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Transcript
PLATE TECTONICS Chapter 17 Drifting Continents Although it is difficult to imagine, the Earth’s surface is constantly changing. – South America is moving away from Africa at a rate of 2-3 cm per year. – What makes up the Hawaiian Islands are migrating northwest at a rate of 8-9 cm per year! – In addition to this, Earth’s highest point (Mt. Everest) continues to slowly rise. When continents slowly move to their current positions, we call this continental drift. Early Observations Early mapmakers first considered the idea of moving landmasses because they found many of the continents had matching coastlines. • 1500’s - Abraham Ortelius (Dutch) noticed fit of continents surrounding Atlantic Ocean. Incorrectly proposed that N America and S America were separated from Europe and Africa by earthquakes and floods. • 1800’s – Eduard Suess (Austrian) proposed southern continents had once been joined by a single landmass. • 1900’s – Alfred Wegener (German) hypothesized the theory of continental drift for the first time in 1912. Continental Drift The theory of continental drift proposed that Earth’s continents were once joined as a single landmass, Pangaea. • Pangaea is a Greek word that means “All Earth.” • Pangaea is proposed to have begun breaking apart approximately 200 million years ago. • Continents then have slowly moved to their current positions. Beginning with a view of Earth with the continents in their present positions, the continents move back in time to reunite as Pangaea, are labeled, and locations of fossil evidence that Wegener used to argue in favor of continental drift are added. Evidence - Rock Formations Wegener reasoned that as Pangaea broke apart, large geologic structures would have fractured. He used this information to hypothesize that there should be similar rock bands separated by oceans. He found this to be true as evidenced by: 1. Appalachian Mountains share similar features with those found in Greenland and Europe as well 2. Other similar groups of rocks were found in South America and Africa, whose coastlines also match almost perfectly Evidence - Fossils Similar fossils of several different animals and plants that once lived on land had been found on widely separated continents. Support is justified as land animals could not have possibly swam across large bodies of water. Glossopteris (fossil fern) representing continental drift due to finding these on several continents despite climates. Reasoned that as ferns require temperate climates, these areas had to have been closer to the equator at one time. Based on his strong meteorological background, he indicated that rocks joining these fossils had to once be joined as he reasoned that the area separating the fossils was too large to have a single climate. Evidence - Climate Evidence of large climate changes were found on some continents: 1. Coal deposits (requires temperate, swampy areas) were found on Antarctica dictating a different climate that we know today. 2. Glacial deposits (estimated to be 290 million years old) were found in Africa, India, Australia and South America suggesting the areas were once covered by thick ice caps. Suggestions for Glacial Deposition Wegener proposed continents were once located near the south pole, before Pangaea began to fracture, making suggestions that: 1. 2. South Pole had shifted position Landmasses had drifted away from the pole (preferred as it is doubtful the Earth changed its axis of rotation Flaws to the hypothesis of Continental Drift 1. Wegener could not satisfactorily explain what was actually causing the continents to move. He suggested the rotation of the Earth; however, physicists were able to show this force was not great enough to move continents. 2. Wegener could not satisfactorily explain how the continents were moving. He proposed that landmasses were plowing through the ocean floor; however, scientists quickly ridiculed this thought and were able to argue that there would be evidence of fracture on the ocean floor… and there was not. Wegener, unfortunately, left these two questions unanswered despite the impressive collection of evidence he provided to support his hypothesis. He did not give up, working to prove his hypothesis to the scientific community of his time. In 1930, he froze to death on a meteorological expedition to Greenland – still firmly believing in the idea of continental drift. The Driving Force of Continental Drift Shortly after Wegener’s passing, a scientist named Arthur Holmes proposed mantle convection as a driving force of continental drift, providing a simplistic version of the idea of convection of the mantle. This was largely ignored by the scientific community for roughly 30 years. Arthur Holmes 1890-1965 Technology Assistance Until the mid-1900’s, most scientists thought that the ocean floor, unlike the continents, was essentially flat. Many people had the misconceptions that oceanic crust was unchanging and was much older than continental crust. During the 1940’s and 1950’s however, technology advancements proved all of these widely accepted reasons to be wrong – largely in part to WWII and submarine warfare. These tools included the following: • Sonar – an echo sounding technique used to map topography. Travel times of waves were used to determine distances to the ocean floor. • Magnetometer – Device used to detect small changes in magnetic fields, originally developed to detect magnetic fields generated by submarine hulls. Additional use of these devices, during more peaceful times, revealed seafloor bathymetry (study of depth or floors of water bodies), which led to the discovery of deep-sea trenches and mid-ocean ridges. These underwater mountain chains and deep depressions went on to puzzle geologists for over a decade after their discovery. Continued Discovery As tools continued to be utilized on the ocean floor, scientists turned their attention to testing rocks and sediments. Findings: • • • Rocks and sediments found near the midocean ridges are younger than those found near deep-sea trenches Oceanic crust rocks are much younger than continental crust rocks Paleomagnetism (study of magnetic record of Earth’s rocks) proved that magnetic field reversal occurs over time, with magnetic patterns being symmetric around the midocean ridges Upon new findings of the ocean floor (to include topographic, sedimentary, age and magnetic data) an American scientist came up with a theory to explain all prior observations pertaining to continental drift. Harry Hess 1906-1969 Seafloor Spreading Hypothesis Harry Hess proposed that new crust was being produced at mid-ocean ridges (mantle upwelling) and consumed (mantle downwelling) at deep-sea trenches based on mantle convection. Each cycle of spreading and intrusion then resulted in the formation of another small section of ocean floor, which slowly moves away from the mid-ocean ridge So, continents are not pushing through the ocean crust but are instead moving with the ocean crust as it pushes away from the ridge Seafloor Spreading Animation Theory of Plate Tectonics The Earth’s lithosphere is broken into large slabs of rock, called plates, which moves in different directions at different speeds. These plates interact with one another at plate boundaries. Each type of boundary has specific characteristics and processes associated with it. Divergent Boundaries Characteristics: • Two tectonic plates move apart, resulting in thinning of the crust and creation of new oceanic crust. • Most are found on the seafloor, where they form mid ocean ridges. • The formation of new ocean crust accounts for high heat flow, resulting in volcanism and earthquakes. • Some create rift valley’s, which occur on land creating long, narrow depressions. • Examples: Mid-Ocean Ridge, Great African Rift Valley Convergent Boundaries Characteristics: • • Two plates come together resulting in thickening of the crust, recycling of oceanic crust into the mantle, and growth of continental crust Three types: 1. Continental / Continental 2. Oceanic / Oceanic 3. Continental / Oceanic • Result in: 1. Very tall, uplifted mountain ranges 2. Subduction Zones, deep sea trenches, volcanic island arc 3. Subduction Zones, trenches, and volcanic island arcs along continental plate • Examples: Himalayas (1), Western Aleutian Islands (2), and the Western Coast of South America (3) Continental / Oceanic Convergence Transform Boundaries Characteristics: • Two plates slip past one another resulting in no net gain or loss of crust • Plates can move in same direction, with one moving at a faster rate • The Earth’s crust is only deformed, not created or destroyed • Results in long faults and/or shallow earthquakes • Most are offset sections of ocean ridges, rarely found on continents • Examples: San Andreas Fault (CA) Causes of Plate Motions – Confirmed! The transfer of thermal energy by the movement of heated matter is called convection. The heating of matter causes it to expand and decrease in density. As it warms, it rises (buoyancy). The cooler part then sinks (gravity). This up and down pattern is a convection current. In the mantle, convection currents are set in motion by the transfer of energy between the Earth’s hot interior and its cooler exterior. Ridge Push / Slab Pull The upper part of the mantle, the asthenosphere, can flow like a soft plastic, which ultimately drives plate movement. During formation of ocean ridges, mantle forces cause the asthenosphere to rise and push oceanic plates toward trenches (ridge push) At subduction zones, a sinking region of a convection current could then suck an oceanic plate downward into a subduction zone, wherein the trailing lithosphere is ultimately pulled into the subduction zone (slab pull) Plate Tectonics - Unanswered Questions Most scientists agree that convection currents in the mantle are related to the movement of plates. Even today, scientists seek answers to the following questions in regard to how these currents originate: 1. Are mantle convections currents permanent features? 2. 3. 4. 5. Do they shift their positions through geologic time? How does a convection current start? What causes the movement to stop? Does convection only take place in the upper mantle, or do they occur in the lower mantle as well? 6. Does the convection current cause subduction or vice versa? 7. What causes an upward convection current to form beneath a continent?