Earth’s Evolution

TWO SAD JOKES What do you get when you cross a vampire with a mosquito? A very itchy neck! What do you get when you cross an elephant with a kangaroo ? Potholes all over Australia!

The Structure of Earth The Earth is made up different types of layers. The Lithosphere is divided into large plates. Convection currents in the asthenosphere move the plates.

What is Plate Tectonics ? the lithosphere is divided into several “plates”

Why is PLATE TECTONICS “exciting” ? - it explains so many observations of past and present geological processes - it’s a recent example of the development and acceptance of a scientific theory 1915 Wegener: continental drift explained matching coastlines, other fossil/plant/rock evidence 1962 Hess: seafloor spreading new seafloor created that fills gap between drifting continents 1970s: Plate Tectonic theory accepted oceanic rocks and continents move together

Observation: Continents seem to be able to fit together!! Coincidence??? Or were they once connected 1858 Geographer Antonio Snider-Pellegrini made these maps

Alfred Wegener 1912 German meteorologist. He was the first to suggest the hypothesis of continental drift to explain Pellegrini’s observations that continents fit together like puzzle pieces.

1. Evidence from Fossils Animals living today on distant continents (ex: Africa & South America) are all different from one another. Fossils show that animals and plants long ago were similar. This implies the continents were connected. In the millions of years since separation, species evolved differently on different continents. Tropical fossils are found in many polar areas suggesting that those places were once closer to the equator.

Wegener’s evidence for Continental Drift

2. Evidence from Glaciation “Scratches” made on bedrock by glaciers on different continents “line” up when the continents are placed together. On some continents, glacial “striations” visible today make “no sense”. The run east-west rather than north-south. Evidence of glaciers is found in tropical areas suggesting that those areas were once further north or south. This suggests that the continents were connected when the striations were made

3. Evidence from rock and mineral formations Coal deposits had been found in Antarctica. Coal deposits only form in the tropics suggesting that Antarctica had once been in the tropics. Rock composition in certain areas does not match surrounding rock but does match rock type of a distant continent. –Ex: Rocks of Northern Newfoundland do not match NA but are similar in structure and composition to those of Scotland. This suggests that a small bit of what is now Scotland became attached to NA when the two split.

1930 Wegener left for Greenland to find more evidence to support his hypothesis. He was never seen again. This is the last photo of Wegener taken with a native guide. His continental drift hypothesis was hotly debated for the next 30 years.

Most active tectonic events occur at plate boundaries. Map shows that most earthquakes (black, grey) and volcanoes (red) are concentrated at plate boundaries

What else is attributed to plate tectonics? Changing landscapes Volcanic eruptions Mountain building Tsunamis New seafloor and mineral deposits

What are the “side effects” of plate tectonics? Mineral formation Sedimentary deposition / erosion Animal/plant life Ocean circulation Climate

Courtesy AGS

Plates meet at Plate Boundaries Boundaries have different names depending on how the two plates are moving in relationship to each other: 1) Convergent Boundary 2) Divergent Boundary 3) Transform Boundary

Convergent Plate Boundary Oceanic plate has crashed into a continental plate. Folding and bending of the plates causes earthquakes. Continental plate folded into a mountain. Oceanic plate is more dense and is forced under the continental plate. Some of the rock in the Oceanic plate melts as the plate sinks. The melted rock rises up causing earthquakes and volcanic eruptions.

Example of a Convergent Plate Boundary South America: Oceanic Nazca Plate collides into the South America plate. Formed the Andes Mountains (volcanoes along the mountain crest) Formed a deep trench off the coast in the Pacific Ocean. SUBDUCTION AT A PLATE BOUNDARY AnimationSUBDUCTION AT A PLATE BOUNDARY

Divergent Plate Boundaries Plates are being pulled apart by convection currents in the asthenosphere. As the plates separate along the boundary, the block between the faults cracks and drops down into the soft, plastic interior (the asthenosphere). New crust is formed along the boundary. Earthquakes occur along the faults, and volcanoes form where the magma reaches the surface.

Example of a Divergent Plate Boundary The Process of Rifting

Mid Atlantic Ridge Mid-Atlantic Ridge is a Divergent Plate Boundary. Over km long. Average height of 3km. Sea-Floor Spreading Animation.Sea-Floor Spreading Formation of Oceanic Crust Animation.Formation of Oceanic Crust

Transform Boundary Places where plates slide past each other. Transform boundaries are not marked by spectacular surface features, their sliding motion causes lots of earthquakes. Example: San Andreas fault, shown in the drawing to the right. The slice of California to the west of the fault is slowly moving north relative to the rest of California.

The San Andreas fault is the border between two tectonic plates—the North American Plate and Pacific Plate. Los Angeles is located on the Pacific Plate, and San Francisco is on the North American Plate. In a few million years, the two geographic areas will be right next to each other because the western side of the fault (the Pacific Plate) is moving northward with respect to the rest of the state. The fault is moving at about 2 centimeters (just under an inch) per year.

HOT SPOT VOLCANOES o/animations/ch2.htm#1 Animationhttp:// o/animations/ch2.htm#1 zzle/look_beneath.html story of plate tectonicshttp:// zzle/look_beneath.html