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Formation of continental crust • Buoyant Material rises to the surface of a molten Earth, • Continental (Felsic) material is less dense, more buoyant, therefore rises and “floats Higher” 3 bya 2.8-2.5 bya 1.6-1.2 bya 1.2 bya to 700 mya Continental Drift • Proposed by Alfred Wegener in 1912 • Hypothesis: 250 million years ago, all of the continents were combined into one super-continent called “Pangaea”, (all land). • The continents gradually drifted apart to where they are today Alfred Wegener • 1880-1930, German meteorologist • Noticed that South America and Africa look as though they could fit together like pieces of a puzzle. • proposed the hypothesis of continental drift in "On the Origin of Continents and Oceans" (1912) • Died while crossing the Greenland ice cap in 1930, exploring. Evidence for Wegener’s hypothesis. “Puzzle Pieces” • A. Continents look like they could be part of a giant jigsaw puzzle • NOTE: Shorelines don’t match perfectly, but the continental shelves do. Distribution of Fossils • B. Plant and animal fossils found on the coastlines of different continents Tie-in to Sequence of Rocks • C. Same rock patterns found in South America, India, Africa, Antarctica and Australia Geological Evidence for Continental Drift • D. geological matches – mountain belts (AppalachiansCaledonians) – age similarities (matching Archean cratons) – – stratigraphic similarities, matching layers of rock (e.g., Nova Scotia and Morocco) igneous provinces, chemical imprints (Mesozoic basalts, Precambrian anorthosites) Ancient Climates • E. Tropical plant remains (coal deposits) found in Antarctica • E(2). Glaciation in Africa, South America, India, and Australia during the same time Paleoclimate evidence Glacial striations Ice sheets covered big areas of southern hemisphere ~ 220-300 million years ago E(3). Climate Evidence Coal deposits and fern fossils in Antarctica Glacial deposits in India, S. America, Africa, Australia hint: These locations must have once been at a different latitude (further north or south!) Problems With The Hypothesis • No mechanism for movement of continents. What force could possibly drive them? • Wind and water currents could possibly move fossils. Some animals could migrate / swim. • Biblical Stories explained marine fossils on mountain peaks, as well as the existence of tropical fossils in Antarctica. • Theory was not accepted by scientists. • Glacial evidence and geologic matches, however, were stumpers. Wegener’s Hypothesis Confirmation of Continental Drift • World War II technology – mapping the ocean floor with sonar – Discovery of the match of continental shelves. They matched up better than previously thought. • Worldwide Standardized Seismic Network 1963- What do you notice? Worldwide Standardized Seismic Network Discovery of Sea-Floor Spreading Sea-Floor Spreading Seafloor Spreading: What does this mean for the evidence of the theory? Now we have a theory. What is the difference between a theory and a hypothesis? • A hypothesis is an idea, an educated guess, or a possible solution derived from logic or common sense. • A theory is an explanation that fits ALL of the facts, observations, and data. • Scientists realized there was a relationship between seafloor spreading, volcanic activity, earthquakes, mountain formation, mid-ocean ridges, the continental shelves, and deep ocean basins. The Plate Tectonics Model