BIOLOGY and GEOLOGY | Printed Edition Plate Tectonics BIOLOGY and GEOLOGY | PLATE TECTONICS | Printed Edition Tectonic Plate Theory
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BIOLOGY AND GEOLOGY | Printed edition PLATE TECTONICS BIOLOGY AND GEOLOGY | PLATE TECTONICS | Printed edition TECTONIC PLATE THEORY The ground we stand on is moving very slowly. It is moving right now. Maybe you cannot notice because it moves slower than your fingernails grow. But, over millions of years, this movement is enough to change the appearance of the surface of the Earth. However, sometimes the movements are more abrupt and earthquakes occur. Origin of the Earth When the Earth was formed, it was a large, white hot sphere of rock melted at high temperatures. Little by little, over millions of years, it cooled off. Like any other object, the Earth lost heat through its surface, which was the first to cool, and thus formed a covering of solid rock: the lithosphere. Today our planet continues to gradually cool off. Therefore, the interior still maintains a great part of that original heat. Underneath the lithosphere there is a great ocean of liquid rock, and the top part of this is called the asthenosphere. Lithosphere: tectonic plates Nowadays we know that the Earth’s surface is like a puzzle of pieces that float on a sea of magma, like boats that drift on a great, white hot ocean. These pieces that divide up the Earth’s lithosphere are called tectonic plates, and they fit together with large cracks between them. In some places they collide and one sinks below another, in others they slide alongside each other, etc. The lithosphere, which is made up of the uppermost solid layer of the mantle and the crust, is not the same everywhere. This is due to there being two types of crust, which are mainly differentiated by their thickness, density, and composition: • Oceanic crust Oceanic crust is thin and dense, which is why it tends to sink and is usually located below sea level. • Continental crust Continental crust is thicker and lighter. It is usually located at sea level or above. The tectonic plates that divide up the lithosphere may only contain oceanic crust (like the Pacific Plate) or combine oceanic crust and continental crust (like the majority). PLATE TECTONICS | 1 BIOLOGY AND GEOLOGY | PLATE TECTONICS | Printed edition Oceanic crust Continental crust Average thickness 8 km 30-70 km Density 3.2 g/cm3 2.7 g/cm3 Rich in Fe, Mg Rich in Si, Al Composition Poor in Si, Al Poor in Fe, Mg Figure 1: Table which shows the thickness, density, and composition of each type of crust. According to the Tectonic plate theory, great geological phenomena such as the formation of mountains, the movement of continents, earthquakes, volcanoes, etc., can all be explained by one common cause: The internal heat of the Earth is a driving force that moves the plates of solid rock that make up the lithosphere. This theory, which was proposed not more than 50 years ago, was as revolutionary for geology as Darwin’s theory of evolution was for biology. MOVEMENT OF THE PLATES Vertical movements It is simply buoyancy equilibrium. Denser plates sink more, and lighter plates float more on the asthenosphere. If, on the contrary, a plate loses weight — because it erodes, for example, — then it experiences an ascensional force. This buoyancy equilibrium is called isostatic equilibrium, and the slow ascending and descending movements that plates consequently experience are called epeirogenic movements. Horizontal movements On the other hand, horizontal movements of plates happen when they slide on the asthenosphere. PLATE TECTONICS | 2 BIOLOGY AND GEOLOGY | PLATE TECTONICS | Printed edition THE DRIVING FORCE OF PLATES In the same way that winds and ocean currents are formed, there are magma currents inside the asthenosphere that push the plates. The cause is exactly the same in all three situations: the differences in temperature that are found at different heights or depths. When an object heats up, it loses density and tends to rise. On the contrary, when an object cools down, its density increases and it tends to fall due to gravity. This causes cyclical currents of matter that heats up and rises, and when it reaches the upper layers, it cools down and falls. Movement inside the asthenosphere 1. It is hottest deep down in the asthenosphere. The magma heats up, reduces its density, and rises toward the upper layers of the asthenosphere. 2. The surface of the asthenosphere, which is in contact with the lithosphere, is cooler, so magma cools down and increases its density. It falls again, thus completing a convection cycle. The internal heat of the Earth is responsible for these circular currents. Convection currents, which are caused by the internal heat of the Earth, are responsible for the movement of tectonic plates. PLATE BOUNDARIES Convection currents can make plates collide at some points, or separate at other points, or even slide alongside each other. This produces three types of boundaries between plates: • Divergent boundaries: plates slide apart. • Convergent boundaries: plates collide. • Transform boundaries: plates slide alongside each other. PLATE TECTONICS | 3 BIOLOGY AND GEOLOGY | PLATE TECTONICS | Printed edition Divergent boundaries: plates slide apart Mid-ocean ridges Wherever convection currents rise, there are two plates that separate. Indeed, rather than separate, they grow. Magma that rises from the asthenosphere pushes them, but at the same time it rises to the surface, cools, and becomes basalt rock which is added to the boundaries of each plate. The plates move in opposite directions as they grow. In these places, there are what we call mid-ocean ridges: underwater mountain ranges which form a continuous system over 64,000 km long throughout all the oceans. Mid-ocean ridges have a characteristic shape: at their center they have a fissure called a rift, which is about 1,300 m deep. These fissures are like wounds that never heal, because magma continuously rises from the asthenosphere. This magma, made of very dense matter, cools down enough to solidify, thus producing new oceanic lithosphere. So mid-ocean ridges are places where oceans grow. They are called constructive boundaries because oceanic lithosphere is continuously created there. The mid-ocean ridge that crosses the Atlantic from north to south has made this ocean grow 10 meters since the time of Christopher Columbus. Scientific demonstration of the existence of mid-ocean ridges • Firstly: These are areas of intense seismic and volcanic activity with a high thermal flow. • Secondly: They have high gravity values, due to the large amount of very dense matter concentrated there. • Thirdly, and most importantly: There is a complete lack of sediments in the spine of the mid-ocean ridges. The layer of sediments is thicker the farther away we are from them, which means that the crust is younger the close we are to these underwater mountain ranges. We can also prove this directly through radiometric dating of rocks. Indeed the igneous rocks on the ocean floors are older the farther away they are from the ridges. There are also transform faults, which are cracks that appear every few hundred kilometers perpendicularly to the ridges. These form because the convection currents below the ridges do not push them uniformly. Convergent boundaries: plates collide This is where the convection currents push two plates together, making them collide. What happens when two plates collide depends on the type of lithosphere of each plate. There are basically three situations: PLATE TECTONICS | 4 BIOLOGY AND GEOLOGY | PLATE TECTONICS | Printed edition • Oceanic lithosphere collides with oceanic lithosphere. • Oceanic lithosphere collides with continental lithosphere. • Continental lithosphere collides with continental lithosphere. Oceanic lithosphere collides with oceanic lithosphere When two plates of oceanic lithosphere collide at a boundary, one of them sinks below the other, creating a subduction zone. In these places we can find long trenches that are several kilometers deep and parallel to island arcs. The enormous friction between the plates causes high volcanic activity which creates an island arc in the plate that is not subducted. This is what created the islands of Japan and New Zealand. Oceanic lithosphere is formed in the mid-ocean ridges, and as it moves away, it cools down, becomes thinner and denser, and tends to sink. Wherever there is oceanic lithosphere at a convergent boundary, there is a a subduction zone, because the tendency of this type of lithosphere is to sink and return to the mantle. Oceanic lithosphere collides with continental lithosphere Oceanic lithosphere is thinner and denser than continental lithosphere. This is why oceanic lithosphere subducts below continental lithosphere. Again there is a subduction zone with a long trench where the oceanic lithosphere disappears. This time, the plate that does not subduct is continental. So instead of forming an island arc along the boundary, a long pericontinental mountain range is created, like the Andes. Continental lithosphere collides with continental lithosphere In a mixed plate, when the oceanic lithosphere has subducted below the continental lithosphere of the other plate, two continental masses can come up against each other. In this situation, instead of forming a subduction zone, large mountains called collision orogens are created. The most clear example is the mountain range of the Himalayas, formed when the Indian continent collided with the Eurasian continent. Transform boundaries: plates slide alongside each other In these situations plates slide alongside each other at the boundaries. Transform faults are created there (like in mid-ocean ridges). Transform faults are areas of high seismic activity (where there are many earthquakes). PLATE TECTONICS | 5 BIOLOGY AND GEOLOGY | PLATE TECTONICS | Printed edition For example, the San Andreas fault in California is due to the Pacific plate moving north alongside the North American plate. HOT SPOTS The places where very hot magma rises from time to time as plumes are called hot spots.