PHSC 1011 Fall 2016 L05 1. Plate Tectonics. This lecture will help you understand: Chapter 21 Lecture

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1 Chapter 21 Lecture Plate Tectonics This lecture will help you understand: : Folds and Faults L05 1

2 Plate tectonics is the unifying theory that explains the dramatic, changing surface features of the Earth. Earth's lithosphere is divided into 7 large plates + medium sized and smaller ones. The continents move because they are embedded within the drifting plates. Plates are sections of Earth's strong, rigid outer layer the lithosphere. Plates consist of uppermost mantle and overlying crust. Plates overlie and ride atop the weaker asthenosphere. L05 2

3 7 major lithospheric plates Plates are in motion and continually changing in shape and size Largest plate is the Pacific Plate Several plates include an entire continent plus a large area of seafloor Earth's plates move in different directions and at different speeds. Continental plates tend to move slowly. Oceanic plates tend to move faster. CHECK YOUR NEIGHBOR Continental plates tend to move slower than oceanic plates, because A. their roots extend deep into the mantle. B. they are heavier. C. they are convergent. D. of gravity. L05 3

4 Interactions between plates occur along plate boundaries. Creation and destruction of lithosphere occurs along plate boundaries. Earthquakes, volcanoes, and mountains occur along plate boundaries and sometimes along former plate boundaries. Map of Earthquake Epicenters, Source: Three Types of Plate Boundaries Divergent Plate Boundaries Magma generation and lithosphere formation Convergent Plate Boundaries Magma generation and lithosphere destruction Transform Fault Boundaries No magma generation, no formation or destruction of lithosphere L05 4

5 Divergent Boundary Features Plates move away from one another As plates move apart, asthenosphere rises and partially melts to form lava New crust is formed as lava fills in the gaps between plates In the ocean, seafloor spreading Mid-ocean ridge On land, continents tear apart Rift valley Shallow earthquakes Convergent Boundary Features Plates move toward each other Oceanic crust is destroyed Continental crust is deformed Deep earthquakes Types of Convergent Boundaries Oceanic-oceanic convergence: Older and denser plate descends beneath the other (subduction). As plate descends, partial melting of mantle rock generates magma and volcanoes. If volcanoes emerge as islands, a volcanic island arc is formed (Japan, Aleutian islands, Tonga islands). L05 5

6 Types of Convergent Boundaries Oceanic continental convergence: Denser oceanic plate subducts beneath the less-dense continental plate. As plate descends, partial melting of subducting rock generates magma. Mountains produced by volcanic activity from subduction of oceanic lithosphere and compression from convergence are called continental volcanic arcs (Andes and Cascades). Types of Convergent Boundaries Continental continental convergence: Continued subduction can bring two continents together. Less dense, buoyant continental lithosphere does not subduct. The result is a collision between two continental blocks. The process produces mountains (Himalayas, Alps, Appalachians). Types of Convergent Boundaries Continental Continental Convergence The continent to continent collision of India with Asia produced and is still producing the Himalayas Sites of the deepest, strongest earthquakes L05 6

7 Transform Plate Boundaries Plates slide past one another and no new lithosphere is created or destroyed Most transform faults join two segments of a mid-ocean ridge Transform faults are oriented perpendicular to mid-ocean ridge Permits plates to move from offset ridge segments Shallow but strong earthquakes Transform- Fault Boundaries Most are located within ocean basins. A few transform fault boundaries, such as the San Andreas Fault, cut through continental crust. There are three type of stress caused by interactions between plate boundaries: Compressional stress slabs pushed together Tensional stress slabs pulled apart Shear stress slabs are both pulled and pushed sliding L05 7

8 Rocks respond to stress in 3 ways: Elastic deformation returning to original shape Brittle deformation breaking Plastic deformation flowing Folds Syncline: Layers tilt in toward a fold axis. Anticline: Layers tilt away from axis. Faults Classified by relative direction of movement (displacement). Footwall Hanging wall L05 8

9 Normal Fault The hanging wall drops down relative to the level of the footwall. Reverse Fault The hanging wall is pushed up relative to the footwall. Strike Slip Fault Blocks of rock slip past one another with very little vertical displacement horizontal movement. Ex: San Andreas Fault. L05 9

10 : Faults CHECK YOUR NEIGHBOR Which type of fault created the state of Nevada? A. Reverse fault B. Normal fault C. Strike-slip fault Earthquakes The plate tectonics model accounts for the global distribution of earthquakes. About 80% of the world's big earthquakes occur in subduction zones of the Ring of Fire. Earthquakes An earthquake can occur on or between plate boundaries. Strain begins at depth as elastic deformation. When the build-up of stress exceeds the rock's elastic limit, the rock breaks. This is how a fault forms. L05 10

11 Earthquake Measurement The Richter magnitude scale measures the energy released in terms of ground shaking. Determined from the logarithm of the amplitude of waves recorded by a seismograph a 1-unit increase on the Richter scale is a 10-fold increase in amplitude. The Moment magnitude scale measures the energy radiated from the earthquake's focus. For each 1-unit increase in magnitude, the seismic energy released by an earthquake increases by about 30 times. L05 11

12 Tsunami A giant sea wave, or series of sea waves, generated by a powerful disturbance that vertically displaces the water column. Reverse fault earthquakes thrust the seafloor upward. Huge, displaced mass of water drops back down to sea level and a large wave is generated. L05 12