Transcript Student Notes 1 -PlateTectonics

The scientific development
of the hypothesis (and
then theory) of Plate
Tectonics
Earthquakes: Seismology and Society
PHY 131
Mr. M. Castner
Fall 2014
History
What did we know
about the cause of
earthquakes in the
1950’s ?
History
What did we know about the cause of earthquakes in the
1950’s ?
Click here: Leet-Physical Geology 1954-1963.pdf
History
What did we know about the cause of earthquakes in the
1950’s ?
Click here: Longwell-Physical Geology - 19321960
Maps of epicenters
before WW II
Maps of epicenters before WW II
Physical Geology
Leet & Judson
Maps of epicenters before WW II
Earthquake History of the United States, Ed. by J L Coffman & C A vonHake, U.S. Department of Commerce, Pub. 41-1, 1973
A problem
During the first half of the 20th century,
seismologist mapped thousands of earthquakes
and discovered elastic rebound as the
immediate cause of earthquakes. But they had
no definitive cause for the buildup of strain
leading to elastic rebound and they had no
unifying theory for the real cause of
earthquakes. See excerpts from historic textbooks.
An answer
In the mid-1960’s seismologists brought
together many different types of data to
produce a new theory.
This new theory, plate tectonics, explains the
mid-ocean ridges, the close fit of widely
separated continents, and the concentration of
earthquakes in long narrow bands.
Adding up the pieces:
Maps of the ocean floor.
live
Adding up the pieces:
Maps of the ocean floor.
Magnetic reversals found in ocean floor rocks.
A Short History of Planet Earth
J. D. Macdougall
Natural Disasters 4th edition, Patrick Abbott
Natural Disasters 4th edition, Patrick Abbott
Adding up the pieces:
Maps of the ocean floor.
Magnetic reversals found in ocean floor rocks.
Maps of world seismicity:
the distribution of earthquakes around the world.
Maps of epicenters after WW II
Maps of epicenters
after WW II
Shallow earthquakes, magnitude 5 & larger, 1963-1988
Earthquakes 5th edition, Bruce Bolt
World seismicity map from IRIS
Incorporated Research Institutions for Seismology
Live
Magnitude greater than 5.5
Lamont-Doherty Earth Observatory of Columbia University, copyright©1999, all rights reserved
See IRIS or USGS
for a map of today’s
Earthquakes.
Adding up the pieces,
continued:
Similar fossils (identical even) found on
continents separated by an ocean.
Similar rock formations on continents separated
by an ocean.
This information was not new. It was discovered by
geologists in the 19th and 20th centuries
Field Guide to the San Andreas Fault, D. Lynch, Thule Scientific, 2006, p. 2
Fossils in New York
and Morocco
In March of 2007, Dr. Gordon Baird of SUNY
Fredonia was a member of an expedition that
spent two weeks in Morocco mapping the rock
strata in the Anti Atlas Mts and collecting
trilobites and other fossils.
The work of the expedition confirmed earlier work,
showing that the limestone/shale strata in
Morocco is identical to the strata in Western New
York. In particular, the Onondaga limestone
(which can be seen from the Kensington
Expressway between Grider St. and the
Scajaquada Expressway) is identical to the
limestone in Morocco from the lower Middle
Devonian period, including trilobites and other
fossils.
Fossils in New York
and Morocco
In addition, a study of the trilobites on the two different
continents shows that they are not only similar in the
two locations, they are identical genus and species.
New York
Phacops rana
Morocco
=
Greenops boothi =
Phacops speculator
Hollardrops mervcristata
70% of the trilobites in both locations are identical Phacops and
25-30% are identical Greenops/Hollardrops.
Thanks to Walter Drabek, paleontologist and staff member in ITS for his help with fossil
and strata identification.
Greenops boothi from
18 Mile Creek, Hamburg, NY
Hollardrops mervcristata, Morocco
Fossils in New York and Morocco are one of
thousands of pieces of evidence that
eventually lead to this description of how
the continents have moved.
350 million years ago (mya) most of the land mass on
Earth was concentrated in two large continents:
1. Near the equator was Laurentia which contained
present day North America, Greenland, and parts of
northern Europe.
2. Just south of Laurentia was Gondwanaland which
contained present day South America, Africa, and
Arabia.
Africa was separated from present day eastern North
America by a shallow sea called the Tethys Sea. The
predominant life forms in this sea were trilobites,
brachiopods, and corals.
From: The Historical Atlas of the Earth, R. Osborne and D. Tarling, Holt and Co. 1996
Description of how the
continents have moved
Over tens of millions of years, trilobites and other
sea creatures died and fell to the sea floor where
they were covered by sediment. The sediment
slowly turned into either limestone or shale,
depending on the chemical composition of the
water in the Tethys Sea and the sediment that fell
to the bottom with the trilobites and other sea
creatures.
By 300 mya the Tethys Sea was closing up due to
the northward movement of Gondwanaland.
Present day Africa ran into present day eastern
North America and mountains began to form.
Today we call those mountains the Appalachians
in the U.S. and the Anti Atlas Mountains in
Morocco.
From: The Historical Atlas of the Earth, R. Osborne and D. Tarling, Holt and Co. 1996
Description of how the
continents have moved
By 250 mya the Tethys Sea was gone and the
Appalachian/Anti Atlas Mountains were
becoming a major mountain range. The
limestone and shale layers were pushed up and
tilted, carrying the fossils with them.
From: The Historical Atlas of the Earth, R. Osborne and D. Tarling, Holt and Co. 1996
The Grand Conclusion
Ocean floor spreading and
plate tectonics.
The theory of plate tectonics asserts
that the earth’s crust is made up of a
puzzle of large, moving crustal blocks
of solid rock (plates) floating on
denser, more plastic rocks below.
Ocean floor spreading makes the
plates move.
Plate Tectonics
The global distribution of earthquakes, mid-ocean
ridges, ocean floor spreading, magnetic striping
of the ocean floor rocks, the fit of the
continental margins, fossil records, and other
evidence mentioned above all lead to the theory
of plate tectonics -- and the maps that follow.
Live
http://pubs.usgs.gov/gip/dynamic/slabs.html
US Geological Survey
Natural Disasters 4th edition, Patrick Abbott
Natural Disasters 4th edition, Patrick Abbott
Moving plates lead to
earthquakes.
Most plates move at a rate of 2 to 10 cm. per year.
6 cm. per year for 100 years = 6 meters
When friction can no longer hold back the strain,
the plates move past each other and produce
seismic waves.
We feel an earthquake.
Field Guide to the San Andreas Fault, D. Lynch, Thule Scientific, 2006, p. 7
Summary - evidence
for plate tectonics:
The continents fit together.
Similar rocks, fossils, and geologic structures
are found on opposite sides of oceans.
Discovery of the mid-ocean ridges.
Evidence of ocean floor spreading.
Summary - evidence
for plate tectonics:
Discovery of parallel bands of magnetized rock
on the ocean floor, spreading out in mirror-like
fashion on both sides of the mid-ocean ridges.
Summary - evidence
for plate tectonics:
Increasing age of the sea floor with increasing
distance from the mid-ocean ridges.
Ocean depths increasing systematically with sea
floor age.
Summary - evidence
for plate tectonics:
Earthquake epicenters that outline and
define the edges of tectonic plates!
Earthquake epicenters that systematically increase
in depth to explain ocean trenches and the
destruction of crust.
Summary - evidence
for plate tectonics:
The existence of ocean trenches at the
boundaries of tectonic plates.
The increasing depth of earthquakes
corresponding to increasing distance from ocean
trenches (subduction zones and Wadati-Benioff
zones).
By about 1970, scientists had
accepted Plate Tectonics as a
scientific theory.
All evidence over the past 45
years has supported the theory
of Plate Tectonics with only
minor adjustments.
Alternate summary
David T. Sandwell. “Plate Tectonics: A Martian View”. Plate Tectonics, An
Insider’s History of the Modern Theory of the Earth. Westview Press; Boulder,
CO; 2001. PP 344-345.
The path to discovery and confirmation of plate tectonics would have been
smoother if we only had the advantage of exploring Earth from another nearby
planet. Nevertheless, the outcome would be the same. Today, the most
important observations related to plate tectonics are provided by space
geodesy, seismology, ship surveys, and geological investigations. I would
rank them as follows:
1. radar altimeter measurements of marine gravity, fit of the continents
2. space geodetic measurements of plate motion
3. shallow earthquakes to define plate boundaries
4. deep earthquakes to prove that slabs penetrate into the deep mantle
5. magnetic reversals at sea to provide plate speed
6. mid-ocean ridge axis topography and symmetric deepening about the ridge
7. dating of reversals on land
8. fossil evidence
9. glacial striations
10. matching of rock types on conjugate continental margins.
You see: this is exactly the reverse of the order in which things actually
occurred.
Ocean floor spreading
Sea floor spreading
Before World War II, we knew more about the Moon's
surface than we did the ocean floor!
The need for better maps of the seafloor for
submarine navigation led to a variety of studies of
the seafloor that led to several amazing discoveries
about the ocean floor:
From careful bathymetric measurements the following
were discovered:
The mid-ocean ridge system
deep sea trenches
fracture zones
John Encarnación, SLU
http://www.eas.slu.edu/People/JPEncarnacion/easa133/outline.htm
Ocean floor spreading
The need for better maps of the seafloor for submarine
navigation led to a variety of studies of the seafloor that led
to several amazing discoveries about the ocean floor:
The ocean floor is made up almost entirely of the volcanic rock
basalt (whereas the continents are made up of lighter-colored
"granitic" rocks), and the basalt is very young at the ridges.
Sediments resting on the basalt are nil on the mid-ocean ridges
and became thicker away from the ridges.
The same sediments resting directly on the basalt are older
away from the ridges.
Measurements of heat flow from the ocean floor show that the
ridges are hotter and heat flow declines away from the ridges.
Magnetic anomalies form symmetrical, striped patterns parallel
to the ridges. This observation was one of the most perplexing
of them all.
John Encarnación, SLU
http://www.eas.slu.edu/People/JPEncarnacion/easa133/outline.htm
Ocean floor spreading
Harry Hess proposed a process called seafloor
spreading and Vine & Matthews explained the striped
magnetic anomalies by combining seafloor spreading
and reversals of the Earth's magnetic field. (Note:
these reversals are a phenomenon different from
polar wander, but they are occurring during polar
wander--north pole becomes south pole and vise
versa. They lead to an ambiguity in whether a
continent was in the northern or southern
hemisphere.)
A website giving a simple explanation of sea floor
spreading can be found here.
John Encarnación, SLU
http://www.eas.slu.edu/People/JPEncarnacion/easa133/outline.htm
Written in Stone
Earthquake Country
Los Angeles
Video by Dr. Pat Abbott
and UCSD.
See chapters 2 & 9
in Canisius iTunes U
for related information.
Questions?
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