Plate Tectonics in a Nutshell

Plate Tectonics in a Nutshell Name: ____________________________________________________ Period: _________ Date: _____________ EQ: How is Earth surface affected by plate movements? The theory of plate tectonics is a relatively new scientific concept. While its forerunner—the theory of continental drift— had its inception as early as the late 16th century, plate tectonics only emerged and matured as a widely accepted theory since the 1960s. In a nutshell, this theory states that the Earth’s outermost layer is fragmented into a dozen or more large and small solid slabs, called lithospheric plates or tectonic plates, that are moving relative to one another as they ride atop hotter, more mobile mantle material (called the asthenosphere). The average rates of motion of these restless plates—in the past as well as the present—range from less than 1 to more than 15 centimeters per year. With some notable exceptions, nearly all the world’s earthquake and volcanic activity occur along or near plate boundaries. There are two basic types of LITHOSPHERE: continental and oceanic. CONTINENTAL lithosphere has a low density because it is made of relatively light-weight granite rocks. OCEANIC lithosphere is denser than continental lithosphere because it is composed of heavier basalt rocks. A lithospheric plate may be made up entirely of oceanic or continental lithosphere, but most are partly oceanic and partly continental. 2. Beneath the lithospheric plates lies the ASTHENOSPHERE, a layer of the mantle composed of denser semi-solid rock. Because the lithospheric plates are less dense than the asthenosphere beneath them, they are floating on top of the asthenosphere. 3. Deep within the asthenosphere the pressure and temperature are so high that the rock can soften and partly melt. The softened but dense rock can flow very slowly (think of Silly Putty) over geologic time. Where temperature instabilities exist near the core/mantle boundary, slowly moving convection currents may form within the semi-solid asthenosphere. 4. Once formed, convection currents bring hot material from deeper within the mantle up toward the surface. 5. As they rise and approach the surface, convection currents diverge at the base of the lithosphere. The diverging currents exert a weak tension or “pull” on the solid plate above it. Tension and high heat flow weakens the floating, solid plate, causing it to break apart. The two sides of the now-split plate then move away from each other, forming a DIVERGENT PLATE BOUNDARY. 6. The space between these diverging plates is filled with molten rocks (magma) from below. Contact with seawater cools the magma, which quickly solidifies, forming new oceanic lithosphere. This continuous process, operating over millions of years, builds a chain of submarine volcanoes and rift valleys called a MID-OCEAN RIDGE or an OCEANIC SPREADING RIDGE. 7. As new molten rock continues to be extruded at the mid-ocean ridge and added to the oceanic plate, the older (earlier formed) part of the plate moves away from the ridge where it was originally created. 8. As the oceanic plate moves farther and farther away from the active, hot spreading ridge, it gradually cools down. The colder the plate gets, the denser (“heavier”) it becomes. Eventually, the edge of the plate that is farthest from the spreading ridges cools so much that it becomes denser than the asthenosphere beneath it. 9. As you know, denser materials sink, and that’s exactly what happens to the oceanic plate—it starts to sink into the asthenosphere! Subduction happens when a denser tectonic plate or lithospheric plate sinks into the asthenosphere. The area where one plate sinks beneath another is called a subduction zone. 10. The sinking lead edge of the oceanic plate actually “pulls” the rest of the plate behind it—evidence suggests this is the main driving force of subduction. Geologists are not sure how deep the oceanic plate sinks before it begins to melt and lose its identity as a rigid slab, but we do know that it remains solid far beyond depths of 100 km beneath the Earth’s surface. 1. 11. Subduction zones are one type of CONVERGENT PLATE BOUNDARY, the type of plate boundary that forms where two plates are moving toward one another. Notice that although the oceanic plate is sinking, the less dense continental plate floats like a cork on top of the denser asthenosphere. 12. When the subducting oceanic plate sinks deep below the Earth’s surface, the great temperature and pressure at depth cause the fluids to “sweat” from the sinking plate. The fluids sweated out percolate upward, helping to melt the overlying solid mantle above the subducting plate to form pockets of liquid rock (magma). 13. The newly generated molten mantle (magma) is less dense than the surrounding rock, so it rises toward the surface. Most of the magma cools and solidifies as large bodies of plutonic (intrusive) rocks far below the Earth’s surface. These large bodies or rocks, when later exposed by erosion, commonly form cores of many great mountain ranges [such as the Sierra Nevada (California) or the Andes (South America)] that are created along the subduction zones where the plates converge. 14. Some of the molten rock may reach the Earth’s surface to erupt as the pent-up gas pressure in the magma is suddenly released, forming volcanic (extrusive) rocks. Over time, lava and ash erupted each time magma reaches the surface will accumulate—layer upon layer—to construct volcanic mountain ranges and plateaus, such as the Cascade Range and the Columbia River Plateau (Pacific Northwest, U.S.A.). Clarifying Questions: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. Lithospheric plates are large, rigid pieces of the Earth's outer shell that float on the semi-fluid asthenosphere below. What are lithospheric plates? __________________________________________________________________ Lithospheric plates move at varying speeds, generally ranging from a few millimeters to several centimeters per year. How fast are the lithospheric plates moving? ______________________________________________________ Most earthquakes and volcanic activities occur at plate boundaries, where the movement and interaction of lithospheric plates generate geological phenomena. Where do most earthquakes and volcanic activities happen? _________________________________________ oceanic continental What are the two types of lithosphere? __________________________ and ____________________________ Continental lithosphere __________________ is less dense that oceanic lithosphere. The continental lithosphere is made up of mainly granite basalt ______________________ rocks while the oceanic lithosphere is made up of ______________________ rocks. The asthenosphere is a semi-fluid layer of the mantle beneath the lithospheric plates. What is the asthenosphere? ___________________________________________________________________ Lithospheric plates float on the asthenosphere because they are less Why do the lithospheric plates float on the asthenosphere? __________________________________________ dense than the semi-fluid rock beneath them. Rocks melt in the asthenosphere due to the high pressure and temperature, allowing them to Why do rocks melt in the asthenosphere? ________________________________________________________ soften and partly melt. Convection currents are circulating movements of a fluid (in this case, the semi-fluid asthenosphere) What is convection current? Research! ___________________________________________________________ caused by temperature differences, leading to the transfer of heat. A divergent plate boundary is where lithospheric plates move away from each other. What is a divergent plate boundary? _____________________________________________________________ The space between diverging plate boundaries is filled with magma What fills up the space between diverging plate boundaries? _________________________________________ from below. When magma comes in contact with seawater, it cools What happens to magma/lava when it comes in contact with seawater? and ________________________________ solidifies, forming new oceanic lithosphere. A mid-ocean ridge or an oceanic spreading ridge is a continuous What is a MID-OCEAN RIDGE or an OCEANIC SPREADING RIDGE? _____________________________________________ underwater mountain range formed by divergent plate boundaries where new oceanic lithosphere is created. ____________________________________________________________________________________________________ 14. What happens to the older oceanic lithospheric plates as new magma/lava is extruded into the mid ocean As new magma/lava is extruded at the mid-ocean ridge, the older oceanic lithospheric plates move away from the ridge. ridge? _____________________________________________________________________________________ colder 15. The oceanic plate gradually becomes ____________ as it moves farther away from the active mid- ocean ridge. sinks subduction The colder and denser plate ___________ into the asthenosphere in the process called _________________. Subduction is the process by which a denser tectonic plate sinks into the asthenosphere beneath another plate. 16. What is subduction? _________________________________________________________________________ The subduction zone is the area where one lithospheric plate sinks beneath another. 17. What is the subduction zone? _________________________________________________________________ The main driving force of subduction is believed to be the sinking lead edge of the oceanic 18. What is the main driving force of subduction? ____________________________________________________ plate, which pulls the rest of the plate behind it. A convergent plate boundary is where two lithospheric plates are moving toward one another. 19. What is a convergent plate boundary? __________________________________________________________ oceanic plate 20. The __________________________________ is the one that sinks into the asthenosphere because it is denser . 21. What melts the solid mantle above the subducting plate into pockets of magma? The fluids sweated from the sinking plate help melt the solid mantle above the subducting plate into pockets of magma. ___________________________________________________________________________________________ Plutonic rocks are igneous rocks formed deep within the Earth's crust by the slow cooling of magma. 22. What are plutonic rocks? ______________________________________________________________________ 23. How can this large plutonic rocks get exposed to the Earth’s surface and become mountains? Large plutonic rocks can be exposed to the Earth's surface through processes such as uplift, erosion, and tectonic activity, eventually forming mountains. ___________________________________________________________________________________________ 24. How is Earth affected by plate movements? Think!Research! Earth is affected by plate movements through various geological phenomena such as earthquakes, volcanic eruptions, mountain formation, and the shaping of ___________________________________________________________________________________________ continents and ocean basins. These movements also influence the distribution of resources, climate patterns, and the evolution of life. ___________________________________________________________________________________________ R. Angat