Earth Structures Day 2: Plate Boundaries and Movement

Goal: ▪ I will be able to differentiate between the types of plate boundaries ▪ I will be able to recognize what a fossil is and how it relates to plate boundaries

Plate Tectonics ▪ Many years ago… in 1915 a scientist by the name of Alfred Wegener suggested that the continents were moving very slowly! ▪ His theory became known as the continental drift.

SHOW ME THE EVIDENCE!!! ▪ Wegener said that he knew his theory to be true because of fossils that were found and because of different rock formations. ▪ What is a fossil you ask? ▪ A fossil is a physical remain or trace of a plant or animal that lived long ago. ▪ Usually found in sedimentary rock

But what about…… ▪ He was unable to explain how the continents moved through the solid crust of the ocean floor. ▪ He also could not explain the forces that move the continents. Thus his theory was REGECTED!

New discoveries every day!

1950’s The lithosphere is not one solid shell of rock ▪ Molten rock was rising from the mantle onto the ocean basins. As the rock cooled it hardened and was being added to the earth’s crust! Because of this discovery: ▪ Scientists now believe that the crust is broken up into giant slabs of rock called plates. ▪ These plates seem to “float” on top of the mantle, much like giant ships float on a sea of molten rock.

Plate Tectonics The idea that giant plates of rock are moving slowly across the Earth’s surface The plates are moving relatively slow: the average speed is about 10 cm or 4 inches a year

Kinds of plates Oceanic PlatesContinental Plates Consist almost entirely of dense ocean floor material Made up of lighter continental rock “riding” on top of denser rock.

Plate Interaction ▪ Plates interact at their edges or plate boundaries.

Converging Boundaries Two plates move toward each other. Eventually the plates will collide. One plate may move under the other one in the process called subduction.

When will subduction occur? Subduction usually occurs when a continental and oceanic plate converge The dense rock of the oceanic plate slides under the lighter rock of the continental plate.

Diverging ▪ Two plates move away or diverge from each other. ▪ Molten rock will rise up between the two rocks forming new crust. ▪ This usually happens in the middle of the ocean floor, so it is also called sea- floor spreading.

Sliding Boundaries ▪ Plates may also slide past each other in opposite directions.

Moving Plates ▪ Whether converging, diverging or sliding, Earth’s plates never stop moving and changing! ▪ New rock is added to the Earth’s crust in some places, Old rock is “lost” to the mantle in other places. ▪ Moving, separating and merging of the continents has been occurring for billions of years. Some of these changes are constructive and others are destructive.

Where is your Evidence Mr. Science Man? ▪ Scientists claim that the plates have been moving for about 2 billion years ▪ Their evidence comes from the rocks at the Earth’s surface. ▪ Layers of sedimentary rocks provide clues to changes that were taking place at the time the layers were forming.

What can these rock layers prove? The layers show the mineral content of the rocks and how the sediment was deposited. The layers may also contain fossils.

How can studying fossils help prove the plates are moving? ▪ Fossils of similar species were buried on opposite sides of the ocean along the edges of different continents. ▪ Scientists have stated that this must have happened during a time that the continents were joined, like PANGEA! ▪ Over time the continents separated, thus the fossils are on different continents.

Fossils ▪ Some fossils seem to also be very far from where they were deposited. ▪ Fossils of tropical plants and animals have been found in polar regions. ▪ Fossils of salt water fish have been found far from oceans ▪ Scientists believe that the moving of the plates carried these fossils to their current location.