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TAMU GEOL 101 - Geology 101 Class Day 3

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Geology 101- 539Class 3-Test 1Tuesday, September 9, 2014Outline- Recap- Rates and history of plate movementso Paleo-magnetism recorded in rockso Seafloor as a magnetic tape recorder- The grand reconstructiono Seafloor isochroneso Reconstructing the history of plate movements- Mechanism of plate tectonicso Mantle convections, mantle plumesPlate Tectonics- Involves the outer portion of earth crust and topmost part ofthe mantle down to the asthenosphereo About 20 distinct plateso Tens of km in thicknesso In relative motion with respect to each other- …Plate motion causes earthquakes, builds mountain ranges, and volcanic activity3 Type of Plate Boundaries- Divergent Boundarieso Oceanic spreading center Rifting and spreading along a mid-ocean ridge createnew oceanic lithosphere Mid-Atlantic ridgeo Continental rift zone Rifting and spreading zones on continents are characterized by parallel rift valleys, volcanism, and earthquakes Great Rift Valley- Convergento Ocean-Ocean Convergence Where oceanic lithosphere meets oceanic lithosphere, one plate is sub ducted under the other and a deep sea trench and volcano is formedo Ocean-Continent convergence Peru-Chile trench When oceanic lithosphere meets continental lithosphere the oceanic lithosphere is sub ducted,and a volcanic mountain belt is formed and the continental margino Continent-Continent Convergence Where two continents converge, the crust crumples and thickens creating high mounts and a wide plateau Himalayas- Transform-Fault Boundarieso Continental Transform Fault Plates slip horizontally past each other San Andres faulto Mid-Ocean Ridge Transform Fault Mid-Ocean ridges are typically offset by transform faults Eurasian plateSuggested Video- Secrets in Stoneo The role of paleomagnetism in the evolution of plate tectonic theoryo http://online.wr.usgs.gov/calendar/2004/jul04.htmlSeafloor As a Magnetic Recorder…- Produced by the convection of the liquid Fe-Ni alloy in the outer core- Similar to the ones produced by a bar magnet or a current coil- Typeso Bar magneto D.c. energized coilo Geodynamo- NO GEOMAGNETIC FIELD-Assume during class experimento Magnetic minerals represented as magnetic needleso When lavas crystallizes each magnetic moment orientationis preservedo Random orientation of the magnetic materialso Every crystal is going to move wherever we wanto Move from liquid to solid phase, direction they were moving get frozen. The needles will point whatever direction it wants. Completely randomo Lets say a few million years pass…- …. GEOMAGNETIC FIELD PRESENT “NORMAL” DIRECTIONo Magnetic minerals orient their magnetization in the direction of the geomagnetic fieldo When lavas crystallizes each magnetic mineral remains aligned- GEOMAGNETIC FIELD PRESENT “REVERSED” DIRECTIONo New lavas preserve magnetization in the new “reversed” directiono Tape recorders are similar-  Very thin plastic ribbon with a thin layer on it, like a credit card strip. Magnetic field that preserves information. Charges one side going one direction. Plays the way the magnetic strip flows.- Paleomagnetism in a nutshello 1. Geologists examine magnetization of volcanic rocks. Their findings show reversals of Earth’s magnetic fieldo 2. Geologists then determine the absolute age of the rock. Every sample is characterized by its magnetization and ageo 3. Large number of rock samples allows geologists to construct a reversal time scale by studying many volcanoes and lava flows (age vs. magnetization)Determining Seafloor Age By Magnetics…- Paleomagnetism in a nutshello 4. Measure magnetic field offshore using ship-borne instruments. Match the magnetic patterns to those determined on land and assign its age.- Zebra stripes represent normal and reverse magnetized ocean crust rocks- Magnetic dating of the sea flooro The magnetic time scale was used to date the magnetic anomalies on the seafloor. The magnetic chrons of the last 5 million years, represented here as blue and brown bands,were named prominent scientists who studied earth’s magnetic field- Seafloor is very young…oldest seafloor is 200 Ma- Atlantic ocean is growing in size- What part of the Atlantic Ocean is the oldest?o When the Americas started splitting, which part of the ocean splitted first? North part of south part? North Part (approximately 60 million years before)- How old is the oldest part of the Pacific Ocean?o Lies West of Marianas Trench, 170-180 million years old- The plate of the Pacific Ocean is wider because it is growing faster- where are the oldest rocks on the planet?o Not in the ocean. They are youngest in the ocean. Older than 180 million years old are located on the continents.The Grand Reconstruction- Geologists use plate motions, and the study of different types of rocks in continents to reconstruct geological past…- 24% of Earth’s historyMechanism for Plate Tectonics…- Mantle Convectiono Whole Mantle Convectiono Stratified Convection- Plastic upper mantle asthenosphere- Gravity field as the driving force- Ridge push, slab pull- How:o Mid-Ocean ridgeo Gravity pushed the plate sliding off the mid-ocean ridgeo Sub ducting plateo The sinking lithospheric slab pulls on the sub ducting plateo Deep sea trench…Mantle Convection Anomalies- Hotspots and mantle plumes- Hotspot is a location on the earth’s surface that has experienced active volcanism for a long period of timeQuestion- Who proposed the theory of continental drift?o A. Charles Darwino B. Harry Hesso C. Alfred Wegenero D. Tuzo Wilson- Identify the zones below with their corresponding number on the diagramo Trencho Subducted Plateo Mountain Range- What type of plate boundary is shown in the diagram?o A. Continento B. Subduction zoneo C. Spreading Center- When a deep sea trench is ocated next to a continent, where would you expect to find active volcanoeso A. on the ocean sideo B. In the deap sea trencho C. on the continent side of the trench- Which of the following is an example of a transform plate boundary?o East African rifto San andreas - On a map of the seafloor, the boundaries between normally magnetized oceanic crust and reversely magnetized oceanic crust are called…o


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TAMU GEOL 101 - Geology 101 Class Day 3

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