Plate Movement and Convection Help
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.
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