Dating The Global Tectonic Rock Cycle

Plate tectonics The theory that the surface of the earth is made of lithospheric plates, which have moved throughout geological time resulting in the present-day positions of the continents. The theory explains the locations of mountain building as well as earthquakes and volcanoes. The rigid lithospheric plates consist of continental and oceanic crust together with the upper mantle, which lie above the weaker plastic asthenosphere.

Today, Europe and North America move about 3 inches (7.5 centimeters) farther apart every year as the Atlantic Ocean grows wider. Plate tectonics is the theory explaining geologic changes that result from the movement of lithospheric plates over the asthenosphere (the molten, ductile, upper portion of the earth’s mantle). The visible continents, a part of the lithospheric plates upon which they ride, shift slowly over time as a result of the forces driving plate tectonics. Even before plate tectonics theory, scientists wondered what forces could create a 2,400-kilometer-long string of volcanoes, all neatly lined up like ducklings in a row .

Another line of evidence in support of plate tectonics came from the long-known existence of ophiolte suites found in upper levels of mountain chains. Beginning in the 1950s, scientists like Victor Vacquier, using magnetic instruments adapted from airborne devices developed during World War II to detect submarines, began recognizing odd magnetic variations across the ocean floor. This finding, though unexpected, was not entirely surprising because it was known that basalt—the iron-rich, volcanic rock making up the ocean floor—contains a strongly magnetic mineral and can locally distort compass readings.

Great uplift, accompanied by rapid erosion, is taking place and large sediment fans are being deposited in the Indian Ocean to the south. With time, water-soluble “cement” will cause the sandy units to become sandstone. Rocks of this kind in the ancient record may very well have resulted from rapid uplift and continent collision. He says another way to look at it is to think of the drifting ocean plate as an airport baggage conveyor, and the microcontinents are like pieces of luggage travelling on the conveyor.

The uplift has left beaches raised by hundreds of metres around the shores of the Great Lakes and Hudson Bay. A brief review of the three types of plate boundaries and the structures that are found there is the subject of this wordless video. The Arabian, Indian, and African plates are rifting apart, forming the Great Rift Valley in Africa. The top limb of the convection cell moves horizontally away from the ridge crest, as does the new seafloor. Hot mantle from the two adjacent cells rises at the ridge axis, creating new ocean crust. The arrows show whether the plates are moving apart, moving together, or sliding past each other.

At transform boundaries, exemplified by the San Andreas fault , the continents create a shearing force as they move laterally past one another. In plate tectonics, the movement of one plate down into the mantle where the rock melts and becomes magma source material for new rock. Convection cells in the mantle bring molten udates io rock to the surface along MORs where it forms new ocean crust. Along transform margins, plate movement produces periodic earthquakes as the two plates slide past one another. The best known example of a transform plate margin is the San Andreas fault in California, which separates the Pacific and North American plates.

Continental-continental plates

Continental-oceanic convergence may form a prominent trench, but no continental-oceanic divergent margins exist today. They are unlikely to form and would quickly become oceanic-oceanic divergent margins as sea floor spreading occurred. The answer is plate tectonics, the name both of a theory and of a specialization of tectonics. Plate tectonics theory brings together aspects of continental drift, seafloor spreading , seismic and volcanic activity, and the structures of Earth’s crust to provide a unifying model of Earth’s evolution. Until the early 1990s, scientists believed that mantle convection, seafloor spreading, and magma intrusion at mid-ocean ridges (called “ridge push”) were the predominant mechanisms that drove plate motion.

When oceanic crust collides with the less-dense, more-buoyant continental crust, the oceanic crust subducts under the lighter continental crust and the continental crust may be wrinkled under compression to form mountain chains (e.g., the Andes). The subducting oceanic crust melts as it penetrates deeper into the asthenosphere, and rising molten material and gases from the melted crust may contribute to the formation of volcanic arcs such as those found along the Pacific Rim. The continuous rearrangement of the size and shape of ocean basins and continents over geologic time, accompanied by changes in ocean circulation and climate, had a major impact on the development of life on Earth.

Transform margins

The minerals wash into the ocean, where tiny ocean creatures use the carbon to build their calcium carbonate shells. Ultimately those creatures die, their shells sinking to the ocean floor and becoming carbonate rocks themselves. As more and more carbon dioxide gets sequestered away from the atmosphere in this way, the planet cools — until, eventually, the slow grind of plate tectonics carries the carbonate into the planet’s interior with a subducting plate. Continental plates collide, the edge of one plate is thrust onto that of the other. The rocks in the lower slab undergo changes in their mineral content in response to heat and pressure and will probably become exposed at the surface again some time later. Rocks converted to new mineral assemblages because of changing temperatures and pressures are called metamorphic.

Continental Hearts

Since it has nowhere to go but up, this creates some of the world’s largest mountains ranges . Magma cannot penetrate this thick crust so there are no volcanoes, although the magma stays in the crust. Metamorphic rocks are common because of the stress the continental crust experiences. With enormous slabs of crust smashing together, continent-continent collisions bring on numerous and large earthquakes. A triple junction involving three ridges is always stable, and the magnetic anomalies within the surface area created have Y-shaped patterns around the spreading ridges.

This volcanic mountain chain, known as a volcanic arc, is usually several hundred miles inland from the plate margin. The Andes Mountains of South America and the Cascade Mountains of North America are examples of volcanic arcs formed by subduction along a continental-oceanic convergent margin. Continental-oceanic convergence may form a prominent trench, but not always. As you can imagine, they are unlikely to form and would quickly become oceanic-oceanic divergent margins as sea floor spreading occurred. Because of its greater density, the oceanic plate easily subducts below the edge of the continental plate.

These stripes are a clear record of the growth of the sea floor in sheets spreading from the ridge; as rock cooled in the crust forming along the line of origin, it recorded the direction Earth’s magnetic field happened to be pointing at the time. When Earth’s magnetic field flipped, as it does occasionally, fresh crust on both sides of the ridge would record an opposite magnetic direction for awhile. As the crust grew and Earth’s field flipped many times over tens of millions of years, two sets of stripes were formed, one to the east of the ridge and another to the west. No plausible explanation for the existence of these stripes exists, other than sea-floor spreading and movement of the continents.

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