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. Subduction occurs when two tectonic plates meet and one moves underneath the other Fig. Oceanic crust is primarily composed of basalt, which makes it slightly denser than continental crust, which is composed primarily of granite.
Because it is denser, when oceanic crust and continental crust meet, the oceanic crust slides below the continental crust. This collision of oceanic crust on one plate with the continental crust of a second plate can result in the formation of volcanoes Fig. As the oceanic crust enters the mantle, pressure breaks the crustal rock, heat from friction melts it, and a pool of magma develops.
This thick magma, called andesite lava, consists of a mixture of basalt from the oceanic crust and granite from the continental crust. Forced by tremendous pressure, it eventually flows along weaker crustal channels toward the surface. The magma periodically breaks through the crust to form great, violently explosive composite volcanoes —steep-sided, cone-shaped mountains like those in the Andes at the margin of the South American Plate Fig.
Continental collision occurs when two plates carrying continents collide. Because continental crusts are composed of the same low-density material, one does not sink under the other. During collision, the crust moves upward, and the crustal material folds, buckles, and breaks Fig. The Himalayan Mountains were formed by the collision between Indian and Eurasian tectonic plates.
Ocean trenches are steep depressions in the seafloor formed at subduction zones where one plate moves downward beneath another Fig. These trenches are deep up to The deepest ocean trench is the Mariana Trench just east of Guam. There are transform faults on either side of the India Plate in this area. The entire northern India and southern Asia region is very seismically active.
Earthquakes are common in northern India, Nepal, Bhutan, Bangladesh and adjacent parts of China, and throughout Pakistan and Afghanistan. Many of the earthquakes are related to the transform faults on either side of the India Plate, and most of the others are related to the significant tectonic squeezing caused by the continued convergence of the India and Asia Plates. That squeezing has caused the Asia Plate to be thrust over top of the India Plate, building the Himalayas and the Tibet Plateau to enormous heights.
Most of the earthquakes of Figure The southernmost thrust fault in Figure There is a very significant concentration of both shallow and deep greater than 70 km earthquakes in the northwestern part of Figure This is northern Afghanistan, and at depths of more than 70 km, many of these earthquakes are within the mantle as opposed to the crust. It is interpreted that these deep earthquakes are caused by northwestward subduction of part of the India Plate beneath the Asia Plate in this area.
This map shows the incidence and magnitude of earthquakes in British Columbia over a one-month period in March and April The string of small earthquakes adjacent to Haida Gwaii H.
As the Atlantic Ocean floor gets weighed down around its margins by great thickness of continental sediments i. A subduction zone will develop, and the oceanic plate will begin to descend under the continent.
Once this happens, the continents will no longer continue to move apart because the spreading at the mid-Atlantic ridge will be taken up by subduction.
If spreading along the mid-Atlantic ridge continues to be slower than spreading within the Pacific Ocean, the Atlantic Ocean will start to close up, and eventually in a million years or more North and South America will collide with Europe and Africa. There is strong evidence around the margins of the Atlantic Ocean that this process has taken place before. The roots of ancient mountain belts, which are present along the eastern margin of North America, the western margin of Europe, and the northwestern margin of Africa, show that these land masses once collided with each other to form a mountain chain, possibly as big as the Himalayas.
The apparent line of collision runs between Norway and Sweden, between Scotland and England, through Ireland, through Newfoundland, and the Maritimes, through the northeastern and eastern states, and across the northern end of Florida.
When rifting of Pangea started at approximately Ma, the fissuring was along a different line from the line of the earlier collision. This is why some of the mountain chains formed during the earlier collision can be traced from Europe to North America and from Europe to Africa. That the Atlantic Ocean rift may have occurred in approximately the same place during two separate events several hundred million years apart is probably no coincidence.
The series of hot spots that has been identified in the Atlantic Ocean may also have existed for several hundred million years, and thus may have contributed to rifting in roughly the same place on at least two separate occasions Figure This map shows the boundaries between the major plates.
Without referring to the plate map in Figure Start with the major plates, and then work on the smaller ones. Finally, using a highlighter or coloured pencil, label as many of the boundaries as you can as divergent, convergent, or transform.
Skip to content Chapter 10 Plate Tectonics. Figure Previous: Next: Share This Book Share on Twitter. This plate includes all of Africa and the surrounding ocean, including the eastern Atlantic Ocean, the surrounding Antarctic Ocean, and the western Indian ocean. The part of the plate around the South America plate is moving northwards and a little east.
The part of the plate around the Australia plate is moving southwards. This plate includes Australia and much of the surrounding ocean. New Guinea and the northern parts of New Zealand are part of the Australia plate. The location data are the distance each earthquake was from the trench and how deep within the Earth it was. Geologists can observe most of the processes occurring at plate tectonic boundaries today earthquakes, volcanoes, mountain building, etc.
However, understanding the plate tectonic activity of the geologic past is more difficult because the events have already happened. Hence, geologists use processes that occur in the present to interpret processes that occurred in the past. This is known as uniformitarianism.
One way geologists can interpret ancient plate tectonic activity is to look at the topography of an area. Topography is the study of shapes and features of the land surface.
When studying features on the seafloor, the topography is instead referred to as bathymetry because this data is referencing how deep a feature is. Below are five topographic profiles showing different plate boundary configurations. A topographic profile is a graph that shows elevation changes as you walk from one point on the Earth to another.
This overemphasizes the changes in topography. In all of these profiles, the 0 value on the vertical axis is at sea level. Geologists can use topography to get a broad sense of the tectonic history of an area. Generally speaking, plate tectonic activity tends to produce elevation changes at or near the plate boundary, especially in convergent settings.
The collision of two plates leads to suturing ; the two plates become one when the collision ends. Evidence of these ancient boundaries is most commonly in the form of linear mountain belts that are not currently near a plate tectonic boundary. For example, an eroded, linear mountain belt in the middle of a continent would indicate that area was part of a convergent boundary deep in the geologic past and likely a continent-continent collision. The Ural Mountains in Russia fit this description Figure 1.
They formed during an orogeny to million years ago and now serve as the boundary between Europe and Asia. When most people think about tectonic plate boundaries, they often visualize parallel, symmetric lines separating the plates. This is not always the case in the real world as many plate boundaries are curved or segmented; this is because Earth is a sphere. Think about this: if you had a ball and tried to wrap it with a flat sheet of paper, would the paper wrap around it perfectly smooth?
The answer is no; the paper will want to fold in some places and tear in other places. The tectonic plates behave similarly to the paper.
Of course, other factors affect the shape of a boundary. Evidence of these plate boundaries is also contained in the topography of continents because not all mountain belts are straight lines.
This area is not near an active plate tectonic boundary today; the closest boundary is in the Gulf of Mexico. However, there is evidence in this topography to indicate it was part of a plate tectonic boundary at least twice in the geologic past.
Hotspots are still a poorly understood geologic phenomenon, but they allow extremely hot mantle material to rise close to the surface. Hotspots are marked by volcanoes, which come from melting of the mantle and crust directly above a hotspot.
If they occur under oceanic crust, they produce basalts. On the other hand, if they are under continental crust, they form both basalts and rhyolites, often called bimodal volcanism. One of the controversies is whether the hot spot is still capable of supereruptions or whether the volume of eruptive material is waning.
Instead, we think of motion as a straight line from point a to point b. These hotspots are on the North American plate which means the plate rotates around a point in the middle of northern Quebec. Since they rotate around a point on a sphere, different locations on the plate move at dissimilar speeds and directions.
Geoscientists call this an Euler pole. Knott, T. Geology, v. Martinod, J. Earth and Planetary Science Letters, v. It is composed of the crust and uppermost, rigid part of the mantle.
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