The Earth's Mechanism PDF

**The Earth's Mechanism** **The Continental Drift** In 1912, Alfred Wegener, a German meteorologist, proposed a theory that about 200 million years ago, the continents were once one large landmass. He called this landmass Pangaea, a Greek word which means "All Earth." Figure 7...

**The Earth's Mechanism** **The Continental Drift** In 1912, Alfred Wegener, a German meteorologist, proposed a theory that about 200 million years ago, the continents were once one large landmass. He called this landmass Pangaea, a Greek word which means "All Earth." Figure 7 shows how Pangaea evolved into how the continents look today. This Pangaea started to break into two smaller supercontinent called Laurasia and Gondwanaland during the Jurassic Period. These smaller supercontinents broke into the continents and these continents separated and drifted apart since then. Wegener searched for evidences to support his claim. He noticed the fit of the edges of the continents on the opposite sides of the South Atlantic. His evidences to the Continental Drift Theory includes the **distribution of fossils in different continents, rock features, and ancient climates**. **Evidence: The Continental Jigsaw Puzzle** Did it really start as one big landmass? It seems very impossible that the seven continents, which are currently thousands of miles away from each other were actually connected pieces of a supercontinent. The most visible and fascinating evidence that these continents were once one is their shapes. The edge of one continent surprisingly matches the edge of another: South America and Africa fit together; India, Antarctica, and Australia match one another; Eurasia and North America complete the whole continental puzzle in the north. **Evidence from Fossils** Fossils are preserved remains or traces of organisms (plants and animals) from the remote past. Fossilized leaves of an extinct plant Glossopteris were found in 250 million years old rocks. These fossils were located in the continents of Southern Africa, Australia, India, and Antarctica, which are now separated from each other by wide oceans. The large seeds of this plant could not possibly travel a long journey by the wind or survive a rough ride through ocean waves. Mesosaurus (shown in Figure 10) and Lystosaurus are freshwater reptiles. Fossils of these animals were discovered in different continents, such as in South America and Africa. It is impossible for these reptiles to swim over the vast oceans and move from one continent to another. Fossils were also found in Antarctica. Could it be possible that they existed in this region where temperature was very low? Or could it be possible that, long before, Antarctica was not in its current position? Evidence from Rocks Fossils found in rocks support the Continental Drift Theory. The rocks themselves also provide evidence that continents drifted apart from each other. From the previous activity, you have learned that Africa fits South America. Rock formations in Africa line up with that in South America as if it was a long mountain range. The folded cape mountains of South America and Africa line up perfectly as if they were once a long mountain range. Coal Deposits Coal beds were formed from the compaction and decomposition of swamp plants that lived million years ago. These were discovered in South America, Africa, Indian subcontinent, Southeast Asia, and even in Antarctica. How is a coal bed formation possible in Antarctica? The current location of Antarctica could not sustain substantial amount of life. If there is a substantial quantity of coal in it, thus, it only means that Antarctica must have been positioned in a part of the Earth where it once supported large quantities of life. This leads to the idea that Antarctica once experienced a tropical climate, thus, it might have been closer before to the equator. The Seafloor Spreading The question as to how the drifting took place left the Continental Drift Theory blurry. Despite the evidences presented by Wegener, his idea that the continents were once joined together was not accepted by the scientific society until the 1960s. He wasn't able to explain how this drifting took place. This made scientists conduct further studies in search for the answer. During the 1950s and 1960s, new techniques and modern gadgets enabled scientists to make better observations and gather new information about the ocean floor. With the use of sonars and submersibles, scientists had a clearer view of the ocean floors. They have discovered underwater features deep within the ocean. Scientists found a system of ridges or mountains in the seafloor similar to those found in the continents. These are called mid-ocean ridges. One of these is the famous Mid-Atlantic Ridge (Figure 11), an undersea mountain chain in the Atlantic Ocean. It has a gigantic cleft about 32-48 km long and 1.6 km deep. The ridge is offset by fracture zones or rift valleys. In the early 1960's, scientist Harry Hess, together with Robert Dietz, suggested an explanation to the continental drift. This is the Seafloor Spreading Theory. According to this theory, hot, less dense material from below the earth's crust rises towards the surface at the mid-ocean ridge. This material flows sideways carrying the seafloor away from the ridge, and creates a crack in the crust. The magma flows out of the crack, cools down and becomes the new seafloor. Overtime, the new oceanic crust pushed the old oceanic crust far from the ridge. The process of seafloor spreading allowed the creation of new bodies of water. For example, the Red Sea was created as the African plate and the Arabian plate moved away from each other. Seafloor spreading is also pulling the continents of Australia, South America, and Antarctica away from each other in the East Pacific Rise. The East Pacific Rise is one of the most active sites of seafloor spreading, with more than 14 centimeters every year. In the place where two oceanic plates collide or where an oceanic plate and a continental plate collide, a subduction zone occurs. As the new seafloor is formed at the mid-ocean ridge, the old seafloor farthest from the ridge is destroyed at the subduction zone. The rate of formation of a new seafloor is not always as fast as the destruction of the old seafloor at the subduction zone. This explains why the Pacific Ocean is getting smaller and why the Atlantic Ocean is getting wider. If subduction is faster than seafloor spreading, the ocean shrinks. When the seafloor spreading is greater than the subduction, then the ocean gets wider. **Findings that support Seafloor Spreading Theory:** 1\. Rocks are younger at the mid-ocean ridge. 2\. Rocks far from the mid-ocean ridge are older. 3\. Sediments are thinner at the ridge. 4\. Rocks at the ocean floor are younger than those at the continents. The Seafloor Spreading Theory contradicts a part of the Continental Drift Theory. According to this theory, continents moved through unmoving oceans and that larger, sturdier continents broke through the oceanic crust. Whereas, the seafloor spreading shows that the ocean is the actual site of tectonic activity. **Magnetic Reversal** Seafloor spreading was strengthened with the discovery that the magnetic rocks near the ridge follow a pattern aside from the fact that rocks near the ridge are remarkably younger than those father from the ridge. A magnetic compass tells us directions on Earth. It also proves that the Earth has a magnetic field. The needle of a magnetic compass usually points to the North Pole of the Earth which is actually the South Magnetic Pole at present. The Earth's magnetic field is generated in the very hot molten outer core and has already existed since the birth of our planet. The Earth's magnetic field is a dipole, one that has a North Pole and a South Pole. What is magnetic reversal? How does magnetic reversal happen and how does it prove seafloor spreading? Magnetic reversal is also called magnetic 'flip' of the Earth. It happens when the North Pole is transformed into a South Pole and the South Pole becomes the North Pole. This is due to the change in the direction of flow in the outer core. Magnetic reversals happened many times in the past. The occurrence of magnetic reversals can be explained through the magnetic patterns in magnetic rocks, especially those found in the ocean floor. When lava solidifies, iron bearing minerals crystallize. As these crystallize, the minerals behave like tiny compasses and align with the Earth's magnetic field. So when magnetic reversal occurs, there is also a change in the polarity of the rocks. This allowed scientists to visualize the magnetic stripes in the ocean floor similar to Figure 14, and to construct a magnetic polarity time scale similar to Figure 15. Over the last 10 million years, there has been an average of 4 to 5 reversals per million years. New rocks are added to the ocean floor at the ridge with approximately equal amounts on both sides of the oceanic ridge. The stripes on both sides are of equal size and polarity which seemed to be mirror images across the ocean ridge. What does this indicate? It indicates that indeed, the seafloor is spreading. **Plate Tectonic Theory** The Plate Tectonic Theory provided an explanation about the movement of the lithospheric plates. This theory evolved from the two former theories and was developed during the first decades of the 20th century. The Earth's lithosphere is divided into several plates. As you have already learned, these plates ride over the weak asthenosphere. There arethree types of plate movements -- separation of two plates (divergent), collision of two plates (convergent) and sliding past each other (transform). What facilitates the movement of the plates? Heat is produced in the core that produces convection in the mantle. This convection causes the plate to move around. To further understand this process, try the following activity. **Convection Current** As a substance like water is heated, the less dense particles rise while denser particles sink. Once the hot less dense particles cool down, they sink, and the other less dense particles rise. This continuous process is called convection current. This is exactly what happens in the Earth's mantle. The hot, less dense rising material spreads out as it reaches the upper mantle causing upward and sideward forces. These forces lift and split the lithosphere at divergent plate boundaries. The hot magma flows out of the mantle and cools down to form the new ocean crust. The downward movement of the convection current occurs along a convergent boundary where the sinking force pulls the tectonic plate downward. The convection currents rotate very slowly, as they move and drag the plates along. Because of convection current, the tectonic plates are able to move slowly along the tectonic boundaries, pushing each other, sliding past each other and drifting away from each other. This process is further illustrated in Figure 16 below. As an oceanic crust moves away from a divergent boundary, it becomes denser than the newer oceanic crust. As the older seafloor sinks, the weight of the uplifted ridge pushes the oceanic crust toward the trench at the subduction zone. This process is called ridge push. Slab pull is the other possible process involved in the tectonic plate movement. The weight of the subducting plate pulls the trailing slab into the subduction zone just like a tablecloth slipping off the table and pulling items with it. Now that you understand what happens inside the Earth and its effects on the Earth's surface, you should be able to realize that the tectonic activities at the surface just like volcanic eruptions and earthquakes are inevitable. You should view the Earth as a dynamic planet and still the most fascinating planet for it offers you a home that no other planet can. Since you can't prevent these tectonic activities from happening, the following performance task will enable you to contribute meaningfully in minimizing the damage that these phenomena can bring.