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Earth
Geology
Earth
Earth
Earth
topography
red=mountain
blue=depression
Earth’s surface looks different from other
planets.
It seems to have defined ‘continents’.
Why is this?
The continents fit together!
After
rearrangement,
rocks and
mountain belts
match.
After rearrangement,
past climates
seem to match (for
example, by looking at
geological evidence of
glaciers, or swamps).
Glacial grooves
After rearrangement, fossils match.
The theory of continential drift was
proposed by Alfred Wegener in the early
1900s to explain these observations.
The theory of continential drift was
proposed by Alfred Wegener in the early
1900s to explain these observations.
However, his ideas weren’t popular. People said:
•How are you going to move entire continents?
•What force is strong enough to move continents?
•How can you move rocks across rocks?
Continental Drift Revived, 1940s & 1950s:
Supported by Magnetism of rocks
• The Earth has a
magnetic field.
• When molten rocks
cool, they lock in
their current
magnetic field
direction.
• If they are moved
from their point of
origin, their
internal magnetic
signal shows it
• The continents
DEFINITELY
moved!
A new theory was postulated to explain
continental drift:
“Plate Tectonics”
•Earth’s surface divided into many plates that move slowly
over the surface and interact with each other
•Their movements are driven from below - internal heat and
convection in the Earth
(What is convection?)
•This was a REVOLUTIONARY idea
Let’s make sure we understand this.
Earth layers
Crust = 7 - 50 km
Mantle = 2,900 km
Core = 3,470 km
Plates move by “sliding” along soft upper mantle
material.
Mantle convection
Upwelling beneath ridges
•Hot, buoyant material causes ridges to sit “high”
Downwelling at subduction zones
•Cooler, denser, material sinks
Sea Floor spreading (Hess, 1962)
New crust created at ridges (divergence)
Old crust consumed at subduction zones (convergence)
Predicts sea floor is young near ridges and ages away
from the ridges
Tectonic Plates
Plates are rigid, pushed along by sea floor spreading.
Plate Boundaries
?
?
?
Once Plate Tectonics was described,
geologists discovered a ton of
evidence to support it.
Age of Sea Floor
Cascade Mountains - Famous volcanoes
Hot spots
Shield (hot spot) Volcanoes are different from other volcanoes...
Hot spots - Hawaii
Plate Tectonics: Take-home messages
• Earth’s geology is dominated by the processes of
plate tectonics. This is UNIQUE to the Earth.
• Earth’s crust and upper mantle are divided into
plates. The plates slide around.
• Plate movement is driven by convection in Earth’s
mantle.
• Crust is continuously being re-generated and
removed.
• Where plates interact, they form earthquakes,
volcanoes and mountains.
Why does the Earth have plate tectonics and not
other planets?
Is plate tectonics necessary for life?