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1 Plate Tectonics I. Tectonics A. Tectonic Forces are forces generated from within Earth causing rock to become. B. 1. The study of the origin and arrangement of Earth surface including mountain belts, continents, and earthquake belts. 2. Plate Tectonics a. This is the basic idea that Earths crust is divided into a few large, thick. (1) Plates are part of the move slowly and change in size. (2) Plates may be: (c) entirely rock both and rock entirely rock (3) Plate boundaries are geologically active with: (c) b. Plate Tectonics combines two preexisting ideas (1) which is the idea that the continents move freely over Earth s surface, changing their positions relative to one another. (2) which is the hypothesis that sea floor forms a mid-oceanic ridge crests and then moves horizontally away from the ridge towards oceanic trenches. II. Sea Floor Spreading A. This is the hypothesis that: 1. Was originally proposed by Harry Hess, a Princeton University geologist 2. Sea-floor forms at the 3. The sea-floor moves from the ridge crest towards and oceanic trench where it 4. The two sides of the ridge move in directions. ESC 101: Professor Vorwald Plate Tectonics Page 1 of 6

2 B. The Mid-Ocean Ridge 1. Hot mantle rock rises beneath the ridge as a result of convection. 2. This explains high heat flow and basaltic volcanic eruptions. 3. a. at the ridge crest results in cracking open of oceanic crust to form a b. shallow focus earthquakes c. (1) Major lines of weakness in Earth s crust (2) Cross the mid-ocean ridge at nearly right angles. (3) Extend for 1000's kilometers across the ocean floor. (4) The mid-ocean ridge was once continuous across the fracture zones but is now offset. d. Faults 4. Ocean (1) Mid-ocean ridges are offset along fracture zones (2) Transform motion of rocks on either side is not always in opposite directions. Rocks move in opposite directions only in the section between two segments of ridge crest. This is the only section that experiences earthquakes instead of along the entire section as would normally be expected. a. Sea floor moving away from the ridge cools b. It becomes denser and, perhaps sinking back into the mantle. c. Trenches are explained by the downward plunge of cooler rock and explains negative gravity anomalies. 5. Young Age of the Sea Floor a. Less than old. b. New sea floor continually is formed by basalt eruptions at the. (1) Basalt is carried horizontally away from the ridge crest where the rock is found. (2) Sea floor is continually destroyed by subduction into the mantle at the oceanic trenches. c. Deep ocean sediment is thin or absent at the ridge crests. ESC 101: Professor Vorwald Plate Tectonics Page 2 of 6

3 III. Causes of Plate Motion. A. Mantle 1. Heat transferred within a fluid due to density differences caused by temperature differences. 2. Source of heat: a. Heat left over from Earth s formation b. The decay of radioactive isotopes. 3. But, rock isn t a fluid so how can if convect? a. Solid rock will creep when subjected to enough heat an pressure. o b. At 1,835 F it behaves like Silly Putty. It behaves plastically like a fluid. 4. Rate of Convective Flow a. On the human scale convection is slow. The hour hand of a clock moves 10,000 times faster (about the rate at which fingernails grow) b. However, this rate is geologically fast. It takes only 58 million year to move the bottom to the top of the mantle. 5. The Pattern of Mantle Flow is not completely understood. a. There are several models... (1) Thermal Convection Model - Restricted to the upper mantle. (2) Convection throughout the entire mantle. b. In both models: (1) Spreading ridges mark the rising limbs of neighboring convection cells. (2) Trenches occur where the convection cells descend back into Earth s interior. c. Most recent model suggests that the lower mantle does not mix with the upper mantle. d. Acting like a lava lamp heat flows across the core-mantle boundary and then variations in the thickness controls mantle plumes. B. Ridge-Push and Slab Pull: Contradict convection models which assume the plates are dragged along by underlying mantle rock Push a. Spreading centers stand high on the sea floor. b. As a plate moves away from a divergent boundary it cools and thickens causing the sea floor to subside as it moves, forming the broad side slops of the ridge. c. As the asthenospheric mantle cools it thickens creating a slope. d. Lithosphere slides down. e. Due to higher elevation at the ridge, a push is imparted to the tectonic plate. ESC 101: Professor Vorwald Plate Tectonics Page 3 of 6

4 IV. 2. -Pull a. The dense, leading edge of a subducting plate pulls the rest of the plate along. b. Subducting plate s density increases due to cooling, loss of water, and phase (pressure) transitions of minerals. c. Motion is rapid along a steep slope. C. Mantle Plumes and Hot Spots 1. : Narrow columns of hot mantle rock that rise through the mantle. 2. : Regions of active volcanism at Earth s surface above plumes 3. Effects of Plumes a. Flood basalts formed when the head of a plume contacts a continent, causing uplift and eruption of basaltic lavas b. Continental Rifting (1) Causes a dome that breaks in a three-pronged pattern. (2) The plume separates the crust along two of the three fractures (3) The third fracture become inactive and eventually fills with sediment (called a failed rift or aulacogen. c. Hawaiian Islands and Emperor Seamount chain. Types of Plate Boundaries A. Boundaries 1. Plates moving from each other. 2. Marked by rifting, basaltic volcanism, and uplift. 3. Tension causes shallow-focus earthquakes along normal faults along which the crust is stretched and thinned. 4. In a continent a forms as a central valley. 5. Found at: a. ridges b. rift valleys (East African Rift) (1) After widening of the rift, eventually the plates separate and seawater floods into the linear basin between the two divergent continents. (2) Eventually opens into an ocean with a mid-ocean ridge in the center. B. Boundaries 1. One plate slides past another. 2. Sites of shallow-focus earthquakes and less likely to have volcanic activity ESC 101: Professor Vorwald Plate Tectonics Page 4 of 6

5 3. Strike-slip motion is common. 4. No new surface is formed or consumed. 5. Locations of transform motion. a. fault in California (between the Pacific Plate and the North American Plate) b. At mid-ocean ridge (not plate boundaries). C. Plate Boundaries 1. Ocean-Ocean Convergence a. Two plates capped by sea floor converge b. One plate beneath the other. (1) The subducting plate bends downward forming the outer wall of an oceanic trench (2) The trench forms a broad curve convex to the subducting plate due to Earth s rounded surface. (3) A Benioff Zone Forms o o Inclined seismic activity (angle of 30 to 60 ) is created.. Above the Benioff zone magma works its way upward and erupts creating volcanic islands of and island arc. i) String of volcanic islands ii) In a curved line parallel to the ocean trench (4) Wedge (c) Inner wall of a trench that is towards the arc Thrust faulted and folded marine sediment along with pieces of ocean crust. Snowplowed off the subducting plate by the overlying plate. (5) Basin c. Examples: Lies between the accretionary wedge and the volcanic arc. Relatively undeformed. (1) Pacific plate subducting under the western Aleutian Islands (2) Japanese Islands formed by the Pacific Plate subducting beneath the Eurasian Plate. 2. Ocean-Continent Convergence a. Oceanic crust is subducted under continental lithosphere resulting in a active continental margin. b. A zone of earthquakes dips under the edge of the continent. c. A new mountain belt is formed. ESC 101: Professor Vorwald Plate Tectonics Page 5 of 6

6 d. Magma rises forming a magmatic arc with the continent (rather than a island arc). e. Andesitic volcanism occurs f. Beneath the volcanoes are large plutons in thickened crust, seen on land as batholiths when exposed by extensive erosion. g. The more buoyant continental plate experiences intense deformation, metamorphism, and melting (1) crust thickens (2) also rises isostatically (3) thrust faults, associated with folds, move slivers of mountain-belt rocks landward over the continental interior. h. Today, this is occurring where: (1) the Juan de Fuca Plate is subducting under North America (2) the Nazca Plate is subducting under South America. i. : Evidence of an Ancient Convergent Boundary (1) Believed to be slivers representing ocean crust and upper mantle. (2) Pieces of oceanic plate that have been thrusted (obducted) onto the edge of continental plates. (3) Similar to the structure of oceanic crust, determined from seismic studies and drilling. 3. Continent-Continent Convergence a. Two continents. b. Continents become welded together along a dipping zone. c. A mountain belt forms at the interior of the new continent. Examples: (1) Himalayas between Eurasia and India (2) Appalalchians - Formed when Pangaea collided with North America d. Originally separated by an ocean floor that is being subducted under one of them, which, thus, has a volcanic arc. e. When the two continents collide, the lithosphere can t subduct. (1) It s density is too low. (2) One plate may partially slip below the other. f. Crust thickens by: (1) Original arc thickening the crust at site of impact (2) Shallow underthrusting of one continent beneath the other ESC 101: Professor Vorwald Plate Tectonics Page 6 of 6