Recall
Continental
Drift?
Seafloor Spreading
• Nazi hunting and better maps of the ocean
floor
– Discovery of Mid-Atlantic Ridge
• Magnetism and more Nazi hunting
– Discovery of magnetic stripes of alternate
polarities on the ocean floor
• Even more Nazi hunting
– Seafloor sediment thickens with distance from the
MAR
The Crust Can’t Grow on and on…
• If new oceanic crust is
constantly added at midocean ridges, why doesn’t
Earth grow?
• Look to the maps…
– Earthquake locations
Odd Patterns…
• Scientists in Japan noticed odd patterns when they plotted
earthquake foci
– Appeared to lie along a down-sloping plane
• Volcanoes and ocean trenches coincided with these
earthquakes
• It seemed like the ocean crust was being pushed down into
the mantle, creating earthquakes as it did so
Red lines represent
Ocean trenches
How Could This Happen?
• Remember the Two Types of Crust?
– Oceanic crust
• Dense, thin (~4-7 km), iron-rich
• Oldest oceanic crust is 180 million years old
– Continental crust
• Less dense, thick (35-70 km), silicon-rich
• Oldest continental crust is 4 billion years old
– Ocean plate forced to sink when encountering
material of lesser density (ocean/continental)
The Grand Unifying Theory…of
Geology
• Theory of Plate
Tectonics
• The rigid
lithosphere is
broken up into
~16 tectonic
plates
– May include both oceanic
and continental crust
– Each plate moves as one
giant mass
– Collisions responsible for
earthquakes and volcanoes
How do plates interact?
Three main types of interactions
• Divergent plate margins:
– Plates move apart, new lithosphere is created
• Convergent plate margins:
– Plates come together and one plate always subducts
beneath the other back into the mantle
• Transform-fault margins:
– Two plates slide past one another
Divergent Plate Margins
• Within an ocean basin, marked by a midocean ridge that exhibits volcanism and
earthquakes
• Also known as spreading centers
Ocean-Ocean Convergent Margin
• When two ocean plates meet, one descends
or subducts beneath the other
►Depends on which is older  colder  denser
Subduction Leads to Melting
• As plate enters the asthenosphere
– Increase in pressure + temp forces out water into
asthenosphere above it causing it to melt
– This creates a chain of volcanoes (island arc)
Ocean-Continent Convergence
• Plates with continental edges override ocean
edges because they are less dense
– Again, the ocean crust melts as it subducts, giving water
to the asthenosphere which also melts
– Coastal trench, huge earthquakes/volcanoes on land
– Melting along continent edges richer in silica (Si) which
makes them viscous and explosive
Continent-Continent Convergence
• Neither plate edge can subduct
– Creates a sort of double crust forming world’s
highest mountain ranges
The Himalayas
Transform-Fault Margins
• When two plates slide
past each other
lithosphere is not
created or destroyed
– The rocks on either side
of the faults are often
different ages due to
displacement
– Large earthquakes
Hot Spots?
• Some volcanoes
occur at distances
from plate
boundaries…
– Hot columns of rock
from the lower
mantle
– Act like a blowtorch
on plate moving
above
– Mostly volcanism
– Ex: Hawaii
Kauai
3.8-5.6
Direction of
plate motion
Oahu
2.2-3.3
Maui <1.0
Molokai
1.3-1.8
Decreasing age
Hawaii 0.8-present
Loihi
Hot Spot