Transcript 5_Ocean126_2006

Continental Drift & Plate
Tectonics
Chapter 3
Continental drift - observations
 1620, Francis Bacon noticed that S America
and Africa appear to fit together and
proposed that they had once been joined
(as were Europe and Africa)
 1885, Edward Suess, S American and Africa
fit and similarities in fossils found on these
continents
 Fit got even better if continental shelves
were included.
 No mechanisms
Evidence that continents were
together
 Fit of continents
 Ecosystems
– Rock weathering (e.g., glacial weathering in
current tropics)
– Fossils (similar across continents, coal in
Antarctica)
Continental drift proposed
 In 1910, Alfred Wegner proposed the idea of continental
drift. Proposed a super-continent, Pangaea, existed 225
MY ago (about the time of dinosaurs in the Cretaceous
period), and that continental movements caused friction
resulting in volcanic activity.
– N part was Laurasia, S part was Gondwanaland, partially separated
by the Tethys Sea
 Proposed mechanism – centrifugal effect from earth’s
spinning and tidal drag from combined effects of sun and
moon.
 Hang-up was the mechanism.
 Idea largely discounted.
 Lack of understanding of the mantle and isostatic support
of crustal material.
Post-WWII
 New instruments make new observations
possible. Lines of evidence
– Seismology
– Volcanoes, hot spots
– Sediment distribution
– Heat flow patterns from earth
– Magnetism
Additional observations
 Better understanding of earth’s structure –
seismology
 Charts of earthquake and volcanic activity (Pacific
Ring of Fire)
 1925, mid-Atlantic ridge was mapped
 Seismological evidence of deformable, non-rigid
upper mantle (asthenosphere)
 Post WWII, Radiometric dating of ocean crust
(max. age of 200 MY old! Young relative to
continental crust more than 4 BY old)
Seismic events worldwide between 1977-1986.
Seismic activity
 Earthquakes and volcanoes near trenches
 Earthquakes at mid-ocean ridges
– Plus transform faulting because of rotational stress
 Focus of earthquakes is below the surface in the
crust or mantle; epicenter is the site on the earth
surface above the focus
– Shallow focus - < 75 km (e.g., transform quakes)
– Deep focus - > 300 km (e.g., trenches)
 Plate formation and destruction explains
worldwide seismic distribution
More observations
 Detailed mapping of ocean floor crust and
sediments – post WWII, echo sounding
– Ridge conformed to coasts
– Sediments thicker near continents and thicker
near mid-ocean ridges.
 Mantle studies – International Geophysical
Year in 1957.
 Lithosphere isostatically balanced on
partially melted upper layer
Age of rocks and sediments
 Rocks increase in age with increasing
distance from the mid-ocean ridge
 Sediment cover increases with distance
from the ridge (older rocks exposed to
sediment rain for longer periods) – this isn’t
foolproof because of sedimentation from
land and movement of sediments.
Hotspots
 Sources of magma other than those that drive the
main convection cells
 Sources static while lithosphere moves over them
 Erupt as volcanoes for awhile and form chains
(e.g., Hawaiian Islands)
 Youngest mountain in the chain is closest to the
hot spot (big island, Hawaii, most active)
 Emperor Seamount chain formed from same hot
spot long ago
Seafloor spreading
 1960 Harry Hess and Robert Dietz proposed seafloor spreading
 Mechanism – conveyor belts of oceanic crust moving up at center and
down at edges; analogous to a convection cell in water
 New crust formed at Mid-Atlantic Ridge (hot and less dense) and
spreads outward (cools, shrinks and collects sediments)
 Continents carried along
Mid-Atlantic Ridge
Mass balance
 Earth either expanding or there is
 Consumption of crust
 Wadati-Benioff zones – subduction zones
where crust is being consumed
– Oceanic trenches – continental crust less dense
and floats on oceanic crust which dives down
– Violent so, very seismically active
– No direct exchange of mantle material but a
window into mantle composition
Magnetism
 Earth behaves like there’s a giant magnet inside.
– Magnetic N pole in Hudson Bay
– Magnetic S pole opposite in the Pacific
 Outer liquid core (Ni, Fe) rotating around inner
solid core. Rotation calculated at 1 mm/sec (90
m/d)
 When magnetic materials melt and then solidify in
the presence of a magnetic field, solid material
lines up with prevailing field.
 Dating of magnetized rocks (radiometric dating)
reveals that rocks of different ages sometimes
have opposite magnetic orientation
Magnetism (continued)
 Earth’s magnetic field must have reversed in
the past
 Causes are unknown
 History of magnetic polarity and reversals
recorded in volcanic (basaltic) rocks
 Ocean crust is high in basalt
 Magnetometers towed across ocean floor
revealed large scale pattern of alternating
polarity parallel to mid-ocean ridge (stripes!)
If pole moved and not the continents
(would have to have been in different
places at the same time)
If continents moved
Magnetic stripes
 Not evenly spaced
 Stripes used to calculate rates of seafloor
spreading
– E.g., rocks 1000 km from ridge are 50 MY old
1000km/50,000,000 = 2 cm/year
Magnetism aside
 Reversals take many years as field collapse and
reorient
 May allow cosmic radiation to penetrate normal
magnetic field and cause changes in surface living
organisms during these periods (fossil records?)
 Magnetosphere protects us from solar wind
(radiation)
 Cause of reversals is unknown
 Current polarity has lasted 730,000 yr
 170 reversals in last 76 MY
 Decrease in field of 7% in the last 160 yr which
would predict a reversal in 1500-2000 years!
Seafloor spreading
 1963 – Vine and Matthews proposed that
magnetic patterns were created by seafloor
spreading
– New ocean floor formed by vulcanism at mid-ocean
ridge
– Rock solidifies and takes on polarity of prevailing field
– Matching patterns on each side of spreading center
– Stripes of alternating polarity with increasing age and
distance from the ridge axis
– Reversal stripe in sea floor correlated with age-dated
reversals on land
– Youngest rocks at mid-ocean ridges and oldest at
margins and on continents
Plate tectonics
 1965 – John Wilson. Lithospheric plates floating
on asthenosphere
– Plates diverge when heated mantle (asthenosphere)
becomes less dense & rises up
– Plates converge where cool, dense crustal rocks collide
and are pulled down
 1966-1967 Debate
 1968 Glomar Challenger drilled deep-crustal cores
 Reexamination of scientific disciplines – e.g.,
similar fossils on different continents, coal in
Antarctica
Plate movement
 Plate movement is slow about 5 cm/yr (2
inches) & powered by mantle’s heat
 Crust is formed and consumed
 Plate boundaries
– Convergence
– Divergence
– Transform
Seven major lithospheric plates
Each have continental or continental plus ocean crust
Ridges, faults and trenches form boundaries
Some highlights
 Lithospheric plates float on asthenosphere
 Asthenosphere is where convection cells
occur
 Lithospheric plates move past each other
but are not created and destroyed as is
oceanic crust
Wegner’s date for Pangaea not so bad!
The Earth: # of Millions of Years Ago
550
136
220
65
190
Grand Canyon
0
Plate boundaries
Major features of plates
 Why no trench on the west coast of N
America? Probably used to be and then
Pacific plate changed direction
 Current spreading has lasted about 200 MY
but, earth is older so previous history
depends on fossils & rocks on land
Take home points
 Lithosphere versus asthenosphere also the Moho
 Continental drift, seafloor spreading and plate
tectonics
 Continental drift – fit of continents & fossil
evidence
 Seafloor spreading – theory supported by several
lines of evidence made possible by post WWII
technology (magnetism, seismology,
sedimentology, heat flow, distribution of volcanoes
and hot spots)
 Plate tectonics – provided a mechanism
 Types and features of plate boundaries