III.

Plate Movement

Geologists study how tectonic plates move relative to a fixed spot in the earth’s mantle and how they move relative to each other. The first type of motion is called absolute motion, and it can lead to strings of volcanoes. The second kind of motion, called relative motion, leads to different types of boundaries between plates: plates moving apart from one another form a divergent boundary, plates moving toward one another form a convergent boundary, and plates that slide along one another form a transform plate boundary. In rare instances, three plates may meet in one place, forming a triple junction. Current plate movement is making the Pacific Ocean smaller, the Atlantic Ocean larger, and the Himalayan mountains taller.

A.

Measuring Plate Movement

Geologists discovered absolute plate motion when they found chains of extinct submarine volcanoes. A chain of dead volcanoes forms as a plate moves over a plume, a source of magma, or molten rock, deep within the mantle. These plumes stay in one spot, and each one creates a hot spot in the plate above the plume. These hot spots can form into a volcano on the surface of the earth. An active volcano indicates a hot spot as well as the youngest region of a volcanic chain. As the plate moves, a new volcano forms in the plate over the place where the hot spot occurs. The volcanoes in the chain get progressively older and become extinct as they move away from the hot spot (see Hawaii: Formation of the Islands and Volcanoes). Scientists use hot spots to measure the speed of tectonic plates relative to a fixed point. To do this, they determine the age of extinct volcanoes and their distance from a hot spot. They then use these numbers to calculate how far the plate has moved in the time since each volcano formed. Today, the plates move at velocities up to 18.5 cm per year (7.3 in per year). On average, they move nearly 4 to 7 cm per year (2 to 3 in per year).

B.

Divergent Plate Boundaries

Divergent plate boundaries occur where two plates are moving apart from each other. When plates break apart, the lithosphere thins and ruptures to form a divergent plate boundary. In the oceanic crust, this process is called seafloor spreading, because the splitting plates are spreading apart from each other. On land, divergent plate boundaries create rift valleys—deep valley depressions formed as the land slowly splits apart.

When seafloor spreading occurs, magma, or molten rock material, rises to the sea floor surface along the rupture. As the magma cools, it forms new oceanic crust and lithosphere. The new lithosphere is less dense, so it rises, or floats, higher above older lithosphere, producing long submarine mountain chains known as mid-ocean ridges. The Mid-Atlantic Ridge is an underwater mountain range created at a divergent plate boundary in the middle of the Atlantic Ocean. It is part of a worldwide system of ridges made by seafloor spreading. The Mid-Atlantic Ridge is currently spreading at a rate of 2.5 cm per year (1 in per year). The mid-ocean ridges today are 60,000 km (about 40,000 mi) long, forming the largest continuous mountain chain on earth. Earthquakes, faults, underwater volcanic eruptions, and vents, or openings, along the mountain crests produce rugged seafloor features, or topography.

Divergent boundaries on land cause rifting, in which broad areas of land are uplifted, or moved upward. These uplifts and faulting along the rift result in rift valleys. Examples of rift valleys are found at the Krafla Volcano rift area in Iceland as well as at the East African Rift Zone—part of the Great Rift Valley that extends from Syria to Mozambique and out to the Red Sea. In these areas, volcanic eruptions and shallow earthquakes are common.

C.

Convergent Plate Boundaries

Convergent plate boundaries occur where plates are consumed, or recycled back into the earth’s mantle. There are three types of convergent plate boundaries: between two oceanic plates, between an oceanic plate and a continental plate, and between two continental plates. Subduction zones are convergent regions where oceanic crust is thrust below either oceanic crust or continental crust. Many earthquakes occur at subduction zones, and volcanic ridges and oceanic trenches form in these areas.

In the ocean, convergent plate boundaries occur where an oceanic plate descends beneath another oceanic plate. Chains of active volcanoes develop 100 to 150 km (60 to 90 mi) above the descending slab as magma rises from under the plate. Also, where the crust slides down into the earth, a trench forms. Together, the volcanoes and trench form an intra-oceanic island arc and trench system. A good example of such a system is the Mariana Trench system in the western Pacific Ocean, where the Pacific plate is descending under the Philippine plate. In these areas, earthquakes are frequent but not large. Stress in and behind the arc often causes the arc and trench system to move toward the incoming plate, which opens small ocean basins behind the arc. This process is called back-arc seafloor spreading.

Convergent boundaries that occur between the ocean and land create continental margin arc and trench systems near the margins, or edges, of continents. Volcanoes also form here. Stress can develop in these areas and cause the rock layers to fold, leading to earthquake faults, or breaks in the earth’s crust called thrust faults. The folding and thrust faulting thicken the continental crust, producing high mountains. Many of the world’s large destructive earthquakes and major mountain chains, such as the Andes Mountains of western South America, occur along these convergent plate boundaries.

When two continental plates converge, the incoming plate drives against and under the opposing continent. This often affects hundreds of miles of each continent and, at times, doubles the normal thickness of continental crust. Colliding continents cause earthquakes and form mountains and plateaus. The collision of India with Asia has produced the Himalayan Mountains and Tibetan Plateau.

D.

Transform Plate Boundaries

A transform plate boundary, also known as a transform fault system, forms as plates slide past one another in opposite directions without converging or diverging. Early in the plate tectonic revolution, geologists proposed that transform faults were a new class of fault because they “transformed” plate motions from one plate boundary to another. Canadian geophysicist J. Tuzo Wilson studied the direction of faulting along fracture zones that divide the mid-ocean ridge system and confirmed that transform plate boundaries were different than convergent and divergent boundaries. Within the ocean, transform faults are usually simple, straight fault lines that form at a right angle to ocean ridge spreading centers. As plates slide past each other, the transform faults can divide the centers of ocean ridge spreading. By cutting across the ridges of the undersea mountain chains, they create steep cliff slopes. Transform fault systems can also connect spreading centers to subduction zones or other transform fault systems within the continental crust. As a transform plate boundary cuts perpendicularly across the edges of the continental crust near the borders of the continental and oceanic crust, the result is a system such as the San Andreas transform fault system in California.

E.

Triple Junctions

Rarely, a group of three plates, or a combination of plates, faults, and trenches, meet at a point called a triple junction. The East African Rift Zone is a good example of a triple plate junction. The African plate is splitting into two plates and moving away from the Arabian plate as the Red Sea meets the Gulf of Aden. Another example is the Mendocino Triple Junction, which occurs at the intersection of two transform faults (the San Andreas and Mendocino faults) and the plate boundary between the Pacific and Gorda plates.

F.

Current Plate Movement

Plate movement is changing the sizes of our oceans and the shapes of our continents. The Pacific plate moves at an absolute motion rate of 9 cm per year (4 in per year) away from the East Pacific Rise spreading center, the undersea volcanic region in the eastern Pacific Ocean that runs parallel to the western coast of South America. On the other side of the Pacific Ocean, near Japan, the Pacific plate is being subducted, or consumed under, the oceanic arc systems found there. The Pacific Ocean is getting smaller as the North and South American plates move west. The Atlantic Ocean is getting larger as plate movement causes North and South America to move away from Europe and Africa. Since the Eurasian and Antarctic plates are nearly stationary, the Indian Ocean at present is not significantly expanding or shrinking. The plate that includes Australia is just beginning to collide with the plate that forms Southeast Asia, while India’s plate is still colliding with Asia. India moves north at 5 cm per year (2 in per year) as it crashes into Asia, while Australia moves slightly farther away from Antarctica each year.