![]() Subduction continued into the Tertiary (the Age of Mammals), when most of the Farallon plate got pushed beneath North America, and the East Pacific rise, which separated the Farallon and Pacific plates, collided with North America. Tilting of the Sierras also exposed the granitic roots beneath the volcanaoes to create the Sierran landscape we know today. Then both the crest of the Coast Ranges, and the volcanic cover of the Sierran Nevadas were then eroded off to fill in the San Joaquin Valley with sediment, and the valley, which had been an inland sea up to this point, became a landlocked basin. As the Farallon plate pushed against North America, compressive forces created the Coast Ranges of California and pushed them up and over the San Joaquin Valley.Ĭompressive forces also tilted the Sierra Nevada volcanic arc on the far side of the valley so that the arc sloped towards the west. When subduction takes place, frictional heating caused by one plate diving beneath another can generate enough heat for volcanoes to form and result in a curved chain of volcanoes called and arc.Ī volcanic arc did indeed develop to the east of the Farallon-North American subduction zone, and created an ancient mountain range that preceded the modern Sierra Nevadas. A similar trench once existed off ths coast of California. Subduction zones are represented by deep ocean trenches, like the modern trenches off the coast of Alaska and South America. This process, which is called subduction, is shown below. The map below shows the present day plate boundaries.ĭuring the Mesozoic Age, more than 65 million years ago when dinosaurs walked the earth, the Farallon plate (which preceded the Pacific Plate) and the North American plate were moving towards each other, with dense oceanic crust (basalt) of the Farallon plate diving beneath the more bouyant continental crust (granite) of North America. The oceanic plate to the west is the Pacific Plate, although in the geologic past there was another ocean plate to the west called the Farallon plate, and the plate to the east is the continent of North America. The west coast of is in fact a plate boundary that separates oceanic crust to the west from continental crust to the east. Beneath the ocean basins there is a layer of dense basalt, in some places less than 3-miles thick, which makes up these plates, whereas the land masses (i.e., continental crust) are made up of a layer of less dense granitic material, up to 20-miles thick, that, in essence, "floats" on a thin underlying layer of oceanic crust. ![]() The San Andreas Fault is one of the best examples of lateral plate motion.The earths crust is divided into several tectonic plates (shown below) that have over time moved across the surface of the earth. When two tectonic plates slide past each other, the place where they meet is a transform or lateral fault. This can lead to the formation of huge, high mountain ranges such as the Himalayas. Since neither plate is stronger than the other, they crumple and are pushed up. About 80% of earthquakes occur where plates are pushed together, called convergent boundaries.Īnother form of convergent boundary is a collision where two continental plates meet head-on. Sometimes the molten rock rises to the surface, through the continent, forming a line of volcanoes. The rocks pulled down under the continent begin to melt. ![]() Subduction causes deep ocean trenches to form, such as the one along the west coast of South America. When a continental plate meets an oceanic plate, the thinner, denser, and more flexible oceanic plate sinks beneath the thicker, more rigid continental plate. The Great Rift Valley in Africa, the Red Sea and the Gulf of Aden all formed as a result of divergent plate motion.Ĭonvergent (Colliding): This occurs when plates move towards each other and collide. The earthquakes that occur along these zones, called spreading centers, are relatively small. Molten rock from the mantle erupts along the opening, forming new crust. The three main types of plate movements include:ĭivergent (Spreading):This is where two plates move away from each other. The movements of the plates help shape the geological features of our planet. Other plates include continents, and some plates include both continents and ocean. Some of the plates have ocean water above them. When the plates finally give and slip due to the increased pressure, energy is released as seismic waves, causing the ground to shake. Most seismic activity occurs at three types of plate boundaries-divergent, convergent, and transform.Īs the plates move past each other, they sometimes get caught and pressure builds up. Movement in narrow zones along plate boundaries causes most earthquakes.
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