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B3: the Theory of Plate Tectonics

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B3: the Theory of Plate Tectonics UNIT B: PLATE TECTONICS 1 B3: The Theory of Plate Tectonics What Happens at Plate Boundaries? Guiding Question: Why has the theory of plate tectonics been so successful as an Plates are constantly in motion. They all explanation for the movement of continents? move at unique speeds. The African plate moves at about 2.15 cm/year (one inch/year), Key Concepts while the South American plate moves about 1.45 cm/year (half an inch/year). Plates con- The lithosphere is broken into plates stantly interact at their boundaries. Scientists that are constantly in motion. have identified three distinct kinds of plate boundary, each of which interacts differently. Interacting plate boundaries are dis- The three types of plate boundaries are di- tinguished by three different types of vergent, convergent, and transform fault. plate interactions. Divergent Boundaries: This is Convection is the fundamental under- where two plates are moving apart lying mechanism driving plate motion. (diverging) from one another. New magma from the mantle flows into the space opened by spreading plates. A narrow rift valley usually forms in this Science Background space. New crust is created here; so divergent boundaries are called con- The Theory of Plate Tectonics structive margins. Divergent bound- The theory of plate tectonics states Earth’s aries are usually underwater where surface is broken into several large chunks of two plates of oceanic crust pull apart, rigid material, or plates, that are constantly in creating a mid-ocean ridge. They can motion. They are composed of oceanic also occur on land where two sections and/or continental crust, plus the rigid upper of continental crust move apart. part of the mantle. These two sections to- gether make up the hard and brittle litho- Convergent Boundaries: This is sphere, which is about 100 kilometers (ap- where two plates move toward one proximately 60 miles) deep. The plates ride another, or converge. Where oceanic on top of the asthenosphere, a layer of rock crust collides with continental crust, in the mantle that is hot enough to be cap- the denser oceanic crust will slide un- able of bending slowly. This hot rock is re- der the lighter continental crust. This ferred to as being plastic. These plates, con- is a subduction zone. Deep ocean taining the oceans and the continents, act trenches usually form as part of this like rafts floating on a very thick liquid. Sci- process. Convergent boundaries are entists have identified seven large plates called destructive margins because (North American, South American, Pacific, crust is lost as a result of the plate in- African, Eurasian, Australian-Indian, and Ant- teraction. Two plates of oceanic crust arctic plates) as well as a number of smaller can also collide and one will subduct plates (e.g., Caribbean and Juan de Fuca under the other. In addition to an plates). oceanic trench, a series of volcanic is- lands known as an island arc can res- Plate tectonics is the dominant theory in ult from this collision. Two plates of Earth science because its unifying nature continental crust can also converge, provides an explanation for many physical as happened to produce the Himalay- processes including earthquakes, volcanoes, as. and mountain formation. The modern theory of plate tectonics combines the continental Transform Fault Boundary: This is drift and seafloor spreading theories. where two plates slide past one an- The Layered Earth UNIT B: PLATE TECTONICS 2 other. The famous San Andreas Fault Lesson Summary is a transform fault boundary where the Pacific plate slides past the North The theory of plate tectonics states American plate. Transform boundar- ies can be visible when they occur on that the upper surface of the Earth is land. The series of zigzag lines offset broken up into a number of large from, and perpendicular to, the mid- chunks of rigid lithosphere plates that oceanic ridges are also transform are constantly in motion. fault boundaries located between two These lithosphere plates ride on top diverging plates. Transform fault of the asthenosphere, a layer of boundaries are called conservative plastic rock in the mantle that can margins because crust is neither cre- flow. ated nor destroyed at these locations. Divergent boundaries, also known as constructive margins, are areas where new crust is being created as two plates move away from each oth- What Drives the Plates? er. Convergent boundaries, also known Convection: This heat transfer mechanism is as destructive margins, are areas driven by a temperature differential within the where crust is destroyed as one plate molten mantle rock, and is believed to be the is forced under another plate. fundamental process underlying plate motion. Transform fault boundaries, also known as conservative margins, are Forces Driving Plate Tectonics areas where crust is neither created nor destroyed as two plates slide past How does convection drive the motion of parallel to one another. plates? Scientists believe there are several Convection is believed to be the un- forces related to convection that have a role derlying mechanism driving the mo- in plate movement. Three of these forces are: tion of plates. Slab pull, caused by sinking plates in Slab pull: the subduction of cold, a subduction zone, is believed to be dense, oceanic lithosphere slabs at the dominant force acting on plates. trenches serves to pull the rest of the plate along. Slab pull is considered the dominant force acting on tectonic plates. Slab suction: friction between plates in a subduction zone causes a down- ward convective flow under both plates, causing both to move down to- ward the ocean trench. Ridge push: the cooling and sinking of new oceanic crust after it is formed at mid-oceanic ridges. The new crust gets denser with age and sinks as it slowly moves away from its point of formation at the center of the mid- oceanic ridge. The Layered Earth UNIT B: PLATE TECTONICS 3 Review Questions 1. What two theories were combined to make the modern theory of plate tectonics? 2. Why is plate tectonics now considered to be the dominant theory in Earth science 3. Please match each term from column A with the most appropriate item from column B. COLUMN A COLUMN B Destructive margin 1) New crust is created Subduction zone 2) Dominant force acting on plates Plate 3) Crust is lost as a result of the interaction Transform fault boundary 4) Movement of particles in a fluid Slab pull 5) Chunk of rigid lithosphere Divergent boundary 6) Crust is neither created nor destroyed Convection 7) One plate is forced under another one The Layered Earth.
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