Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Supercontinent wikipedia , lookup
Cimmeria (continent) wikipedia , lookup
Cascade Volcanoes wikipedia , lookup
Baltic Shield wikipedia , lookup
Algoman orogeny wikipedia , lookup
Izu-Bonin-Mariana Arc wikipedia , lookup
Abyssal plain wikipedia , lookup
Oceanic trench wikipedia , lookup
Mantle plume wikipedia , lookup
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. 1. 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. 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.). http://volcanoes.usgs.gov/about/edu/dynamicplanet/nutshell.php Clarifying Questions: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. What are lithospheric plates? __________________________________________________________________ How fast are the lithospheric plates moving? ______________________________________________________ Where do most earthquakes and volcanic activities happen? _________________________________________ What are the two types of lithosphere? __________________________ and ____________________________ __________________ is less dense that oceanic lithosphere. The continental lithosphere is made up of mainly ______________________ rocks while the oceanic lithosphere is made up of ______________________ rocks. What is the asthenosphere? ___________________________________________________________________ Why do the lithospheric plates float on the asthenosphere? __________________________________________ Why do rocks melt in the asthenosphere? ________________________________________________________ What is convection current? Research! ___________________________________________________________ What is a divergent plate boundary? _____________________________________________________________ What fills up the space between diverging plate boundaries? _________________________________________ What happens to magma/lava when it comes in contact with seawater? ________________________________ What is a MID-OCEAN RIDGE or an OCEANIC SPREADING RIDGE? _____________________________________________ ____________________________________________________________________________________________________ 14. What happens to the older oceanic lithospheric plates as new magma/lava is extruded into the mid ocean ridge? _____________________________________________________________________________________ 15. The oceanic plate gradually becomes ____________ as it moves farther away from the active mid- ocean ridge. The colder and denser plate ___________ into the asthenosphere in the process called _________________. 16. What is subduction? _________________________________________________________________________ 17. What is the subduction zone? _________________________________________________________________ 18. What is the main driving force of subduction? ____________________________________________________ 19. What is a convergent plate boundary? __________________________________________________________ 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? ___________________________________________________________________________________________ 22. What are plutonic rocks? ______________________________________________________________________ 23. How can this large plutonic rocks get exposed to the Earth’s surface and become mountains? ___________________________________________________________________________________________ 24. How is Earth affected by plate movements? Think!Research! ___________________________________________________________________________________________ ___________________________________________________________________________________________ R. Angat Which are the types of plate boundaries where volcanic eruptions can occur? Answer 1: Volcanos are most commonly found at subduction zone boundaries. This is where the thinner oceanic plates get pulled under the thicker continental plates. As the plates go deeper, they melt and form pockets of magma which can cause mountains to be built and as the hot magma rises, it erupts out of volcanos. One area that this is very evident is along the Andes Mountains in South America. Also, the Sierra Nevada was formed this way and the granite of the Sierra Nevada was formed by the very slow cooling of silica rich magma. Volcanos are also found along spreading centers (under water and at the surface in Iceland) where the thinner oceanic crust is being pulled apart the the hot magma is bubbling up (remember heat rises). A third place is where these are weakness in the earth's crust, commonly called hot spots. Hawaii and Yellowstone are excellent examples of this. These weaknesses are not associated with any plate boundary, but are holes in the earth's crust that move as the plate they are associated with moves. That is why there is a chain of islands in Hawaii. Each island was formed as the hot spot hole allowed magma to come up and form new land above the crust. Yellowstone's movement is noted by where the active geysers are. Answer 2: Volcanism occurs regularly at two of the three types of plate Boundaries. The most volcanism occurs at diverging plate boundary where plates separate. Volcanoes also form at converging plate boundaries where one plate dives beneath the other at subduction zones. Answer 3: Usually volcanoes are found near subduction zones, where oceanic crust sinks beneath continental crust. In fact, they're quite often about 200 km inland (on the continental plate) from where the subducting plate sinks below the overriding plate. As the subducting plate sinks, it melts. That rock melt, mixed with water that is pulled down into the subduction zone with the subducting plate, erupts as a volcano at the surface.You have a nice animation to the volcano in the next link: animation Answer 4: Volcanoes occur along both divergent sea-floor-spreading plate boundaries and convergent subduction boundaries, but the volcanoes are very different in character between them (the divergent plate boundaries having fissure flow eruptions and the convergent plate boundaries making strata volcanoes). Transform plate boundaries and continent-continent collision plate boundaries do not normally form volcanoes. http://scienceline.ucsb.edu/getkey.php?key=1960