Changing Earth: Plate Tectonics
Static Earth Theory People used to believe that the Earth and its continents could not change since it was created They thought that earthquakes and volcanoes were caused by periodic expansion and contraction of the earth.
In 1915, Alfred Wegener proposed his Theory of Continental Drift based on several observations:
1. The continents could be fit together (like a jigsaw puzzle). He proposed that the continents were once all joined together in a supercontinent he called Pangaea.
2. Geographic distribution of fossils from the same time period. Mesosaurus is a freshwater reptile found in both S. America and Africa. Before the idea of continental drift was accepted, it was difficult to explain how fossils of Mesosaurus could be found on both sides of the Atlantic.
3. Rock layers in mountain chains on one continent match up with the rock layers in mountain chains on another continent.
4. Rocks and fossils in many places don t match the present climate or conditions. E.g. Canada - We can find evidence of tropical forests in the Arctic and evidence of ocean reefs and marine fossils on mountain tops,, coral reefs in Alberta.
A fossil tree trunk on Axel Heiberg Island in Arctic Ocean
Modern Evidence for Plate Tectonics Wegener couldn t t explain what could cause the continents to move and scientists could come up with alternate theories. His theory was forgotten until other pieces of evidence were discovered that supported his idea: The use of sonar in WWII led to the discovery of underwater trenches, fractures and mid-ocean ridges under oceans.
Mid ocean ridges have been found at the bottom of all oceans
The ocean floor on the opposite sides of the ridge is moving in opposite directions
Paleomagnetism: : The magnetic record of the ocean floor is a mirror image on either side of seams. and the field is sometimes reversed Dark areas: rocks are magnetized in the same direction as Earth s magnetic field light areas: rocks are magnetized in the opposite direction to Earth s magnetic field
the age of rocks increases the further away from the mid-ocean ridges
Satellites have been used to measure the spreading of the ocean floor. The Pacific ocean spreads apart at an average of 10 cm/year. Atlantic ocean spreads apart at an average of 1.3 cm/year
Main Features of Plate Tectonics The Earth s crust consists of seven giant pieces called plates plus a few smaller plates.
Movement occurs because of convection currents in the molten magma under the plates. Hot magma from near the centre of the Earth rise at the ridges, cool and sink
Plate boundaries are where earthquakes and volcanoes occur most often.
Types of Plate Boundaries Diverging Boundaries (rift zones or spreading ridges) new crust is created. E.g. Mid-Atlantic Ridge
Converging Boundaries (subduction( zones) where old crust is destroyed. There are two types of subduction zones: Two continental plates meet. The older, denser, mass wedges under the younger, less dense mass pushing up mountain ranges. E.g. The Himalayas
Oceanic and continental plates meet. Rising magma E.g. The Andes in South America and Cascade Mountains (BC and Washington State) Ocean plate The older, cold, and heavy ocean floor dives under the continent forming a trench. The subducting plate sets off deep earthquakes, crumples the continental crust, building mountains and melts the overlying plate creating volcanoes.
Hydrothermal vent near a spreading centre
Mountain building exerts great stresses on strata. a sandstone unit (S- shaped light band) has been folded and snapped. The break is known as a fault. The pressure came from the right toward the left.
Lateral Boundaries (strike slip zones or transform faults) where crustal plates slide past one another. As plates slide they create many small earthquakes or single huge ones. E.g. San Andreas Fault.
No volcanoes form on transform boundaries Not enough heat is generated to melt the rocks
Shortcut to Paleo3.mov the evolution of the Earth's surface due to plate tectonics over the past 600 million years.