Earth’s Evolution

The Lithosphere The Lithosphere is divided into several large plates. Convection currents in the asthenosphere move the plates.

PLATE TECTONICS - explains observations of past and present geological processes - example of the development and acceptance of a scientific theory, as some revolutionary scientific theories may take years or decades to win general acceptance. A theory explaining the structure of the earth's crust and many associated phenomena as resulting from the interaction of rigid lithospheric plates that move slowly over the underlying mantle.

Moving Plates Plates are in constant motion, travelling at a few centimetres per year. The ocean floors are continually moving too. Convection currents beneath the plates move the plates in different directions. The source of heat driving the convection currents is radioactive decay which is happening deep in the Earth.

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. PANGEA Continental Drift: states that parts of the Earth's crust slowly drift atop a liquid core LINK

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 however. 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. This suggests that the continents were connected when the striations were made Evidence of glaciers is found in tropical areas suggesting that those areas were once further north or south.

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 North America, 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

1) Convergent Plate Boundary 1.Oceanic plate has crashed into a continental plate. 2.Folding and bending of the plates causes earthquakes. 3.Continental plate folded into a mountain. 4.Oceanic plate is more dense and is forced under the continental plate. 5.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.

2) 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

Mid Atlantic Ridge Mid-Atlantic Ridge is a Divergent Plate Boundary. Over km long. Average height of 3km.

3) Transform Boundary Places where plates slide past each other. Transform boundaries are not marked by spectacular surface features, but their sliding motion causes lots of earthquakes. Example: San Andreas fault. 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.

ANIMATIONS OF:ANIMATIONS Three types of boundaries Rifting Formation of earth’s crust Hot Spot Volcanoes