3Layered Earth and Plate Tectonics I. A Layered Earth a. Because deep layers of the earth are denser than the above layers, we say that the earth is density stratified. i. Solid inner core- 6,370 km deep ii. Liquid outer core- 2,900 km deep iii. Mantle – 100-200 km deep iv. Brittle crust- Surface to 100-200 km b. Evidence came from Seismic Waves generated from earthquakes i. P Waves are compressional waves ii. S Waves are transverse waves 1. S Waves cannot pass through liquid iii. During earthquakes, the waves are bent, indicating density gradient iv. SWaves do not pass through the earth’s core, indicating that it is liquid v. P Waves slow down through the core, also indicating that it is liquid II. Plate Tectonics a. Continents i. Continents are made of low density crust rock (like granite) that floats on denser crust 1. Oceanic crust is more dense than continental crust so it sits lower b. Continental Drift and Seafloor Spreading i. Continental drift is the theory that the continents move 1. Proposed by Alfred Wegener in 1912. People thought he was nuts ii. Evidence to support this came from similarities between eastern South America and Western Africa 1. Coal deposits 2. Fossils 3. Other geological formations iii. Seafloor Spreading 1. 1960- Harry Hess of Princeton and Robert Dietz of Scripps 2. Found that new crust was being created at the mid-Atlantic ridge 3. This was caused by convection currents 4. Also found old oceanic crust was being subducted or pushed under continental crust in areas called subduction zones iv. Plate tectonics is the theory that explains Continental drift. c. The lithosphere, which contains the crust, is a strong, solid outer shell riding on a weak, partially molten asthenosphere. i. The continents are raft-like inclusions embedded in the lithosphere.4ii. Over most of the planet, only a thin crust, the oceanic crust, tops the lithosphere. d. The lithosphere is broken into about twelve large rigid plates, each plate moving as a distinct unit. i. Plates spread apart along divergent junctions, typified by a rift, or a crack-like, valley. 1. The void between the receding plates is filled by melted, mobile material that rises from below the lithosphere. The material solidifies in the crack and the plates grow as they separate. a. Because new seafloor is created in this process, it is known as seafloor spreading. b. Seafloor spreading was important in the formation of the theory of plate tectonics. i. Magnetic bands found running parallel to the Mid-Atlantic ridge, must have formed at different times from the reverse magnetized bands. ii. The sea floor was therefore formed at different times in stripes parallel to the mid-ocean ridge. 2. The Atlantic Ocean provides a good example of this a. A rift formed in the middle of Pangaea, about 150 mya, which split the continent in two. b. As the land masses began to move apart from each other, the ocean flowed in between them, forming the Atlantic. c. A similar scenario is taking place in Africa right now. ii. Plates come together head-on along convergent junctions, or subduction zones, such as is found along our coast. 1. At these convergent junctions, the heavy oceanic lithosphere descends into the mantle, beneath the lighter continental lithosphere. a. An offshore trench marks downbuckling in the continental plate. b. The edge of the overriding plate is crumpled and uplifted to form a mountain chain parallel to the trench. i. In our case, this is the Coast Range. c. The denser, oceanic plate is subducted into the mantle where it melts. Because it contains lots of water, this melted rock is lighter than the rock above it and therefore rises through the continental rock, forming a chain of volcanic mountains some distance inland.5i. This is how the Cascades are formed. iii. Transform plate boundaries are boundaries in which two plates slide past each other 1. These plate boundaries are characterized by a transform, or a strike-slip fault. a. The San Andreas fault is an example of