Download Theme 8 – The Dynamic Earth: Plate Tectonics

Theme 8 – The Dynamic Earth:
Plate Tectonics
ASTR 101
Prof. Dave Hanes
The Earth is Active
On small, fairly local scales:
…and Also Globally
with enormous plates moving about the surface
[from the Greek ‘tekton’ = carpenter, or builder]
Q: What Energy Drives This?
A: the Earth’s internal heat.
Outflowing heat can cause
convective motions – in
churning soup, and in the
Earth.
‘Continental Drift’
Meet Alfred
Wegener, who
first proposed
this notion in
1912.
He had some
good arguments.
A ‘Jig-saw’ Fit
[showing, on the right, where the pieces used to be]
Common Fossils and Stratigraphy
‘Recent’ History
Wegener’s Problem
He hypothesized
entire continents
plowing through the
Earth’s crust like an
Arctic icebreaker
No real physics.
Sadly…
Wegener died in 1930,
on a Greenland
expedition.
He did not live to see
his (modified) theory
vindicated!
The Evidence First
(In addition to what Wegener had noticed!)
1. 
2. 
3. 
4. 
5. 
6. 
Undersea ridges, with upwelling material
Rift valleys.
Sea floors that have no accumulated silt
Ages of sea-floor rocks
‘Magnetic remanence’ in sea-floor rocks
Actual measurements of movement!
1. The Undersea Ridges
The longest mountain chains on Earth, with active
volcanoes and so on. Note the mid-Atlantic ridge.
Activity in the Mid-Atlantic
Undersea ‘black smokers’
Iceland
2. Rift Valleys
(Kenya)
3. Bare Sea-Floor Rocks
Where Does the Silt Go?
4. Ages of Seafloor Rocks
Sea-Floor Spreading
5. Magnetic Remanence
Molten rock, when it solidifies and cools below the
‘Curie temperature’, imprints a record of the
magnetic field at the time
So what?
Well, the Earth’s magnetic field spontaneously
reverses every million years or so (irregularly)
Sea-Floor Spreading
Remember the Age Differences Too
6. Actual Measurements!
Plates – Not Just Continents!
http://www.ucmp.berkeley.edu/geology/tectonics.html
One Problem
The Earth has a finite surface area.
How can upwelling material from the mantle
continue to create ‘new sea floor’?
Where does it go?
Subduction
[“sub” = “under”]
This Explains…
The uplifting of mountains at plate boundaries
Volcanic mountain ranges along boundaries
- the ‘Ring of Fire’
Earthquake activity there
Deep trenches
Fish Fossils on Upraised Mountains!
‘Ring of Fire’
To the Challenger Deep
Picard, 1960; Cameron, 2013
Convective Motions in What?
Your first thought might be that the churning
convective motions must be in the outer core,
since it is fluid and could clearly move in this way.
So, the obvious question: Are big slabs of the
overlying mantle-plus-crust ‘floating’ on the outer
core, and being carried and moved around by it?
No.
The mantle has three regions:
n  a stiff, rocky upper section
(which, with the crust, is
called the ‘lithosphere’)
n  a ‘plastic’ intermediate zone
(the ‘aesthenosphere’)
n  a much more rigid lower part
(the ‘mesosphere’)
It is the material in the
aesthenosphere that
circulates and carries
the plates.
Can ‘Solid Rock’ Flow?
http://smp.uq.edu.au/content/pitch-drop-experiment
Rheology
Materials that seem
completely rigid and
solid can slowly ‘flow’
-- especially when under
great pressure, and if heated
Earth’s Activity:
The Importance for Life
If the Earth had no dynamic geology (attributable
to its interior heat) then:
n 
n 
n 
erosion / weather would wear down the mountains and
continents
the Earth would become very smooth overall (slightly
flattened by rotation)
its solid surface would be covered by ~4 km-deep
oceans everywhere!
One More Implication
Material circulating in the molten iron-nickel core is the source of
the global magnetic field. This helps to protect us from
incoming cosmic rays and bursts of particles from the sun (by
deflecting them towards the poles).
Excess bombardment can lead to enhanced mutuation rates and
‘accelerated’ evolutionary effects.