The Dynamic Earth
Ground she's movin' under me
Tidal waves out on the sea
Sulphur smoke up in the sky
Pretty soon we learn to fly
-Jimmy Buffet, “Volcano”,Volcano
Geological History in 5 minutes
• Catastrophism:
– Assumes that history is marked by big, bad
occurrences we can’t imagine: floods, earthquakes,
etc.
– Big deal is that the conditions on earth were different
back then, so we can’t
understand those happenings
using the same techniques we
apply to today
– Supported by church orthodoxy,
very popular
He remembers when
rock was young.
Uniformitarianism
• Forces acting today are the same as the
forces that acted way back in the old days
when MTV still showed videos
• If we can explain something today, then
we can use the same explanation for the
same thing that happened way back then
• These forces act constantly and slowly so
that changes occur only over long periods
of time
Which is right?
• Most scientists today accept uniformitarianism
– Doesn’t require special explanations or circumstances
– Allows us to make predictions without requiring a
Deus Ex Machina
• Catastrophic events still occur (volcanic
eruptions, floods, asteroid impacts) but constant
forces (erosion, uplift) contribute more
Continental Drift
How do you explain the following?
•Looks like the continents fit
together like puzzle pieces
•Fossils of identical animals and
plants found in South America and
Africa living at the same time
•Identical rock features on edges of
continents thousands of miles
apart
•Some cited a “land bridge” between Africa and
South America that had disappeared
•Alfred Wegener suggested that continents had
moved over time. (continental drift)
Plate Tectonics
• Wegener’s ideas were ridiculed at the
time, but today are widely accepted, with
some modifications:
– Continents (the lithosphere, a rigid body) float
on the aesthenosphere, a partially molten
section of the mantle.
– The lithosphere is composed of large “plates”
that fit together at joints all over the surface of
the earth
Plates!
Wha? The Continents Move?
Motion of plates is driven primarily by
convection in the mantle. Hot molten
earth goo rises, cools, and sinks. This
creates motion in the material on which
the plates float.
What do moving plates mean to
me?
Moving plates create different results. They can
•collide
•Slide over/under one another
•Spread apart
Spreading Plates
Convection drives the plates
apart, creating a space (rift) in
between them. Magma from
the mantle flows up into that
space, hits the cool water, and
solidifies into basaltic rock.
This creates mountains at the
place where the gap forms.
Above, a radar map of the depth of the atlantic
ocean. The Mid-Atlantic Ridge is shown in the
middle.
Colliding Plates
When plates collide, they
can push each other
upward, creating large
mountain ranges
(Himalayas, Andes)
Note where the mountain
ranges are in this map.
More Colliding Plates
•Rift motion is happening in the middle of the Pacific, but the edges are
more interesting.
•When a denser plate sinks under a less dense one, this is called
subduction.
•Subduction zones are marked by volcanoes and earthquakes.
Islands
Subduction can
also cause island
(seamount)
formation
•Hawaii
•Japan
How do we know?
Age strips
(left) can
be dated
Magnetic
Striping
can be used
to coordinate
dates with
earth’s
magnetic field
reversals
Summary
• Plate Tectonics
– Continents move by floating on
aesthenosphere
– Geologic formations are the results of plates
moving
• Rift Motion – plates separate, creating mountain
ranges
• Subduction – plates pass over/under, creating
trenches, mountains, and islands
More Pictures and Movies
•
•
•
Fast
Controllable
India takes the lead
Pangea
After Pangea…
The
continents
get moving
Buy
beachfront
property in
Rwanda!
Unlike the movie, the Earth’s core does
not suck.
Earth’s core composition:
Mostly Fe, some Ni.
Inner core rotates 0.3-0.5 degrees
faster annually than the outer core.
By comparison, crust moves 3-5
cm/year compared to mantle.
Mantle? Core? Wha?
Manticore.
Geochemical Periodic Table
Cross Section
•Crust (solid)
•Mantle (plastic)
•Outer Core (molten)
•Inner Core (solid)
•Where does the heat come from?
•leftover from formation
•Differentiation
•Latent heat from phase transitions
•Decay of U, K
Rotating Core = Magnetic Field
•
•
•
•
Inner and outer
cores have
different fields
Competition
between them
leads to
changes in
polarity over
time
Magnets—how
do they work?
Why do we
care about
having a
magnetic field?