Chapter 7 Section 2 Pages 198-201 Continental Drift

Continental Drift Scientist Alfred Wegener had a theory called continental drift. His 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. Glaciers: 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 and Puzzle Theory

Wegener’s Pangaea

What is the Mid-Ocean Ridge? 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 at the Mid-Ocean Ridge 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, showing the time of formation of the Atlantic Ocean.

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.                                                                       

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.

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). 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

KEY: WEGENER’S EVIDENCE Glaciers Rocks and Fossils Mountain Belts