Plate Movement and Convection Help (page 2)
Plate tectonic geologists are always chasing their work!
Though they are always changing in size, the Earth has seven major continental plates. The outer crustal layer, the lithosphere, is a puzzle of moveable parts that mold to each other according to the different pressures put on them. For the last two or three hundred years scientists have studied mountains, valleys, volcanoes, islands, earthquakes, and many other geologic happenings, but each study was done independently. Each individual study was thought to be unique and not connected to other geological sites or processes. Classification of rocks and land types was done apart from the other land types. It was not until widespread travel and communication began that geologists began to compare notes.
Geophysicist, J. Tuzo Wilson, was the first scientist to put it all together. He knew that tectonics , the large-scale movement and folding of the Earth’s outer layers were ongoing. What Wilson pieced together from the ideas of Wegener and others was the concept of plate tectonics ; the study of geology and physics. The little understood idea of continental drift made sense to Wilson when combined with the idea of large plate movement and pressures.
Plate tectonics ( tektonikos is Greek for “builder”) describe the formation and movement of ocean and continental plates.
Plate tectonics is the umbrella theory that explains the Earth’s activity and the creation, movement, contact, and flattening of the solid rock plates of the lithosphere.
But Wilson didn’t just sit around his office thinking about plate movement, he led expeditions to remote areas of Canada and was the first to climb Mount Hague in Montana in 1935. The majestic range of mountains in Antarctica, the Wilson range, was named for this inventive and adventurous man.
Since Wilson’s first push toward the idea of plate tectonics, geologists began matching up plate measurements and found that plates moved farther over and around the planet than first thought. Most plates aren’t even close to their original positions! Fossils of tropical plants, once located at the equator, have been found in Antarctica. Deep rock in the Sahara desert, sliced by the heavy passage of glacier travel, was frozen and frosty long before traveling to its hot and dry retreat of today.
Most importantly, plates continue to move, sliding along at rates of up to eight inches per year in some areas. Measurements made around the active Pacific plate shows lots of overall movement. The Pacific and Nazca plates are separating as fast as 16 cm/year, while the Australian continental plate is moving northward at a rate of nearly 11 cm/year.
Plates seem to move more slowly in the Atlantic where plates crawl along at 1–2 cm/year. Since the time of the first European explorers westward in the 15th century, the Atlantic plate has expanded by about 10 m.
Plates are affected most often by the movement of magma filling the cracks in mid-ocean floor ridges as the plates move apart. When this happens, magma pours out creating new ocean floor and edging along the existing plate margin. Across the oceans, there is an arrangement of ridges where new material is being formed. When enough new material is deposited, plates slant, slide, collide, and push over, under, and alongside their neighbors. Continental and ocean plates ride over or dive under each other, forcing movement down and back into the mantle and liquid core.
The regular arguing and conflict between plates causes and releases pressure buildup deep within the crust. Plate borders, sites of the highest volcanic and tectonic activity, are well known for their violent personalities. Ask anyone living in southern California, where the Pacific and the North American plates collide, about their many earthquakes and the Earth’s constant rumblings!
A subduction zone is an area where two lithospheric plates collide and one plate is forced under the other into the mantle.
Figure 4-4 illustrates the subduction of the lithosphere between plates. The lithospheric plate sometimes induces volcanism on the overriding plate. A crustal plate that is subducted then dives deep into the mantle. Note: Mountains and lithosphere not to scale.
The circulation of material caused by heat is called convection . In the Earth system, convection is affected by gravitational forces within the planet as well as heat and radioactive recycling of elements in the molten core.
All tectonic processes within the Earth involve movement of solid or malleable matter. Convection in the mantle, driven by the thermal gradient between the core and lithosphere, takes place by deformation (creep) of the rocks and minerals that comprise the upper/lower mantle and the transition zone. Think of it like those square, hand-held puzzle games where one piece is left out and you can only slide one square into the open place at a time. In order to complete a number sequence or picture, you must keep sliding the squares around (one-at-a-time) until you are able to slide all of them around to their correct spots to complete the puzzle.
Mantle creep is like that. Because of imperfections in the crystalline structures of minerals and rocks, there are gaps. When pressure is applied, the atoms in the structure shift (creep), one atom at-a-time to a new position.
Plate tectonics, as seen in mountain building, earthquakes, and volcanoes, takes place by plastic (malleable) or brittle bending of the rocks and minerals that make up the oceanic and continental lithosphere. Temperature, pressure, and rate of deformation to a large extent define the nature of deformation for most minerals and rocks in the interior of the Earth. However, the chemical environment (presence or absence of water, oxygen, silica, and other elements) may also have a big impact. By understanding the mechanisms by which rocks and minerals move and change shape under extreme temperature and pressures, we will add to our understanding of the processes that shape our planet.
The steady movement of magma deep within the Earth depends on differences in temperature and differences in density within large “pockets” of molten matter. Depending on conditions, magma rises in the pockets of hotter temperatures and falls in pockets of cooler temperatures. Since the Earth’s center is still hot, this endless thermal activity keeps the tectonic process going.
On a smaller scale, convection happens in liquids or gases, like the swirling currents of a pot of boiling soup. In the depths of the Earth, convection moves flowing magma that is heated from below by the core and then pushed upward over time and cooled from above. This solid flow movement is much slower than the liquid flow we saw earlier. Remember the lava lamp?
Convection is the process of heat transfer that causes hot, less dense matter to rise and cool matter to sink.
Convection affects rocks of different densities as well. Lighter density lithospheric rock tends to ride along above sea level, while denser asthenospheric rock sinks below sea level. The hard, rigid lithosphere is an unyielding outer shell, while the softer, wax-like asthenosphere is moldable and fluid when pushed.
When hot matter is forced up and out, it cools and adds to the outer crustal rock. As more material moves up, the earlier matter is pushed out of the way. The pressure from underlying rock is removed as it comes to the surface and the material in the magma chamber, a “crystalline mush” heats as it gets closer to the surface. This activity expands the area between the plates by a few centimeters per year. After a while, this new surface rock comes to another plate that will not yield. When this happens, plates argue and new rock gets pushed back down by subduction to be melted over again. Subduction occurs between opposing plates (mountains and magma chamber are not to scale), while cooling, rising magma causes spreading at ocean ridges as is shown in Fig. 4-5.
Fig. 4-5. Magma creates new land at ocean ridges.
Practice problem of this concept can be found at: Plate Tectonics Practice Test
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