C.W. Post
Department of Earth and Environmental Science 

Why are There Mountains? The Development of the Theory of Continental Drift
geosynclinal theory of mountain-building
Wegener 1912, DuToit 1921-1937, and Continental Drift
Paleomagnetism

Hall: great accumulations of sediments cause crustal downbuckling
Dana: crustal downwarp causes
a cooling, shrinking Earth causes compression and folding along continent edges?
problems: what causes extensional terranes; what causes mountain belts in middle of continents?

- Wegener 1912, DuToit 1921-1937, and Continental Drift

1. identical fossils on trans-oceanic continents (eg. Glossopteris flora, Mesosaurus)
2. paleoclimate indicators (glacial sediments, tropical coal deposits, etc.)
3. jigsaw puzzle fit of the Atlantic-bordering continents
4. truncated
(eg. Parana [S.Amer.] and Etendeka [Afr.] basalts, Appalachian [N.Amer.] and Caledonide [Eur.] Mountains)

- Paleomagnetism

the Earth's dipole magnetic field (secular variation not withstanding)
  - declination - direction to the pole)
  - inclination (horizontal to vertical) - proportional to latitude or distance to the pole
  - intensity (how strong)

rock magnetism and natural remanent magnetization
  - rocks can record the Earth's magnetic field
  - the direction to the pole and the distance to the pole (paleolatitude)
  - magnetic polarity reversals
  - apparent polar wander - wandering poles or wandering continents?

The Plate Tectonic Revolution
marine magnetic anomalies, seafloor spreading, transform faults

- Cox et al. (1963) derive magnetic polarity timescale from reversals sequences of dated lava flows
- Vine and Matthews (1963);
stated the formal hypothesis explaining marine magnetic anomalies by seafloor spreading
- Pitman and Heirtzler (1966); Magnetic anomalies over the Pacific-Antarctic Ridge
bilaterally symetric magnetic anomalies and similarity of anomalies in two widely separated
places along the midocean ridge gave convincing evidence that seafloor spreading actually occurs
- Opdyke et al (1966); Paleomagnetic study of Antarctic deep-sea cores supporting magnetic reversals
and indirectly supporting Pitman interpretation of magnetic anomalies and seafloor spreading
- Wilson (1965); A new class of faults and their bearing on continental drift proposes transform faults
offseting spreading ridge segments contrary to old view of "transcurrent faults"
- Sykes (1967); Mechanism of earthquakes and nature of faulting on the midoceanic ridges provided
via earthquake first motion studies for the sense of motion on spreading ridges and transform faults

Plate Boundaries

- global distribution of earthquakes and volcanoes
most concentrated near midocean ridges, transforms, and deep ocean trenches, and continental collision belts
- divergent boundaries:
midocean ridges (eg. the Mid-Atlantic Ridge); 1) plates spread apart, 2) new crust forms
continental rifts (eg. East Africa)
- convergent boundaries
ocean-ocean (eg. the Mariannas Trench and the Mariannas Islands) - subduction zones (Benioff Zones)
ocean-continent (eg. Peru-Chile Trench and the Andes Mts)- subduction zones
continent-continent (eg. Alps, Himalayas) - collisional mountain belts
- transform boundaries
oceanic transforms offset midocean ridge spreading centers
continental transforms (e.g., San Andreas Fault)

Earthquake Seismology

P and S Waves: how they travel through the Earth
seismic waves are refracted as they travel through material with varying seismic velocity
ray paths

The Earth's Structure

tools and observations used to evaluate the composition and structure of the mantle and core

ophiolites, xenoliths, seismic velocities, lab studies of Earth materials, magnetism, Earth mass, composition of mantle-derived lavas...

seismic structure of the Earth

Mohorovicic seismic discontinuity or "the moho": crust-mantle boundary

oceanic vs. continental crustal thickness, composition, and structure (vs. compared to mantle composition)

low velocity zone (& seismic wave attenuation)
zone of partial [~1%] melting
asthenosphere
410 and 660 km discontinuities
phase changes to denser crystal structure
olivine -> spinel -> perovskite
Gutenberg seismic discontinuity (core-mantle boundary)
S & P wave shaddow zones
composition of the core (reasoning)
Lehman seismic discontinuity: velocity increase - the inner core

interesting questions

Do the upper and lower mantle mix?
What is the composition of the core?
How is the Earth's magnetic field produced?

what is a plate?

the crust (oceanic or continental) plus the uppermost part of the mantle
above the partly molten asthenosphere
the crust plus the uppermost mantle are called the lithosphere

isostacy - the crust floats on the mantle

using density and thickness of crustal blocks to determine how deep the bottom sinks into the mantle

rheology - how rocks behave in the Earth's interior

brittle vs. ductile behavior

Midocean Ridges

ocean crustal structure (sediments, pillow basalts, dikes, gabbro, layered gabbro, cummulate ultramafics)
seismic moho and petrologic moho
depth age relationship of seafloor and why (cooling profile)
normal faults
fast vs. slow spreading ridge characteristics
how is magma produced at midocean ridge? (decompression melting)
extent of magma chambers beneath ridge
hydrothermal alteration of ocean crust near midocean ridge (black smokers and vent communities)
thickening of the lithosphere away from a ridge

Oceanic Transforms

sense of motion (and type of EQs) on oceanic transform faults
fracture zones: description of topography and why; why no motion/EQ
euler poles
why transform offsets of midocean ridges; original breakup features?

Continental Transforms

examples: San andreas, Anatolian, Asia
restraining and releasing bends (e.g., Ridge Basin, California)
rotated features (e.g., Transverse Ranges, California)

Continental Rifts

Why? membrane stresses, trench suction, heating & weakening, hotspots???
passive vs. active rifts
rift-rift-rift triple junctions
propogating rifts
aulocogens (failed rifts)
model for active rift: upwelling mantle, thinning of lithosphere, doming of crust, stretching & normal faulting, volcanism
rift basalt formed by partial melting of upwelling mantle - decompression melting
caused by mantle plume heads?

Additional topics related to continental rifts and to subduction zones covered in lecture on March 9 may also be on the exam.