A New theory in geology

DEVELOPMENT OF A NEW THEORY
Geology, as an organized science, is relatively young;
however, some of the first scientific observations were
geologically based (observations of volcanic activity for
example). With all of this in mind, our understanding of
how and why geology works was not well understood until
the 1960’s! This resulted in the theory of plate tectonics;
but what lead us to this theory was the incorrect theory of
continental drift.
WEGENER’S ARGUMENTS FOR CONT. DRIFT
•
MATCHING SHORELINES (actually, he used edge of continental
shelves) ON OPPOSITE SIDES OF OCEANS
•
MATCHING BOUNDARIES BETWEEN ROCK PROVINCES ON
OPPOSITE SIDES OF OCEANS
•
SIMILAR STRATIGRAPHY ON OPPOSITE SIDES OF OCEANS
•
PALEOCLIMATE (Late Paleozoic glaciation)
•
MATCHING pre-CENOZOIC FOSSILS
•
GRAVITY (isostasy) ARGUMENT AGAINST LAND BRIDGES TO
EXPLAIN DISTRIBUTION OF FOSSILS OF LAND ANIMALS &
PLANTS (will cover isostasy later in course)
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Wegner’s vision of how the continents fit together into Pangaea, and how they
subsequently breakup and dispersal into today's positions.
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Problems with Wegener’s Pangea reconstruction
- continents distorted in order to make the fit
- MEXICO doesn’t fit!
1960’s best fit of edge of
continental shelves (= edge of
cont. crust) made using a
computer
Gray = cont. shelves
Black = overlap
Mexico is still a problem!
Possibly the rocks that now
form Mexico were located
farther to the north, and
subsequently moved south
along a transform (some
geologic evidence for this, but
as yet not fully resolved).
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Matching Mountain Belts that record deformation at similar times.
OLD BASEMENT
ROCKS (CRATONS)
(Scientific American)
1968 matching of geologic
features including old
mountain ranges and basins
(some labeled “geosynclines”
here).
Note good matching of 1000 mdepth contour, which
approximates edge of
continental crust, except for
significant gap to right of
Madagascar (can you see a
possible alternative placing of
Madagascar relative to Africa
and/or India?). We’ll see later
that seafloor magnetic
anomalies provide additional
constraints on this
reconstruction.
(Scientific American)
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Paleoclimate evidence: Cold climate indicators in late Paleozoic
Glossopteris flora. These plant
fossils were interpreted to
indicate a cool climate.
Similar Late Paleozoic to Jurassic stratigraphy in areas in orange.
Note glacial deposits (mostly tan) in these sections. G = glossopteris plant fossils
Glacial scratches on bedrock beneath very young till, Alps.
Such scratches are exposed in glaciated valleys where glaciers
have retreated during this century. They are also common on
bedrock beneath till in the Albany region (Pleistocene)
Scratches on bedrock beneath a
Paleozoic tillite, Sahara Africa. These
scratches indicate the overlying poorly
sorted rocks are indeed glacial in origin.
Glacial till near Albany (~11,000
years old)
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Note that direction
of ice flow (red
arrows, from
striations) is away
from ocean!
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Explanations for distribution of Carboniferous-Permian glacial
deposits and associated cold-climate Glossopteris flora:
Wegener: Continents were together & since drifted apart
• What evidence indicates whether or not entire Earth was cold (a
“snowball Earth”)?
• Why couldn’t the south pole simply have been in a different place
(in the Indian Ocean)? That is, the whole earth rotated around such
that rotation axis was in a different place than now?
Later data (1930’s to 1980’s): striated bedrock beneath tillites
shows that the direction of ice flow was primarily away from the
present area of oceans.
Can glacial ice form on oceans?? (recall that ice floats). How is
the answer relevant to Wegener’s arguments?
Paleontologic Evidence
Distribution of modern anteaters. These animals evolved from very different ancestors to fill a similar
ecologic niche (parallel evolution) -- similarities in their shape (e.g, elongate snout) reflect their
adaptation to eat ants. This is the result of geographic isolation.
This was well known by Wegener’s time. Thus, paleontologists agreed that if the same land animal or
plant is found as a fossil in distant parts of the world today, it implies that their was a geographic
connection. For example, if East Africa split into two continents in the future, the presence of
Aardvark fossils in both would indicate they were once connected.
Wegener’s most famous example. The fossil reptile Mesosaurus in Permian sedimentary rocks.
Interpreted by paleontologists to be aquatic:
What features might suggest a fossil was aquatic?
It is unlikely that Mesosaurus could swim across an ocean or it would have been much more widely
distributed, and it occurs in rocks that appear to have been deposited in fresh or shallow marine water.
enough to cross Atlantic, why isn’t it distributed more widely in world?
Several examples of late Paleozoic land mammals and plants (including Mesosaurus and
Glossopteris) whose distribution makes sense when continents are reassembled.
Does this evidence alone, however, require that continental drift occurred?? Why?
Paleontologists agreed that the occurrence
of the same animals such as Mesosaurus or
plants such as Glossopteris indicated the
continents were once connected. But
instead of continental drift, they proposed
land bridges. A modern land bridge
between two continents include the Panama
canal and, during low sea level, the Bering
strait (Alaska-Asia).
These maps show an example of what
happens when a link is established between
two previously isolated land masses. Here,
North and South America were joined about
4 m.y. ago after having isolated for the
entire Tertiary! Nearly all animals in South
America were marsupials, most of which
became extinct. Some of the surviving
marsupials migrated into North America
(armadillo, opossum).
The marsupial vs. placental type of
mammals in North & South America during
the Tertiary illustrates what happens when
land masses are isolated and evolution
proceeds independently.
Examples of land bridges
proposed to explain the
distribution of fossils of
similar land animals and
plants. These are an
alternative explanation to
continental drift. After
all, there are modern land
bridges as an example.
Wegener pointed out that
such land bridges violated
the well established
theory of isostasy (gravity
measurements indicate
crust “floats” on a fluid
layer -- realize that fluid
does not necessarily mean
liquid in that minerals
can flow such as the
mineral ice in a glacier)
Here are the OBSERVATIONS that led to
the theory of isostasy.
Vertical
(line pointing to center
of the Earth)
CRUST
MANTLE
Isostasy. The explanation for the small effect of the Himalayas was that the a crust, lower in
density that the underlying mantle, is thicker beneath mountains (this has been shown to be the
case for many mountain ranges). This was proposed to be the result of the crust “floating” on a
fluid (can be a deformable solid), like ice floating on water (inset). According to the theory of
isostasy, it would be impossible for land that was once above sea level to sink to great depth, as
proposed by paleontologists for land bridges.
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Iceberg’s are the classic example of isostasy. The ice is not as dense as the water,
therefore the amount of water needed to be displaced to compensate for the
weight of the ice allows for 20% of the ice to be exposed above the water.
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As a load is placed on the surface of the Earth, the lithosphere and the
asthenosphere behave differently. The more ridged lithosphere will bend (and
sometimes break) while the more plastic asthenosphere will flow.
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The different densities of the continental and oceanic crust cause them to
isostactily effect the lithosphere and asthenosphere differently.
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TWO DOMINANT ELEVATIONS.
NOT RANDOM AS PREDICTED BY
CONTRACTING EARTH THEORY
PROBLEMS WITH WEGENER’S CONTINENTAL DRIFT THEORY
•
•
•
•
•
SIMILARITIES IN LAND FOSSILS could be explained by past land
bridges (like modern Panama) that have sunk beneath ocean
(Wegener correctly argued this violated isostasy theory – see quote)
PERSONALITY/COMMUNICATION
highly critical of those who opposed his theory (e.g.,
see quote regarding biologists/paleontologists)
MATCHING CONTINENTS
- some large overlaps (e.g., Mexico)
- proposed that terminal moraines in Europe & N.
America were once continuous (WRONG!)
DRIVING MECHANISM
- basaltic rocks (ocean crust) stronger than continental crust
so how could weaker rocks plow through stronger rocks?
See quote by Sir Jeffreys (now known that rocks can be
weak when at elevated T and deformed very slowly)
- proposed forces (e.g., tidal) shown to be too weak
TRIED TO MEASURE DIRECTLY RATE OF DRIFT (now possible)
obtained 19 m/year(!): over-interpreted data
THE LAST THREE ITEMS DAMAGED WEGENER’S CREDIBILITY!
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Some rocks record the place on
earth that they formed by what is
called paleomagnetics. This is
the alignment of magnetic
minerals in a rock with that of
the earth’s magnetic field.
THE MAGNETIC FIELD AT ANY ONE
LOCALITY ON EARTH IS A VECTOR,
WHOSE ORIENTATION IS GIVEN BY
DECLINATION AND INCLINATION
(SAME AS TREND AND PLUNGE,
EXCEPT INCLINATION IS UP (+) OR
DOWN (-)
DECLINATION, SHOWN HERE, ALWAYS
POINTS TO MAGNETIC NORTH
DECLINATION TODAY ON
EARTH (from Cox & Hart)
NMP = north magnetic pole
Note how inclination of magnetic field changes with latitude,
as shown by compass needles
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Apparent polar-wander paths can be constructed using the paleopole (longitudes
where rocks magnetized) from locations within a continent.
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Fig. A is the apparent polar-wander paths for North America, Africa, and
Europe over the past several 100 million years drawn on top of what the earth
looks like today. Note that the modern poles all converge!
B) shows that the apparent polar-wander paths for North America and Europe
fit back together, meaning that they were once joined, and then broke apart.
This was further evidence that the continents were once joined together.