Biogeography Chapter 8
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- Looking at index fossils.
- Largely limited to determine geological events.
- Determines sequential order which a series of events occured
- Using radiometric dating techniques.
- Computes numerical age.
- Example: Carbon 14 (organic matter) 1/2 life=5730 years to upper limit 40,000 years.
Thought the supercontinent was fragmented by the Great Flood in the Bible
Europe thinking before drift and Tectonic Theory
- Contraction theory: Earth cooled, shrank, and deformed creating mountains and ocean basins.
- Arching pressure under compression formed mountains.
US thinking before drift and Tectonic Theory
- Permanence Theory: Locations of continents and ocean basins were permanent.
- Positions of continents may have changed, but not in relation to each other.
Alfred Wegener and Tectonic Theory
- Contraction/Permanence ignored facts about geology and biogeography.
- Movements of continents show paleonotolgical evidence of species distribution.
- Agreement between paleontologists that continents connected during Carboniferous.
- Due to isotasy, it's impossible for a landbridge to sink (proves contraction theory wrong)
How did Alfred Wegener prove Contraction Theory wrong?
- Radioactive materials produced heating (not cooling)
- Places where tension (not compression) produced topographic highs
- Alps had overthrust--multiple layers of rock (not arching)
- Most marine rock on continents was shallow (not deep having been thrust up from ocean basins).
Multiple lines of evidence of continental drift
- Fit of continents
- Plants and animals match
- Rocks match
- Ice movement matches
- Climates were different positions do not match
Skepticism for Wegener's theory
- Biogeographic evidence weak and could be explained without moving continents.
- Rates of movement were too high and geologic evidence was still unconvincing
- He couldn't identify clear mechanism by which solid continents move through ocean basins. He suggested molten convective currents, but didn't have evidence.
Ultimate Acceptance of continental drift
- In the 1950s and 1960s ocean cores were recovered and showed reversing polarity.
- rock was much younger than expected
- proved seafloor spreading
Theory of Plate Tectonics
Earth's crust is broken into many pieces, called 'plates' and these move in specific ways to create all the earth's features.
Interactions between plates
- Transform Faults- 2 plates move past each other and create linear scars (Ex San Andreas Fault).
- Continental Collision- 2 plates collide, form high mountains (converging)
- Subduction- create linear volcanic mountain ranges like the Cascades in OR and WA. Subducts underneath other crust.
- Seafloor Spreading
- Fixed locations where magma comes through the mantle and creates volcanoes.
- Do not occur at plate boundaries
- Show us the direction of plate movement because the active volcano is over the hot spot and extinct volcanoes show the direction that the plate is moving.
- ex: Hawaii
Pangea, Gondwanaland, Laurasia
- Pangea- Supercontinent (late Permian)
- Gondwana- South America, Africa, Australia, India, Antarctica
- Laurasia- North America, Asia, Europe
Effects on Climate: Solar
- Changes in total solar irradiance
- Indirect effects of ultraviolet radiation on stratosphere
Effects on Climate: Orbital (Milankovitch's theory of Orbital forcing)
- Variations in Earth's orbit- the shape around the orbit around the sun.
- Changes in obliquity (Tilt) changes the angle that Earth's axis makes with the plane of the Earth's orbit
- Change in direction of the Earth's axis rotation behaves like the spin axis of a top that is winding down.
Effects on Climate:Configuration of land masses
Surface albedo (reflectance of sun on land masses)
Effects on Climate:Atomospheric turbidity (aerosols)
- Primary source of variation: volcanoes and asteroid impacts. Longer residence time and also a net cooling effect.
- Tropospheric (short residence time)- Salt from ocean waves, soot from combustion, or meteor impact, pollen volcanoes (cooling effect)
Evidence of past climates
- Ice cores drawn from Greenland Antacrtica, and tropical mountain glaciers
- Show the Earth's climate responds to changes in solar output, in the Earth's orbit, and in greenhouse gas levels.
- Show large changes in climate have happened very quickly, geologically speaking.
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