The rift valley is a classic example of a divergent plate boundary. There are a few handfuls of major plates and dozens of smaller, or minor, plates. Six of the majors are named for the continents embedded within them, such as the North American, African, and Antarctic plates. Though smaller in size, the minors are no less important when it comes to shaping the Earth. The tiny Juan de Fuca plate is largely responsible for the volcanoes that dot the Pacific Northwest of the United States.
The plates make up Earth's outer shell, called the lithosphere. This includes the crust and uppermost part of the mantle. Churning currents in the molten rocks below propel them along like a jumble of conveyor belts in disrepair. Most geologic activity stems from the interplay where the plates meet or divide. The movement of the plates creates three types of tectonic boundaries: convergent, where plates move into one another; divergent, where plates move apart; and transform, where plates move sideways in relation to each other.
Where plates serving landmasses collide, the crust crumples and buckles into mountain ranges. India and Asia crashed about 55 million years ago, slowly giving rise to the Himalaya , the highest mountain system on Earth.
As the mash-up continues, the mountains get higher. Mount Everest, the highest point on Earth, may be a tiny bit taller tomorrow than it is today. These convergent boundaries also occur where a plate of ocean dives, in a process called subduction, under a landmass.
As the overlying plate lifts up, it also forms mountain ranges. In addition, the diving plate melts and is often spewed out in volcanic eruptions such as those that formed some of the mountains in the Andes of South America. At ocean-ocean convergences, one plate usually dives beneath the other, forming deep trenches like the Mariana Trench in the North Pacific Ocean, the deepest point on Earth.
These types of collisions can also lead to underwater volcanoes that eventually build up into island arcs like Japan. At divergent boundaries in the oceans, magma from deep in the Earth's mantle rises toward the surface and pushes apart two or more plates. Mountains and volcanoes rise along the seam. The process renews the ocean floor and widens the giant basins. Magma is the molten rock below the crust, in the mantle. Tremendous heat and pressure within the earth cause the hot magma to flow in convection currents.
Based on evidence that has been found at plate boundaries, make some hypotheses about the movement of those plates. The earth has changed in many ways since it first formed 4. They have gradually moved over the course of hundreds of millions of years—alternately combining into supercontinents and pulling apart in a process known as continental drift. The supercontinent of Pangaea formed as the landmasses gradually combined roughly between and mya.
It is widely accepted by scientists today. Earthquakes and volcanoes are the short-term results of this tectonic movement. The long-term result of plate tectonics is the movement of entire continents over millions of years Fig.
The presence of the same type of fossils on continents that are now widely separated is evidence that continents have moved over geological history. Evaluate and interpret several lines of evidence for continental drift over geological time scales. The shapes of the continents provide clues about the past movement of the continents. The edges of the continents on the map seem to fit together like a jigsaw puzzle. For example, on the west coast of Africa, there is an indentation into which the bulge along the east coast of South America fits.
The shapes of the continental shelves—the submerged landmass around continents—shows that the fit between continents is even more striking Fig. Some fossils provide evidence that continents were once located nearer to one another than they are today. Fossils of a marine reptile called Mesosaurus Fig. Another example is the fossil plant called Glossopteris, which is found in India, Australia, and Antarctica Fig.
The presence of identical fossils in continents that are now widely separated is one of the main pieces of evidence that led to the initial idea that the continents had moved over geological history. Evidence for continental drift is also found in the types of rocks on continents. There are belts of rock in Africa and South America that match when the ends of the continents are joined. Mountains of comparable age and structure are found in the northeastern part of North America Appalachian Mountains and across the British Isles into Norway Caledonian Mountains.
These landmasses can be reassembled so that the mountains form a continuous chain. Evidence from glacial striations in rocks, the deep grooves in the land left by the movement of glaciers, shows that mya there were large sheets of ice covering parts of South America, Africa, India, and Australia. These striations indicate that the direction of glacial movement in Africa was toward the Atlantic ocean basin and in South America was from the Atlantic ocean basin. This evidence suggests that South America and Africa were once connected, and that glaciers moved across Africa and South America.
There is no glacial evidence for continental movement in North America, because there was no ice covering the continent million years ago.
North America may have been nearer the equator where warm temperatures prevented ice sheet formation. Mid-ocean ridges or spreading centers are fault lines where two tectonic plates are moving away from each other. Mid-ocean ridges are the largest continuous geological features on Earth. They are tens of thousands of kilometers long, running through and connecting most of the ocean basins.
Oceanographic data reveal that seafloor spreading is slowly widening the Atlantic ocean basin, the Red Sea, and the Gulf of California Fig.
The gradual process of seafloor spreading slowly pushes tectonic plates apart while generating new rock from cooled magma. Ocean floor rocks close to a mid-ocean ridge are not only younger than distant rocks, they also display consistent bands of magnetism based on their age Fig. Geomagnetic reversal allows scientists to study the movement of ocean floors over time.
Paleomagnetism is the study of magnetism in ancient rocks. In other words, the particles will point in the direction of the magnetic field present as the rock was cooling.
Seafloor spreading gradually pushes tectonic plates apart at mid-ocean ridges. When this happens, the opposite edge of these plates push against other tectonic plates. Where plates are being pushed together, the crust can either rise up to form mountains or one plate is shoved under the other and is sucked back into the mantle. That heat is ebbing away as Earth ages, and this was expected to slow plate motion.
A study last year by Martin Van Kranendonk at the University of New South Wales in Sydney, Australia, and colleagues measured elements concentrated by tectonic action in rocks from around the world, and concluded that plate motion has been slowing for 1. Now Kent Condie , a geochemist at the New Mexico Institute of Mining and Technology in Socorro and his colleagues have used a different approach and concluded that tectonic activity is increasing.
They looked at how often new mountain belts form when tectonic plates collide with one another. They then combined these measurements with magnetic data from volcanic rocks to work out at which latitude the rocks formed and how quickly the continents had moved.
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