Download I. Continental Drift a. Alfred Wegener—German meteorologist i

I.
II.
Continental Drift
a.
Alfred Wegener—German meteorologist
i.
Suspected continents were once joined across the Atlantic
Ocean—hypothesis suggested by obvious fit of coastlines
ii.
Supported by restricted range of coastal swamp reptile
Mesosaurus—
1.
appearing on both South America and Southern
Africa, in Permian age rocks, but not elsewhere in the
same age rocks
2.
if it could swim the Atlantic Ocean to get to the other
continent, why was it not more widespread?
iii.
Set out to find additional evidence to support hypothesis
1.
2.2 billion year old granite in Brazil and Africa
2.
similar aged mountain belts from North America to
Europe
3.
Glacial deposits in tropical areas
4.
Equatorial swamp deposits in temperate areas
iv.
Rejoined land masses into Pangaea in the early Jurassic
Period
b.
Challenged to find a mechanism for continents to plow through the
ocean basins
i.
Suggested tidal forces, but this was easily countered by
physicists
ii.
But just because there were holes, and some
misconceptions (plowing through ocean basin) doesn’t make
the supporting evidence wrong.
Plate Tectonics
a.
Additional data about the nature of our world came to be known
throughout the middle of the twentieth century
i.
Sonar mapped the sea floor
1.
showed oceanic ridge system, crossed by fracture
zones, and presence of deep arcuate trenches
ii.
Earthquake studies showed distinct patterns of locations and
depths of earthquakes
iii.
Magnetic data from continents and the sea floor
1.
sea floor showed parallel bands of greater and lesser
magnetism symmetric across the mid-ocean ridge
2.
continents showed location of North Pole relative to
the land area had changed through time: Polar
Wandering. Each continent had a different path of
polar wandering unique to itself.
b.
Plate Tectonics Paradigm is a ‘super theory’: so well supported and
able to explain large amounts of apparently unrelated data
i.
Earth’s surface is composed of rigid lithospheric plates—
from a few km to a few hundred km thick
1.
ii.
These move as coherent units with respect to one
another
2.
Each plate has little to no internal deformation
3.
i.e.: New York moves with respect to London, but the
distance between New York and Denver remains
essentially the same.
There are seven major lithospheric plates, most of which
contain both continental crust and oceanic crust, and
numerous smaller plates
1.
Lithospheric plates
a.
Contain both crust and uppermost rigid mantle
b.
Slide over the asthenosphere, which is
composed of rocks that are nearly melted
2.
crust types
a.
continental crust is granitic, less dense, more
silicic (about 80%)
b.
oceanic crust is basaltic, more dense, about ½
silica
3.
seven major plates
a.
Pacific
b.
North American
c.
South American
d.
Eurasian
e.
Australian-Indian
f.
Antarctic
4.
intermediate plates
a.
Juan de Fuca
b.
Cocos
c.
Nazca
d.
Caribbean
e.
Scotia
f.
Philippine
g.
Arabian
5.
over a dozen small plates have been identified of
local significance
6.
Plate boundaries are defined by earthquake locations
iii. plates interact on their edges
1. divergent boundaries—move apart
a. mantle material partially melts and magma fills
in the gap remaining
b. constructive, because building new plate
2. convergent boundaries—move toward one another
a. oceanic plate can be overridden and
subducted—destructive margin
b. continental plate remains buoyant
i. will subduct oceanic plate
ii. two continental plates collide to create
mountain range
3. transform boundaries—slide past one another
4. each plate is bounded by a combination of these
types of margins—Example: Juan de Fuca Plate
a. eastward margin—Cascadia Subduction zone
b. western edges—Gorda Ridge, JdF Ridge
c. southern sides (connecting ridges to ridges or
to trench) are transform boundaries—
i. Mendocino Fault: most southerly
ii. Blanco Fault: between Gorda and JdF
III. Divergent Boundaries
a. Moving apart
b. Sea Floor spreading at Oceanic Ridges
i. Ridge is a relative term
1. 2000-3000 m higher than surrounding sea floor
2. 1000-4000 km wide
3. often contain central ‘rift valley’
ii. ridge exists because of newly created lithosphere
1. made from upwelling, hot melt from mantle
2. hotter things are less dense
3. as sea floor moves away from ridge, it cools
a. contracts as it cools, becomes more dense
b. increase in lithosphere thickness because
cooling strengthens underlying mantle
iii. spreading occurs from 2 to 15 cm/yr: average ~5 cm/yr
1. could have created all ocean basins in existence in
200 million years
2. no oceanic crust has been discovered that is over 180
million years old
c. divergent boundaries can occur on continents also
i. examples
1. East African Rift Valley
2. Rio Grande Rift
ii. Characterized by volcanism, faulting, down-dropped areas
within uplifted area
iii. Continued rifting results in splitting of continent
1. fill with mantle-derived material—dense
2. new lithosphere created becomes oceanic
a. floats at level below ocean surface
b. Red Sea, for example, or Gulf of California