The document outlines the theory of continental drift proposed by Alfred Wegener, suggesting that all continents were once a single landmass called Pangea and have since moved to their current positions, supported by fossil and geological evidence. It then discusses Harry Hess's theory of seafloor spreading, which explains how oceanic plates move away from mid-ocean ridges due to rising molten material, leading to the formation of new crust. Finally, the text introduces the combined theory of plate tectonics, detailing how Earth's lithosphere is divided into plates that move on the asthenosphere due to convection currents, with various interactions occurring at plate boundaries.

1. Plate Tectonics Section 1: Continental Drift

2. Alfred Wegener suggested that all continents were joined together at some time in the past. This is called the hypothesis of “ continental drift” . According to continental drift, continents have moved slowly to their current locations. He called this large landmass as Pangea (means all land)

4. Besides the puzzlelike fit of continents, fossils provided support for continental drift. For example; fossils of reptile Mesosaurus have been found in south America and Africa.

5. Another fossil that supports the continental drift is Glossopteris which has been found in Africa, Australia, India, South America, and Antarctica.

6. Climate Clues Fossils of warm weather plants were found in arctic ocean. Glacial deposits and rock surfaces scoured and polished by glaciers are found in South America, Africa, India and Australia. Similar rock structures are found on different continents. For example, parts of Appalachian mountains are similar to those found in Greenland and western Europe.

7. Although Wegener provided evidence to support his hypothesis, he couldn’t answer two questions What was causing the continents to move? Wegener said that this force might be the rotation of Earth, however, physicists were able to show that this force was not great enough to move continents.

8. How were the continents moving? Wegener proposed that the continents were plowing through a stationary ocean floor. But, his peers argued that continents could not push through the ocean floor without fracturing, because crustal rock is too brittle. And no evidence of fracturing had been found.

9. Plate Tectonics Section 2: S eafloor S preading Harry Hess

10. Mapping The Ocean Floor Scientists began using sound waves on moving ships to map large areas of ocean floor in detail. Sound waves echo off the ocean bottom – the longer the sound waves take to return to the ship, the deeper the water is.

11. In the early 1960s, Harry Hess proposed that hot, less dense material below Earth’s crust rises toward the surface at the mid-ocean ridges. Then, it flows sideways, carrying seafloor away from the ridge in both directions. This theory is known “ seafloor spreading ”.

12. In 1968, a research ship, Glomar Challenger, began gathering rock samples from mid-ocean ridges. They made a remarkable discovery as they studied the ages of rock samples. The younger rocks are closer to the mid-ocean ridges and older rocks are farther from the ridges.

13. Magnetic Clues Iron-bearing minerals, such as magnetite, that are found in the rocks of the seafloor can record Earth’s magnetic field direction when they form. The magnetic alignment in the rocks reverses back and forth over time in strips parallel to the mid-ocean ridges.

14. Seafloor Spreading

15. Section 3: Theory of Plate Tectonics In the 1960s, scientists developed a new theory that combined continental drift and seafloor spreading . According to the theory of plate tectonics , Earth’s crust and part of upper mantle are broken into sections. These sections are called plates, move on a plasticlike layer of the mantle.

16. Earth’s crust and a part of upper mantle combined are the lithosphere . (100km=62 mile thick) The plasticlike layer below the lithosphere is called asthenosphere. The rigid plates of the lithosphere float and move around on the asthenosphere.

17. Causes of Plate Tectonics Convection currents cause the movements of plates Hot, less dense liquid or gas is forced upward, as it reaches the surface, it cools down and sinks back down. This entire cycle of heating, rising, cooling and sinking is called a convection current .

20. Plate Boundaries When plates move, they can interact in several ways. Three different moving types of plates are: Plates moving apart Plates moving together Plates slide past each other