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Plate Tectonics Erin Johnson. Brief History. Since the Theory of Continental Drift was first proposed by Alfred Wegener in 1912, it has had a long, shaky history.
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Brief History • Since the Theory of Continental Drift was first proposed by Alfred Wegener in 1912, it has had a long, shaky history. • The theory was strongly challenged and widely ignored, however with the discovery of sea-floor spreading in the late 1950's and early 60's, the idea was reinvigorated, this time as the Theory of Plate Tectonics.
Tectonic Plate Boundaries • The theory of Plate tectonics states that Earth's outermost layer, the lithosphere, is broken into 7 large, rigid pieces called plates: the African, North American, South American, Eurasian, Australian, Antarctic, and Pacific plates. • Several minor plates also exist, including the Arabian, Nazca, and Philippines plates.
Tectonic Plate Boundaries (cont.) • The plates are all moving in different directions and at different speeds (from 2 cm to 10 cm per year.) • The plates are moving around, which means they sometimes crash together, pull apart, or sideswipe each other. • The place where the two plates meet is called a plate boundary. • Boundaries have different names depending on how the two plates are moving in relationship to each other -crashing: Convergent Boundaries, pulling apart: Divergent Boundaries, or sideswiping: Transform Boundaries.
Layers of the Earth • Earth is divided into three chemical layers: the core, the mantle and the crust. • The core is divided into two layers: a solid inner core and a liquid outer core. • The middle layer of the Earth, the mantle, is made of minerals rich in the elements iron, magnesium, silicon, and oxygen.
Layers of the Earth (cont.) • The crust is rich in the elements oxygen and silicon with lesser amounts of aluminum, iron, magnesium, calcium, potassium, and sodium. • There are two types of crust. Oceanic crust is made of relatively dense rock called basalt. Continental crust is made of lower density rocks, such as andesite and granite. • The outermost layers of the Earth can be divided by their physical properties into lithosphere and asthenosphere.
The lithosphere (from the Greek, lithos, stone) is the rigid outermost layer made of crust and uppermost mantle. The lithosphere is the "plate" of the plate tectonic theory. The asthenosphere (from the Greek, asthenos, devoid of force) is part of the mantle that flows, a characteristic called plastic behavior. A good example of a solid that flows, or of plastic behavior, is the movement of toothpaste in a tube. The flow of the asthenosphere is part of mantle convection, which plays an important role in moving lithospheric plates. Layers of the Earth (cont.)
Continental Drift • Continental drift was originally proposed by Alfred Wegener, a German meteorologist, in 1912. • He used the fit of the continents, the distribution of fossils, a similar sequence of rocks at numerous locations, ancient climates, and the apparent wandering of the Earth's polar regions to support his idea. • He used his observations to hypothesize that all of the present-day continents were once part of a single super continent called Pangaea.
Continental Drift - Fossils • Fossils of the same species were found on several different continents. • Wegener proposed that the species dispersed when the continents were connected and later carried to their present positions as the continents drifted.
Continental Drift – Fossils (cont.) • For example, Glossopteris, a fern, was found on the continents of South America, Africa, India, and Australia. If the continents are reassembled into Pangaea, the distribution of Glossopteris can be accounted for over a much smaller contiguous geographic area. • The distribution of other species can also be accounted for by initially spreading across Pangaea, followed by the breakup of the super continent, and movement of the continents to their present positions.
Continental Drift - Rock Sequences • Rock sequences in South America, Africa, India, Antarctica, and Australia show remarkable similarities. • Wegener showed that the same three layers occur at each of these localities. • The bottom (oldest) layer is called tillite and is thought to be a glacial deposit.
Continental Drift - Rock Sequences (cont.) • The middle layer is composed of sandstone, shale, and coal beds. Glossopteris fossils are in the bottom and middle layers. • The top (youngest) layer is lava flows. • The same three layers are in the same order in areas now separated by great distances. • Wegener proposed that the rock layers were made when all the continents were part of Pangaea. Thus, they formed in a smaller contiguous area that was later broken and drifted apart.
Summary • The plate tectonic model has been rigorously tested since the mid-1960s. Because the model has been successfully tested by numerous methods, it is now called the plate tectonic theory and is accepted by almost all geologists.
Sources • http://volcano.und.edu/vwdocs/vwlessons/plate_tectonics/introduction.html • http://www.cotf.edu/ete/modules/msese/earthsysflr/plates1.html
