2 Wegener’s Continental Drift Hypothesis In the early 1900’s scientist Alfred Wegener developed the hypothesis of Continental DriftContinental Drift is the hypothesis that states that the continents once formed a single land mass, broke up, and drifted to their present locations.This hypothesis explained a lot including how well the continents fit together.
3 Continental DriftAlso explained why fossils of the same plant and animal species are found on continents that are on different sides of the Atlantic Ocean.In addition to fossils, similar types of rock and evidence of the same ancient climatic conditions were found on several continents.
4 The Breakup of PangaeaWegener thought that all of the present continents were once joined in a single, huge continent that he called Pangaea.We now know form the theory of plate tectonics that Pangaea existed about 245 million years ago.Pangaea also split into two huge continents, Laurasia and Gondwana, about 180 ,million years ago.These two continents split again into the continents that we know today. (65 million years ago).
5 Wegener’s Hypothesis Wegener’s hypothesis was rejected at first Scientists rejected it because from the calculated strength of rocks it did not seem possible for the crust to move this way.It was not until many years after Wegener’s death that evidence provided clues that forces moved the continents.
6 Sea Floor SpreadingMid-Ocean Ridges are underwater mountain chains that run through Earth’s ocean basinsMid-Ocean ridges are also where sea-floor spreading takes place.Sea-Floor Spreading is the process by which new oceanic lithosphere forms magma rises toward the surface and solidifies.As the tectonic plates move away from each other, the sea floor spreads apart and magma fills the gap.
7 Sea-Floor SpreadingAs new crust forms, the older crust gets pushed away from the mid-ocean ridgeTherefore, the older crust is farther away from the mid-ocean ridge than the younger crust
8 Evidence for Sea Floor Spreading Some of the most important evidence comes from magnetic reversals recorded in the ocean floorThroughout Earth’s history the north and south poles have changed places man timesWhen the poles change places, the polarity of Earth’s magnetic poles changesThis change is called magnetic reversal
9 Magnetic Reversals and Sea Floor Spreading The molten rock at the mid-ocean ridges contains tiny grains of magnetic materialsThese minerals contain iron and are like compassesThey align with the magnetic field of the EarthWhen the molten rock cools, the record of these tiny compasses remains in the rockThe record is then carried slowly away from the spreading center of the ridge as sea-floor spreading occurs
10 Magnetic Reversals and Sea Floor Spreading When the Earth’s magnetic field reverses, the magnetic mineral grains align in the opposite directionThis record of magnetic reversals was the final proof that sea-floor spreading does occur.
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