2 how do we explain the geological activity of the earth? 2-1 many pieces of information had to come...
TRANSCRIPT
2 How do we explain the geological activity of the
earth?
2-1
Many pieces of information had to come together...
2Age of the Earth
•uniformitarianism vs catastrophism (1800’s)
•about 4.6 billion years old
2Continental Drift
• Alfred Wegener, 1912
• based on Amazing fit of Africa & South America & fossils evidence
2
Figure 2.2
2
Figure 2.6
2
Figure 2.5
2Continental Drift
•single super continent = Pangea
– split 200 million years ago & are still drifting
•Wegener’s proposed mechanism = centrifugal force of spinning earth and tidal drag of moon & sun
– He was WRONG about this
•His idea was criticized through his death in 1930
2
Fig. 3-18, p. 63
Discovery of Mid-Atlantic Ridge in 1925
2
Patterns of Earthquakes and
Volcanoes•Plotted by Wadati & Benioff in late
1930’s
– Related to continental drift?– Pacific Ring of Fire
•a world wide pattern that followed orderly lines
– many corresponded to oceanic ridges (first plotted in 1925)
2
Figure 2.13a
2
Youth of Ocean Floor
•Maximum age of ocean floor was 200 million years
•centers of continents were much older (3.9 billion years)
2
Figure 2.12
2
Mantle Studies
•Seismographic evidence showed that the upper mantle was deformable and plastic
•perhaps the continents could move
2
Sea Floor Spreading
•1960, Harry Hess and Robert Dietz
•new seafloor (basaltic crust) develops at mid-oceanic ridges and then spreads outward
•continental drift would be caused by the same forces
2
Sea Floor Spreading
•powered by convection currents in the asthenosphere
2
Fig. 3-11, p. 57
2
Figure 2.10
2
Sea Floor Spreading
•explained...
– why ridges were hot– why ocean was deeper away from ridges– why sediments were thicker and older
away from ridges
2Since the earth is not
increasing in diameter...
2
Fig. 3-13, p. 59
2Subduction Zones
•Subduction zones (Wadati-Benioff zones) were discovered where the crust plunges into the mantle
•crust is destroyed here
•explains why the ocean floor is so young
2
Figure 2.20a
2Plate Tectonics
•1965, primarily by Wilson
• lithosphere is divided into plates that float & drift on top of the plastic asthenosphere
2
Figure 2.13b
2Plate Tectonics
•plates move about 5 cm per year
•Plate movement is powered by:
– downward pull of descending plates leading edge
– friction of asthenosphere convection currents
– outward push of new seafloor at spreading centers
2
Fig. 3-11, p. 57
2
2
Plate Tectonics•plates interact at boundaries
– diverge, converge or slip past each other
2
Divergent Plate Boundaries
•spreading centers, mid-oceanic ridges
•a line along which two plates are moving apart
•new oceanic crust forms
2
Figure 2.17
2
Figure 2.15
Divergence along Mid-Atlantic Ridge
2
Figure 2.16
Rift Valley of Iceland
2
Figure 2.18a
East African Rift Valley
2
Fig. 3-16d, p. 61
The Red Sea = a divergent plate boundary
2
Convergent Plate Boundaries
•aka subduction zones, Wadati-Benioff zones
• regions where plates are coming together
•crust is destroyed
2
Convergent Plate Boundaries
•Oceanic-continent
– denser oceanic crust is subducted
Figure 2.20a
2
Figure 2.21
2
Convergent Plate Boundaries
•Oceanic-oceanic
– older crust is more dense & is subducted
– deep oceanic trenches & volcanic islands
2
Convergent Plate Boundaries
•Continent-continent
– neither plate is completely subducted– they compress & uplift forming
mountains
2
Figure 2.22
2
Transform Plate Boundaries
•Plates slide laterally past each other
•Crust is neither created nor destroyed
2
Figure 2.23
2
Table 2.1
2
Confirmation of Plate Tectonics
•Paleomagnetism
– Fossil magnetic field
2
Fig. 3-27, p. 69
2
Fig. 3-28, p. 70
Age of the ocean floor
2
Confirmation of Plate Tectonics
•Hot Spots
– Surface expressions of plumes of magma rising from stationary heat sources in the mantle
2
Fig. 3-30, p. 71
2
Figure 2.25
2
Figure 2.24
2
Fig. 3-12, p. 58
2
Figure 2.33
2
Figure 2.31a
2
Figure 2.31b
2
Figure 2.31c
2
Figure 2.31d
2
Figure 2.31e
2
Figure 2.31f
2
Figure 2.31g
2
Figure 2.31h
2
Figure 2.31i
2
Figure 2.31j
2
Figure 2.32
What Earth may look like in 50 million years…