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The Solid Earth
Crust
Lithosphere
(crust+mantle)
Asthenosphere
(mantle)
Mantle
Mesosphere
(lower mantle)
Post-glacial rebound in Scandinavia:
about 9 cm/100years
Solid, lower density lithosphere floats atop
plastic, denser asthenosphere
lithosphere
asthenosphere
Pangaea ~ 200 million years ago
Movement of continents from late Paleozoic to
the present
Wegener identified fossils of a variety
of plants and animals
Paleoclimatic evidence for Continental Drift
Paleoclimatic evidence for Continental Drift
Mid Ocean Ridge system
Sea-floor spreading
Stages of a Divergent Plate Boundary
Great Basin
East African Rift
Red Sea
Atlantic
Ocean
Magnetic reversals
occur at random
intervals in Earth’s
history…
The last reversal
occurred ~
730,000 years ago
Rekyjanes Ridge, North Atlantic: stripes
represent magnetic anomalies – Note symmetry
Development of magnetic stripes
As new crust cools it locks in the prevailing
magnetic field orientation
Thickness of sediment increases with distance
from the mid-ocean ridge
Age of sediment is approximately the same as the
crust on which it lies
You
nge
r
Crust (basalt)
Olde
r
Age of ocean crust increases with distance from
the mid-ocean ridge system – oldest crust is ~ 180
million years old (purple)
Relative plate velocities
Convergent plate boundaries (destructive margins)
Oceanic-oceanic convergence
Two oceanic slabs converge and one descends
beneath the other
Oceanic-continental convergence
Denser oceanic slab sinks into the asthenosphere
Continent-continental convergence
Lighter continental lithosphere can’t sink into mantle
Continent-continental convergence
Lighter continental lithosphere can’t sink into mantle
Collision produces folded mountains
Deepest (and largest) earthquakes
associated with subduction zones
EQ >100 km can only occur on subducting slab
Magnitude 8.5+ only occur at convergent boundaries
Distribution of earthquake foci
Transform fault at the mid-ocean ridge,
perpendicular to the spreading center
Plate tectonics:
Hot spots
•  Rising plumes of mantle material
•  Volcanoes form over them
Guyots extinct
islands, eroded flat
by wave action
Volcanic
islands
(Active)
(Inactive)
Atoll – Guyot formed in tropical regions, fringed
by growing coral reefs as they erode and sink
Increasing time
Maldive islands in the Indian Ocean