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Transcript
2.4 Plate Tectonics and Ocean Floor Geology 1 Earth’s lithospheric crust: - is divided into large and small crustal plates - makes up the ocean floor and the continental land masses, along with the upper mantle. The oceanic crust averages 8 km deep but is dense enough to support the continental crust riding on it. 2 The continental crust averages 35 km. deep both are relatively thin. The crust covers a thick, molten, moving mantle (68% of Earth’s volume) and a heavy core, possibly composed of nickel and iron. The Earth from the Moon. The plates are moving in different directions further cracking the crust. 3 By looking at seismic activity around the world, scientists can identify the plate boundaries because of the volatility of these areas. 4 These are the major plates: North American, South American, Pacific, Eurasian, African, Nazca, Australian, and the Antarctic plate. 5 3 TYPES OF PLATE BOUNDARIES 1. Divergent – meaning to move apart ex: boundary to two oceanic plate edges a. Both plates have the same high density rock. b. The result is a vent where lava extrudes and solidifies to form new ocean floor. c. These volcanoes with rift valleys form a 40,000-long mountain ridge found in many places other than the middle of the Atlantic Ocean. ex: Mid-ocean ridge (MOR) 6 NOAA Photo Library Mid-Ocean Ridge About 15 percent of all volcanoes occur at divergent boundaries such as the midocean ridges. As the rift expands, magma flows in and may deposit enough material to build an island. Iceland is a large section of the MOR extending above the water. NOAA 7 Islands can also form in the middle of plates in places called hot spots. These are high-temperature plumes of mantle material rising from great depths to eventually melt through the crust to form volcanoes, seamounts and island arcs. NOAA When the plate over-riding the hot spot is moving, a plume will break through the plate in several places causing island chains. The Hawaiian Islands are a chain of islands formed in this way and smaller volcanic seamounts (future islands) are still submerged. 8 Hawaii’s volcano, Mauna Kea, is actually the highest mountain on Earth. It’s 31,000 ft. from the Pacific floor - although only 13,823 ft. of it are above water. Earth Observatory NASA NASA 9 Islands rising from the ocean floor are all volcanic seamounts. When enough material is accumulated, they extend out of the water. Natural islands on the continental shelf are generally from deposits of sediment, sands and coral. island Other features on the ocean floor are similar to surface topography but are larger underwater because erosion is less due to slower moving water. 10 2. Convergent - meaning to come together Ex #1: Continental to Continental boundary a. plates have the same low density rocks b. The result is that edges are forced up into mountains and seismic activity is common. ex: Himalayas, Atlas Mountains 11 Ex #2: Oceanic to Continental boundary - also convergent, where plate edges over ride each other (called subduction) a. Lighter continental plate edge may override the denser oceanic plate. b. Oceanic plate edges are subducted down into the asthenosphere and are remelted. c. About 80 percent of all volcanic activity occurs in these subduction zones. 12 d. The resulting deep valleys in the ocean lithosphere are called trenches. Trenches are so common around the boundary of the Pacific plate that the area is known as the Ring of Fire because of its seismic activity. Mt. Everest, at 29,000 ft., could fit into the deepest Pacific trench (Mariana) and still be 7,000 ft. below sea level. NASA 13 3. Transform Boundary - convergent Here plates slide past each other and as they hang, stick and jolt, earthquakes result. Mountains, like these along the San Andreas Fault, form as sediments are folded and faulted. 14 In 1912, Alfred Wegener proposed the idea that the continents were once part of a huge land mass, called Pangaea (all lands), that later broke apart. He was convinced because: 1. the continent’s shape roughly fit together like a puzzle 2. the rocks and fossils on different continents matched 15 Wegener’s theory of continental drift was ignored because of thin data – ocean exploration was still in its infancy. As a German meteorologist, he lacked credentials, and he couldn’t explain how huge continents could move. 16 1969 - The Glomar Challenger drilled a series of holes in the Mid-Atlantic ridge. What they found revolutionized ocean geology! Continental rocks date the Earth at about 5 billion years old. Since the ocean floor is lower in the lithosphere, scientists expected to find older rocks at those depths. Continental geology’s Law of Superposition states that oldest rocks are laid down first and should be found horizontally lowest in a bed unless uplifted. 17 Instead, they found no rock older than 3 billion years old and most were much younger. How could the ocean floor be younger than the continents riding on it? The Deep Sea Drilling Project showed that rocks became older as they moved away from the MOR. Oceanic geology showed rock layers are created vertically not according to the Law of Superposition. 18 Core samples revealed matching magnetic striping in rocks on both sides of MOR that were parallel distances from continents. NOAA NOAA In 1977, Project FAMOUS, using Alvin, photographed magma squeezing out of the MOR and creating new ocean floor. Today, satellites can track that the Atlantic Ocean is widening by about 1 inch per year. Seismic activity marks the edges of these plates. The sea floor is spreading - moving the continents with it. The theory of sea floor spreading explains continental drift. Wegener was right. 19