3.2: Tectonic Plates, Plate Motions, and Plate Boundaries

Earth's Tectonic Plates

Earth is covered with eight major tectonic plates: Eurasia, Pacific, India, Australia, North America, South America, Africa, and Antarctica. There are also numerous small plates (e.g., Juan de Fuca, Cocos, Nazca, Scotia, Philippine, Caribbean), and many very small plates or sub-plates. For example the Juan de Fuca Plate is actually three separate plates (Gorda, Juan de Fuca, and Explorer) that all move in the same general direction but at slightly different rates (Figure \(\PageIndex{1}\)).

Plates can include continental crust, oceanic crust, or both; their boundaries do not necessarily coincide with the edges of continents or ocean basins. For example, the North American Plate includes most of North America, plus half of the northern Atlantic Ocean. Similarly the South American Plate extends across the western part of the southern Atlantic Ocean, while the European and African plates each include part of the eastern Atlantic Ocean. The Pacific Plate is almost entirely oceanic, but it does include the part of California that is west of the San Andreas Fault.

Figure \(\PageIndex{1}\): Earth’s fifteen largest tectonic plates. Red arrows show how plates move relative to each other. "Earth's Tectonic Plates and Relative Plate Motion" by Steven Earle (2015), a derivative of "Tectonic Plates", is licensed under CC BY 4.0.

Rates of motions of the major plates range from less than 1 cm/year to over 10 cm/year (~4 inches/year). The arrows on Figure \(\PageIndex{1}\) are velocity vectors that represent the direction and speed of plate motion (the length of the arrow indicates the speed, which can be determined from the scale bar). For example, the Pacific plate, which has one of the highest relative plate velocities, is moving to the northwest at a velocity of 7 and 11 cm/year (~3-4 inches/year). The North American Plate is one of the slowest, averaging around 2.3 cm/year (1 inch/year).

A plate boundary exists anywhere where two plates meet and move in opposing directions. Boundaries between the plates are of three types: divergent (i.e., plates move apart or away from one another), convergent (i.e., plates move toward one another), and transform (plates move side by side, parallel to the boundary).

Query \(\PageIndex{1}\)

Divergent Boundaries

Divergent boundaries are locations where two plates move away from one another. These are areas of tension and normal faulting, as well as crustal thinning.

As the plates move away from one another, new oceanic plate material is formed, derived from partial melting of the mantle below the boundary. The cause of melting here is decompression as hot mantle rock from depth is moved toward the surface (Figure \(\PageIndex{2}\)). Because new oceanic lithosphere is created at these boundaries, and plates are moved away from them, they are also referred to as spreading centers.

Most divergent boundaries are located at oceanic ridges (although some continue into continents), and the crustal material created at a spreading boundary is always oceanic in character; in other words, it is mafic igneous rock (e.g., basalt or gabbro, rich in ferromagnesian minerals). The rate at which new plate material is created (spreading rates) varies considerably, from 2 cm/year to 6 cm/year (~1 inch/year to 2 inches/year) in the Atlantic, to between 12 cm/year and 20 cm/year (~5 inches/year to 8 inches/year) in the Pacific. (Note that spreading rates are typically double the velocities of the two plates moving away from a ridge.)

Figure \(\PageIndex{2}\): Mantle upwelling taking place along an oceanic divergent plate margin. Here, the spreading of ocean crust creates space for mantle rock to move upward. This mantle rock experiences partial melting and is eventually erupted at the surface on the seafloor. "Mantle Upwelling" by Karla Panchuk, a derivative of the original, is licensed under CC BY-NC-SA.

Some of the processes that take place in a divergent boundary setting include:

• Magma from the mantle pushes up to fill the voids left by divergence of the two plates
• Pillow lavas form where magma is pushed out into seawater
• Vertical sheeted dikes intrude into cracks that developed from the spreading
• Magma cools more slowly in the lower part of the new crust and forms gabbroic bodies

Figure \(\PageIndex{3}\): At spreading centers, the crust is actively pulling apart. This creates tension and results in normal faulting near the plate boundary. A predictable sequence of rocks occurs at these locations: Gabbro, overlain by sheeted dikes, overlain by pillow basalts. " Spreading Center" by Steven Earle is licensed under CC BY 4.0.

Creating a Divergent Boundary

The formation of a divergent boundary is hypothesized to start within a continental area with up-warping or doming related to an underlying mantle plume or series of mantle plumes. The buoyancy of the mantle plume material creates a dome within the crust, causing it to fracture in a radial pattern, with three arms spaced at approximately 120° (Figure \(\PageIndex{4}\)).

Figure \(\PageIndex{4}\): Dome and three-part rift formation (left) and continental rifting (right) between the African and South American parts of Pangea at around 200 Ma. "Rift Formation" by Steven Earle is licensed under CC BY 4.0.

When a series of mantle plumes exists beneath a large continent, the resulting rifts may align and lead to the formation of a rift valley (such as the present-day Great Rift Valley in eastern Africa). It is suggested that this type of valley eventually develops into a linear sea (such as the present-day Red Sea, or the Gulf of California between Baja California and mainland Mexico), and finally into an ocean (such as the Atlantic). It is likely that as many as 20 mantle plumes, many of which still exist, were responsible for the initiation of the rifting of Pangea along what is now the mid-Atlantic ridge.

Convergent Boundaries

There are three possible combinations of tectonic plates that can move toward each other at a convergent boundary, depending on the type of lithosphere at the boundary. The combinations are ocean-ocean, ocean-continent, and continent-continent. All of these boundaries are associated with compression, crustal thickening, and thrust or reverse faulting.

Ocean-Ocean Convergence

At an ocean-ocean convergent boundary, one of the plates (oceanic crust and lithospheric mantle) is pushed, or subducted, under another plate of oceanic lithosphere because older rock is colder and therefore more dense than younger, warmer rock, it is the older and colder plate that is denser and subducts beneath the younger and hotter plate. An oceanic trench forms along the subduction boundary due to the bending of the downgoing plate.

The subducted plate and its overlying oceanic sediments contain a significant volume of water and other fluids within their minerals. These fluids are released as the subducted plate is heated. These fluids rise and mix with the overlying mantle rock, lowering the mantle rock's melting point and leading to the formation of magma. This process is termed flux melting (Figure \(\PageIndex{5}\)).

The magma is more buoyant than the surrounding mantle material. It rises through the mantle and the overlying oceanic crust to the ocean floor where it creates a chain of volcanic islands parallel to the subduction boundary known as a volcanic island arc. A mature island arc develops into a chain of relatively large islands (such as modern Japan or Indonesia) as more and more volcanic material is extruded and sedimentary rock accumulates around the islands.

Figure \(\PageIndex{5}\): Configuration and processes of an ocean-ocean convergent boundary. As a plate of oceanic lithosphere subducts beneath another, a trench develops. Flux melting is generated along the subducting slab, causing buoyant magma to rise through the overlying plate. "Ocean-Ocean Convergence" by Steven Earle is licensed under CC BY 4.0.

Examples of ocean-ocean convergent zones are subduction of the Pacific Plate beneath the North America Plate south of Alaska (Aleutian Island Arc) and beneath the Philippine Plate west of the Philippines, subduction of the India Plate beneath the Eurasian Plate south of Indonesia, and subduction of the Atlantic Plate beneath the Caribbean Plate.

Ocean-Continent Convergence

At an ocean-continent convergent boundary, the oceanic plate is subducted under the continental plate in the same manner as at an ocean-ocean boundary. Some of the sediment that has accumulated on the downgoing oceanic plate, along with sediment that has accumulated on the continental slope, is thrust up into an accretionary wedge. Compressional forces associated with convergence leads to thrust faulting within the overriding continental plate (Figure \(\PageIndex{6}\)).

As with the ocean-ocean boundary, flux melting of mantle rocks creates a mafic magma. This magma rises to the base of the continental crust and leads to partial melting of the overlying crustal rock. The resulting magma ascends through the crust, producing a mountain chain with many volcanoes called a volcanic arc. Examples of ocean-continent convergent boundaries are found where the oceanic Nazca Plate subducts under western South America (which has created the volcanic arc of the Andes Range) and subduction of the Juan de Fuca Plate under western North America (creating the Cascade Range, which includes Mt. Shasta and Mt. Lassen in California).

Figure \(\PageIndex{6}\): Cross section of an ocean-continent convergent boundary. As oceanic lithosphere subducts beneath continental lithosphere, an accretionary wedge of seafloor sediment develops near the trench. Flux melting is generated above the subducting slab, causing buoyant magma to rise through the overlying plate. "Ocean-Continent Convergence Illustrating Magma Formation" by Karla Panchuk, a derivative of "Ocean-Continent Convergence", is licensed under CC BY 4.0.

Continent-Continent Convergence

Continent-continent convergence (or collision) occurs when a continent or large island collides with another continent (Figure \(\PageIndex{7}\)). In this case, the colliding continental material will not be subducted because it is too buoyant (i.e., because it is composed largely of less dense, more felsic, and more buoyant continental rocks) and much thicker than oceanic crust.

At this type of plate boundary, tremendous deformation of the pre-existing continental rocks dramatically thickens the crust and builds high-standing mountain ranges consisting of any sediments that had accumulated along the shores of both continental masses, and commonly also some ocean crust and upper mantle material. These large mountain ranges consist of fold-and-thrust belts, as well as a suture zone, which marks the boundary between the original plates. The oceanic lithosphere (slab) subducted before this collision is believed to founder beneath these zones as it is recycled into the mantle.

The formation of this type of plate boundary is how oceans are destroyed and how supercontinents, such as Pangea, are formed. Examples of continent-continent convergent boundaries are found where the India Plate meets the Eurasian Plate and creates the Himalayan Mountains, and where the African Plate meets the Eurasian Plate, creating the series of ranges extending from the Alps in Europe to the Zagros Mountains in Iran.

Figure \(\PageIndex{7}\): Configuration and processes of a continent-continent convergent boundary. "Continent-Continent Convergence" by Steven Earle is licensed under CC BY 4.0

Query \(\PageIndex{1}\)

Transform Boundaries

Transform boundaries are named for the fact that they “transform” the motion at one plate boundary into the motion at another, forming the connections between areas of convergence and areas of divergence, or areas that are diverging or converging at different rates. Plate motion at transform boundaries occurs along very large strike-slip fault systems that are parallel to the boundary: one plate slides past another without the production or destruction of crustal material. While small transform faults occur at divergent plate boundaries, where they connect spreading segments of mid-ocean ridges, actual transform plate boundaries, such as the San Andreas fault system, are much larger. The San Andreas fault system (Figure \(\PageIndex{8}\)), for example, connects the convergence on the Cascadia trench offshore of the Pacific Northwest with the divergence at the East Pacific Rise, where it enters the Gulf of California.

Figure \(\PageIndex{8}\): The San Andreas Fault extends from the north end of the East Pacific Rise in the Gulf of California to the southern end of the Juan de Fuca Ridge. All of the red lines on this map are strike-slip transform faults. Black lines are divergent margins, and the blue line is a convergent margin. "Example of a Transform Plate Boundary - The San Andreas Fault of California" by Steven Earle (2015) is licensed under CC BY 4.0.

Query \(\PageIndex{1}\)

Box \(\PageIndex{1}\): Plate Tectonics and Supercontinent Cycles

As originally described by Alfred Wegener in 1915, the present continents were once all part of a supercontinent, which he termed Pangea (meaning “all land”). More recent studies of continental matchups and the magnetic ages of ocean-floor rocks have enabled us to reconstruct the history of the break-up of Pangea.

Pangea began to rift apart along a line between Africa and Asia and between North America and South America at around 200 Ma. During the same period, the Atlantic Ocean began to open up between northern Africa and North America, and India broke away from Antarctica. Between 200 and 150 Ma, rifting started between South America and Africa and between North America and Europe, and India moved north toward Asia. By 80 Ma, Africa had separated from South America, most of Europe had separated from North America, and India had separated from Antarctica. By 50 Ma, Australia had separated from Antarctica, and shortly after that, India collided with Asia.

The following silent video demonstrates how continents have moved from the time of Pangea to present day:

Within the past few million years, rifting has taken place in the Gulf of Aden and the Red Sea, and also within the Gulf of California. Incipient rifting has begun along the Great Rift Valley of eastern Africa, extending from Ethiopia and Djibouti on the Gulf of Aden (Red Sea) all the way south to Malawi.

Over the next 50 million years, it is likely that there will be full development of the east African rift and creation of new ocean floor. Eventually Africa will split apart. There will also be continued northerly movement of Australia and Indonesia. The western part of California (including Los Angeles and part of San Francisco) will split away from the rest of North America, and eventually sail right by the west coast of Vancouver Island, en route to Alaska. Because the oceanic crust formed by spreading on the mid-Atlantic ridge is not currently being subducted (except in the Caribbean), the Atlantic Ocean is slowly getting bigger, and the Pacific Ocean is getting smaller. If this continues without changing for another couple hundred million years, we will be back to where we started, with one supercontinent.

Pangea, which existed from about 350 to 200 Ma, was not the first supercontinent. It was preceded by Pannotia (600 to 540 Ma), by Rodinia (1,100 to 750 Ma), and by others before that.

In 1966, Tuzo Wilson proposed that there has been a continuous series of cycles of continental rifting and collision; that is, break-up of supercontinents, drifting, collision, and formation of other supercontinents. At present, North and South America, Europe, and Africa are moving with their respective portions of the Atlantic Ocean. The eastern margins of North and South America and the western margins of Europe and Africa are, at present, passive margins because there is no subduction taking place along them, or any other types of plate boundary nearby.

Box Figure \(\PageIndex{1}\): A diagrammatic passive margin resembling what western North America looked like. Continental crust that has been rifted is broken up by west-dipping listric normal faults. It is overlain by thick ocean sediments. Note that this diagram is very vertically exaggerated to emphasize these different features. "Passive Margin" by Allison Jones, is a derivative of the original, is in the public domain.

This situation may not continue for too much longer, however. As the Atlantic Ocean floor gets weighed down around its margins by great thickness of continental sediments, it will be pushed farther and farther into the mantle, and eventually the oceanic lithosphere may break away from the continental lithosphere (Box Figure \(\PageIndex{1}\)). A subduction zone will develop, and the oceanic plate will begin to descend under the continent. Once this happens, the continents will no longer continue to move apart because the spreading at the mid-Atlantic ridge will be taken up by subduction. If spreading along the mid-Atlantic ridge continues to be slower than spreading within the Pacific Ocean, the Atlantic Ocean will start to close up, and eventually (in a 100 million years or more) North and South America will collide with Europe and Africa. These cycles of collision and rifting proposed by Wilson are called Wilson cycles. The following video illustrates a single Wilson cycle from continental rifting to continental collision:

References

1. Sinton, J. M., and Detrick, R. S. (1992). Mid-Ocean Ridge Magma Chambers. Journal of Geophysical Research 97(B1), 197-216.
2. J. Tuzo Wilson (1966), “Did the Atlantic Close and then Re-Open?” Nature 211, 676–681. https://doi.org/10.1038/211676a0