Pangaea • Alfred Wegener (1912) proposed a revolutionary theory of geography. He suggested that the continents move around over time and that millions of years ago, they were all one continent (Pangaea)
Wegener suggested that Pangaea split into two continents. • The northern one, Laurasia, eventually became present day North America, Europe, and Asia. • The southern one, Gondwanaland, became present day South America, Africa, Australia, and Antarctica
Trivia • Wegener’s theory was laughed at, and discarded. He died penniless and insane.
What was Wegener’s Evidence? • 1. continental fit • Wegener traced the west coast of Africa and the east coast of South America. He found they fit like pieces of jigsaw puzzle. His argument was, they were once joined.
2. Fossils • Wegener discovered that fossils of Mozasaursu (an aquatic reptile) have been found only in Brazil and South Africa. Another fossil, glossopterus, is a plant whose remains exist in Antarctica, Australia, Argentina, and South Africa. No one plant could possibly survive in such different climates. The conclusion is that these places once were connected.
3. Rock patterns • Similar to fossil remains, the strata from sedimentary rocks in Ireland, eastern Canada, and Greenland are identical, yet different from those in other places. Furthermore, the strata in Alaska are very different from strata in Canada a few miles away.
4. Age of the sea floor • Using absolute dating, rocks on the continents seem to be several billion years old, yet the oldest rocks from the sea floor were only 200,000,000 years old. This might seem pretty old, but somehow, ocean rocks were younger than continental rocks.
Why did everybody laugh at this idea? • Wegener could never suggest a mechanism for continental drift. • Why would the continents move around? • What was pushing them?
Explanation • Magnetic polarity: mafic magma contains magnetite (Fe3O4). If molten magma cools below 600°C, the magnetite will point toward the magnetic north pole. Even if the magnetic north pole changes position, the magnetite will still point in the same direction.
Continued • Every several thousand years, the magnetic polarity of the poles reverses. In about 2000 years, our compasses will point south. • Frederick Vine sampled the rocks on either side of the Mid Atlantic Ridge (a long mountain range in the Atlantic Ocean) at 50°north latitude. The pattern of polarity reversals on either side of the ridge were mirror images. • Vine’s conclusion: Mid ocean ridges are places where new ocean floor is formed.
Current plate tectonic theory • We now believe the earth’s crust (lithosphere) is made of several patches called plates which move around. • But what makes them move???
The mantle is a very hot liquid at its bottom and a much cooler solid at its top. Because of the difference in temperature, convection currents occur in the liquid portion of the mantle (asthenosphere).
Crust (Cool) Hot magma flows out of the crust, forming new sea floor Magma cools. This pulls crust down Cools magma is more dense, so it falls Hot magma is less dense, so it rises Mantle (Hot)
As convection currents pull up on the crust, new sea floor is created. As the currents pull down on the crust, old sea floor is melted.
Types of lithospheric plates • 1. continental plates: • Composed of felsic rocks • Relatively low density • Relatively older rocks • Relatively thicker plates (32 km)
Oceanic plates • 2. Oceanic plates • Composed of mafic rocks • Relatively high density • Relatively younger rocks • Relatively thinner plates (8 km)
Most geographical features on the planet occur at plate boundaries. What features occur at plate boundaries? That depends on: • The types of plates that meet • What direction they are moving