TOPIC 6 - PLATE TECTONICS
I. Theories to explain the origin of the ocean basins - Scientists have long considered why there are extensive areas of the Earth's surface under water. How were these basins formed?
A. One idea was that the major Earth features formed several billion years ago as the crust formed and has been the same ever since. If this was true then the oceans should have a sediment record of all of the Earth's history.
B. George Darwin (Charles' son) suggested that the Pacific Ocean basin was formed when the Moon was ripped out of the Earth's crust. Thus the Pacific was the oldest ocean basin.
C. Vertical Tectonics was a general model in the 19th Century to explain why some mountainous regions were elevated far above most land while other regions, including the ocean basins, were much lower. Scientists thought that internal forces within the Earth caused intermittent uplift or down warping of the different regions.
II. The Theory of Continental Drift: In 1912, Alfred Wegener started to revolutionize geologic thinking when he announced that the continents are not stationary, but instead drift around Earth's surface. The ocean basins were then the holes left behind as the continents moved.
A. He suggested that at one time in the Earth's geologic past: all the continents were joined into a single land mass, which he called Pangaea.
1. Pangaea began to split apart around 200 million years ago, forming two new continents.
2. He called the northern continent Laurasia; it included what is now North America, Europe, and most of Asia.
3. The southern continent was called Gondwana; it included South America, Africa, India, Australia, and Antarctica.
B. Wegener presented the following evidence to support his theory of continental drift:
1. It has been noted since the first maps of the Atlantic Ocean were made that the two sides of the Atlantic Ocean could be fitted together like a jig-saw puzzle. Did that mean that the continents had once been much closer together?
2. After determining relative rock ages from the fossils in them, boundaries between rock units of very different ages in South America match up with the same boundaries between rocks of very different ages in Africa when the continents are put back together.
3. Mountain ranges on the west side of the Atlantic Ocean match up with mountain ranges on the east side of the Atlantic Ocean when the continents are put back together as Pangaea. (Appalachians and Caladonides in Scotland)
4. Some land plant fossils (Glossopteris sp.) are found in the glacier- and river-deposited rocks of South America, Africa, India and Australia. (They are even under the ice of Antarctica, but Wegener didn't know that.) Those same plant fossils are absent from North America, Europe, and Asia. How could the plant spread itself all over the southern continents and stay out of the northern continents?
5. The distribution of Permian glacial deposits and the orientations of glacial striations indicate that the southern continents shared episodes of glaciation with a common point from which the glaciers spread out. Glacial striations are scratches in rock, made as glaciers grind across the land surface, that indicate the direction of glacier movement the way slickensides indicate the direction of fault movement.
6. The problem with Wegener's hypothesis was to find a mechanism for moving the continents around. Without some reasonable way to explain how the continents moved about, few people believed in his ideas.
C. Recent studies of the Earth's magnetic field have yielded another set of observations that are consistent with the idea of continental drift. These observations are embodied in what is termed Apparent Polar Wander (APW).
1. We know from historic studies that the magnetic north pole determined from magnetic measurements coincides with the north geographic pole (on average); i.e., compasses generally point toward North.
2. Paleomagnetic studies of rocks of different ages on individual continents, however, suggest that the north magnetic pole has moved its position in a systematic manner over geologic time. This is termed apparent polar wander. Whats more, the pattern of apparent polar wander is different for each continent. (Rocks contain magnetic minerals that act like compasses and lock-in their orientation at the time the rocks were formed.)
3. It is easiest to explain APW by movement of individual continents. Each continent records a path of apparent polar wander for its geological past. The APWPs of different continents are different. Therefore it is more likely that the continents moved than that the pole moved. In some cases, the APWPs of two continents can be fit together for hundreds of millions of years suggesting that they were linked and moving together over that time interval.
III. The Theory of Sea Floor Spreading: In the last 30 years as evidence for apparent polar wander and continental drift has developed, there has also been an incredible amount of research in the oceans which has led to another theory - sea floor spreading.
A. Recent observations of the ocean basins that are best explained by sea-floor spreading.
1. Ocean crust topography: The large-scale ocean basins tend to have symmetric oceanic ridges and long narrow trenches. Ridges usually occur in the middle of ocean basins and have high heat flow; trenches usually occur at the edges of ocean basins and have low heat flow. (Heat flow is the rate of heat loss from the Earth's interior. Remember, the center of the Earth is at temperatures near 5000 K. That heat must escape somehow!)
2. Magnetic stripes: Ocean crust is made up mostly of basalt and gabbro. These rocks are rich in magnetic minerals. When geophysicists started to measure magnetic field anomalies in ocean crust they saw startling patterns emerge - 'stripes'. These stripes run parallel to ridges and their patterns are symmetrical about the ridges. These stripes were explained by sea-floor spreading. The idea is that oceanic crust records times of normal (white regions below) and reversed (black regions below) magnetic field polarity. As spreading continues, rocks that record the same polarity are split and moved apart from one another.
3. Geophysicists have noted for a long time that earthquakes in the ocean basins have a distinctive pattern. Earthquakes occur mostly at oceanic ridges and trenches. The ridge earthquakes are always shallow and occur near the ridge crest. The trench earthquakes may be shallow to very deep, but the shallow earthquakes occur near the trench while the deeper earthquakes towards continents.
B. The theory of sea-floor spreading centers around the idea that the mid-ocean ridges are lines of volcanoes on the ocean floor. As magma comes to the Earth's surface and is emplaced in oceanic crust at the mid-ocean ridge volcanoes, the oceanic crust on both sides of the ridges moves way (spreads) to make room for more crust. To balance the growth of new oceanic crust, there must be a place where oceanic crust is consumed. (Otherwise the Earth would continually grow, and we have good scientific evidence that is not the case.) The trenches are locations where the oceanic crust is pushed underneath the continents and consumed at depth. (Actually the oceanic crust is re-melted in the mantle and reused to form more oceanic crust later.)
C. There is quite a lot of recent corroborating evidence in support of sea floor spreading.
1. The possibility that sea-floor spreading really occurs can be (and repeatedly has been) tested by 1) dating the basalt at different places in the oceanic crust and 2) by dating the oldest fossils above any place in the oceanic crust. (DSDP, ODP - deep sea drilling projects). These results showed that the oldest crust is far from oceanic ridges and the youngest crust is at the ridges!
2. Even so, there is no oceanic crust older than about 200 Ma. By contrast, the oldest continental rocks are older than 3.5 Ga. Thus the ocean floor must be continually recycled.
IV. Plate Tectonics is the theory that unifies the continental drift and sea-floor spreading theories. Tectonics is the study of large-scale deformation and movement of the Earth's layers. Plates are rigid crustal blocks that move over semi-molten mantle material.
A. The elements of plate tectonics.
1. There are six large crustal plates and several small plates that comprise Earth's solid surface. the plates are rigid to a depth of about 50 km. Below that is a plastic layer of earth materials, the asthenosphere, that flows in convection cells that extend to a depth of 300 km or so.
2. There are three types of plate boundaries: constructive (or divergent), destructive (or convergent), and conservative. Mid ocean ridges are the constructive boundaries, the trenches are the destructive boundaries, and transform faults (like the San Andreas fault) are conservative boundaries that let one plate slide past another.
3. Major plates: North American, South American, Pacific, Eurasian, Australia/India, Antarctica.
4. What drive plate tectonics? There are three ways of dissipating heat from an area of higher temperature to an area of lower temperature: conduction, radiation, and convection. The Earth's interior is very hot, and the most efficient way for it to transfer heat to the surface is through convection.
Undersea mountain ranges, the mid-ocean ridges, are over the rising parts of convection currents in Earth's mantle. the currents, like conveyor belts, carry ocean crust away from the ridges. The pulling-apart stresses put cracks in the graben at the ridge crest. Lava oozes out of the cracks, cooling to form new oceanic crust rocks.
Since new Earth surface is created at the mid-ocean ridges, and since Earth isn't getting any bigger, crust must be destroyed someplace, right? Right - at trenches, crust goes back into the mantle, melting as it does so.
5. There are three types of convergent plate boundaries, depending on which kinds of crust are colliding. If ocean crust converges with ocean crust, one plate is subducted and the other rides over it. This type of boundary is characterized by a trench, and volcanoes on the overriding plate. If ocean crust and continental crust converge, the heavier ocean plate is subducted and the lighter continental plate overrides it. This type of boundary is characterized by a trench, volcanoes on the overriding plate, and coastal mountain ranges composed of sediments pushed up from the trench. If continental crust converges with continental crust, neither plate subducts. Instead, they just keep crunching together, building up mountains and generating earthquakes. The highest Richter magnitude earthquake ever recorded occurred in the Himalayas, which are the product of a continent-continent collision.
B. Implications of plate tectonics for surface oceanic and environmental processes.
1. Plate tectonics theory explains the distribution of volcanoes, earthquakes, and heat flow on Earth. For the most part, linear trends in earthquakes and volcanoes mark plate boundaries.
2. It also explains the function of the San Andreas fault and the geology of the western margin of North America.
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