Our Changing Earth Alfred Wegener was a German scientist and arctic explorer who suggested the concept of continental drift. Continental drift is gradual movement of the continents across the earth's surface through geological time.

Wegener thought that the continents we know today had once been part of a giant supercontinent. He called this super continent Pangaea

The Earth is made up of many different pieces called plates, that are similar to pieces in a puzzle. Plate tectonics describes how these pieces move on Earth’s surface

Moving Pieces of the Earth: Lithospheric Plates Scientists realized that large pieces of Earth’s surface moved about like rafts on a river. These “rafts” are pieces of lithosphere called lithospheric plates.

There are two kinds of lithospheric plates: oceanic plates and continental plates.

What causes these plates to move? Convection currents in Earth’s lower mantle move the lithospheric plates on the surface. Heated lower mantle material, which is less dense, rises toward Earth’s surface.

Cooling makes the material denser and it sinks deeper into the lower mantle. This sinking process is called subduction.

Plate Boundaries – Sketch in notes Imagine a single plate, moving in one direction on Earth’s surface. One edge of the plate called the divergent boundary, moves away from things. The opposite edge, called convergent boundary bumps into anything in the way.

An edge of a lithospheric plate that slides by another plate is called a transform fault boundary. There are three scenarios shown below:

Divergent Boundaries Features A divergent boundary is the line between two plates where they are moving apart. This type of boundary is found over the rising plume of a mantle convection.

Divergent Boundaries Examples Divergent boundaries can also be found on continents as rift valleys. When a rift valley forms on land, it may eventually split the landmass. Divergent boundaries are found in the ocean as mid-ocean ridges.

Transform Boundaries Features A good clue for locating transform faults is offsetting. When seen from above, the feature will appear to make a zig-zag.

Transform Boundaries Example San Andreas Fault

Convergent Boundaries Features When oceanic plates collide, one subducts under the other. This forms a valley in the ocean floor called a trench.

Think About This… What happens if an oceanic plate and a continental plate collide? Which plate would subduct? The oceanic plate must subduct under the continental plate. WHY -- A continental plate is simply too buoyant to subduct under an oceanic plate.

Mountains and Convergent Boundaries examples Mountain ranges are formed when continents collide.

How are islands used as evidence? A single hot rising plume, called a mantle plume, can cause a volcanic eruption in the plate above it. The volcanic eruption may form an island. After the island forms, the movement of the plate carries it away from the origin. Scientists determine the direction and speed of plate movement by measuring distance of the chain of islands.

Fossils can provide some evidence of continental drift. Matching plant fossils are found in South America, Africa, India, Australia, and Antarctica. Matching reptile fossils are found in South America and Africa. Matching mammal fossils are found in South America and Africa.

Sea Floor Spreading In the 1960's a gentleman by the name of Harry Hess proposed that not only were the continents moving, but the sea floor was also moving.  Motion of the sea floor in a conveyor belt fashion explains the why the only youngest rocks are being found only at the mid-ocean ridges, and the rocks get progressively older as you move away from the ridge.

The key to Hess’ discovery are “magnetic patterns” in the rocks on either side of the mid-ocean ridges. Matching magnetic patterns and the age of rocks on either side of mid-ocean ridges provided strong evidence for sea-floor spreading.