Plate Tectonics CHAPTER 17

Transcription

1 Plate Tectonics CHAPTER 17

2 Layers of the Earth A. Crust- solid, 5-70 km thick Moho Two Types of Crust: Oceanic- ocean floor, more dense then because of more iron Continental-dry land (mostly silicates which are lighter) B. Moho- boundary between mantle and crust Lithosphere- upper part of the mantle (solid), and the crust Asthenosphere- softer layer under lithosphere C. Mantle-(lower) melted, plastic magma D. Outer Core- liquid metal E Inner Core- solid metal (iron and nickel) Moho is short for the Mohorovicic Discontinuity and is the boundary between the crust and the mantle.

3 Continental Drift: ALFRED WEGNER- (A GERMAN METEOROLOGIST) IN 1912 HE PROPOSED THE CONTINENTS WERE ONCE CONNECTED. HE NAMED IT PANGAEA (GREEK FOR ALL EARTH) HOWEVER, STATED ONLY THE CONTINENTS MOVED. HE BASED THIS OFF THE FIT OF THE CONTINENTS AS WELL AS ROCK, FOSSIL, CLIMATIC AND FOSSIL EVIDENCE. THIS BECAME THE THEORY OF CONTINENTAL DRIFT. Pangaea broke up 200 mya. The northern half of Pangaea was referred to as Laurasia and the southern portion is known as Gondwanaland.

4 Wegner s Evidence in the Rocks Wegner believed that the rocks on either side of the Atlantic should be the same. After examining mountain ranges, he found that the Appalachian Mountains were similar as mountains in Greenland and Europe. These ranges were also all formed 200mya, during the time of his proposed Pangaea.

5 Wegner s Climatic Evidence: Wegner used his background in Meteorology to support his theory. He stated the fossils of the plant Glossopteris (a deciduous forest shrub) could not have thrived on continents such as Antarctica, South America and India if those currents were at their current locations. Therefore, since fossils of the plant was found on all of these continent, the continents MUST have been closer to the equator for this plant to thrive. Fossils of Glossopteris

6 Wegner s Fossil Evidence: Wegner studied fossils from all over the world and simply stated that land-dwelling animals such as the Cynognathus and Lystrasaurus could NOT have swum across vast oceans. He also noted that the Mesosaurus was a freshwater reptile and would not have favored salt water to swim across an ocean.

7 Wegner s Glacier Deposits as evidence: Glacial deposits were discovered by Wegner in parts of Africa, Australia and South America. The deposits were located in regions that are far too warm to have glaciers today. The glacial deposits were dated to be present around 290 million years ago, which, puts them during the time of Pangaea. These glacial deposits were as thick as the deposits that are currently over Antarctica and therefore took hundreds of thousands to millions of years to be produced.

8 Wegner s Coal Deposits as Evidence: Tropical Coal Deposits were discovered in regions that are now too cold to carry this type of plants. Places like; northern North America and northern and central Europe. Therefore, these places had to be along the equator to form these deposits. Coal deposits, made accumulation of tropical swamp plants, were also found on Antarctica.

9 From Continental Drift TO Plate Tectonics! In the early 1900 s Wegner s theory was rejected by the Scientific community. Continental Drift had two major flaws: A. Wegener could provide a satisfactory force that would be powerful enough to move the earth s crust such great distances. B. Wegner could not state HOW the continents were moving. TWO Devices changed our perception of the ocean floor: A. Magnetometer- a device that can be attached to ships that can detect and record the magnetic field given off by rocks on the ocean floor. B. Sonar- showed us a picture of the topography and elevation of the ocean floor (which was always believed to by flat) so we could see that there are mountains, trenches and even volcanoes on the ocean floor. Ocean Floor Mapping Changes the world!

10 Ocean Discoveries: SONAR AND MAGNETOMETER REVEALED: A MID OCEAN RIDGE IS THE LARGEST MOUNTAIN RANGE ON THE PLANET. FORMED ALONG A DIVERGENT BOUNDARY AND IS ALMOST 50,000 MILES LONG AND PLACES ARE UP TO 1.5 MILES ABOVE SEA LEVEL! SCIENTISTS ALSO FOUND TRENCHES THAT WERE MILES DEEP. THE MARIANA'S TRENCH, CREATED FROM A SUBDUCTING PLATE, IS 6 TIMES DEEPER THAN THE GRAND CANYON. SCIENTISTS ALSO COLLECTED ROCK AND SEDIMENT FROM THE OCEAN FLOOR WHICH REVEALED TWO THINGS: 1. ROCKS WERE YOUNGER ALONG THE RIDGES (CENTER) AND OLDER ALONG TRENCHES (WHERE OCEANIC CRUST HITS CONTINENTAL CRUST). 2. THE OCEAN CRUST WAS MUCH THINNER THAN CONTINENTAL CRUST (OCEANIC= M, WHILE CONTINENTAL IS 20,000M THICK!)

11 Paleomagnetism THE EARTH S MAGNETIC FIELD IS CREATED BY THE FLOW OF MOLTEN IRON IN THE EARTH S OUTER, LIQUID CORE. MAGNETIC REVERSAL OCCURS WHEN THE FLOW OF THE OUTER CORE CHANGES, GENERALLY EVERY 10-15,000 YEARS.(IS CURRENTLY CHANGING) SINCE OCEAN CRUST IS MOSTLY BASALT (WHICH HAS A HIGH IRON CONTENT) THE MAGNETIC REVERSAL IS SHOWN EASILY. NORMAL AND REVERSE POLARITY FORM AS STRIPES ALONG RIDGES AND SCIENTISTS FOUND THAT AGES OF ROCKS ARE DIRECTLY RELATED TO THE PALEOMAGNETISM. THEREFORE, NEWER ROCKS FORM ALONG RIDGES AND OLDER ROCKS ARE FOUND ALONG THE CONTINENTS, SUPPORTING PLATE MOVEMENT. Older, subducting rock Younger newly formed rock

12 Seafloor Spreading A. SEAFLOOR SPREADING IS THE THEORY THAT EXPLAINS HOW NEW OCEAN CRUST IS FORMED AT OCEAN RIDGES AND DESTROYED AT DEEP-SEA TRENCHES. DURING SEAFLOOR SPREADING, HOTTER LESS DENSE MAGMA, IS FORCED UPWARD TOWARDS THE CENTER OF THE OCEAN RIDGE. (CONVECTION CURRENTS) THEN, THIS MAGMA COOLS WITHIN THE GAP OF THE RIDGE TO CREATE NEW OCEAN FLOOR. OLDER OCEAN FLOOR IS THEN PUSHED OUTWARD, SUBDUCTS UNDER THE LIGHTER CONTINENTAL CRUST AND MELTS INTO THE MANTLE.

13 Types of Plate Boundaries: Tectonic Plates are huge pieces of crust and rigid upper mantle that fit together at their edges to cover Earth s surface. Divergent A. DIVERGENT-WHEN TWO PLATE BOUNDARIES ARE SEPARATING AND MOVING AWAY FROM EACH OTHER. Mid ocean Ridges Rift Valleys B. CONVERGENT- WHEN TWO PLATE BOUNDARIES ARE COLLIDING OR COMING TOGETHER. Creates mountain ranges, land volcanoes or island arc (oceanic volcanoes) C. TRANSFORM- WHEN TWO PLATE BOUNDARIES SLIDE PAST EACH IN TWO DIFFERENT DIRECTIONS. Can cause large earthquakes as the plates slowly slide past each other Convergent Transform

14 Details of types of plate boundaries:

15 Tectonic Plates of the World The Ring of Fire- Created by the subducting Pacific Plate. Mt. Saint Helens- O-C Convergent San Andreas Fault (transform) Mid-Ocean Ridge and Iceland being separated by a divergent boundary Himalayan Mts. Created from C-C Convergence

16 Divergent Boundaries DIVERGENT BOUNDARIES: A. MOST DIVERGENT BOUNDARIES ARE FOUND ALONG THE SEAFLOOR IN RIFT VALLEYS. B. THE MID OCEAN RIDGE APPEARS TO BE A CONTINUOUS MOUNTAIN CHAIN ON THE OCEAN FLOOR. A. Example is the Mid Atlantic Oceanic Ridge RIFT VALLEY- IS WHEN CONTINENTAL CRUST SEPARATES, THEN THE STRETCHED CRUST FORMS A LONG NARROW DEPRESSION. Example is the African Rift Valley African Rift Valley

17 Convergent Boundaries Convergence- when two plates collide, the denser plate will be pulled by gravity and subduct. Since oceanic crust is basaltic (mostly iron) and continental crust is mostly silica and less dense, oceanic crust subducts under the lighter continental crust. A. OCEANIC-OCEANIC- Subduction creates a trench Creates island arc (volcanic island) B. OCEANIC-CONTINENTAL- When two oceanic plates converge, one is denser The denser subduct under the other oceanic plate creating volcanic islands C. CONTINENTAL-CONTINENTAL- Continental crust cannot subduct due to its low density Therefore, when two continental pieces of crust converge, they crumble and uplift creating mountains.

18 Examples of Convergent Boundaries A. Continental-Oceanic: Examples: Chilean Andes Mountains and Mariana Trench, Mount Saint Helens B. Oceanic-Oceanic: Examples: Fiji, Japan (NOT Hawaii) C. Continental-Continental: Examples: Himalayas, Appalachian

19 Transform Plate Boundary TRANSFORM BOUNDARY WERE NAMED FOR THE WAY EARTH S CRUST TRANSFORMS. CRUST IS ONLY DEFORMED OR FRACTURED SOMEWHAT ALONG TRANSFORM BOUNDARIES. HOWEVER, TRANSFORM BOUNDARIES CAN CAUSE LARGE EARTHQUAKES. Example: San Andreas Fault

20 Causes for plate motion: Convection currents- are created in the mantle from heat rising heated magma along the core and cooled magma that surfaces closer to the surface.

21 Motion of the Earth s Plates Convection currents within the earth's mantle PUSH our tectonic plates.