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Lecture 2
The earth is not a static, but a dynamic system with predictable changes.
The major theory covering the geologic processes at the surface of the earth is summarized in the Theory of Plate Tectonics
•
Basic Theorem: the crust is made up of individual
plates which move independently of each other.
There are 8 major plates (e.g. North American Plate, Pacific Plate) and
about 20 minor plates (e.g. Juan de Fuca Plate off the coast of
• Consequence: Three types of boundaries between plates
– Divergent boundary: new oceanic crust is formed; basaltic volcanism; shallow earthquakes
– Convergent boundary: crust is recycled into mantle; andesitic volcanism; major earthquake zones
– Tangential boundary: crust is neither formed nor recycled; strong earthquakes
Observation: Histogram of altitudes has a bimodal distribution à Presence of two types of crust (Fig. 2):
• Continental Crust: Granitic, 30à100 km thick, density 2700 kg/m3
– Average height ~800 m
–
Maximum height:
• Oceanic crust: Basaltic, 6à15 km thick, density 2900 kg/m3
– Average depth ~ 3800 m
– Maximum depth Mariana Trench 11,000 m
The break in the distribution is somewhat below sealevel, indicating that part of the continental crust is flooded as this time (continental shelf).
Three types of converging boundaries:
Oceanic-oceanic
à
trench + island arc:
à
Marianas
Oceanic-continental
à
trench + volcanic mountain range:
à
Continental-continental
à
mountain range + plateau:
à
Fig. 3 summarizes the processes of Plate
Tectonics
History of the
Earth system:
•
Geologic
times scales
•
Two
dating methods:
–
Geologic
times scale: Relative
–
Radiometric
time scale: Absolute
The Geologic time
system (Fig. 4)
•
Age of
Earth: 4.56 Ga
•
Turnover
time for Mantle: 1 Ga
•
Opening
of
•
Extinction
of Dinosaurs: 65 Ma
•
Onset
of subduction off
Focus on Metals:
•
Categories:
•
Iron
and Ferro-alloy metals (Fe, Ni, Mn, Mo)
•
Base
metals: Cu, Zn, Pb and Sn
•
Light
metals: Al, Mg
•
Noble
Metals: Au, Ag, Pt
•
Special
use metals: Ti, Ga, Ge
•
Fuels:
Th, U
History of mine production and population (Fig. 5):
Observations:
Increase in mine
production steeper than population increase
More Fe mined
than other metals combined
Increase in
production is parallel in most cases
Exceptions: Al,
Sn, Pb
Large minima in
all production lines
