Geology Class, Week 2 Notes (Week of 8/29/16)
Geology Class, Week 2 Notes (Week of 8/29/16) Geos 1113
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This 4 page Class Notes was uploaded by Brandon Notetaker on Monday September 5, 2016. The Class Notes belongs to Geos 1113 at University of Arkansas taught by Mohamed Aly in Fall 2016. Since its upload, it has received 52 views. For similar materials see General Geology in Geology at University of Arkansas.
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Date Created: 09/05/16
Intro to Geology Review Week 2 (8/29/16) 1. Difference between Physical and Historical Geology a. Physical – The processes that shape the Earth over time b. Historical – How we can reconstruct the record of the change in Earth 2. Important component of Geology a. Earth is changing 3. Difference between Hutton’s idea of Uniformitarianism and the idea of Catastrophism a. Uniformitarianism – Requires change over long periods of time b. Catastrophism – Requires the view of a young Earth that has rapid change 4. Thickest zone on Earth a. Geosphere 5. Mud and worms on the edge of the lake a. Example of Combination of Spheres 6. Nebular Hypothesis a. All planets are formed at the same time, and out of the same matter as the sun 7. Inner Planets a. Contain heavy elements because it is very hot and a weak gravitational field that cannot hold lighter elements 8. Chemical Differentiation a. Led to the formation of layers in earth based on density of the materials that make up those layers 9. Mapping internal structure of earth a. Use Seismic waves caused by Earthquakes 10. Ocean floor vs. Continental floor a. Continents are less dense than the ocean floor and both float on weak mantle material. Plate Tectonic Notes Week 2 (8/29/16) 1. Continental Drift – 200 million years ago there was a supercontinent, named Pangea. a. A German meteorologist Alfred Wegener explain continental movement with this hypothesis. This idea was initially rejected by North American geologists. 2. Evidence a. Continental Jigsaw puzzle – All the continents on Earth seem to fit together b. Fossil evidence identical fossil organisms are found on continents now separated by vast oceans c. Rock types and geologic features – Identical land features seen on separate continents d. Ancient climates – Similar historical climates 3. Objections a. Wegener incorrectly suggested that the gravitational forces of the Moon and Sun were capable of moving the continents b. Wegener also incorrectly suggested that continents broke through the ocean crust c. Wave erosion and coastal deposition have changed the shape of the continents 4. Plate Tectonics a. Lithosphere – Strong outer layer of the Earth. Thicker the farther it gets from the midocean ridge. b. Asthenosphere – Dethatched from the lithosphere, the asthenosphere is a hotter, weaker region of the upper mantle (under the lithosphere) c. Earth’s Major Plates – The lithosphere is broken into approximately two dozen smaller sections called lithospheric plates – These plates are in constant motion 5. Plate Boundaries – Where most interactions among individual plates a. Divergent plate boundaries (constructive margins) plates move apart, and a new ocean floor is created. Located on the crust of mid oceanic ridges. Warmer material near the ridge is less dense, so floats higher on the mantle. Where Ridge Push is most important. i. Seafloor spreading the mechanism that operates along the ridge to create new ocean floor (Average spreading rate is 5 cm/year). ii. Continental Rifting – Forms new ocean basins when a divergent plate boundary occurs within a continent. A landmass will split into two or more smaller segments. Ex: East African Rift Valley b. Convergent plate boundaries (destructive margins) Two plates move toward each other, and the older (the colder / the denser) portions of oceanic plates are returned to the mantle. The leading edge of one plate is bent downward, as it slides beneath the other at subduction zones. i. Oceanic Continental Convergence denser oceanic slab sinks beneath the continental block. Results in continental volcanic arc. Ex: North American & Juan de Fuca Plates ii. Oceanic–oceanic convergence – When two oceanic slabs converge, one (the older / the colder / the denser) descends beneath the other. Results in volcanic island arc. Ex: Pacific & Philippine Plates iii. ContinentalContinental Convergence – Two continents come together through subduction producing mountains. Ex: Indian & Eurasian Plates c. Transform plate boundaries or Transform Fault (conservative margins) Plates grind past each other without creating or destructing the lithosphere. Majority are on the ocean floor offsetting segments of oceanic ridge 6. The Future of Plate Tectonics a. The total surface of Earth does not change, despite movement along plate boundaries – Plates can grow or shrink depending on the plate boundaries surrounding each plate that can also be created or disappear. b. Geologists use present plate motions to predict what will happen in the future. Baja and southern California will eventually slide past the North American Plate Africa will continue to collide with Eurasia i. Evidence from Ocean Drilling – The oldest sediments are furthest from the spreading center. Thickness of oceanfloor sediments verifies seafloor spreading. The ocean floor is indeed youngest at the ridge axis. ii. Evidence from Hot Spots and Mantle Plumes – A mantle plume is a cylindrically shaped volcanic upwelling. As a plate moves over this hot spot, a chain of volcanoes, known as a hotspot track. The age of each volcano indicates the time that passed since the volcano was over the mantle plume. Radiometric dating of the Hawaiian Islands shows that volcanic activity increases in age, and moves away from the Big Island of Hawaii. iii. Evidence from Paleomagnetism – Basaltic rocks contain magnetite, an ironrich mineral affected by the Earth’s magnetic field – When the basalt cools below the Curie point (~585 °C), the magnetite aligns toward the position of the north pole – The magnetite is then “frozen” in position and indicates the position of the north pole at the time of rock solidification. iv. Evidence from Polar Wandering – The movement of magnetic poles indicates that the continents have moved. v. Evidence from Magnetic Reversals and Seafloor Spreading – During a magnetic reversal, the Earth's magnetic field periodically reverses polarity. Rocks that exhibit the same magnetism as the present magnetic field exhibit normal polarity • Rocks that exhibit the opposite magnetism exhibit reverse polarity – The magnetic time scale shows the sequence of shifts in the polarity of Earth’s magnetic field determined from lava flows 7. Ocean Floor as a Magnetic Recorder a. Magnetic intensities are recorded when a magnetometer is towed across a segment of the oceanic floor. i. Geologic Evidence for Plate Motion – By knowing the age of the seafloor and the distance from the spreading center, an average rate of plate motion can be calculated ii. Measuring Plate Motion from Space – Accomplished by using the Global Positioning System (GPS) to measure various points on Earth’s surface – GPS data are collected over various points repeatedly for many years to establish plate motion 8. Forces That Drive Plate Motion a. Convective flow in the mantle: researchers agree this is the basic driving force of plate tectonics b. Slabpull force: the subduction of cold oceanic lithosphere. Contributes the most. c. Ridgepush force: elevated lithosphere on an oceanic ridge will slide down due to gravity 9. Models of Plate – Mantle Convection a. Wholemantle convection model: cold oceanic lithosphere descends into the lowermost mantle, while two types of hot mantle plumes transport heat toward the surface. b. Layercake model: consists of two convection layers — an upper dynamic convective layer (~660 – 1000 km depth) overlying a sluggish layer below.
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