Lecture Outline Friday January 12 Friday January 19, 2018

Transcription

1 Lecture Outline Friday January 12 Friday January 19, 2018 Questions?? Key Points for today The structure of the Earth Introduction to Plate Tectonic Theory and Convection Chapter 2 Development of the Plate Tectonic Theory - Continental Drift and Alfred Wegner Plate Tectonics Readings Review end of Chapter 1 starting on page 25 and Chapter 2 If you are interested here is a website for you to look at: This Dynamic Earth, USGS Review Chapter 2 - Plate Tectonics: A Scientific Revolution Unfolds Plate Tectonic System - the Earth is zoned chemically as well as by strength. The forms the outer most rigid layer of the lithosphere distinguished by composition - continental or oceanic The (lithos = stone) forms the strong outer shell and includes the crust and part of the upper mantle to ~ 100km The (athenos = weak) forms a moldable or ductile solid and is the lower part of the upper mantle Deep mantle solid but hotter, contains most of volume of the Earth liquid solid Development of Plate Tectonic Theory The concept that large scale movement of continents has been around since the 16th century. Alfred Wegner (1915) Continental Drift Hypothesis Pangaea Continental Drift Hypothesis What evidence supports Wegener s hypothesis of Pangaea? Patterns of present day animal life Fossil evidence (Mesosaurus) The Theory of Plate Tectonics consists of four basic concepts. The outer portion of the earth the lithosphere, is not a shell but is broken in units called plates. There are about. The plates move! Plates act as rigid units that ride on the asthenosphere which is also in motion. Plates move at rates of a few centimeters per year. Most geologic activity (such as earthquakes and volcanoes) is concentrated near. Plate interiors are relatively quiet (less active).

2 Evidence for plates: Both earthquakes and volcanoes tend to occur near the edges of continents There are 3 major types of plate boundaries each is defined by its relative plate motion. Plates move by convection A process of from the interior of the earth to the exterior (hot material rises due to lower density and cool material sinks). Lecture Outline Wednesday January 17, 2018 Review Key Points for today How is the magnetic striping of the ocean floor formed? What does it imply about how the continents move? Know and understand the different plate boundary types and be able to identify examples from around the world Divergent boundary Convergent oceanic-continental oceanic-oceanic continental-continental What cause the plates to move? Use the above diagram to help you understand the different type of plate boundaries Types of Plate Boundaries Divergent boundaries - plates move away from one another Convergent boundaries plates move towards one another Transform boundaries plates slide past one another Hot spot (special case) stationary thermal plume burning through a plate Divergent Boundaries and the Origin of Ocean Basins from one another (tension) by the process of rifting. Sea Floor Spreading - New oceanic lithosphere is created where partially molten mantle material rises to fill the gap in the rift. Rift - A crack like valley which allows molten rock to erupt from below

3 Examples of divergent plate boundaries: Iceland, Sea of Cortez (Gulf of California) -as hot mantle material rises the drop in pressure results in melting of the rock How does the magnetic signature get locked into the rock record? As mantle material rise toward the divergent plate boundary the pressure is drastically reduced which causes melting (decompression melting). The Geodynamo system controls the Earth s magnetic field and reverses direction on average of half million years with shorter events which last thousands to 200,000 years. As new oceanic crust is formed (solidifies from molten rock) it locks in the current magnetic field, so the further away from the ridge the older the rock. The change in the magnetic polarity of the rocks indicates that the Earth s magnetic field has switched. The paired magnetic signature observed on each side of this rift is evidence of seafloor spreading. The detailed mapping of the magnetic reversals on land and in the ocean, combined with absolute age dating of the rocks allow a time scale to be developed for the ocean crust. The oldest ocean crust dates to about 200 my. The oldest Continental crust dates to about 4 by. Why the big difference? What was happening around 200my on the Earth? Where would you find Black Smokers? Does anything live down that deep? Examples of divergent plate boundaries (continental rifting): Baja California Sea of Cortez/Gulf of California How fast is crust created at the divergent boundaries? What type of plate boundary occurs off the coast of Washington and Oregon? Convergent Boundaries Plates move towards one another ( ) Collision or Subduction occurs. is where one plate sinks beneath the other plate. What determines which plate will sink? 3 types of convergent plate boundaries 1. Continental Oceanic Convergence trench

4 Subduction Zone ( ) Chain of Volcanoes along subduction zone Earthquakes Features of a subduction zone (add a few comments that will help you recognize these terms) Deep sea trench Where do earthquakes occur? Accretionary prism (wedge) Subduction zone (Benioff zone) Zone of partial melting (flux melting What causes the plates to move? Convection at a divergent boundary and is considered more of passive upwelling Bulging of the continent due to mantle upwelling gives rise to (gravity-driven mechanism) - initiates rifting (Pangea?) by cold crust as it sinks into the mantle at a convergent boundary as the subducting slab descends it induces mantle circulation pulling the plate towards the trench All are part of convection process in the mantle Examples The (Washington and Oregon). The Juan De Fuca plate is subducting under N. American plate. The (west coast of South America). The Nazca plate is subducting under the S. American plate. How deep does plate recycling occur? 2. Oceanic-Oceanic Convergence Deep sea trench Subduction Zone (consumes old oceanic crust) Chain of Volcanoes = Island Arc (Igneous Rocks) Earthquakes Examples:, Philippines, Aleutian What determines which oceanic plate will subduct? The older the crust the. 3. Continent-Continent Convergence Continental Collision - Suture No Subduction = No Volcanoes Earthquakes Rocks of any type may convert into metamorphic rocks from increased temperatures and pressures Ancient Example: formed during the collision of E. North America with Africa Active Example:. India converging with Eurasian Plate pushing the Himalayas higher Orogeny = a mountain building event

5 Lecture Outline Friday January 19, 2018 Questions?? Lecture Quiz 1 Friday January 26, 2087 Type of question: Multiple choice Identification from slides Few fill in the blank and short answer Length questions (20 pts) Covers material thru Wed. January 24 Key Points for today Know and understand the different plate boundary types and be able to identify examples from around the world Transform Special case Hot Spots Where and how are new oceanic and continental lithosphere created? How did plate tectonics shape North America? Review Transform Boundaries Plates shift past one another laterally ( ) and generates earthquakes!) Volcanoes present ( is created or destroyed) Commonly offset divergent boundaries in the ocean but also form on continents. Continental Example: Fault, CA. What is the fate of California? Will LA fall into the ocean? Do you think a portion of California is going to break off and form an island after a major earthquake as shown in some movies? So what is going to happen? Fault = Fracture in the crust Hot Spots, surface expression of volcanic activity. Result of a, a localized source of rising heat energy from the mantle-core (?) boundary. So hot and intense it will burn holes in plate and erupt at the surface Can you name two which are tourist attractions in the US?, So what s up with Iceland, Galapagos and Afar? Example: Hawaii hot spot Hawaii is located in the middle of the Pacific Plate and forms a series of islands.

6 Plate velocity can be determined by tracking the rate that plates move over a stationary hot spot. Understand how to determine the direction and rate of plate motion using Hot spots. Which way did the plate move during the formation of the Emperor Seamount Chain? Which way did the plate move during the formation of the Hawaiian Ridge? Example: Yellowstone Hot Spot Yellowstone is located in the North American Plate. A whole chain of Yellowstones have been left behind as the N America plate has moved west. Yellowstone is the largest volcano in the United States. How will the continents positions change over time will they stay as they appear today? How did the crust of the Earth form? Divergent boundaries Decompression melting forming oceanic crust Convergent boundaries (with subduction zones) Flux melting partial melting in a subduction zone This forms new land through volcanism Convergent boundaries (without subduction zones) Continents change shape during collisions by suturing landmasses together. What about transform boundaries? and Hot spots? Accretion is the process of continental growth in which buoyant fragments (accreted terranes) of crust are during plate motions Have there been other mountain building events in N. America? Give some examples: