Transcript Lecture 5b (Plate Tectonics)

CSC 599: Computational
Scientific Discovery
Lecture 5b: The Plate
Tectonics Paradigm
Outline
Importance
Pre-Plate Tectonics
Big Picture
Ramifications
Complications
Earthquake Patterns
Earthquake Database
Importance
Plate tectonics gives geologists there first
wholestic view of the Earth over time and
space . . .
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Most earthquakes
Volcanos
Mountains
Continents
but it does not explain everything:
1. Earthquakes in middle of continent?
2. Early, early Earth?
3. Maybe ancient Mars, but today's Venus?
Pre-Plate Tectonics
Theory 1: Earth made by God
Evidence: Bible
Explains: Mountains formed during flood
Theory 2: Earth was hot, cooled off, shrunk
Evidence: Further go down in mine hotter it gets
Explains: Mountains are shrink ripples
Theory 3: Geomorphic Cycles
Evidence:
Appalachian mountains built out of mountains
Those mountains were built out of earlier mountains
Geomorphic Cycles
1. Mountain is lifted
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No idea what lifts rock
2. Erodes into sea
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Sand, mud forms plain
at sea level
3. Eroded rock solidifies
underground
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Sand to sandstone
Mud to shale
4. Solidified rock lifted
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Cycle starts again
What lifts rock?
Rebound?
Geomorphic Cycles (2)
Explains Appalachian Mountians
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Taconic Orogeny (550 – 440 MYA)
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Explains carbonate (ie. Marine shell) rock in NY, New
England
Acadian Orogeny (360 – 320 MYA)
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Explains:
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Angular unconformities (nonparallel strata)
Igneous intrusions,
Regional metamorphism
Deformation of older rock
Alleghenian Orogeny (299 - 251 MYA)
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Also called “Appalachian”
Created “piedmont” from NJ to Alabama
Continental Drift (1)
“Hey! Africa matches South American”
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Abraham Ortelius (1596)
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Francis Bacon (1620)
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British Philosopher, Statesman
Benjamin Franklin (1700s)
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Dutch/Belgian Cartographer & geographer
British/American Publisher, Scientist and Statesman
Antonio Snider-Pellegrini (1858)
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French geographer
Continental Drift (2)
Franklin Coxworthy
Roberto Mantovani (1889-1909)
Italian Geologist
William Henry Pickering (1907)
US astronomer
Frank Bursley Taylor (1908)
Amateur US geologist
Alfred Wegener (1912)
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German meteorologist
Compared
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Coastlines
Rock types
Fossils
Continental Drift (3)
Fossil Evidence for joined continents
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Laurasia (N. supercontinent) broke up before
Gondwanaland (S. continent)
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Dinosaurs more diverse in N. hemisphere than S.
Coal fields in Europe and N. America line up
Evidence stronger in S. hemisphere than N.
Continental Drift (4)
Continental Drift's Downfall
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Anglo-American geologists dismissed Wegener
Fuddy-duddies: “How can rocks move?”
Wegener: “Continents move by centrifugal force to
equator”
Fuddy-duddies: “It takes way more force to plow thru
ocean rock”
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Wegener died in Greenland in Greenland, 1930s
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Doing experiments with balloons & jet stream
Plate Tectonics
Explains most
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Earthquakes
Volcanoes
Magnetism
Fossils among continents
Rock ages among continents
Over WHOLE planet for MUCH of its life!
Plate ideas of geology and plate tectonics
1. Geological time
2. Rocks flow!
3. Continents come and go
Geological Time
Earth way old
MYA =
Millions of
Years Ago
Don't know
most history
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No fossils
before
Cambrian
Explosion
Old rocks
cycled
Big Picture (2)
2. Rocks flow!
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Continents move at
a couple of cm/year
Continents float on
top of and moved by
mantle
Mantle gently
churning
Liquid outer core
churning
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Causes magnetic
field
Big Picture (3)
3. Life of Plates
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Spreading ridges
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Subduction zone
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Ocean recycled in mantle
Transform fault
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New (ocean) plate made
Plates slide past each other
Plates come and go
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Continental plates
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light, old and float (N. America)
Ocean plates
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heavy, new and sink (Juan de Fuca)
Evidence (beyond Wegener's)
Magnetism and spreading ocean ridges
Closest to ridge = youngest
Farthest from ridge = oldest
Symmetric magnetism both sides
Ramifications
Subduction zones in more detail:
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Ocean under continent
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Ocean under ocean
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Continent under continent
Complications
1. Hot spots
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Upwelling of mantle rock punch thru continent
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As continent moves above hot spot volcano
moves on continent
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Get volcanic mountain
Get volcanic rock mtn chain eroding away from active
volcano
Hot spots may be necessary to keep plates
buoyant and moving
Complications (2)
2. Plate boundaries not always clear
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India and Australia?
3. Plates deform internally
China = Australia smashing into Eurasia
Chinese rocks are a mess, very few level strata
Basin and Range in N. America
Problems
1. Earthquakes in
middle of
continents?
New Madrid faulting
system
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Two estimated mag 8
quakes, 1811 & 1812
Problems (2)
2. Early-early Earth?
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Pangaea only 500 MY Old
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Earth 4500 MY Old
Much more hotter way back when
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More initial heat
Several times more radioactive decay
When Iron melted it fell to core
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Released 1022 megatons of gravitational energy
What was “crust” like then?
Isolated island arcs?
Problem (3)
3. If plate tectonics such a good theory why don't
we see it on other Earth-like planets?
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Mars
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Maybe at one time
Some evidence of ancient ridge-like bands
Mars is smaller --> cooled faster --> no tectonics now
Venus
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Magellan spacecraft radar-imaged whole planet
“Know more about surface of Venus than of Earth”
Expected “ocean crust” (spreading ridge, subduct zone)
See “continental crust” (mountains, strike-slip faults)
Explanation: “Venus' surface so hot that is naturally
buoyant”
Earthquakes
Earthquake patterns (1)
In space:
Along faults
Transform faults
Subduction zones
Wadati-Benioff zone = seismically active portion of fault
Earthquake patterns (2)
In Time:
Measured in months:
Fore shocks
Main shocks
After shocks
Measured in centuries (?)
“Elastic rebound theory”
BIG QUAKE (releases most energy)
(many years of many small quakes)
BIG QUAKE (releases most energy)
(many years of many small quakes)
BIG QUAKE (releases most energy)
Is it true?
Not enough data to say
Japanese have longest record of (large) quakes:
several centuries
Earthquake patterns (3)
In magnitude
By definition “main shock” bigger than:
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fore shocks
after shocks
Gutenberg-Richter Relationship
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Lower magnitude ->
more earthquakes
Log(N(M)) = a-bM
M = magnitude
N(M) = # quakes of mag M & greater
a,b = constants
Modeling earthquakes
Spring-slider block model
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2 moving plates
matrix of blocks between them w/friction
blocks connected by springs
when one block moves, can trigger others
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Reproduces Gutenberg/Richter stats!
Earthquake Databases
USGS records (http://neic.usgs.gov/neis/epic/)
Where:
longitude: -180 to 180 degrees
latitude: -90 to 90 degrees
depth: 0 to 700 (?) km
When: time
since 1973
How big: Magnitude
ML: “Richter Scale”
Just a measure of earth-shaking in California
MS: Travel along surface
Mb: Travel deep
MW: Seismic moment = (rock rigidity)(fault area)(slip dist)
Has more “rational” relationship w/the energy
Problems with db
Location
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Missing if too small/under sea
Inaccurately placed if too small/under sea
Time
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Missing if too small/under sea
Magnitude
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Inconsistent scale
Missing if too small/under sea