continents and Supercontinents. New York: Oxford University Press, 2004.

Simpson, Sarah. “Violent Origins of Continents.” Scientific American, vol. 302, 2010, pp. 60-67.

Stanley, Steven M. Earth and Life Through Time. 2d ed., New York: W. H. Freeman, 1989.

Strickland, Ashley. “A Lost Continent has been Found under Europe.” CNN, 23 Sept. 2019, www.cnn.com/2019/09/23/world/lost-continent-europe-scn-trnd/index.html. Accessed 20 July 2024.

Taylor, S. Ross, and Scott M. McLennan. “The Evolution of Continental Crust.” Scientific American, Jan. 1996, p. 274.

Wegener, Alfred. The Origin of Continents and Oceans. Dover Publications, 1966.

Weiner, Jonathan. Planet Earth. Toronto: Bantam Books, 1986.