Plate Tectonics Chapter 9 Notes

Continental Drift German Climatologist and Geophysicist named Alfred Wegener. Proposed the idea of continental drift. Wrote a book in 1912 called the Origin of Continents and Oceans.

Matching a Puzzle Others before him noticed the matching patterns of the continental plates. Abraham Ortelius (1596) Frances Bacon (1625) Benjamin Franklin (Sometime in the 1700s?) Antonio Snider-Pelligrini (1858) picture above

Pangaea Wegener proposed that at one time in the distant past, all of the continental plates converged into one big supercontinent. He called this supercontinent Pangaea.

Evidence for Continental Drift Matching fossils found on adjacent continents across the ocean. Matching rock types found on adjacent continents across the ocean. Erosion and depositional features found on adjacent continents across the ocean. Magnetic patterns found on the ocean floor (not discovered until the 1960s). Hawaiian Islands produced by Hot Spots.

Matching Fossils Mesosaurus – Extinct crocodile-like reptile. Glossopteris – Extinct seed-fern type plant. Lystrosaurus – Extinct dinosaur. Cynognathus – Extinct dinosaur.

Matching Rock Types Matching Rock Types found on adjacent continents across the ocean. Appalachian and Caledonian Mountains

Erosion and Deposition Deposits that resemble glacial till found on separate continents. Large striations found in solid bedrock. This could also be explained by erosion and deposition caused by submarine landslides.

Magnetic Reversals In the 1960’s, scientists discovered that basalt on the ocean floor shows that the magnetic field has reversed polarity in the past. It is not understood how this happens and what affect it might have on living organisms. It does appear to be very strong evidence that the bottom of the ocean has spread.

Hawaiian Hot Spots

Hawaiian Hot Spots

Hawaiian Hot Spots

Geologic Time Uniformitarianism vs. Catastrophism Current seafloor spreading is causing the plates to move about a few centimeters per year. That is about the speed at which your fingernails grow. So where is all that lithosphere (Earth’s crust) going? Subduction and Mountain building. The current rates of mountain erosion (shrinking) and uplift (rising) vary depending on the mountain range. Some mountains are slowly growing due to uplift, and some are slowly shrinking due to erosion. Appalachians, Alps, Rockies, Andes, Himalayas

Modern Plate Tectonics In spite of the numerous evidences, most scientists rejected Wegener’s idea of seafloor spreading until the 1960’s. With the aid of sonar technology and the ability to map the ocean floor, substantial evidence was finally accepted.

Plate Boundaries The Earth’s lithosphere (crust) is made up of several plates which rest upon a hot molten magma called the asthenosphere. Wherever these plates come in contact with another plate is called a boundary. There are three types of plate boundaries. Divergent Boundaries: Plates move apart Convergent Boundaries: Plates come together Transform Boundaries: Plates slide past each other

Plate Boundaries At divergent boundaries, new lithosphere is rising up and being produced. At convergent boundaries, old lithosphere is either subducted or compressed to build thicker regions of lithosphere (mountains). At transform boundaries, two plates slide past one another. (San Andreas Fault).

Divergent Plate Boundaries Divergent plate boundaries can be found on both oceanic crust and continental crust. Sometimes a rift valley that begins on land can develop into an ocean ridge on the bottom of the ocean. New lithosphere expands and forms a ridge on the bottom of the ocean.

Convergent Plate Boundaries At convergent Plate Boundaries, one plate slides and descends downward toward the mantle and eventually begins to melt. Some of the lithosphere gets compacted into the crust, causing the plates to thicken. This process is called accretion. This thickening of the crust creates a difference in topography at the surface. Lithosphere that descends gets recycled.

Convergent Plate Boundaries Three types of convergent plate boundaries. Ocean– Continental Ocean–Ocean Continental-Continental Differences in density is a major factor contributing to subduction of lithosphere. As dense ocean crust comes in contact with buoyant continental crust, the dense ocean crust descends. Where oceanic crust descends below continental crust you find an ocean trench at the surface of the Earth.

Ocean–Continental Convergence Oceanic crust is made of dense basalt. Dense basalt descends below buoyant continental crust rich in silicates (quartz, feldspar, granite etc.) Continental Volcanic Arcs form adjacent to Ocean-Continental Convergent Plate Boundaries.

Oceanic-Oceanic Convergence This plate boundary forms volcanic chains on the bottom of the ocean floor located adjacent to the plate boundary. When the submarine volcanoes reach the ocean surface, they form island chains called volcanic island arcs.

Continental-Continental Convergence Sometimes two continental plates collide. Continental plates resist subduction due to their low density, which makes them very buoyant. The crust becomes very thick. Isostasy and isostatic adjustment and equilibrium takes place. This creates uplift and builds the mountains. The largest mountains of the world were formed in this way. (Himalayas, Everest)

Isostatic rebound (readjustment)

Continental-Continental Convergence Example: The collision of India and Asia to form the Himalayan mountains.

Transform Plate Boundaries Plates slide past one another horizontally. Most of these are found along divergent plate boundaries in the ocean (mid-ocean ridge). Some are found on the continents, like the San Andreas Fault in California.

Mechanism for Plate Tectonics One of the main reasons why scientists remained skeptical of plate tectonics until the 1960s was because of a lack of a mechanism that could explain it.

Convection Currents The earth’s mantle is solid, but behaves like a ductile fluid. Uneven heat throughout the earth’s mantle causes convection currents. It is believed that these convection currents cause the earth’s plates to move. Two forces that drive plate movement are the ridge-push and slab-pull