PLATES TECTONICS

A world to explain and some strange connections

Plate tectonics is a scientific theory developed during the 1960s and 1970s, that describes the large-scale motion of Earth's lithosphere. This theoretical model builds on the concept of continental drift, a theory first proposed by scientist Alfred Wegener in Plate tectonics is the unifying theory of geology, capable to explain seismic activity, volcanism and mountain formation. Prior to the 1960s, most geologists held the view that the ocean basins and continents had fixed geographic positions and were of great antiquity. Less than a decade later researchers came to realize that Earth’s continents are not static; instead, they gradually migrate across the globe. What is plate tectonics?

Three Parts of Earth’s Interior A knowledge of earth's interior is essential for understanding plate tectonics. The Earth is composed of three layers: the crust, mantle and the core.

Earth’s crust There are two different types of crust: thin oceanic crust, that underlies the ocean basins, and thicker continental crust, that underlies the continents. These two different types of crust are made up of different types of rock. The thin oceanic crust is composed primarily of basalt and the thicker continental crust is composed primarily of granite. The low density of the thick continental crust allows it to "float" in high relief on the much higher density mantle below

Earth’s mantle Earth's mantle is thought to be composed mainly of ultrabasic rock. It has different temperatures at different depths. The temperature is lowest immediately beneath the crust and increases with depth. The highest temperatures occur where the mantle material is in contact with the heat- producing core. This steady increase of temperature with depth is known as the geothermal gradient. The geothermal gradient is responsible for different rock behaviors and the different rock behaviors are used to divide the mantle into two different zones. Rocks in the upper mantle are cool and brittle, while rocks in the lower mantle are hot and soft (but not molten). Rocks in the upper mantle are brittle enough to break under stress and produce earthquakes. However, rocks in the lower mantle are soft and flow when subjected to forces instead of breaking.

Earth’s core Earth's Core is thought to be composed mainly of an iron and nickel alloy. The core is earth's source of internal heat because it contains radioactive materials which release heat as they break down into more stable substances. The core is divided into two different zones. The outer core is a liquid because the temperatures there are adequate to melt the iron-nickel alloy. However, the inner core is a solid even though its temperature is higher than the outer core. Here, tremendous pressure, produced by the weight of the overlying rocks is strong enough to crowd the atoms tightly together and prevents the liquid state.

Lithosphere The crust and the rigid, outer zone of the mantle make up a layer that is called the lithosphere. The zone directly under the lithosphere is made of a flowing, denser layer called the asthenosphere.

Lithospheric plates The lithosphere is composed of about two dozen segments having irregular sizes and shapes called lithospheric plates or tectonic plates that are in constant motion with respect to one another. Seven major lithospheric plates are recognized. These plates, which account for 94 percent of Earth’s surface area, include the North American, South American, Pacific, African, Eurasian, Australian-Indian, and Antarctic plates. Plates are bounded by three distinct types of boundaries, which are differentiated by the type of movement they exhibit: divergent boundary, convergent boundary, transform boundary.

Convergent plate boundaries are locations where lithospheric plates are moving towards one another. The plate collisions that occur in these areas can produce earthquakes, volcanic activity and crustal deformation. Convergent boundaries 1 When continental and oceanic plates collide the thinner and more dense oceanic plate is overridden by the thicker and less dense continental plate. The oceanic plate is forced down into the mantle in a process known as "subduction". As the oceanic plate descends it is forced into higher temperature environments. At a depth of about 100 miles (160 km) materials in the subducting plate begin to approach their melting temperatures and a process of partial melting begins. If a magma chamber rises to the surface without solidifying the magma will break through in the form of a volcanic eruption. The Andes Mountain Range of western South America is an example of a convergent boundary between an oceanic and continental plate. Here the Nazca Plate is subducting beneath the South American plate.

When two thick continental plates collide neither of them will subduce because both of them have a density that is much lower than the mantle. Effects found at a convergent boundary between continental plates include: intense folding and faulting, the formation of a broad folded mountain range (orogenesis), shallow earthquake activity, shortening and thickening of the plates within the collision zone. The Himalaya Mountain Range and Alps are the best active example of this type of plate boundary. Convergent boundaries 2 Folding and mountain building faulting

Divergent plate boundaries are locations where plates are moving away from one another. This occurs above rising convection currents. The rising current pushes up on the bottom of the lithosphere, lifting it and flowing laterally beneath it. This lateral flow causes the plate material above to be dragged along in the direction of flow. At the crest of the uplift, the overlying plate is stretched thin, breaks and pulls apart (rifting). Magma from the mantle flows into the rift forming new oceanic crust. The Mid-Atlantic Ridge and the East African Rift Valley are a classic examples of this type of plate boundary. Divergent boundaries convection currents rift new oceanic crust

Transform boundaries Transform Plate Boundaries are locations where two plates slide past one another. The fracture zone that forms a transform plate boundary is known as a transform fault. Most transform faults are found in the ocean basin and connect offsets in the mid-ocean ridges. The most famous example of this is the San Andreas Fault Zone of western North America. The earthquakes are usually shallow because they occur within and between plates that are not involved in subduction. Volcanic activity is normally not present because the typical magma sources of an upwelling convection current or a melting subducting plate are not present.

Evidence for Plate Tectonics Evidence based upon fossil record Evidence based upon earthquake patterns Evidence based upon sea-floor spreading Evidence based upon “hot spot” tracks

Evidence based upon fossil record indicates that once continents were united

Evidence based upon earthquake patterns shows location of plate boundaries

Evidence based upon sea-floor spreading support the separations of continents and the creation of new oceanic crust

Evidence of plate movement from Hot-Spot tracks

What Drives Plate Motions? Scientists still continue to study and debate the mechanisms that move the plates. They believe, however, that a major driving force behind plate tectonics is thermal convection in the mantle: 1. Hot mantle from the two adjacent cells rises at the ridge axis, creating new ocean crust. 2. The top limb of the convection cell moves horizontally away from the ridge crest, as does the new seafloor (sea-floor spreading). 3. The outer limbs of the convection cells plunge down into the deeper mantle, dragging oceanic crust as well. This takes place at the deep sea trenches. 4. The material sinks to the core and moves horizontally. 5. The material heats up and reaches the zone where it rises again

A closer look at the Mediterranean

Lesson Summary Most of the Earth’s geologic activity takes place at plate boundaries. Plates interact at three types of plate boundaries: divergent, convergent and transform. At a convergent boundary with at least one oceanic plate, an ocean trench, a chain of volcanoes develops and many earthquakes occur. At a convergent boundary where both plates are continental, mountain ranges grow and earthquakes are common. At a divergent boundary, rifting occurrs and volcanic activity produces a mid ocean ridge and small earthquakes. At a transform boundary, there is a transform fault and massive earthquakes occur but there are no volcanoes. Plate tectonics is based on numerous evidence, including sea-floor spreading, fossil distribution and pattern of earthquake epicenters. Plates of lithosphere move because of convection currents in the mantle.