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Geology - Plate Tectonics

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    Study Guide Geology – Plate Tectonics 1. How Plates Move What Are Tectonic Plates? Earth’s surface is made up of large pieces called tectonic plates . Most of these plates include both continental crust (land) and oceanic crust (seafloor) . Even though the plates seem solid and unmoving, they are slowly shifting over time. 1.1 Di ff erent Ways Plates Move Plates can move in three main ways, depending on how they interact with nearby plates: • Divergent boundaries (spreading centers): Here, plates move away from each other . As they separate, new crust can form between them. • Convergent boundaries: At these boundaries, plates move toward each other . When this happens, one plate may sink beneath another. • Transform boundaries: These are fault zones where two plates slide past each other in opposite directions . The plates do not create or destroy crust, but their movement often causes earthquakes.

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    Study Guide 1.2 What Happens During Subduc Ɵ on? When plates collide at a convergent boundary, oceanic crust often sinks beneath another plate. This process is called subduction and usually happens beneath continents or in deep ocean trenches. Continental crust , however, is less dense and lighter than oceanic crust. Because of this, it does not subduct easily. 1.3 Plate Interiors vs. Plate Boundaries The interior of a tectonic plate is usually calm and stable. These areas experience: • Few earthquakes • Very little volcanic or mountain - building activity However, there are some exceptions. Flood basalts and mantle plume “hot spots” can sometimes form within plate interiors. In contrast, the edges of plates are much more active. Most of Earth’s: • Earthquakes • Volcanoes • Mountain ranges occur along plate boundaries , where plates interact most strongly.

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    Study Guide 2. Types of Plate Boundaries Tectonic plates interact with each other in different ways. These interactions happen at plate boundaries , and each type of boundary produces distinct landforms, earthquakes, and volcanic activity. Let’s explore them step by step. 2.1 Convergent Boundaries: Plates Move Toward Each Other At convergent boundaries , plates collide. They may meet head - on or at an angle. There are three main types of convergent boundaries, depending on the type of crust involved. 1. Con Ɵ nent – Con Ɵ nent Convergence This occurs when two continental plates collide . • At first, the continents were separated by oceanic crust . • That oceanic crust was gradually subducted beneath one continent. • As long as subduction continued, the continent above the subduction zone developed a magmatic arc (volcanic activity). Eventually, the oceanic crust disappeared completely, and the two continents collided . Because continental crust is less dense than oceanic crust, it does not sink easily . One continent may briefly slide over the other, but soon the two plates become welded together along a complex suture zone , which marks the original collision line. This collision causes: • Thickening of the crust • Isostatic uplift • Mountain building • Thrust faulting 2. Ocean – Con Ɵ nent Convergence This happens when oceanic crust subducts beneath continental crust . • The result is an active continental margin .

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    Study Guide • Along the edge of the continent, steep andesitic mountain ranges commonly form. • Earthquakes occur along dipping Benioff zones beneath the continent. Magma Ɵ c (Volcanic) Arcs A magmatic arc is a belt of volcanoes and mountains that forms above a subduction zone. These include: • Island arcs • Inland volcanic mountain ranges These arcs are built on thickened crust , created by large underground bodies of magma called batholiths , which form when rocks partially melt above the subducting plate. Volcanic arcs form due to: • Isostatic uplift • Compressional forces • Thrust faulting Thrust faults push slices of rock inward over the continent, forming backarc thrust belts . The added weight bends the crust downward inland, creating a foreland basin . This basin fills with sediment eroded from the mountains or, if flooded, with marine sediments. 3. Ocean – Ocean Convergence This occurs when two oceanic plates collide . • One plate is subducted beneath the other at a deep ocean trench . • The trench curves toward the subducting plate. • Earthquake data show that the angle of subduction increases with depth . Subduction likely continues to depths of at least 670 km (400 miles) , where the plate probably becomes plastic .

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    Study Guide Island Arcs Above these subduction zones, andesitic volcanoes often form curved chains of islands called island arcs . • These form between the trench and nearby landmasses. • Partial melting occurs about 100 km (60 miles) deep in the asthenosphere. • Rising magma may mix with silica - rich rocks, producing andesitic magma that erupts at the surface. The distance between the trench and the island arc depends on the steepness of the subduction zone : • Steeper subduction → arc forms closer to the trench • Shallower subduction → arc forms farther away 2.2 Accre Ɵ onary Wedges, Forearc, and Backarc Regions As subduction continues: • Marine sediments slide off the descending plate. • These sediments pile up at the trench, forming an accretionary wedge (also called a subduction complex). • This wedge is slowly pushed upward, forming a ridge. Between the wedge and the volcanic arc lies the forearc basin , which is relatively undisturbed. Behind the arc, toward the continent, is the backarc region . 2.3 Backarc Ri Ō ing and Basin Forma Ɵ on Sometimes the backarc basin is stretched by new extensional forces, causing backarc 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 and moves seaward (Japan is an example).

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    Study Guide This rifting may be caused by a mantle plume rising and spreading beneath the crust, stretching it until it breaks. 2.4 Shi Ō ing Oceanic Trenches Oceanic 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 subduction zone, pulling it away and allowing the trench to shift position. 2.5 Divergent Boundaries: Plates Move Apart At divergent boundaries , plates are pulled apart by tensional forces . • The crust stretches, thins, and eventually breaks . • The central block drops down, forming a graben . • Basaltic volcanism is common along the faults. • Hot mantle material rises beneath the rift, pushing the plates farther apart.

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    Study Guide Where Divergence Occurs • Most active divergent boundaries are mid - ocean ridges , where new seafloor forms. • Divergence can also occur on land, as it did when Pangaea broke apart about 200 million years ago. 2.6 Failed Ri Ō s Not all continental rifts become oceans. • Some rifting stops before full separation occurs. • These failed rifts become basins or seas filled with sediment. An example is the Midcontinental Rift in the United States, formed about 2 billion years ago . Its rugged terrain filled with sediments and lava flows and was later buried beneath thick layers of sedimentary rock.

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