Chapter 7 Section 2 Pages 198-201 Continental Drift Continental Drift Scientist Alfred Wegener had a theory called continental drift. A hypothesis that states that the continents once formed a single land mass, over a LONG PERIOD OF TIME broke apart and eventually drifted to their present location. This land mass was called Pangaea. Wegener’s Theory Wegener’s hypothesis seemed to explain several things: Puzzle Theory: How well the continents fit together, like a puzzle. Fossil Pattern: Fossils of the same plant and animal species are found on continents that are on different side of the Atlantic. Rocks: Rock formations and rock dating showed that these rocks and formations were the same age, thus leading scientists to believe that they were once connected into similar rock formations, such as mountain chains. Wegener’s Theory: Evidence Fossil Evidence Fossils of the same plant and animal species are found on continents that are on different side of the Atlantic. • In Wegener's mind, the drifting of continents after the break-up of Pangaea explained not only the matching fossil occurrences but also the evidence of dramatic climate changes on some continents. For example, the discovery of fossils of tropical plants (in the form of coal deposits) in Antarctica led to the conclusion that this frozen land previously must have been situated closer to the equator, in a more temperate climate where lush, swampy vegetation could grow. Other mismatches of geology and climate included distinctive fossil ferns (Glossopteris) discovered in now-polar regions, and the occurrence of glacial deposits in present-day arid Africa, such as the Vaal River valley of South Africa. Glaciation •Wegener was aware that a continental ice sheet covered parts of South America, southern Africa, India, and southern Australia about 300 million years ago. Glacial striations on rocks show that glaciers moved from Africa toward the Atlantic Ocean and from the Atlantic Ocean onto South America. Such glaciation is most likely if the Atlantic Ocean were missing and the Continents were joined. Fossil Evidence Wegener’s Pangaea Evidence: Map Key for Class work Geologic Province Glaciation/Glaciers Mountain Belts Rock and Fossil Oceanic Crust Map of Oceanic Crust Ages in the Atlantic Since Wegener's day, scientists have mapped and explored the great system of oceanic ridges, the sites of frequent earthquakes, where molten rock rises from below the crust and hardens into new crust. We now know that the farther away you travel from a ridge, the older the crust is, and the older the sediments on top of the crust are. The clear implication is that the ridges are the sites where plates are moving apart (click on the picture at the left to see a map of the age of the ocean crust). This map shows the ages of the crustal rocks that make up the floor of the Atlantic Ocean. Red represents the youngest rocks; the deepest red marks the Mid-Oceanic Ridge, where continental plates are pulling apart and new crust is being formed. Older rocks are yellow, green, and blue: the deepest blue rocks, along the coastlines of Europe, Africa, and the Americas, are Jurassic in age, showing the time of formation of the Atlantic Ocean. •Where plates collide, great mountain ranges may be pushed up, such as the Himalayas; or if one plate sinks below another, deep oceanic trenches and chains of volcanoes are formed. Earthquakes are by far most common along plate boundaries and rift zones: plotting the location of earthquakes allows seismologists to map plate boundaries and depths (click on the picture at the right to view a map of quake epicenters). Paleomagnetic data have allowed us to map past plate movements much more precisely than before. It is even possible to measure the speed of continental plates extremely accurately, using satellite technology. Nevertheless, Wegener's basic insights remain sound, and the lines of evidence that he used to support his theory are still actively being researched and expanded. Mid-Ocean Ridge & Sea-Floor Spreading A chain of submerged mountain ranges that runs through the center of the Atlantic Ocean. Sea-floor spreading takes place along the mid-ocean ridge Sea- Floor Spreading Sea-floor spreading is the process by which new oceanic lithosphere forms as magma rises toward the surface and solidifies or hardens. As tectonic plates move away from one another, the sea floor spreads apart and magma fills in the gaps, creating new ocean floor.