Plate Tectonics PDF

SES4U​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Name:​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Date: Plate Tectonics By the end of this lesson, I will be able to de...

SES4U​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Name:​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Date: Plate Tectonics By the end of this lesson, I will be able to describe plate tectonics and explain phenomena related to their existence Introduction Earth’s lithosphere is broken up into plates that move. In some places, the plates are moving toward each other, and in other places the plates are moving away from each other. Plate tectonics is a theory that encompasses the formation, movements, and interactions of these plates. Continental Drift In 1912, a German scientist named Alfred Wegener proposed that the Earth’s continents had once been joined as a single landmass or super continent he called Pangaea. He believed that Pangaea began to break apart about two hundred million years ago. Since that time, the continents have slowly drifted apart moving to their current positions. He based his hypothesis on the following pieces of evidence: ​ The puzzle piece structure of the continents ​ Matching rock types found on opposite sides of the Atlantic ​ Matching fossils found on opposite sides of the Atlantic ​ Ancient climatic evidence stored in sedimentary rock which suggested huge climate changes in certain continents Wegener’s hypothesis of continental drift was originally rejected due to two unanswered questions: What forces could move something as massive as a continent, and how could continents move without shattering? Our inability to answer these questions resulted from, in part, our lack of understanding of the ocean floor. Plate Tectonic Theory In the 1950s and 60s, additional evidence gathered about earthquakes, magnetism, and the age of rocks on the ocean floor provided further support for Wegener's hypothesis. From these ideas, the theory of plate tectonics was born. According to this theory, the continents are embedded in lithospheric plates. As these plates move, they carry the continents with them. This theory is supported by a surfeit of evidence regarding the location of volcanoes and earthquakes, as well as the formation of new crust along the ocean floor. SES4U​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Name:​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Date: Locations of Earthquakes and Volcanoes Data indicates that volcanic and seismic activity do not occur randomly across the planet. Instead, they are concentrated along belts, as depicted in the diagram. Earthquake and volcano belts support the theory of plate tectonics because they exist along plate boundaries. These boundaries describe locations where two plates meet, pull away from each other, or slip past one another. Large amounts of strain build up in these regions and when the strain becomes too great, fractures form and earthquakes occur. Boundaries also mark areas of high heat flow where molten rock is capable of moving upwards, thereby resulting in volcanic activity. Magnetism and the Age of the Ocean Floor During the 1940’s and 1950’s advances in technology allowed us to finally study the ocean floor with some degree of accuracy. Sound Navigation and Ranging (S.O.N.A.R.) uses sound waves to determine the depth of the ocean. It was used to map the topography of the ocean floor. By releasing a sound and timing how long it takes to hear the sound’s echo, the distance between a boat and the sea floor can be calculated given the constant speed at which sound travels in water. What was found were huge mountain ranges (mid ocean ridges) and deep valleys (deep-sea trenches). The Mariana Trench is over 11 km deep, while Mount Everest is only 8.8 km high. At the time, this was quite surprising. The assumption was that the seafloor would be rather flat and uninteresting. Eventually, scientists obtained rock samples from the ocean floor. In doing so, two key observations were made. Rock samples taken from near an ocean ridge were found to be much younger than those taken near deep-sea trenches. Furthermore, the seafloor rock, as a whole, was found to be much younger than continental rock. Sea floor rock, on average, is 180 million years old, while continental rock can be as old as 3.8 billion years old. SES4U​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Name:​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Date: Magnetometers and the Ocean Floor These devices allowed for scientists to study the history of Earth’s magnetic field (and the creation of a new branch of science called paleomagnetism). Magnetometers were used to measure variations in Earth’s magnetic field strength around volcanoes. Here is how this works: ​ Magma/lava contains iron-bearing minerals. Iron atoms have tiny magnetic fields around them. (same is true for nickel and cobalt). As lava flows out of a volcano, these iron atoms respond to Earth’s magnetic field just like a compass needle and point to magnetic south. When the lava solidifies forming basalt, these iron atoms become locked into place leaving behind a permanent record of Earth’s magnetic field at the time of the eruption. ​ By investigating different layers of basaltic rock (and therefore different eruptions), we can reconstruct Earth’s magnetic field history. In doing so, it was found that our magnetic field has flipped, or reversed, several times throughout Earth’s existence. (i.e., Compasses would not have always pointed to geographic north.) ​ This same process was used on the ocean floor - specifically, along mid ocean ridges. It was found that the magnetic records on either side of the ridge were mirror images of one another. Furthermore, the magnetic data collected from the ocean floor matched the pattern of magnetic reversals found in basalt floors on land. ​ This information led scientists to conclude that mid-ocean ridges mark boundaries where lithospheric plates are moving apart. New rock is formed by lava rising between the spreading plates. As the new rock forms, the older rocks spread farther away from the ridge on either side. This means rock closest to the ridge is youngest, while rock far away from the ridge is oldest. SES4U​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Name:​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Date: Practice! 1.​ How do observations of volcanic and seismic activity support the theory of plate tectonics? 2.​ What observations support the continental drift hypothesis? 3.​ What evidence in support of plate tectonics is provided by studies of the ocean floor? 4.​ What would you expect to find if you measured the temperature of rocks on the ocean floor at various distances from a mid-ocean ridge? Explain your thinking. 5.​ A boat uses SONAR to map out the topography of the seafloor beneath it. If the time between the sound being emitted and the echo being received is 3.5 s, how deep is the water? (Assume that the sound travels through the water with a constant speed of 1500m/s.) [ANS: 2625 m]