Transcript Chapter 1 - Cloudfront.net

Mountains
Describe what you see…
Olympic Mtns., WA
1
2
Mountains
Where are they located?
6
3
5
4
Mountains
What are they made of?
Half Dome, Yosemite Valley, CA
Barrovian Facies Series of the Southern Appalachians
The Barrovian Facies Series occurs in the southern Appalachians, extending from Central Virginia to
Alabama. Interpretation of the relationship between deformation and metamorphism is complicated by
the fact the region has experienced at least three mountain building events, and thrust faults cut the
area.
http://www.tulane.edu/~sanelson/eens212/regionalmetamorph.htm
Vertical movement of the crust !
James D. Dana
Permanence of oceans
(1813-1895)
Wrote “Geological results of the
Earth’s contraction in
consequence of cooling” - 1847
Oceans are permanent,
continents are stable
Albert Heim
Alpine terrain, fold nappes
(1849-1937)
Worked in the Alps, documented mountain-scale
folds and large horizontal displacements;
constructed terrain models of Swiss alps
Mountainside fold nappe in Norway
Isostasy suggested that
mountains formed by
vertical movement of the crust
However…
• What is the heat source for metamorphism or
melting?
• What produces the petrologic complexity
among mountain ranges?
• How do we account for kilometers of horizontal
displacement in mountain ranges like the alps?
James Hall
Geosynclinal theory
State Paleontologist of New York; proposed
geosynclinal theory in 1857
(1811-1898)
GEOSYNCLINES
• fixed linear troughs at margins of, or within, continents
• deep water sediments and volcanic rocks in center,
(eugeosyncline: high deformation)
• shallow water sedimentary rocks on flanks
(miogeosyncline: mild deformation)
• deformation is symmetrical with thrusting away from center
onto adjacent continental platforms to produce orogens
George Marshall Kay
Geosynclinal theory
(1904-1974)
Wrote “North American
Geosynclines” - 1951
What part of the southern
Appalachians would be
the miogeosyncline?
Geosynclinal theory
Geosynclinal theory
Geosynclinal theory
So now we have a mechanism for both
horizontal and vertical displacements
in mountains
But what about fossil correlations
between continents?
Other geological evidence??
Eduard Suess
Gondwanaland
Documented the Glossopteris
fern on present-day continents:
(1831-1914)
Glossopteris
But
isostasy says
that by
land
bridges
sink !
Gondwanaland
connected
ancient
land shouldn’t
bridges
Antonio Snider-Pellegrini
Continental fit and “drift”
1858 - “Creation and its Mysteries Resolved”
Noah’s flood caused the continents to break up and drift apart
S. America and Africa break up and drift apart
Frank Bursey Taylor
Drift of continents toward equator
1910 - “Bearing of the Tertiary mountain belt on the origin
of the Earth’s plan”
Tidal forces induced movement of continents toward equator
Alfred Lothar Wegener
Paleoclimatic evidence
Evidence for Permo-Carboniferous inland
glaciation on several present-day continents
(1880-1930)
Alfred Lothar Wegener
Continental Drift
1915 - “Die Entstehung der Kontinente und Ozeane”
1924 - “The Origin of Continents and Oceans”
(1880-1930)
“Scientists still do not appear to understand sufficiently that all earth
sciences must contribute evidence toward unveiling the state of our
planet in earlier times, and that the truth of the matter can only be
reached by combing all this evidence…It is only by combing the
information furnished by all the earth sciences that we can hope
to…find the picture that sets out all the known facts in the best
arrangement and that therefore has the highest degree of probability.
Further, we have to be prepared always for the possibility that each
new discovery, no matter what science furnishes it, may modify the
conclusions we draw.”
Emile Argand
Gondwanaland reconstruction
(1879-1940)
1916 - “Sur l'arc des Alpes
occidentales”
1922 - “La Tectonique de l'Asia”
Wegener’s paleogeography
Iron/Magnesum-rich rocks
Silica/Aluminum-rich rocks
Alexander Du Toit
South America - Africa
geologic connections
1927
“A Geological Comparison
of South America with
South Africa”
1937
“Our Wandering Continents”
The same fossils and
the same rock units in
Africa and South America
(1878-1948)
Reginald A. Daly
Gravity sliding
1926 - “Our Mobile Earth”
(1871-1957)
Arthur Holmes
Convection in a molten substrate
1945 - “Principles of Physical Geology”
(1890-1965)
Complications for Continental Drift
• Causal mechanism for drift?
• Tidal forces?
• Rotation-induced movement towards the equator?
• Gravity sliding?
• Molten substrate?
• And what about isostasy??
• How can more felsic continent crust move through
denser, more mafic oceanic crust?
• Why isn’t there any evidence for this movement on
the ocean floor?
The Accumulation of Anomalies
The Submarine Cable Project
Samuel Morse invented the telegraph in the 1830’s, but it would only work on land.
Messages to/from Europe still required ships, but in 1858, a transoceanic cable was laid
But they found that the ocean floor was not as deep as expected in the middle.
The Challenger Expedition of 1872
HMS Challenger was the first dedicated
oceanographic expedition
Soundings not only determined ocean depth, but returned samples from
the ocean floor
Depths varied widely, sediments varied, fossils varied, contrary to current thought
So What?
- Conventional theory stated that the ocean floor
was simply submerged land;
- The shallowing in the mid-ocean seemed to
support this, but only marine fossils were found
in the mid-ocean;
- Areas of incredible depth were not accounted
for by this theory.
Sonar
• During WWI, German U-boats nearly destroyed
the British merchant fleet. As a result, the
British sought a technological edge, using
passive hydrophones to listen for U-boats;
• In the 1920’s the British feared a repeat in the
future, forming the Anti-Submarine Detection
Investigative Committee (ASDIC) to create an
active system;
• A similar device was invented in the US, called
SONAR (Sound Navigation and Ranging);
• Sound waves were sent out and timed for the
return reflected waves.
Sonar
• Harry Hess, naval
reserve officer, called
up during WWII, used
the SONAR on his
ship to collect data
on shape of the sea
floor;
• Not only did the
depth vary, but so did
the thickness of the
sediment layers on
top of the sea floor
Gravity Surveys
• As stated earlier, the force of gravity varied in
unexpected ways, being stronger or weaker in
ways that could not be accounted for by current
theory;
• Sensitive torsional balances could detect
variations in mass, but were sensitive to
vibrations;
• Pendulums were less sensitive to vibration, but
more sensitivity was needed.
Gravity Surveys
• A Dutch scientist, Felix
Vening Meinesz developed
such a gravitometer,
detecting differences in the
period of co-planar
pendulums;
• A difference in T meant a
difference in g.
• Traveling in the US
submarine, S-21, he found
variations in gravity to be
unexpected - less in areas
of sedimentation and more
in deep ocean basins.
Gravity Surveys
• A later cruise in the S48 included a young
geologist and naval
reserve officer, Harry
Hess;
• A Caribbean cruise
found very strong
negative anomalies in
the deep ocean
trenches.
George Airy, revisited
George Airy, revisited
Fg= G
m1m
2
Fg= G
d2
If Fg > Fg , then the difference is a positive
anomaly
If Fg < Fg , then the difference is a negative
anomaly
An anomaly is the difference
between the predicted and the
observed measurement.
mEm2
d2
Fg= G
mEm2
d2
Difference between predicted and
observed measurements come from:
• Higher or lower density rocks, i.e. basalts &
gabbros vs. sediments and granites;
• Displacement of higher density material by
lower density material, i.e. roots of mountains
displacing mantle material, subduction zones;
• Large masses of rock, such as mountains or
ultramafic concentrations.
http://www.csr.utexas.edu/grace/gravity/gravity_definition.html
Magnetic Variations
• During WWII, the renewed U-boat threat
lead to improvements in SONAR, but also
other detection methods;
• RADAR could detect surfaced subs, but
deep-diving subs could escape even
SONAR;
• The disruption of the local magnetic field
was sought as a way of detecting
submerged U-boats.
Magnetic Variations
Local field
Magnetic Variations
• The first such
detectors were
towed behind a ship,
but later were made
more compact to fit
in aircraft;
• More sensitive
magnetometers
began to detect
reversals in the local
magnetic field
preserved in the sea
floor rocks.
Magnetometer boom
Magnetic Variations
• Vine and Matthews
found linear patterns
of these variations,
with mirror images of
the patterns on
opposite sides of
ocean ridges;
• These same rocks
were of similar age
on opposite sides of
the ridge;
• Patterns were also
offset by apparent
faults.
Magnetic Poles
• Sensitive magnetometers on
land were also used to
detect the position of the
magnetic poles over time,
looking for past shifts of the
pole;
• But different continents had
apparently different pole
positions for the same point
in geologic time;
Magnetic Poles
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
So What?
• Ocean floor topography more highly variable than
expected;
• No submerged land bridges found;
• Sediment thickness and type more variable than
expected;
• Variations in gravity not accounted for by vertical
movement alone;
• Crust was likely more rigid than previously throught;
• Magnetic field signature in rocks varied in consistent
pattern;
• Paleomagnetic poles did not line up for static continents;
• Vertical movement could not account for such variations
alone.
Differences?
• Paleomagnetic
measurements on land
revealed differing polar
wandering paths on different
continents;
• Paleomagnetic variations
and ages of seafloor
material indicated new crust
was formed with polarity the
same as when it was
created;
• Geometric solutions of
plates as rigid bodies could
be resolved to past
positions;
• Seismic data indicated
variations in structure of the
subsurface
• Fossil evidence across
oceans suggested prior
interchange among land
species;
• Similarities in stratigraphic
layers across the ocean
suggest prior close
connection;
• Climate change indicators,
including glacial patterns,
could not be accounted for
by warm or cool periods
alone;
• Continental shelves
exhibited closer fit than
current coastlines.