Plate Tectonics(Part-I)
INTRODUCTION
Plate Tectonics, theory that the outer shell of the earth is made up of thin, rigid plates that move relative to each other. The theory of plate tectonics was formulated during the early 1960s, and it revolutionized the field of geology. Scientists have successfully used it to explain many geological events, such as earthquakes and volcanic eruptions as well as mountain building and the formation of the oceans and continents.
Magma Upwelling Mid-ocean ridges occur along boundaries between plates of Earth’s outer shell where new seafloor is created as the plates spread apart. As plates move apart under the ocean, molten rock, or magma, wells up from deep below the surface of the seafloor. Some of the magma that ascends to the seafloor produces enormous volcanic eruptions. The rest solidifies on the edges of the plates as they spread apart, creating new rocky seafloor material
Plate tectonics arose from an earlier theory proposed by German scientist Alfred Wegener in 1912. Looking at the shapes of the continents, Wegener found that they fit together like a jigsaw puzzle. Using this observation, along with geological evidence he found on different continents, he developed the theory of continental drift, which states that today’s continents were once joined together into one large landmass.
Plate tectonics is a relatively new theory that has revolutionized the way geologists think about the Earth. According to the theory, the surface of the Earth is broken into large plates. The size and position of these plates change over time. The edges of these plates, where they move against each other, are sites of intense geologic activity, such as earthquakes, volcanoes, and mountain building. Plate tectonics is a combination of two earlier ideas, continental drift and sea-floor spreading. Continental drift is the movement of continents over the Earth's surface and in their change in position relative to each other. Sea-floor spreading is the creation of new oceanic crust at mid-ocean ridges and movement of the crust away from the mid-ocean ridges.
The lesson starts with a description of the chemical and physical layers of the Earth. Then the historical development of the plate tectonic theory is described. The lesson concludes with descriptions of the location and types of plate boundaries. This lesson is longer than most . A concise classroom presentation might describe the layers of the Earth, location of plate boundaries, and types of plate motion.
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. 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 supercontinent, 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. 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.
Continental Drift - Glaciation
Glaciation in South America, Africa, India, and Australia is best explained if these continents were once connected. Glaciers covered all or part of each of these continents during the same time period in the geologic past.
If the continents were in their present position, a major glaciation event that covered nearly all of the continents and extended north of the equator would be required. Geologists have found no evidence of glacial action in the northern hemisphere during this time period. In fact, during this time period, the climate in North America was warm.
Wegener proposed that the continents were adjacent to each other during the glacial event. Therefore, glaciers spread over a much smaller area in the southern hemisphere and probably did not influence the climate of the northern hemisphere.
Wegener used the distribution of specific rock types to determine the distribution of climate zones in the geologic past. For example, glacial till and striations (scratches on the rock), sand dunes, and coral reefs, indicate polar, desert, and tropical climates, respectively. The present climate zones are shown in the above figure. Note how the distribution of reefs, deserts, and glacial ice constrain the position of the rotational pole of the Earth.
Using the distribution of rock types, Wegener reconstructed the distribution of climates zones at specific times in the geologic past. He found that, unlike the present distribution, in which zones parallel the equator, the past zones occupied very different positions. This implies that the rotational pole was in very different locations relative to today. Wegener proposed an alternative interpretation. He believed that the climate zones remained stationary and the continents drifted to different locations. The drift of the continents caused the apparent movement of the climate zones.
Wegener used the distribution of climate zones to determine the location of the poles at different times in the geologic past. He found that the rotational pole appears to gradually change location, arriving at its present position only in the very recent geologic past. The apparent movement in the pole position over time is called polar wandering. Wegener offered an alternative explanation. He suggested that the poles remained stationary and that the continents changed their positions relative to the poles.
Problems with Wegener's Model of Continental Drift
Wegener's model was not accepted by all geologists. Some thought that dispersion by winds or ocean currents could explain the distribution of fossil species. Other geologists thought the poles might wander and continents remain stationary. Many geologists thought Wegener's evidence was insufficient.
The greatest shortcoming, at least in the eyes of American geologists, was the lack of an adequate mechanism for moving the continents. Wegener proposed that the Earth's spin caused the continents to move, plowing through the oceanic plate and producing mountains on their leading edges. Geologists at that time understood enough about the strength of rocks to know that this was highly unlikely. Wegener's work was largely unaccepted in the northern hemisphere. In the southern hemisphere, where geologists were familiar with the rocks that Wegener used to support his hypothesis, continental drift was generally accepted.
A mechanism to move continents was proposed by Arthur Holmes, Scottish geologist in 1928. He believed heat trapped in the Earth caused convection currents, areas where fluids beneath the Earth's crust rise, flow laterally, and then fall. The currents would rise beneath continents, spread laterally, then plunge beneath the oceans. (Geologists now know that solid rock, not fluids, convect in the mantle). Unfortunately, Wegener died in 1930 while exploring the Greenland ice cap. He never had the opportunity to adapt Holmes' ideas to his views of continental drift.
Subscribe to:
Post Comments (Atom)
0 comments:
Post a Comment