Drifting Continents Continental Drift• (1912) Wegener proposed that continents began to break apart about 200 million years ago• He called this continental drift, where continents continue to slowly move to their present positions.• Pangaea, a Greek word that means “all the earth,” refers to the combined landmass. Wegener proposed that the continents were joined as a single landmass called Pangaea.
Drifting Continents A Rejected Hypothesis• In the early 1900s, most scientists rejected Wegener’s hypothesis of continental drift.• Two unanswered questions—what forces could move continents and how continents could move without shattering—were the main reasons that the hypothesis of continental drift was rejected.
Drifting Continents Continental Drift Evidence from Rock Formationsa) Wegener reasoned mountain ranges would have fractured as the continents separated.b) Using this reasoning, Wegener hypothesized that similar rock types are on opposite sides of the Atlantic Ocean.c) That similar groups of rocks were observed in the South America, Africa, United States, Greenland, and Europe supported Wegener’s idea.
Evidence of Continental Drift Fossil RecordSimilar animal and plant fossils were found on oppositesides of the Atlantic Ocean.
Drifting ContinentsContinental DriftEvidence from Fossils
Drifting Continents Continental DriftAncient Climatic Evidence • Coal deposits found in Antarctica. • Glacial rock deposits in Africa.
Seafloor Spreading Seafloor Spreading• Seafloor spreading states that new ocean crust is formed at ocean ridges and destroyed at deep-sea trenches.• An American scientist named Harry Hess proposed the theory of seafloor spreading. – Magma is forced toward the crust along an ocean ridge and fills the gap that is created.
Seafloor Spreading Seafloor Spreading– When the magma hardens, a small amount of new ocean floor is added to Earth’s surface. – Each cycle of spreading and the intrusion of magma results in the formation of another small section of ocean floor, which slowly moves away from the ridge.
Theory of Plate Tectonics Theory of Plate Tectonics• The theory of plate tectonics states that Earth’s crust is broken into slabs called plates.• There are a dozen major plates and several smaller ones.• Tectonic plates move in different directions and at different rates over Earth’s surface.
Theory of Plate TectonicsTheory of Plate Tectonics
Theory of Plate Tectonics Plate Boundaries• Tectonic plates interact at places called plate boundaries.• At some plate boundaries:– Plates come together, or converge– Plates move away from one another, or diverge– Plates move horizontally past one another, or transform.
Theory of Plate Tectonics Plate Boundaries (3) 1. Divergent Boundaries– Divergent boundaries are places where two tectonic plates are moving apart.– Most divergent boundaries are found in rifts, or fault-bounded valleys, which form along the axis of an ocean ridge.– A rift valley, which is a narrow depression, is created when a divergent boundary forms on a continent.
Divergent Boundaries cont…– Some of the magma forms new oceanic crust at the ridge or is forced back to the surface, forming an arc of volcanic islands that parallel the trench.– Oceanic plates are always YOUNGER than continental plates.
Theory of Plate Tectonics Plate Boundaries 2. Convergent Boundaries– Convergent boundaries are places where two tectonic plates are moving toward each other.– There are three types of convergent boundaries:a. Oceanic crust converging with oceanic crustb. Continental crust converging with continental crust.c. Oceanic crust converging with continental crust
Plate BoundariesConvergent Boundaries cont…– Subduction occurs when one of the two converging plates descends beneath the other.– A subduction zone forms when one oceanic plate, which has become denser as a result of cooling, descends below another plate creating a deep-sea trench.– The subducted plate descends into the mantle and melts.
Theory of Plate Tectonics Plate Boundaries3. Transform Boundaries– A transform boundary is a place where two plates slide horizontally past each other, deforming or fracturing the crust.– Transform boundaries are characterized by long faults and usually offset sections of ocean ridges.– The San Andreas Fault is an exception to the fact that transform boundaries rarely occur on continents.
Plate Boundaries Stress/Force Description/Facts1. Divergent TENSION -Plates move apart. -Lava creates new oceanExample: floor.Mid Ocean Ridge (MOR)2. Convergent COMPRESSION -Plates move together.a.Ocean – Continent -Ocean floor is destroyed byExample: Andes Mtns. subduction. -Mountains, volcanoes, andb. Ocean – Ocean islands can be formed.Example: AleutianIslands, Alaska. Japan.c. Continent – ContinentExample: Himalayas.3. Transform SHEARING -Plates move past eachExample: other.-San Andreas Fault, CA-MOR.
Theory of Plate Tectonics Plate BoundariesConvergent Boundaries
Activity: When Plates CreateYou need:4 Pages: Instructions, 114, 115, & 116.5 colored pencils: orange, gray (or black), blue, brown, and red.1. COLOR and LABEL puzzle pieces:a. Orange (Magma)b. Gray or Black(Oceanic Crust)c. Gray or Black (Oceanic Crust)d. Gray or Black(Oceanic Crust)e. Orange (Magma)f. Blue (water) Brown (Islands)g. Gray or Black (Oceanic Crust)h. Orange (Magma)i. Blue (water) Brown (Islands/volcanoes) Orange (Magma)2. Answer questions and STAPLE on your “Plates,” and turn in.
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