Earth’s Crust

Earth’s Crust Tiffany Conrad 2023 Name: _________________________________________________________ Period: ______ Date: ____________ Earth has three layers: the crust, the mantle, and the core. The crust is made of solid rocks and minerals. Beneath the crust is the mantle, which is also mostly solid rocks and minerals, but punctuated by malleable areas of semi-solid magma. At the center of the Earth is a hot, dense, mostly iron and nickel core. Earth’s layers constantly interact with each other, and the crust and upper portion of the mantle are part of a single geologic unit called the lithosphere. The lithosphere’s depth varies, and the Mohorovicic discontinuity (the Moho)— the boundary between the mantle and crust—does not exist at a uniform depth. Isostasy describes the physical, chemical, and mechanical differences between the mantle and crust that allow the crust to “float” on the more malleable mantle. Earth’s crust is divided into two types: oceanic crust and continental crust. The transition zone between these two types of crust is sometimes called the Conrad discontinuity. Silicates (mostly compounds made of silicon and oxygen) are the most abundant rocks and minerals in both oceanic and continental crust. Oceanic crust, extending 5-10 kilometers (3-6 kilometers) beneath the ocean floor, is mostly composed of different types of basalts. Oceanic crust is dense, almost 3 grams per cubic centimeter (1.7 ounces per cubic inch). Oceanic crust is constantly formed at mid-ocean ridges of the Atlantic Ocean, where tectonic plates are tearing apart or diverging from each other. As magma that wells up from these rifts in Earth’s surface cools, it becomes young oceanic crust. The age and density of oceanic crust increases with distance from mid-ocean ridges. Just as oceanic crust is formed at mid-ocean ridges, it is destroyed in subduction zones. Subduction is the important geologic process in which a tectonic plate made of dense lithospheric material melts or falls below a plate made of less-dense lithosphere at a convergent plate boundary. At convergent plate boundaries between continental and oceanic lithosphere, the dense oceanic lithosphere (including the crust) always subducts beneath the continental. In the northwestern United States, for example, the oceanic Juan de Fuca plate subducts beneath the continental North American plate. At convergent boundaries between two plates carrying oceanic lithosphere, the denser (usually the larger and deeper ocean basin) subducts. In the Japan Trench, the dense Pacific plate subducts beneath the less-dense Okhotsk plate. As the lithosphere subducts, it sinks into the mantle and melts, becoming more plastic and ductile. Through mantle convection, the rich minerals of the mantle may be ultimately “recycled” as they surface as crust-making lava at mid-ocean ridges and volcanoes.cLargely due to subduction, oceanic crust is much, much younger than continental crust. The oldest existing oceanic crust is in the Ionian Sea, part of the eastern Mediterranean basin. Continental crust is mostly composed of different types of granites. The continental crust is thicker but slightly less dense (about 2.7 grams per cubic centimeter (1.6 ounces per cubic inch). As with oceanic crust, continental crust is created by plate tectonics. At convergent plate boundaries, where tectonic plates crash into each other, continental crust is thrust up in the process of orogeny, or mountain-building. For this reason, the thickest parts of continental crust are at the world’s tallest mountain ranges. Like icebergs, the tall peaks of the Himalayas and the Andes are only part of the region’s continental crust—the crust extends unevenly below the Earth as well as soaring into the atmosphere. Continental crust is almost always much older than oceanic crust. Because continental crust is rarely destroyed and recycled in the process of subduction, some sections of continental crust are nearly as old as the Earth itself. Clarifying Questions The three layers of the Earth are the crust, mantle, and core 1. What are the three layers of earth? _______________________________________________________________ 2. Describe the layers of earth. The outermost layer of the Earth, consisting of solid rock. It is divided into oceanic and continental crust a. crust: _________________________________________________________________________________ The layer beneath the crust, composed of semi-solid rock that flows slowly over time b. mantle: _______________________________________________________________________________ The innermost layer of the Earth, consisting primarily of iron and nickel c. core: __________________________________________________________________________________ 3. What is the lithosphere? ________________________________________________________________________ The lithosphere is the rigid outer layer of the Earth, including the crust and uppermost part of the mantle _______________________________________________________________________________________________ The Mohorovicic discontinuity, or the Moho, is the boundary 4. What is the Mohorovicic discontinuity or the Moho? _________________________________________________ between the Earth's crust and mantle, marked by a change in seismic wave velocity _______________________________________________________________________________________________ oceanic crust continental crust 5. Earth crust is divided into two types: _____________________________and______________________________ The Conrad discontinuity is a minor discontinuity within the Earth's continental crust, 6. What is the Conrad discontinuity? ________________________________________________________________ marking the transition between the upper and lower crust _______________________________________________________________________________________________ 7. What are the most abundant rocks and minerals in both oceanic and continental crust? The most abundant rocks and minerals in both oceanic and continental crust are silicate minerals such as quartz, feldspar, and mica _______________________________________________________________________________________________ 8. How thick and dense is the oceanic crust? Oceanic crust is typically thinner and denser than continental crust, with an average thickness of about 5-10 kilometers _______________________________________________________________________________________________ the mid-ocean ridges world's 9. New oceanic crust are constantly formed at the _______________________ of the __________________ ocean. divergent constructive boundaries 10. The tectonic plates at the mid ocean ridges are _________________________ or ________________________. 11. Which rocks are younger the one closer to the mid ocean ridge or the one farther from the mid ocean ridge? Rocks closer to the mid-ocean ridge are younger than those farther from the ridge Think! ______________________________________________________________________________________ divergent boundaries at convergent boundaries 12. New oceanic crust is created at the _________________________ and destroyed ________________________. Subduction is the process by which one tectonic plate sinks beneath another into the Earth's mantle 13. What is subduction? __________________________________________________________________________ _______________________________________________________________________________________________ denser oceanic plate 14. In a convergent boundary, the __________________________ always subducts beneath the continental plate. an oceanic plate 15. Juan de Fuca plate is a ___________________________ plate. In the Northwestern United States, Juan de Fuca North American plate sunducts beneath the _________________________________________________________________________. 16. What happens to the lithosphere that subducts into the mantle? The lithosphere that subducts into the mantle undergoes partial melting and assimilation into the mantle ______________________________________________________________________________________________. 17. How does subduction results to the recycling of rocks? Think! Subduction results in the recycling of rocks as oceanic crust is destroyed in subduction zones, leading to the formation of new _______________________________________________________________________________________________ crust at divergent boundaries _______________________________________________________________________________________________ The oceanic crust is much younger than the 18. Why is the oceanic crust much younger than the continental crust? _____________________________________ continental crust because it is constantly being created and destroyed through the process of plate tectonics _______________________________________________________________________________________________ basalt granite 19. Oceanic crust are made mostly of ______________ while continental crust are mostly made of ______________ Orogeny is the process of mountain building, typically caused by the collision of tectonic plates 20. What is orogeny? _____________________________________________________________________________ _______________________________________________________________________________________________ 21. What are the thickest parts of the continental crust? The thickest parts of the continental crust are found beneath mountain ranges and continental plateaus _______________________________________________________________________________________________ _______________________________________________________________________________________________ 22. Why is the continental crust almost always much older than the oceanic crust?Think!______________________ _______________________________________________________________________________________________ The continental crust is almost always much older than the oceanic crust because it is not constantly recycled through subduction. Continental crust can be billions of years old, whereas oceanic crust is typically less than 200 million years old. This is because oceanic crust is continually being formed and destroyed at plate boundaries