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Sea-floor spreading is the process in which the ocean floor is extended when two plates move apart. As the plates move apart, the rocks break and form a crack between the plates. Earthquakes occur along the plate boundary. Magma rises through the cracks and seeps out onto the ocean floor like a long, thin, undersea volcano. • As magma meets the water, it cools and solidifies, adding to the edges of the sideways-moving plates. As magma piles up along the crack, a long chain of mountains forms gradually on the ocean floor. This chain is called an oceanic ridge. http://library.thinkquest.org/17457/platetectonics/4.php
The boundaries where the plates move apart are 'constructive' because new crust is being formed and added to the ocean floor. The ocean floor gradually extends and thus the size of these plates increases. • As these plates get bigger, others become smaller as they melt back into the Earth in the process called subduction. http://library.thinkquest.org/17457/platetectonics/4.php
Plate Boundaries • Convergent • Divergent • Transform-Fault
Convergent Boundaries • Boundaries between two plates that are colliding • There are 3 types…
Type 1 • Ocean plate colliding with a less dense continental plate • Subduction Zone: where the more dense plate slides under the less dense plate • VOLCANOES occur at subduction zones
Oceanic-Continental Convergence When an oceanic plate pushes into and subducts under a continental plate, the overriding continental plate is lifted up and a mountain range is created. Even though the oceanic plate as a whole sinks smoothly and continuously into the subduction trench, the deepest part of the subducting plate breaks into smaller pieces. These smaller pieces become locked in place for long periods of time before moving suddenly and generating large earthquakes. Such earthquakes are often accompanied by uplift of the land by as much as a few meters.Oceanic-Oceanic Convergence
Type 2 • Ocean plate colliding with another ocean plate • The more dense plate slides under the less dense plate creating a subduction zone called a TRENCH
Oceanic-Oceanic Convergence When two oceanic plates converge one is usually subducted under the other and in the process a deep oceanic trench is formed. The Marianas Trench, for example, is a deep trench created as the result of the Phillipine Plate subducting under the Pacific Plate. Oceanic-oceanic plate convergence also results in the formation of undersea volcanoes. Over millions of years, however, the erupted lava and volcanic debris pile up on the ocean floor until a submarine volcano rises above sea level to form an island volcano. Such volcanoes are typically strung out in chains called island arcs.
Type 3 • A continental plate colliding with another continental plate • Have Collision Zones: • a place where folded and thrust faulted mountains form.
Continental-Continental Convergence When two continents meet head-on, neither is subducted because the continental rocks are relatively light and, like two colliding icebergs, resist downward motion. Instead, the crust tends to buckle and be pushed upward or sideways. The collision of India into Asia 50 million years ago caused the Eurasian Plate to crumple up and override the Indian Plate. After the collision, the slow continuous convergence of the two plates over millions of years pushed up the Himalayas and the Tibetan Plateau to their present heights. Most of this growth occurred during the past 10 million years.
Divergent Boundaries • Boundary between two plates that are moving apart or rifting • RIFTING causesSEAFLOOR SPREADING
Features of Divergent Boundaries • Mid-ocean ridges • Rift valleys • Fissure volcanoes
Divergent Boundaries Places where plates are coming apart are called divergent boundaries. When Earth's brittle surface layer (the lithosphere) is pulled apart, it typically breaks along parallel faults that tilt slightly outward from each other. As the plates separate along the boundary, the block between the faults cracks and drops down into the soft, plastic interior (the asthenosphere).
The sinking of the block forms a central valley called a rift. Magma (liquid rock) seeps upward to fill the cracks. In this way, new crust is formed along the boundary. Earthquakes occur along the faults, and volcanoes form where the magma reaches the surface. Plate separation is a slow process. For example, divergence along the Mid Atlantic ridge causes the Atlantic Ocean to widen at only about 2 centimeters per year.
Transform Fault Boundaries • Boundary between two plates that are sliding past each other • EARTHQUAKES along faults
Transform BoundariesPlaces where plates slide past each other are called transform boundaries. Since the plates on either side of a transform boundary are merely sliding past each other and not tearing or crunching each other, transform boundaries lack the spectacular features found at convergent and divergent boundaries. Instead, transform boundaries are marked in some places by linear valleys along the boundary where rock has been ground up by the sliding. In other places, transform boundaries are marked by features like stream beds that have been split in half and the two halves have moved in opposite directions.
Perhaps the most famous transform boundary in the world is the San Andreas fault, shown in the drawing above. The slice of California to the west of the fault is slowly moving north relative to the rest of California. Since motion along the fault is sideways and not vertical, Los Angeles will not crack off and fall into the ocean as popularly thought, but it will simply creep towards San Francisco at about 6 centimeters per year. In about ten million years, the two cities will be side by side!
Although transform boundaries are not marked by spectacular surface features, their sliding motion causes lots of earthquakes. The strongest and most famous earthquake along the San Andreas fault hit San Francisco in 1906. Many buildings were shaken to pieces by the quake, and much of the rest of the city was destroyed by the fires that followed. More than 600 people died as a result of the quake and fires. Recent large quakes along the San Andreas include the Imperial Valley quake in 1940 and the Loma Prieta quake in 1989.
Animations • How Ocean Crust Forms • Plate Boundaries • Seafloor Spreading • Seafloor Spreading
Animations • Seafloor Spreading • Seafloor Spreading Video
