Geology - Plate Tectonics - Quick Guides

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Study GuideGeology–Plate Tectonics1. How Plates MoveWhat Are Tectonic Plates?Earth’s surface is made up of large pieces calledtectonic plates.Most of these plates includeboth continental crust (land)andoceanic crust (seafloor). Eventhough the plates seem solid and unmoving, they are slowly shifting over time.1.1 Different Ways Plates MovePlates can move in three main ways, depending on how they interact with nearby plates:•Divergent boundaries (spreading centers):Here, plates moveaway from each other. As they separate, new crust can form between them.•Convergent boundaries:At these boundaries, plates movetoward each other. When this happens, one plate may sinkbeneath another.•Transformboundaries:These arefault zoneswhere two platesslide past each other in opposite directions. Theplates do not create or destroy crust, but their movement often causes earthquakes.

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Study Guide1.2 What Happens During SubducƟon?When plates collide at a convergent boundary,oceanic crustoften sinks beneath another plate. Thisprocess is calledsubductionand usually happens beneath continents or in deep ocean trenches.Continental crust, however, isless dense and lighterthan oceanic crust. Because of this, it doesnotsubduct easily.1.3 Plate Interiors vs. Plate BoundariesTheinterior of a tectonic plateis usually calm and stable. These areas experience:•Few earthquakes•Very little volcanic or mountain-building activityHowever, there are some exceptions.Flood basaltsandmantle plume “hot spots”can sometimesform within plate interiors.In contrast, theedges of platesare much more active. Most of Earth’s:•Earthquakes•Volcanoes•Mountain rangesoccuralong plate boundaries, where platesinteract most strongly.

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Study Guide2. Types of Plate BoundariesTectonic plates interact with each other in different ways. These interactions happen atplateboundaries, and each type of boundary produces distinct landforms, earthquakes, and volcanicactivity. Let’s explore them step by step.2.1 Convergent Boundaries: Plates Move Toward Each OtherAtconvergent boundaries, plates collide. They may meet head-on or at an angle. There arethreemain typesof convergent boundaries, depending on the type of crust involved.1. ConƟnent–ConƟnent ConvergenceThis occurs whentwo continental plates collide.•At first, the continents were separated byoceanic crust.•That oceanic crust was graduallysubductedbeneath one continent.•As long as subduction continued, the continent above the subduction zone developed amagmatic arc(volcanic activity).Eventually, the oceanic crust disappeared completely, and thetwo continents collided.Because continental crust isless densethan oceanic crust, it doesnot sink easily. One continentmay briefly slide over the other, but soon the two plates becomewelded togetheralong a complexsuture zone, which marks the original collision line.This collision causes:•Thickening of the crust•Isostatic uplift•Mountain building•Thrust faulting2. Ocean–ConƟnent ConvergenceThis happens whenoceanic crust subducts beneath continental crust.•The result is anactive continental margin.

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Study Guide•Along the edge of the continent, steepandesitic mountain rangescommonly form.•Earthquakesoccur along dippingBenioff zonesbeneath the continent.MagmaƟc (Volcanic) ArcsAmagmatic arcis a belt of volcanoes and mountains that forms above a subduction zone. Theseinclude:•Island arcs•Inland volcanicmountain rangesThese arcs are built onthickened crust, created by large underground bodies of magma calledbatholiths, which form when rocks partially melt above the subducting plate.Volcanic arcs form due to:•Isostatic uplift•Compressional forces•Thrust faultingThrust faults push slices of rock inward over the continent, formingbackarc thrust belts. The addedweight bends the crust downward inland, creating aforeland basin. This basin fills with sedimenteroded from the mountains or, if flooded, with marine sediments.3. Ocean–Ocean ConvergenceThis occurs whentwo oceanic plates collide.•One plate is subducted beneath the other at a deepocean trench.•The trench curves toward the subducting plate.•Earthquake data show that theangle of subduction increases with depth.Subduction likely continues to depths of at least670 km (400 miles), where the plate probablybecomesplastic.

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Study GuideIsland ArcsAbove these subduction zones,andesitic volcanoesoften form curved chains of islands calledisland arcs.•These form between the trench and nearby landmasses.•Partial melting occurs about100 km (60 miles)deep in the asthenosphere.•Rising magma may mix with silica-rich rocks, producingandesitic magmathat erupts at thesurface.The distance between the trench and the island arc depends on thesteepness of the subductionzone:•Steeper subduction→arc forms closer to the trench•Shallower subduction→arc forms farther away2.2 AccreƟonary Wedges, Forearc, and Backarc RegionsAs subduction continues:•Marine sediments slide off the descending plate.•These sediments pile up at the trench, forming anaccretionary wedge(also called asubduction complex).•This wedge is slowly pushed upward, forming a ridge.Between the wedge and the volcanic arc lies theforearc basin, which is relatively undisturbed.Behind the arc, toward the continent, is thebackarc region.2.3 Backarc RiŌing and Basin FormaƟonSometimes thebackarc basinis stretched by new extensional forces, causingbackarc rifting.•A small spreading center may develop.•The island arc can be pushed away from the continent.•In some cases, part of the continent splits off andmoves seaward (Japan is an example).

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Study GuideThis rifting may be caused by amantle plumerising and spreading beneath the crust, stretching ituntil it breaks.2.4 ShiŌing Oceanic TrenchesOceanic trenches do not stay in one place forever.•The overlying plate pushes the trench back over the subducting plate.•This happens due to tectonic forces and gravity.•Some geologists suggest the sinking plate descends at a steeper angle than the subductionzone, pulling it away and allowing the trench to shift position.2.5 Divergent Boundaries: Plates Move ApartAtdivergent boundaries, plates are pulled apart bytensional forces.•The crust stretches, thins, and eventuallybreaks.•The central block drops down, forming agraben.•Basaltic volcanismis common along the faults.•Hot mantle material rises beneath the rift, pushing the plates farther apart.

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Study GuideWhere Divergence Occurs•Most active divergent boundaries aremid-ocean ridges, where new seafloor forms.•Divergence can also occur on land, as it did whenPangaea broke apartabout 200 millionyears ago.2.6 Failed RiŌsNot all continental rifts become oceans.•Some rifting stops before full separation occurs.•Thesefailed riftsbecome basins or seas filled with sediment.An example is theMidcontinental Riftin the United States, formed about2 billion years ago. Itsrugged terrain filled with sediments and lava flows and was later buried beneath thick layers ofsedimentary rock.

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