Plate Tectonics: the theory which proposes the Earth's outer shell consists of individual plates which interact in various ways and thereby produce earthquakes, volcanoes, mountains and the crust itself.
Continental Drift: a forerunner of plate tectonics, proposed movement of land masses but lacked a mechanism.
Alfred Wegener (1880-1930): German climatologist and geophysicist, wrote The Origin of Continents and Oceans in 1915 wherein he set forth the radical hypothesis of continental drift.
Pangaea: super continent (means "all land") 200 million years ago Pangaea began to break apart.
Fit of the continents: Wegener's fit of the continents was crude and was challenged because scientists argued that erosion modifies coastlines.
Today the fit is done along the continental shelf several hundred meters below sea level. South America and Africa fit well together.
Fossil types match on separate continents: Mesosaurus is found in South America and Africa. The question is "how could it have traveled across the ocean". Evidence of land bridges has never been found.
Rock Types: Rocks should match in age and type across the ocean if the continents wre once joined. The Appalacian-British Isles - Caledonian Mountains have been studied and it is found that they create a continuous belt.
Ancient Climate Evidence of glaciers in tropical latitudes and coal fields in Siberia indicate that ancient climate has been dramatically different from present climate in many regions.
Wegener proposed Africa was once centered over the South Pole where all land masses were joined.
The theory of continental drift was ridiculed. The major objection was the lack of a plausible mechnism.
Due to major advances in technology that rendered extensive data on the ocean floor, Earth's magnetic field, and seismic activity a new theory was put forth in 1968.
The Earth's outer shell consists of 20 rigid slabs called plates that move with respect to each other creating mountains, volcanoes, earthquakes and creating and destroying crust in a cyclic process.
The Pacific plate is the largest plate. Many plates include both continental and oceanic crust. This is a major difference between plate tectonics and continental drift.
Lithosphere: crustal rocks and a portion of the upper mantle.
Asthenosphere: hotter "plastic" zone beneath the lithosphere.
The lithosphere floats on the asthenosphere.
Divergent boundaries: Plates move apart. Upwelling of material from mantle.
Convergent boundaries: Plates move together.
Transform boundaries: Plates grind past each other. Connects divergent and convergent boundaries.
Seafloor spreading: rate ranges from 2 to 10 centimeters per year.
Mid-Atlantic Ridge: submerged mountain range from the Arctic Ocean to the southern tip of Africa
Iceland: part of the mid-Atlantic ridge that has grown above sea level.
Other spreading centers: Red Sea (linear sea), Gulf of California
Rift valleys: East-African rift valley (Volcanic mountains - Kilimanjaro, Mount Kenya)
oceanic-continental convergence - oceanic crust descends into the asthenosphere; deep ocean trench is formed by subduction.
Andes mountains formed in this way.
oceanic-oceanic convergence creates island arcs; subduction also occurs.
continental-continental convergence (occurs after all intervening oceanic crust is subducted); no further subduction; crust buckles and is pushed upward; extensive folding and faulting. Himalayas formed in this way.
connects convergent zones to divergent zones
San Andreas Fault
Magnetite is abundant in basaltic lava flows. When this material cools it becomes aligned with the Earth's magnetic field. In this way the rocks record the position of the Earth's magnetic field when they form.
A study in the 1950s showed that the alignment varied widely "Pole Wandering". Either the poles had moved or the land masses had moved. If the magnetic poles remain stationary, their apparent movement was produced by the movement of the continents.
Earth's magnetic field periodically reverses polarity. Currently we have what we have defined as "Normal Polarity". The opposite polarity is called "Reverse Polarity".
Seafloor spreading produces alternating bands of normal and reverse polarity rock.
Deep-focus and shallow-focus earthquakes and their distribution with respect to deep ocean trenches supports the theory that the ocean plate plunges into the mantle.
The age of the sediment just above the basaltic seafloor increases as distance from the mid-Atlantic ridge increases. The oldest ocean crust is 160 million years old. Continental crust is as old as 3.9 billion years.
Chain of Hawaiian islands becomes older as they occur farther than Hawaii. Stationary hot spots create volcanic islands as crust moves over the hot spot.
None of the driving mechanisms proposed can account for all of the major facets of plate motion.