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Continental Drift

Plate Tectonics The shapes of the continents suggest that they could be joined like pieces of a jigsaw puzzle. This observation led to the suggestion, made in 1924, that in the distant past there had been one super-continent (pangaea) that broke up, with the various sections drifting apart to form the present-day continents. This concept, called continental drift is supported by the theory of plate tectonics - a theory that offers a comprehensive explanation of the distribution of continents, mountain chains, volcanoes, earthquake sites, and ocean trenches.

Figure 09-06a Plate Tectonics
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Tectonics Theory Earth is the only planet that has plate tectonics. Models of the Earth have shown that the lithosphere (crust + mantle) is too thick for smaller planet, while the gravitational force for larger planet would squeeze any plates together. Even when the size criterion is met, it needs a way to crack the lithosphere. Numerous computer models fail to simulate conditions in which a break in the crust would spontaneously occur. It is suggested that perhaps asteroid or comet strikes may have led to the creation of the subduction process as shown in Figure 09-06b.

Figure 09-06b Plate Tectonics Theory
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    There are four types of plate boundaries as shown in Figure 09-06a:

  1. Divergent boundaries - where new crust is generated as the plates pull away from each other. In mid-ocean, this movement results in seafloor spreading and the formation of ocean ridges; on continents, crustal spreading can form rift valleys.
  2. Convergent boundaries - where crust is destroyed as one plate dives under another. In mid-ocean, this causes ocean trenches, seismic activity, and arcs of volcanic islands. Where oceanic crust is subducted beneath continental crust or when continents collide, land may be uplifted and mountains formed.
  3. Transform boundaries - where crust is neither produced nor destroyed as the plates slide horizontally past each other such as the San Andreas fault (Figure 09-06c). Such movement produces earthquakes.
  4. Plate boundary zones - broad belts in which boundaries are not well defined and the effects of plate interaction are unclear. Because plate-boundary zones involve at least two large plates and one or more micro-plates caught up between them, they tend to have complicated geological structures and earthquake patterns.
San Andreas Fault Hawaiian Volcanos The movement of the Earth is induced by the convection currents of molten magma deep down in a zone called the mantle. These currents rise, then turn sideways below the solid crust. The crust is divided into nine major plates in the lithosphere (Figure 09-06e, the 9th one is uncertain). Slowly, at rates of a few centimeters per year, the rising current moves these plates. If the plate moves over a localized hot spot (Figures 09-06a and c) in the mantle, volcano will form until the plate carries it away from this source of magma.

Figure 09-06c San Andreas Fault [view large image]

Figure 09-06d Hawaiian Volcanos [view large image]

Earth Plates Earth Quake Zones For example, the Hawaiian group of volcanic islands, which lie in the middle of the Pacific plate, has been built up while the plate has been drifting over a hot spot (Figure 09-06c). But volcanoes occur most commonly along the boundaries of crustal plates (Figure 09-06f). Crustal movement on continents may result in earth-quakes, while movement under the sea bed can lead to tidal waves (tsunami). In term of destructive power, the quake is more devastating, when the stress

Figure 09-06e Crustal Plates [view large image]

Figure 09-06f Earth Quake Zones
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builds up over a long period, than the one occurs in short interval. Such intuitive idea is embodied into a mathematical formula known as the Gutenberg-Richter Law: N = N010-bM
Gutenberg-Richter Law where M is the magnitude of earthquake (see Figure 09-06g for effects of earthquake magnitudes);
b is a parameter with a value close to 1 depending on the tectonic environment, a low value of b signifies slow variation, and at the limit b = 0 strong or weak quake occurs at random;
N0 is the total number of quakes in a given period of time and in certain size of area, the example in Figure 09-06g cover the Earth's entire surface in one year;

Figure 09-06g Gutenberg-Richter Law

N is the number of events greater than or equal to magnitude M within the specific area and time. Its meaning is different from the usual definition by including counts greater than the variable M.
For example, Figure 09-06g indicates that N = 104 for M = 4 giving N0 = 108, then N = 106 cab be derived for M = 2 etc. The formula can be interpreted as the probability of quake occurrence if it is normalized by dividing N0 to both sides. Thus at M = 0, (N/N0) = 1 which means earthquake is bound to happen if N0 > 0.

When a disturbance such as Earth quake or explosion occurs in the Earth's crust, many kinds of wave are generated and propagate through the Earth. There are two main kinds - the P wave, which is longitudinal and the S wave, which is transversal (thus absorbed by the liquid outer core as shown in Diagram a, Figure 09-06h). The most destructive kind is the S wave generated near the surface. Depending on the density of the medium, the velocity is about 5 km/sec (higher than the sound wave of 0.34 km/sec in the air). The velocity and frequency of the P wave is about twice as much as the S wave, and thus enables pinpointing of the epicenter. The different patterns of seismic waves are used to monitor compliance of the moratorium of nuclear testing. As shown in Diagram b, Figure 09-06h, the ratio of P wave to S wave (amplitude) for genuine Earth quake is much lower than a nuclear explosion. The severity of disturbance is measured by the seismometer on seismograph in Richter scale ML(or its variances) by the formula :
ML = log10(A/A0)
Earth Plates where A0 is defined by displacement of 1 m (0.00004 in) on a seismograph recorded using a Wood-Anderson torsion seismometer 100 km (62 mi) from the earthquake epicenter, and A is the corresponding maximum amplitude after adjustments to compensate for the variation in the distance between the various seismographs and the epicenter of the earthquake. The relative energy released by the disturbance is (A2/A1)3/2.

Figure 09-06h Seismic Waves [view large image]

As shown in Figure 09-06i and j, continental drift has altered the face of the Earth for nearly a billion years. The land and sea were mainly separated until about 500 million years ago when some land masses spread into the middle of the ocean. The process of shifting continued on
Continental Drift Plate Tectonics Key with the formation of a supermassive continent called pangaea about 200 million years ago. This supercontinent broke up subsequently leading to the present geological distribution. The animation in Figure 09-06j shows the change starting from 740 million years ago in steps of 10 million years. To see continental locations during a particular period, click (Esc) to stop, (Refresh) to resume.

Figure 09-06i Continental Drift [view large image]

Figure 09-06j Continental Drift
[view animation]


Plate Tectonics Plate tectonics recycles water, carbon and nitrogen, creating an environment that is perfect for life. It makes oceans open and close, mountains rise and fall and continents gather and split. Every 500 to 700 million years, plate tectonics brings the continents together to form a supercontinent. When these supercontinents slowly break up, separating landmasses and forming shallow seas, evolution goes into overdrive, forming countless new species which colonise the new habitats. A tectonic plate, for example, can move a continent from a tropical to a polar latitude, where the organisms will experience new patterns of competition. The life forms present or absent in a particular part of the world help to define the evolutionary fate of all the other organisms in the community. Land and sea barriers generated by continental drift have, by restricting movements, influenced zoogeographical distribution patterns on the face of the Earth. Organisms that arose and diversified on an ancient landmass, such as Gondwana, have been prevented by large sea barriers from colonizing other landmasses. Figure 09-06k shows the different life forms living in different land mass over the last 560 million years.

Figure 09-06k Life and Continental Drift

The Earth's climate is remarkably stable, and has remained in a narrow, live-able, range for almost 4 billion years. The key appears to lie in the interplay between plate tectonics, carbon dioxide and the oceans (see Figure 09-06l). Carbon dioxide is released into the atmosphere by volcanic activities. Too much of CO2 will warm up the air, and cause more seawater to evaporate. Acidic rain reduces the amount of CO2 by producing
Earth's Thermostat Birth of an Island carbon-containing minerals, which is carried into the mantle by plate tectonics, and eventually returns to the atmosphere through volcanoes to repeat the cycle again. This mechanism of climate regulation may not work very well if the carbon dioxide released by human activities becomes too much for the slow process of plate tectonics. Figure 09-06l1 captures the moment of birth of an island in January 2015 about 60 km north of the capital of Tonga.

Figure 09-06l Earth's Thermostat
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Figure 09-06l1 Birth of an Island [view large image]

Sea-floor Map A new sea-floor map was unveiled on October 2014. It is compiled with data from two different satellites both carrying altimeters - instruments that measure the precise distance between the satellite and the surface of the land or ocean below. The elevation of the sea floor is computed by subtracting the surface level of the ocean from the temporary phenomena such as wave. The sea level change corresponds to the gravitational pull of the underwater features such as mountain ranges. The latest map is at least twice as good as the previous one. It shows underwater volcanoes (known as seamounts) as low as 1.5 km high. The red dots in Figure 09-06l2 show the location of past earthquakes of magnitude 5.5 or higher.

Figure 09-06l2 Sea-Floor Map

See the original article and a video of the gravity map for the whole globe.

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