UNIVERSITY OF SOUTH ALABAMA
GY 112: Earth History
Lecture 5:
Clever Thinking 2: Plate Tectonics
Instructor: Dr. Douglas W. Haywick
Last time.....
1. The first geologists and their ideas
•Pliny, Ussher, Steno, Strachy, Hutton, Smith, Lyell,
Darwin, Wegener etc.
•Geological “principles” necessary for stratigraphic
correlation
•Unconformities (non-conformity, disconformity,
angular unconformity)
(Web Lecture 4)
Geological Thinking
First “geologists” date back to the
days of early hominids
http://ijanpromoters.blogspot.com/2009/02/human-apes.html
http://arizonagourds.homestead.com/supplies.html
Clever thinkers:
1638-1687
1) The principle of superposition that states in any sedimentary succession that
has not been overturned, the oldest strata occur on the bottom.
2) The principle of original horizontality that states sedimentary layers are
originally deposited as horizontal sheets
3) The principle of original lateral continuity that states that sedimentary
layers are originally deposited as laterally continuous sheets that naturally
terminate against basin margins or barriers or which grade into other
sedimentary layers.
Clever thinkers:
1797-1875
Principle of cross-cutting relationships in a succession
penetrated by other rock units (e.g., an igneous dike cutting
across sedimentary strata) the cross-cutting body is younger
than the stuff it is cutting.
Clever thinkers:
1797-1875
Principle of Inclusions: A rock containing fragments of
“country rock” (e.g. a pluton might rip off some of the
surrounding rock that it is pushing through and incorporate it
into the magma before cooling), must be younger than the
country rock it contains.
Clever thinkers:
1671-1743
1) Angular Unconformity
2) Disconformity (sedimentary rock on top of sedimentary rock)
3) Non-conformity (sedimentary rocks on top of “basement”)
Clever thinkers:
1671-1743
unconformity
One of Hutton’s water colour cross-sections
James Hutton: The “Father of Geology” and the originator
of “Uniformitarianism”
Clever thinkers:
1769-1839
Affectionately called “The Map”
William “Strata” Smith: the Father of Stratigraphy (and English Geology)
Clever thinkers:
1797-1875
Charles Lyell: Wrote Principles of Geology. Formulated the Principles
of Cross-cutting Relationships and Inclusions
Clever thinkers:
1824-1907
Lord Kelvin: determined the age of the Earth based upon how
long it would take to cool an entirely molten Earth (20-400
million years)
Clever thinkers:
1880-1930
Alfred Wegener: first scientific proposal for “continental drift” (1912)
and the concept of Pangaea
Today’s Agenda
1. Alfred Wegener and “Drifting Continents”
2. The Plate Tectonic Revolution
3. Plate Tectonics Mechanisms
Clever thinkers:
1880-1930
Alfred Wegener: first scientific proposal for “continental drift” (1912)
and the concept of Pangaea
The Earth
Even back in the 1700’s, mariners
commented on the apparent jigsaw puzzle
fit of South American and Africa.
Wegener’s hypothesis:
Wegener argued that 300 million
years ago, the continents were
all grouped together into a
“supercontinent” he called
Pangaea
His supporting evidence?
Matching rock types and
fossils*
* types and ages
And the reaction?
And the reaction?
…Widespread Rejection
Redemption would eventually occur for Wegener
Redemption would eventually occur for Wegener
…after World War II
The Plate Tectonics Revolution
Technological developments during the war increased our
understanding of the world’s oceans.
The Plate Tectonics Revolution
Radar, sonar, and depth sounding revealed that the ocean
bottoms were not flat planes
The Plate Tectonics Revolution
And the single most important piece of
evidence for plate tectonics was also
discovered around the end of WWII…
The Plate Tectonics Revolution
And the single most important piece of
evidence for plate tectonics was also
discovered around the end of WWII…
…. Paleomagnetism
The Plate Tectonics Revolution
Earth has a
magnetic field
(and it isn’t
constant)
The Plate Tectonics Revolution
1669 lava flow
magnetic field
(same as today +/declination shifts)
Mt Etna eruption of 1669
Source: http://boris.vulcanoetna.com/gifs/image/Etna1669a.jpg
The Plate Tectonics Revolution
Older lava flows (1
million years)
magnetic field
Mt Etna eruption of 1669
Source: http://boris.vulcanoetna.com/gifs/image/Etna1669a.jpg
Reversed compared
to today
The Plate Tectonics
Revolution
Paleomagnetism shows that
the ocean floor youngest near
the ridges and oldest near the
continents
Paleomagnetism
The Plate Tectonics Revolution
The outer part of the Earth is broken up into several large
tectonic plates
The Plate Tectonics Revolution
And where they “rub” against one another, you get…
Earthquakes
The Plate Tectonics Revolution
…Volcanoes (active, dormant and extinct)
The Plate Tectonics Revolution
http://shadow.eas.gatech.edu/~anewman/classes/geodynamics/misc/Worldmap.gif
...mountain belts and deep trenches (along convergent plate boundaries)
The Plate Tectonics Revolution
1963-1968 J. Tuzo Wilson was the first to
describe global tectonics in terms of rigid
surface "plates“, and recognized ocean
evolution (“Wilson Cycle”).
He characterized three basic plates boundaries
The Plate Tectonics Revolution
1963-1968 J. Tuzo Wilson was the first to
describe global tectonics in terms of rigid
surface "plates“, and recognized ocean
evolution (“Wilson Cycle”).
He characterized three basic plates boundaries
So how does it all work?
Internal “guts” of the Earth
Internal “guts” of the Earth
Four Major “Geophysical” Layers
1) The Crust (5-35 km thick; rigid rock)
Internal “guts” of the Earth
Four Major “Geophysical” Layers
1) The Crust
2) The Mantle (2865 km; rigid to ductile rock)
Internal “guts” of the Earth
Four Major “Geophysical” Layers
1) The Crust
2) The Mantle
3) The Outer Core (2200 km; liquid metal)
Internal “guts” of the Earth
Four Major “Geophysical” Layers
1) The Crust
2) The Mantle
3) The Outer Core
4) The Inner Core (1270 km; solid metal)
Heat exchange gives rise to
Convection Currents
More History of the Theory of Plate Tectonics
1928 - Arthur Holmes proposed that convection currents in the
mantle driving “plate” motion
Cooler
Asthenosphere
Hotter
The Plate Tectonics Mechanism
Rising convection currents stress the rigid outer layer
of the Earth. This layer consists of the crust and the
outer most mantle
Geophysical
layers
Crust
Mantle
The Plate Tectonics Mechanism
Collectively, the rigid outer most part of the Earth
(about 100 km thick) is called the Lithosphere
Tectonic
layers
Lithosphere:
Athenosphere:
Geophysical
layers
Crust
Mantle
The Plate Tectonics Mechanism
It is postulated that the convection currents can
eventually break up the lithosphere into separate plates
Tension
Cooler
Hotter
The Plate Tectonics Mechanism
Hence the term “plate tectonics”
Asthenosphere
Divergent Plate
Boundaries
Convergent Plate
Boundaries
San Andreas Fault
Transform Fault
Plate Boundaries
Hot Spots
Hot Spots
Today’s Homework
1. Download and read web notes 5
2. Study
Next Time
1. Introduction to dating (lectures 7, 8….long day)
GY 112: Earth History
Lecture 5: Clever thinking part 2
Instructor: Dr. Doug Haywick
dhaywick@southalabama.edu
This is a free open access lecture, but not for commercial purposes.
For personal use only.