We don’t perceive that the continents we live on are moving, and at a rate of as much as 4 inches per year, how could we?.
It’s not as if an airplane flight between Europe and Africa takes five hours one year but only three hours the next. But the continents actually are shifting, very slowly, relative to one another.
In the early 20th century, a scientific theory called continental drift was proposed about this migration of the continents. That theory was initially ridiculed, but it paved the way for another theory called plate tectonics that scientists have now accepted to explain how Earth’s continents move.
Plate tectonics is the theory that Earth's land masses are in constant motion. The realization that Earth's land masses move was first proposed by Alfred Wegener, which he called continental drift. He is shown here in Greenland.
The story begins with Alfred Wegener (1880–1930), a German meteorologist and geophysicist who noticed something curious when he looked at a map of the world. Wegener observed that the continents of South America and Africa looked like they would fit together remarkably well—take away the Atlantic Ocean and these two massive landforms would lock neatly together. He also noted that similar fossils were found on continents separated by oceans, additional evidence that perhaps the landforms had once been joined.
Wegener hypothesized that all of the modern-day continents had previously been clumped together in a supercontinent he called Pangaea (from ancient Greek, meaning “all lands” or “all the Earth”). Over millions of years, Wegener suggested, the continents had drifted apart. He did not know what drove this movement, however. Wegener first presented his idea of continental drift in 1912, but it was widely ridiculed and soon, mostly, forgotten. Wegener never lived to see his theory accepted—he died at the age of 50 while on an expedition in Greenland.
Only decades later, in the 1960s, did the idea of continental driftresurface. That’s when technologies adapted from warfare made it possible to more thoroughly study Earth. Those advances included seismometers used to monitor ground shaking caused by nuclear testing and magnetometers to detect submarines. With seismometers, researchers discovered that earthquakes tended to occur in specific places rather than equally all over Earth. And scientists studying the seafloor with magnetometersfound evidence of surprising magnetic variations near undersea ridges: alternating stripes of rock recorded a flip-flopping of Earth’s magnetic field.
Together, these observations were consistent with a new theory proposed by researchers who built on Wegener’s original idea of continental drift—the theory of plate tectonics. According to this theory, Earth’s crust is broken into roughly 20 sections called tectonic plates on which the continents ride. When these plates press together and then move suddenly, energy is released in the form of earthquakes. That is why earthquakes do not occur everywhere on Earth—they’re clustered around the boundaries of tectonic plates. Plate tectonics also explains the stripes of rock on the seafloor with alternating magnetic properties: As buoyant, molten rock rises up from deep within Earth, it emerges from the space between spreading tectonic plates and hardens, creating a ridge. Because some minerals within rocks record the orientation of Earth’s magnetic poles and this orientation flips every 100,000 years or so, rocks near ocean ridges exhibit alternating magnetic stripes.
Plate tectonics explains why Earth’s continents are moving; the theory of continental drift did not provide an explanation. Therefore, the theory of plate tectonics is more complete. It has gained widespread acceptance among scientists. This shift from one theory to another is an example of the scientific process and the use of the scientific method: As more observations are made and measurements are collected, scientists revise their theories to be more accurate and consistent with the natural world.
By running computer simulations of how Earth’s tectonic plates are moving, researchers can estimate where the planet's continents will likely be in the future. Because tectonic plates move very slowly—only a few centimeters per year, on average—it takes a long time to observe changes. Scientists have found that the planet’s continents will likely again be joined together in another 250 million years. Researchers have dubbed this future continental configuration “Pangaea Proxima.”
One intriguing aspect of Pangaea Proxima is that it will likely contain a new mountain range with some of the world’s highest mountains. That is because as Africa continues to migrate north it will collide with Europe, a collision that will probably create a Himalaya-scale mountain range. However, Christopher Scotese, one of the scientists who developed these simulations, cautions that it is difficult to predict exactly how the continents will be arranged in millions of years. “We don’t really know the future, obviously,” Scotese told NASA. “All we can do is make predictions of how plate motions will continue, what new things might happen, and where it will all end up.”
Plate tectonics is the theory
dealing with the dynamics
’s outer shell—the lithosphere
—that revolutionized Earth science
by providing a uniform context
for understanding mountain-building processes
, and earthquakes
as well as the evolution of Earth’s surface and reconstructing its past continents and oceans.
The concept of plate tectonics
was formulated in the 1960s. According to the theory, Earth has a rigid outer layer, known as the lithosphere
, which is typically about 100 km (60 miles) thick and overlies a plastic (moldable, partially molten) layer called the asthenosphere
. The lithosphere is broken up into seven very large continental- and ocean-sized plates, six or seven medium-sized regional plates, and several small ones. These plates move relative to each other, typically at rates of 5 to 10 cm (2 to 4 inches) per year, and interact along their boundaries, where they converge
, diverge, or slip past one another. Such interactions are thought to be responsible for most of Earth’s seismic and volcanic activity, although earthquakes and volcanoes can occur in plate interiors. Plate motions cause mountains
to rise where plates push together, or converge, and continents
to fracture and oceans to form where plates pull apart, or diverge. The continents are embedded in the plates and drift passively with them, which over millions of years results in significant changes in Earth’s geography
The theory of plate tectonics is based on a broad synthesis of geologic and geophysical data. It is now almost universally accepted, and its adoption represents a true scientific revolution, analogous
in its consequences to quantum mechanics
or the discovery of the genetic code
. Incorporating the much older idea of continental drift
, as well as the concept of seafloor spreading
*, the theory of plate tectonics has provided an overarching framework in which to describe the past geography of continents and oceans
, the processes controlling creation and destruction of landforms
, and the evolution of Earth’s crust, atmosphere
, and climates
. During the late 20th and early 21st centuries, it became apparent that plate-tectonic processes profoundly influence the composition
of Earth’s atmosphere and oceans, serve as a prime cause of long-term climate change
, and make significant contributions to the chemical and physical environment
in which life
Earth's lithosphere and upper mantle.
In essence, plate-tectonic theory is elegantly simple. Earth
’s surface layer, 50 to 100 km (30 to 60 miles) thick, is rigid and is composed of a set of large and small plates. Together, these plates constitute
, from the Greek lithos
, meaning “rock
.” The lithosphere rests on and slides over an underlying partially molten (and thus weaker but generally denser) layer of plastic partially molten rock
known as the asthenosphere
, from the Greek asthenos
, meaning “weak.” Plate movement is possible because the lithosphere-asthenosphere boundary is a zone of detachment. As the lithospheric plates move across Earth’s surface, driven by forces as yet not fully understood, they interact along their boundaries, diverging, converging, or slipping past each other. While the interiors of the plates are presumed
to remain essentially undeformed, plate boundaries are the sites of many of the principal processes that shape the terrestrial surface, including earthquakes, volcanism
, and orogeny
The process of plate tectonics may be driven by convection
in Earth’s mantle, the pull of heavy old pieces of crust into the mantle
, or some combination of both.
For a deeper discussion of plate-driving mechanisms, see Plate-driving mechanisms and the role of the mantle
Crustal generation and destruction
Knowledge of Earth’s interior is derived primarily from analysis of the seismic waves
through Earth as a result of earthquakes. Depending on the material they travel through, the waves may either speed up, slow down, bend, or even stop if they cannot penetrate the material they encounter.
Collectively, these studies show that Earth can be internally divided into layers on the basis of either gradual or abrupt variations in chemical and physical properties. Chemically, Earth can be divided into three layers. A relatively thin crust
, which typically varies from a few kilometres to 40 km (about 25 miles) in thickness, sits on top of the mantle. (In some places, Earth’s crust may be up to 70 km [40 miles] thick.) The mantle is much thicker than the crust; it contains 83 percent of Earth’s volume and continues to a depth of 2,900 km (1,800 miles). Beneath the mantle is the core, which extends to the centre of Earth, some 6,370 km (nearly 4,000 miles) below the surface. Geologists maintain that the core is made up primarily of metallic iron
accompanied by smaller amounts of nickel
, and lighter elements, such as carbon
. (See also Earth
The effect of the different densities
of lithospheric rock
can be seen in the different average elevations of continental and oceanic crust. The less-dense continental crust has greater buoyancy, causing it to float much higher in the mantle. Its average elevation above sea level
is 840 metres (2,750 feet), while the average depth of oceanic crust is 3,790 metres (12,400 feet). This density difference creates two principal levels of Earth’s surface.
itself includes all the crust as well as the upper part of the mantle (i.e., the region
directly beneath the Moho), which is also rigid. However, as temperatures increase with depth, the heat
causes mantle rocks to lose their rigidity. This process begins at about 100 km (60 miles) below the surface. This change occurs within the mantle and defines the base of the lithosphere and the top of the asthenosphere
. This upper portion of the mantle, which is known as the lithospheric mantle, has an average density
of about 3.3 grams per cubic cm (0.12 pound per cubic inch). The asthenosphere, which sits directly below the lithospheric mantle, is thought to be slightly denser at 3.4–4.4 grams per cubic cm (0.12–0.16 pound per cubic inch).
In contrast, the rocks
in the asthenosphere are weaker, because they are close to their melting
temperatures. As a result, seismic waves
slow as they enter the asthenosphere. With increasing depth, however, the greater pressure
from the weight of the rocks above causes the mantle to become gradually stronger, and seismic waves increase in velocity, a defining characteristic of the lower mantle. The lower mantle is more or less solid, but the region is also very hot, and thus the rocks can flow very slowly (a process known as creep).
During the late 20th and early 21st centuries, scientific understanding of the deep mantle was greatly enhanced
by high-resolution seismological studies combined with numerical modeling
and laboratory experiments that mimicked conditions near the core-mantle boundary. Collectively, these studies revealed that the deep mantle is highly heterogeneous
and that the layer may play a fundamental role in driving Earth’s plates.
At a depth of about 2,900 km (1,800 miles), the lower mantle gives way to Earth’s outer core, which is made up of a liquid rich in iron and nickel
. At a depth of about 5,100 km (3,200 miles), the outer core transitions to the inner core. Although it has a higher temperature than the outer core, the inner core is solid because of the tremendous pressures that exist near Earth’s centre. Earth’s inner core is divided into the outer-inner core (OIC) and the inner-inner core (IIC), which differ from one another with respect to the polarity
of their iron crystals. The polarity of the iron crystals of the OIC is oriented in a north-south direction, whereas that of the IIC is oriented east-west.
In a simplified example of plate motion shown in the figure, movement of plate A to the left relative to plates B and C results in several types of simultaneous
interactions along the plate boundaries. At the rear, plates A and B move apart, or diverge, resulting in extension and the formation
of a divergent margin. At the front, plates A and B overlap, or converge, resulting in compression and the formation of a convergent margin. Along the sides, the plates slide past one another, a process called shear. As these zones of shear link other plate boundaries to one another, they are called transform faults
As plates move apart at a divergent plate boundary
, the release of pressure produces partial melting of the underlying mantle. This molten material, known as magma
, is basaltic
and is buoyant. As a result, it wells up from below and cools close to the surface to generate new crust. Because new crust is formed, divergent margins are also called constructive margins.
Upwelling of magma
causes the overlying lithosphere
to uplift and stretch. (Whether magmatism [the formation of igneous rock
from magma] initiates the rifting or whether rifting decompresses the mantle and initiates magmatism is a matter of significant debate.) If the diverging plates are capped by continental crust, fractures develop that are invaded by the ascending magma, prying the continents farther apart. Settling of the continental blocks creates a rift valley
, such as the large present-day East African Rift Valley
. As the rift
continues to widen, the continental crust becomes progressively thinner until separation of the plates is achieved and a new ocean is created. The ascending partial melt cools and crystallizes to form new crust. Because the partial melt is basaltic in composition, the new crust is oceanic, and an ocean ridge
develops along the site of the former continental rift. Consequently, diverging plate boundaries, even if they originate within continents, eventually come to lie in ocean basins of their own making.
As upwelling of magma
continues, the plates continue to diverge, a process known as seafloor spreading
. Samples collected from the ocean floor
show that the age of oceanic crust
increases with distance from the spreading centre
—important evidence in favour of this process. These age data
also allow the rate of seafloor spreading to be determined, and they show that rates vary from about 0.1 cm (0.04 inch) per year to 17 cm (6.7 inches) per year. Seafloor-spreading rates are much more rapid in the Pacific Ocean
than in the Atlantic
and Indian oceans
. At spreading rates of about 15 cm (6 inches) per year, the entire crust beneath the Pacific Ocean (about 15,000 km [9,300 miles] wide) could be produced in 100 million years.
In 1912 German meteorologist Alfred Wagener
, impressed by the similarity of the geography of the Atlantic coastlines, explicitly presented the concept of continental drift. Though plate tectonics is by no means synonomous with continental drift, the term encompasses this idea and derives much of its impact from it.