Theory of Plate Tectonics The Theory of Plate Tectonics Plate tectonics is the theory that the Earth's lithosphere is divided into tectonic plates that move around on top of the asthenosphere. The plates move slowly relative to each other, and their movement is driven by the convection currents in the Earth's mantle. 1
Types of Plate Boundaries Plate boundaries are where tectonic plates touch. Convergent Boundaries: Are formed where the plates converge or move together. Often cause mountains to form. Divergent Boundaries: Are formed where the plates diverge or move apart. Create mid ocean ridges, new land, or can produce volcanoes on land. Transform Fault Boundaries: occur where two plates grind past each other. Causes earthquakes. The type of boundary depends on how the tectonic plates move relative to one another. Convergent Plate Boundaries Divergent Plate Boundaries Transform Fault Boundaries 2
Intro to Plate Tectonics.notebook Types of Convergent Boundaries Continental Continental: two plates with continental crust collide and push upward, creates mountain ranges, sometimes earthquakes. Oceanic Continental: Because it is denser, the oceanic plate is subducted beneath a continental plate, creates mountain range Oceanic Oceanic: one oceanic plate is subducted beneath another oceanic plate, creates a trench, undersea volcanoes > islands The subduction zone is where the denser oceanic crust sinks below the continental plate. Old ocean crust gets pushed down into the asthenosphere, where it is remelted and recycled. http://www.wwnorton.com/college/geo/earth/flash/4_2.swf image from tasaclips.com/illustrations.html 3
Intro to Plate Tectonics.notebook Divergent Plate Boundaries Where two plates separate and move apart. Most common are the mid ocean ridges, where new sea floor forms Example: Rhine Valley in Northwestern Europe. http://www.wwnorton.com/college/geo/earth/flash/4_2.swf image from tasaclips.com/illustrations.html Transform Fault Boundaries Tectonic plates slide past each other horizontally. Because the plates have irregular edges, they grind and jerk, producing earthquakes. http://www.wwnorton.com/college/geo/earth/flash/4_2.swf Ex. San Andreas fault in California image from www.lincolnhigh.net/faculty/jdewolf/ 4
Three Types of Boundaries: Transform Divergent Convergent Label the boxes below to indicate the movement of the three types of boundaries. What type of plate movement or boundaries are shown? Click on the flashing images to begin. 5
Driving Forces of Plate Tectonics A) Convection Hot rock from within the Earth rises, while cooler rock near the surface sinks, which causes the oceanic lithosphere to move sideways, away from the mid ocean ridge B) Ridge Push Since the oceanic crust is higher at the mid ocean ridge than where it sinks into the asthenosphere, oceanic crust slides downhill because of gravity C) Slab Pull Oceanic crust is denser than the asthenosphere, so as the edge of that plate sinks, it pulls the rest of the tectonic plate with it The solid rock of the asthenosphere moves because of changes in the density of the asthenosphere. The density changes because of Earth's thermal energy. Heated rock expands, becomes less dense and rises to the surface of the Earth. Cooled rock at the surface becomes more dense, and sinks. 6
Typically, plates only move a few centimeters per year, but can still cause earthquakes, volcanoes, and the formation of mountains. The December 26, 2004, Indonesian earthquake that caused devastating tsunamis all around the Indian Ocean and South Asia was unusual because the plates moved 10m (33ft) horizontally and 4 5m (13 16 ft) vertically! Indonesian tsunami pics It has taken 169,000,000 years (169 million years) for New York and Africa to reach their current locations: 6760 km 1000 m 100 cm = 676,000,000 cm 1 km 1 m 676,000,000 cm / 4 cm per year = 169,000,000 years GPS Since tectonic plates move so slowly, scientists must use a system of satellites called the global positioning system (GPS) to measure the rate of movement. Radio signals are continuously beamed from satellites to GPS ground stations which record the exact distance between the satellites and the ground station. Over time, these distances change slightly. Scientists can measure the speed of tectonic plates by recording how long it takes the GPS ground station to move a certain distance. 7