Understanding Earth's Crust Movements & Deformations: Plate Tectonics & Structural Geology, Lab Reports of Geology

Download Understanding Earth's Crust Movements & Deformations: Plate Tectonics & Structural Geology and more Lab Reports Geology in PDF only on Docsity! ESS 210 Lab 10: Plate Tectonics and Structural Geology Lab 10: Plate Tectonics and Structural Geology Objective: Students will learn about plate tectonics, which is the process that produces continents and ocean basins, plateaus and mountains, and drives deformation in the earth's crust, folds of strata, and faults. Introduction The planet can be divided into discrete layers based on either their mechanical (material) properties (how they move) or their chemical properties (what they’re made of). If we look at the chemical properties of the Earth, we can divide the planet into four sections: inner core, outer core, mantle, and crust. When we divide the layers of the Earth based on their material properties we get different divisions: the lithosphere, asthenosphere, and core. The crust and outer mantle behave more rigidly, “floating” on the more ductile (but still solid) middle mantle. This rigid exterior layer of the earth, known as the lithosphere, does not form an unbroken shell around the planet but is divided into discrete plates. Our understanding of the movements and interactions of these plates – the theory of plate tectonics – provides a framework that helps to explain many of our geologic observations including locations of mountains, volcanoes, earthquakes, different rock types and many others. Plate Tectonics Plate tectonics is a comprehensive, unifying theory that connects many aspects of geology. There are essentially two types of crust that make up tectonic plates: oceanic and continental. Oceanic crust is generally mafic in composition, more dense than the continental crust, and thinner (~6-60 km thick). In contrast, continental plates are generally felsic in composition, less dense than oceanic crust, and much thicker (~100-150 km thick). It is important to note that the boundaries between these plates do not necessarily correspond with the boundaries of continents. Tectonic plates may be composed of oceanic crust, continental crust, or combinations of oceanic and continental crust. Tectonic plates move in different directions and at different velocities, driven by heat from the planet’s interior. Because the size of the earth remains constant, as these plates move, they must interact with each other at their margins. There are three possible types of plate margins. First, there are divergent margins (Figure 10.1), where plates move away from each other and new crustal material is created between them. This new crust, formed by mafic magma rising from the underlying mantle, is generally oceanic-type crust and forms mid-ocean ridges (between oceanic crustal plates) or rift zones (between continental crustal plates). Second, there are convergent margins, where plates come together (or collide). There are three types of convergent margins depending on the type of plates involved. When an oceanic plate collides with a continental plate, one plate subducts under (goes underneath) the other. Generally, the more dense oceanic plate will go under the less dense continental plate. Continental plates collide with other continental plates sometimes and neither plate likes to subduct under the other. The result is often large mountain chains like the Himalayas. Oceanic plates can collide with other oceanic plates and usually one of these plates will subduct under the other. The south 1 ESS 210 Lab 10: Plate Tectonics and Structural Geology islands of Japan are a good example of this. Finally, there are transform margins, where plates slide past each other. These margins are basically large strike slip faults. Figure 10.1: A cross-section of a divergent plate margin at a mid-ocean ridge. Oceanic lithosphere is generated at the ridge axis, cooling and increasing in thickness as it is pushed away from the axis. The direction and speed of plate motion can be characterized in two ways: relative motion and absolute motion. Relative motion measures the velocity of the movement of one plate relative to another moving plate. Rates of relative motion across mid-ocean ridges are often measured using the magnetic reversals recorded in the oceanic crust. Since the ages of these magnetic reversals have been determined by radiometric dating, we can measure the distance between the ridge and a known (dated) magnetic reversal and calculate a rate of spreading. This rate is called a half spreading rate, since it only represents the growth on one side of the ridge, and can be doubled to determine the full spreading rate. Absolute motion measures the velocity of the movement of one plate relative to a fixed, stationary reference point deep in the earth’s interior. Absolute motion is often measured using hot spots since they are areas of igneous activity with sources in the deep mantle, below the drifting plates. The location of a hot spot is assumed to be fixed and does not change as plates move above it, so the surface expression of the hot spot is a line of volcanoes that increase in age away from the hot spot. The distance, direction, and age of a volcano with respect to its hot spot allow us to calculate the absolute speed and direction of plate movement. 2