Download Ch.2 Tectonics

Environmental Geology Chapter 2 -­‐ Plate Tectonics and Earth’s Internal Structure • Earth’s internal structure -­‐ Earth’s layers are defined in two ways. 1. Layers defined by composition and density o Crust-­‐Less dense rocks, similar to granite o Mantle-­‐More dense rocks, similar to peridotite o Core-­‐Very dense-­‐mostly iron & nickel 2. Layers defined by physical properties (solid or liquid / weak or strong) o Lithosphere – (solid crust & upper rigid mantle) o Asthenosphere – “gooey”&hot -­‐ upper mantle o Mesosphere-­‐solid & hotter-­‐flows slowly over millions of years o Outer Core – a hot liquid-­‐circulating o Inner Core – a solid-­‐hottest of all-­‐under great pressure • There are 2 types of crust ü Continental – typically thicker and less dense (about 2.8 g/cm3) ü Oceanic – typically thinner and denser (about 2.9 g/cm3) The Moho is a discontinuity that separates lighter crustal rocks from denser mantle below • How do we know the Earth is layered? That knowledge comes primarily through the study of seismology: Study of earthquakes and seismic waves. We look at the paths and speeds of seismic waves. Earth’s interior boundaries are defined by sudden changes in the speed of seismic waves. And, certain types of waves will not go through liquids (e.g. outer core). • The face of Earth -­‐ What we see (Observations) Earth’s surface consists of continents and oceans, including mountain belts and “stable” interiors of continents. Beneath the ocean, there are continental shelfs & slopes, deep sea basins, seamounts, deep trenches and high mountain ridges. We also know that Earth is dynamic and earthquakes and volcanoes are concentrated in certain zones. • How can we explain what we see? Continental drift-­‐An idea before its time. Alfred Wegener first proposed his continental drift hypothesis in 1915. He named a supercontinent called Pangaea (Pan-­‐jee-­‐
ah) that began breaking apart about 200 million years ago & the continents then drifted into their current positions. Evidence used in support of continental drift hypothesis • Fit of the continents – like puzzle pieces •
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Fossil evidence – fossils match up across separate continents Rock type and structural similarities – (e.g. Appalachian and Caledonian Mtns Paleoclimatic evidence •
There were objections to the continental drift hypothesis. There was a Lack of a mechanism for moving continents. Wegener incorrectly suggested that continents broke through the ocean crust, much like ice breakers cut through ice. Wegener’s hypothesis was correct in principle, but contained incorrect details and there was strong opposition to the hypothesis from all areas of the scientific community – although some thought his ideas were plausible. During the 1950s and 1960s technological strides permitted extensive mapping of the ocean floor and the Seafloor Spreading hypothesis was proposed by Harry Hess in the early 1960s. He hypothesized that the mid-­‐
oceanic ridges were spreading centers where new crust is formed. •
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Evidence for Seafloor spreading •Age of seafloor rocks: Progressively younger toward the mid-­‐oceanic ridge – evidence from drilling ocean floor •Thickness of seafloor sediments: Progressively thinner toward the ridge – evidence from drilling ocean floor •Paleomagnetic reversals – the most convincing evidence • Earth has a dipolar magnetic field • Earth’s magnetic polarity reverses periodically • Geomagnetic reversals are recorded in the iron bearing rocks igneous rocks of the ocean crust as symmetrical magnetic anomalies (stripes). • In 1963 Vine and Matthews tied the discovery of magnetic stripes in the ocean crust near ridges to Hess’s concept of seafloor spreading • Plate tectonics -­‐ A unified theory: Study the dynamic creation, movement, and destruction processes of plates. There are 7 major plates associated with Earth's strong, rigid outer layer (the lithosphere-­‐ uppermost mantle and overlying crust) The lithosphere overlies a weaker region in the mantle called the asthenosphere. The plates move relative to each other at a very slow but continuous rate and the rates are different on each plate -­‐ Average 5 centimeters (2 inches) per year (approx. growth rate of human fingernails). The rates and direction of movement changes over time. Dynamic actions are concentrated along plate boundaries: Defined by the areas of concentrated seismic and volcanic activities, rifts, faults, and mountain ridges. •
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There are 3 major types of plate boundaries • Divergent: plates moving apart and new lithosphere is produced at mid-­‐oceanic ridge. Divergent boundaries are also associated with continental rifting as two landmasses split apart due to extensional forces acting on the plates (e.g. East African rift valley). • Convergent: plates collide, subduction (Cooler, denser slabs of oceanic lithosphere descend into the mantle) and mountain building. Old crust is “crumpled” or recycled back into the mantle. There are 3 types of convergent boundaries. • Oceanic-­‐Oceanic: Formed when two oceanic slabs converge and one descends beneath the other. Volcanoes are often formed on the ocean floor. If the volcanoes emerge as islands, a volcanic island arc is formed (Japan, Aleutian islands, Tonga islands) • Oceanic-­‐Continental: Formed when ocean crust is subducted beneath continental crust. Melting adds material to the continental crust and volcanic arcs are formed on the continent (e.g. Cascades of the Pacific Northwest U.S.) • Continental-­‐continental: Continued subduction can bring two continents together and the less dense, buoyant continental lithosphere does not subduct. The resulting collision between two continental blocks produces mountains (Examples: Himalayas, Alps, Appalachians) • Transform: two plates slide past one another and no new lithosphere is created or destroyed. Most join two segments of a mid-­‐ocean ridge along breaks in the oceanic crust known as fracture zones. A few (the San Andreas fault and the Alpine fault of New Zealand) cut through continental crust. Hot Spots are caused by rising plumes of mantle material (mantle plume). The magma source may be near the core-­‐mantle boundary. Volcanoes can form over them (Hawaiian Island chain). They are generally stationary and can be located beneath continents (Yellowstone) and oceans (Hawaii). The age of volcanoes/islands (e.g. Hawaii) can be used to determine the direction and rate of plate movement. The Hawaii-­‐Emperor Seamount formed over a hot spot. The bend of this seamount chain represents a change in direction of plate motion. Paleomagnetism in rocks on the ocean floor provides a method for determining plate motions, and bBoth the direction and rate of seafloor spreading can be established • Plate motions can also be measured from space by establishing exact locations on opposite sides of a plate boundary and measuring relative motions. What drives plate motions? Researchers agree that convective flow in the mantle is the basic driving force of plate tectonics. Forces that drive plate motions include Slab-­‐pull – probably the most significant force, Ridge-­‐push and Slab suction. •
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• The Wilson Cycle – shows the cyclic nature of Plate Tectonics Tectonics builds mountains. What happens to mountains when they erode? Erosion and isostacy are interrelated. Because of isostacy, if ice is floating in water and some is removed above the waterline the ice rises so that 10% of the ice is always above the waterline. The erosion of mountains from continents result in a similar but slower “bobbing up” of the continents to “isostatic equilibrium”& with more material bobbing up, they take a long time to erode down. • Plate Tectonics & Environmental Geology –The Significance of tectonic cycle – Global zones of resources (oil, gas, and mineral ores)-­‐processes and Earth’s structures help create and localize resources in certain areas) – Global belts of earthquakes and volcanic activities & safety issues (e.g. U.S. East Coast-­‐quiet vs. West Coast-­‐active) – Geologic knowledge on plate tectonics: Foundation for urban development and hazard mitigation – Impacts on the landscape and global climates (e.g. mountainous vs. low/flat and climate change over time due to drift) WSE 8/2012