Earth Science 9.3 Theory Tectonic Plates

Earth’s Moving Plates During the 1960s, scientists realized that the theory of sea-floor spreading explained the idea of continental drift. It explained how ocean basins could open and close over time. Canadian geologist J. Wilson came up with a theory that led to a revolution in geology. Wilson suggested that the lithosphere was broken into several huge pieces, called plates. Deep faults, like cracks in an eggshell, separate the different plates.

Earth’s Moving Plates In the theory of plate tectonics, Earth’s lithospheric plates move slowly relative to one another, driven by convection currents in the mantle. Previously scientists had failed to explain how the lithosphere could move. The theory of plate tectonics identified a force that could set Earth’s outer shell in motion. According to Wilson, convection currents within Earth drive plate motion. Hot material deep within the mantle moves upward by convection. At the same time, cooler, denser slabs of oceanic lithosphere sink into the mantle

Earth’s Moving Plates Effects of plate motion: Plate motion averages about 5 centimeters per year; about as fast as your fingernails grow. The results of plate motion include earthquakes, volcanoes, and mountain building.

Earth’s Moving Plates Effects of plate motion: Interactions among different plates happen along plate boundaries. Three types of plate boundaries exist Convergent boundaries Divergent boundaries Transform boundaries

Earth’s Moving Plates Divergent Boundaries: Divergent boundaries are found when two of Earth’s plates move apart. Oceanic lithosphere is created where divergent boundaries occur and sea-floor spreading happens.

Earth’s Moving Plates Convergent Boundaries: Convergent boundaries happen when two plates move together towards each other. Lithosphere can be destroyed at convergent boundaries when oceanic lithosphere sinks into the mantle during subduction

Earth’s Moving Plates Transform Boundaries: Transform boundaries occur when two plates grind past each other. Along transform boundaries, lithosphere is neither created nor is it lost.

Earth’s Moving Plates Plates may shrink or grow depending on the locations of the convergent and divergent boundaries. Slowly over time some plates grow over others, pushing them under through subduction when borders converge. Some plates slowly expand while others shrink due to this.

Earth’s Moving Plates Divergent Boundaries: Along divergent boundaries, plates move apart. Because they are the areas where sea-floor spreading begins, divergent boundaries are called spreading centers. Most divergent boundaries occur along the crests of the mid-ocean ridges. Some spreading-centers occur on land on the continents. We think of these plate boundaries as constructive plate margins because this is where new oceanic lithosphere is produced.

Divergent Boundaries When a spreading center forms along land, the process can literally split a continent apart. The process begins when forces of plate motion begin to stretch the lithosphere. At the same time, plumes of hot rock rise from the mantle. The rising plumes bend the crust upward, weakening and fracturing it. The fractures allow magma to reach the surface. The result is a new floor of a rift valley

Divergent Boundaries Examples of rift valleys include the Rhine Valley in Europe and the Great Rift valley in East Africa The Great rift valley in East Africa may represent the first step in the process of the breakup of Africa. This process may take millions of years If the sides of the rift valley continue to move apart, the valley could become a narrow sea like the Red Sea shown at right.

Convergent Boundaries At convergent boundaries, plates collide and interact, producing features including trenches, volcanoes and mountain ranges Along convergent boundaries, older portions of oceanic plates return to the mantle. As a result, Earth’s total surface remains the same, even though new lithosphere is constantly being added at mid-ocean ridges.

Convergent Boundaries Because lithosphere is destroyed at convergent boundaries, they are also called “destructive plate margins”. As two plates slowly converge, the leading edge of one plate is bent downwards, allowing it to slide beneath the other plate. We call this sliding under the other plate subduction. At destructive plate margins, oceanic crust is subducted into the mantle of the Earth.

Convergent Boundaries The type of lithosphere involved and the forces acting upon it determine what happens at convergent boundaries. Convergent boundaries can form between two pieces of oceanic lithosphere, between oceanic lithosphere and continental lithosphere, and between two pieces of continental lithosphere.

Oceanic-Continental When the leading edge of continental lithosphere converges with oceanic lithosphere, the less dense continental lithosphere remains floating. The denser oceanic slab sinks into the asthenosphere. When a descending plate reaches about 100 to 150 kilometers below the Earth, some of the asthenosphere above the descending plate melts. The newly formed magma, being less dense than the rock mantle, rises. Eventually some of it reaches the surface and becomes volcanic activity.

Oceanic-Continental A continental volcanic arc is a range of volcanic mountains produced in part by the subduction of oceanic lithosphere. The volcanoes of the Andes in South America are the product of magma formed during subduction of the Nazca Plate Everywhere around the world, volcanic activity occurs where these subduction zones occur creating a “ring of fire” along these plate boundaries

Oceanic-Oceanic When two oceanic slabs converge, one descends beneath the other. This causes volcanic activity similar to what happens in oceanic-continental. The volcanoes form on the ocean floor instead of on land, however. If this activity continues, it will build a chain of volcanic structures that become islands. This newly formed land we call a volcanic island arc.

Continental-Continental When oceanic lithosphere is subducted beneath continental lithosphere, a continental volcanic arc develops along the margin of the continent. However, if the subduction plate also contains continental lithosphere, the subduction eventually brings two continents together. The result is a collision between the two continental plates. Since neither sinks below the other, collision results and mountains form.

Continental-Continental Before continents collide they are separated by an ocean basin. As the continents move toward each other, the sea-floor between them is subducted beneath one of the plates. When the continents collide, the collision folds and deforms the sediments along the margin as if they were placed in a giant vice. A new mountain range forms that is composed of deformed and metamorphosized sedimentary rocks.

Continental-Continental This kind of collision occurred when India rammed into Asia and produced the Himalayas. Mountain systems such as the Alps, the Appalachians, and the Urals were formed by such a process.

Transform Fault Boundaries The third type of plate boundary system is the transform fault boundary. Pieces of lithosphere move past each other horizontally along a transform fault boundary. At a transform fault boundary, plates grind against each other without destroying or creating lithosphere. Most transform faults join two sections of mid-ocean ridge. These faults occur about every 100 kilometers along the ridge axis.

Transform Fault Boundaries Active transfer faults lie between the two offset ridge segments. The seafloor produced at one ridge axis moves in a direction opposite to that of the seafloor produced at the next ridge segment. Between the ridge segments, these slabs of oceanic crust are sliding past each other along a transfer fault.

Transform Fault Boundaries Although most transform faults are located within the ocean basins, a few cut through continental lithosphere. One example is the San Andreas Fault in California, where the Pacific Plate is moving past the North American Plate. If this movement continues, the part of California west of the fault zone will become an island someday. The more immediate concern is the earthquake activity that results from the tension created by this fault.