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PLATE TECTONICS Tectonic Plate = huge pieces of Earth’s crust that cover its surface and fit together at their edges. Plate Boundaries = the places where tectonic plates fit together (at the edges). There are three types of boundaries that are named for how the plates interact with one another. PLATE BOUNDARIES DIVERGENT = DIVIDE place where two tectonic plates are moving apart (or away from each other) This process is known as seafloor spreading they produce volcanoes, earthquakes, and lots of heat almost always found on the ocean floor; are rarely found on a continent In the ocean, on the ocean floor, this forms mid-ocean ridges. PLATE BOUNDARIES Mid-Ocean Ridge = long & wide mountain chains formed by divergent volcanism are very tall, but they start on the ocean floor, so they are mostly under water they can get so tall they come above the surface of the ocean’s waters PLATE BOUNDARIES On a continent, this forms rift valleys. Rift Valley long, narrow depression that forms when crust begins to separate on a continent will eventually (over millions of years) form a new ocean floor PLATE TECTONICS How were divergent boundaries discovered? In the 1960s, new technologies began to be used to study the ocean floor. What they were used to discover new features on the ocean floor This would eventually lead us to the discovery of seafloor spreading 1. Ocean floor topography was mapped using SONAR = Sound Navigation and Ranging = uses sound waves to determine depth. 2. SONAR revealed underwater mountain chains (the mid-ocean ridges) and deep depressions called deep sea trenches on the ocean floor; they run along the Earth like seams on a baseball. 3. It was also discovered that the thickness of the ocean-floor sediments increases with distance from the mid-ocean ridges PLATE TECTONICS Another new technology was the magnetometer. It measures and records magnetic fields/patterns. This would also lead us to the discovery of seafloor spreading Magnetic patterns on the seafloor showed: 1. Earth’s magnetic field reverses every few thousand years. 1. Iron rich minerals align with the Earth’s magnetic field as new rock is formed. PLATE TECTONICS A symmetrical pattern of the magnetic fields is created as the crust moves out and away from the mid-ocean ridge. These magnetic field patterns were used to figure out the age of the ocean floor in different areas. Ocean crust is young right at the middle of the mid ocean ridge, and older farther away. PLATE TECTONICS How does seafloor spreading work? New ocean crust is formed at mid-ocean ridges when magma rises and fills in the gaps when plates separate; then the magma solidifies into rock and new seafloor is created. Over time, that crust is pushed to the sides, and the oldest ocean crust is destroyed at deep-sea trenches. PLATE TECTONICS CONVERGENT = COLLIDE place where two tectonic plates are moving towards each other Subduction – when 2 plates collide, the denser plate descends (or sinks down) below the other, less -dense plate This does not occur at all convergent boundaries PLATE TECTONICS 1) Oceanic – Oceanic two ocean plates (both have high density) collide resulting in subduction of the denser plate forms a deep-sea/ocean trench on the side that subducts, and volcanic islands on the other plate PLATE TECTONICS 2) Oceanic – Continental one ocean plate and one continental plate collide (the continental plate has low density and the ocean plate has high density) resulting in subduction of the ocean plate (it’s more dense!) forms a deep-sea trench on the ocean side, and volcanic mountain ranges on the continental side PLATE TECTONICS 3) Continental – Continental two continental plates (both have low density) collide forms tall, folded mountain ranges PLATE TECTONICS TRANSFORM = SLIDE place where two tectonic plates slide past each other forms faults with shallow earthquakes PLATE TECTONICS Why do plates move? RECALL these parts of the Earth: Lithosphere -the solid, rigid outermost part of the Earth (crust + uppermost mantle). Asthenosphere - the partially melted layer of the mantle that is just underneath the lithosphere. The composition of oceanic lithosphere is basalt (heavy, dense), while the continental portion of the lithosphere is granite (light, less-dense). PLATE TE PLATE TECTONICS So, Why Do the Plates Move? There is a flowing movement in the asthenosphere called CONVECTION CURRENTS. Convection is a circulatory motion that occurs in fluids that are not the same temperature throughout. Hot and Cool areas create dif ferences in density. Convection currents occur when heated material expands & rises (because it is less dense) but then sinks when it cools & contracts (because it is more dense). HEATING = DECREASE IN DENSIT Y → FLOATS/RISES COOLING = INCREASE IN DENSIT Y → SINKS Plate Tectonics PLATE TECTONICS This movement is VERY SLOW. The movement of the tectonic plates happens because of the convection currents in the asthenosphere. The plates are moved by the moving material below. The rising mantle material spreads out at the top and forces the plates apart. (divergent, mid-ocean ridge) As the crust moves far away from the divergent boundary, it cools and sinks back down into the mantle. (convergent, subduction) PLATE TECTONICS Early Observations Late 1500’s, Abraham Ortelius, a Dutch cartographer noticed and described the apparent fit of the continents . From 1500-1800, many scientists and writers made similar observations. PLATE TECTONICS In 1912, a scientist named Alfred Wegener was the first to propose a hypothesis called Continental Drift to the scientific community. Continental Drift says: The Earth’s continents were once joined as a single landmass that broke apart and sent the continents adrift. Wegener called the supercontinent Pangaea (Greek word meaning All the Ear th) PLATE TECTONICS Pangaea began to break apart 200 MYA (million years ago) Continents have been slowing moving since then, to the positions they are in now. Wegener is important because he was the first to have more evidence than just the fit of the continents. PLATE TECTONICS Rock Formations – Layers of rock in the Appalachian Mountains (U.S.) are identical to layers of rock in similar mountains in Greenland and Europe. Fossils – Identical land-dwelling plant and animal fossils were found on widely separated continents. Glossopteris (plant) Cynognathus and Lystrasaurus Mesosaurus (freshwater reptile) Fossils were older than 200 MY PLATE TECTONICS Mesosaurus PLATE TECTONICS Climate Glossopteris (fossil) – seed fern (a plant) that grows in temperate climates. Fossils were found very wide-spread and in places that are not temperate today. Coal Deposits – were found in Antarctica; coal forms from the compaction and decomposition of ancient swamp plants (that grow in tropical climates). Wegener said that Antarctica must have been closer to the equator in the geologic past. Glacial Deposits – were found in Africa, India, Australia, and South America (all places that today are much too warm for thick ice to accumulate). PLATE TECTONICS PLATE TECTONICS During Wegener’s lifetime, his hypothesi s was not accepted. Although he had lots of suppor ting data and evidence, scientists of the time rejected the hypothesi s because of the following: Many people in the scientific community considered the continents and ocean basins to be fixed features. Hypothesis did not explain what kind of force could move such large masses over such great distances. Hypothesis did not explain how the continents were moving. How could continents move through solid ocean floor? (They did know the ocean floor is made of rock!) There were two problematic, unanswered questions: What force could move the continents? How could continents move through solid rock? PLATE TECTONICS When Seafloor Spreading was discovered in 1960 by Harry Hess, he and other scientists started to answer those questions. SONAR was used to discover the features that form at the boundaries. Magnetometers were used to discover new, young ocean crust at mid-ocean ridges and old crust at deep -sea trenches. As scientists learned more, and collected more evidence, the ideas of Continental Drift and Seafloor Spreading became the THEORY OF PLATE TECTONICS.