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 DriftHow 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 PlatesConvection 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 PlatesWhen 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?