8th Grade Earth Science

Earth Science Plate Tectonics: History of Plate Tectonics: The theory of plate tectonics was an idea describing how the Earth’s tectonic plates are moving. This theory was proven by scientists Alfred Wegener, Harry Hess, and John Tuzo Wilson. Over one century ago, scientist Alfred Wegener came to the conclusion that all seven continents used to be one large continent, therefore creating the idea called continental drift. Continental drift was the theory that Pangea was what used to once be. Pangea was the name given to the supercontinent or massive landmass that was once composed of every continent. Wegener’s proof that his theory of Pangea was true included the following...South America and Africa seemed to fit together like puzzle pieces when considering the shapes of their edges ❖ Matching plant and animal fossils were found on the coasts of South America and Africa that were closest to each other ❖ Matching landforms-folded mountain belts-were present on the coasts of South America and Africa that were closest to each other ❖ Climates were found in places that they wouldn’t normally be present in. Coal deposits, which are present in Africa, were found under the ice in Antarctica, and glacial deposits, common in Antarctica, were found in tropical areas of Africa. ❖ The data Wegener collected about Greenland’s latitude/longitude was different from the data collected in the past Unfortunately, the rest of the science community did not believe him and often ridiculed Wegener for his theory, saying the evidence he provided was not enough. During World War II, though, oceans were explored more with the help of new technology such as SONAR (Sound Navigation and Ranging). Discoveries from SONAR showed that the seafloor was not as flat as it was originally thought to be, reawakening Alfred Wegener’s theory of continental drift and allowing modern scientists to prove that Pangea was real. The observations using SONAR that helped prove continental drift were made by many, one of them being geologist Harry Hess. Hess discovered the theory of seafloor spreading, the idea that the new ocean floor is constantly being created on one end of the planet, and the old floor is constantly being pushed back into the planet through subduction on another end to ensure that the planet does not grow in size. This showed that the movement of tectonic plates was because of something happening deep inside the earth that was causing the seafloor to spread. The final piece to the theory of plate tectonics was discovered by geophysicist John Tuzo Wilson. Wilson proposed that the tectonic plates were being moved through convection currents in the mantle. The convection currents push up against the Earth’s crust, causing the plates to shift around. Plate Tectonics and Boundaries: Tectonic plates are portions of the earth that move to create landforms. There are seven large plates and multiple smaller ones. The major plates, shown on the right along with the minor ones, include the African, Antarctic, Eurasian, Indo-Australian (made up of the Indian and Australian plates), North American, Pacific, and South American plates. There are three types of plate boundaries: convergent, divergent, and transform. The two types of crusts, continental and oceanic, are what move when the plates move. The continental crust is less dense than the oceanic crust, meaning whenever they collide, the continental crust will subduct underneath the oceanic one. Subduction is the movement of one plate going below another. Landforms are made based on the movement of tectonic plates. Continental/Continental Convergent: ❖ Movement: Plates move towards each other and push up against each other. ❖ Landforms/Natural Disasters Formed: ➢ Folded mountains ➢ Mountain ranges ➢ Earthquakes Oceanic/Continental Convergent: ❖ Movement: Plates move towards each other and the oceanic plate subducts underneath the continental plate. ❖ Landforms/Natural Disasters Formed: ➢ Volcanoes ➢ Mountains ➢ Trenches ➢ Earthquakes Oceanic/Oceanic Convergent: ❖ Movement: Plates move towards each other and one oceanic plate subducts underneath the second oceanic plate. ❖ Landforms/Natural Disasters Formed: ➢ Volcanic island arcs ➢ Underwater volcanoes ➢ Trenches ➢ Earthquakes Divergent: ❖ Movement: Plates pull or move away from each other. ❖ Landforms/Natural Disasters Formed: ➢ Rift valleys (on land) ➢ Mid-ocean ridges (in water) ➢ Earthquakes (on land and in water) Transform: ❖ Movement: Plates slide against each other in a sideways motion. ❖ Landforms/Natural Disasters Formed: ➢ Faults ➢ Earthquakes Topographic Maps: The study of the arrangement of the land is called topography. Topographic maps are made so scientists can see elevation and latitude/longitude from a bird’s-eye view. A contour line is a connected line that shows where points of equal elevation are located on a topographic map. ❖ When contour lines are placed closer together, they indicate a steep slope ❖ When contour lines are placed farther apart, they indicate a gentle slope lines A contour interval is a distance between the contour lines on a topographic map.