plate tectonics part1
TRANSCRIPT
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PLATE TECTONICS
Geology 105
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SCIENTIFIC REVOLUTION
• According to Thomas Kuhn (1962), science does not proceed by slowly accumulating knowledge toward ultimate truth
• Instead, it is marked by stable “paradigms” of accepted assumptions about the world, which are modified but rarely challenged by scientific research
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SCIENTIFIC REVOLUTION• Eventually, enough inconsistent observations
accumulate that cannot be explained by the paradigm
• Then a revolutionary new model is proposed which explains the old data as well as the new, and a scientific revolution is born
• Since scientists are human, it takes almost a generation or more for acceptance. The “old guard” resist the new idea, but the young accept it and build their careers with it
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SCIENTIFIC REVOLUTION
Kuhn’s (1962) ideas have been controversial, but do
in fact apply to most of the sciences
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SCIENTIFIC REVOLUTION• For example, Copernican astronomy
completely replaced Ptolemaic astronomy, because it explained planetary motions better
• Newtonian physics replaced Aristotelian ideas, and in turn was replaced by Einsteinian physics for extreme conditions
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SCIENTIFIC REVOLUTION
• Darwinian evolution completely revolutionized biology, replaced old “natural theology”
• Chemistry has not had a comparable revolution as total as these other examples
• Geology’s scientific revolution: plate tectonics
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CONTINENTAL DRIFT• Old school of of thought assumed fixed, stable
continents, but had a lot of unsolved problems• First ideas proposed in 1915 by Wegener, but
were not accepted• In the 1950s to 1963, enough new kinds of data
pushed the hypothesis from crazy to widely accepted
• Some of the “old guard” never accepted it to the day they died; others embraced it; but the major “players” in plate tectonics were all young scientists at the time
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ALFRED WEGENER• German meteorologist, gifted
with a diverse range of interests and wide imagination
• BUT not a conventional geologist by training, so dismissed by professional geologists
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ALFRED WEGENERLed one of the first major meteorological expeditions to Greenland in 1930
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ALFRED WEGENERDied on Oct. 30, 1930, returning from a supply drop in bad weather
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ALFRED WEGENERPublished The Origin of
Continents and Oceans in 1915, but his arguments
were scoffed at by geologists, because he was
not formally trained in geology, and because he
provided no mechanism for how continents could move
through oceanic crust
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WEGENER’S EVIDENCE FOR CONTINENTAL DRIFT
1. Fit of the continents2. Permian Pangea glaciations3. Permian climatic belts4. Identical Permian deposits5. Matching bedrock across the Atlantic6. Distinctive fossils on the Permian
continents
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1. FIT OF THE CONTINENTS
Originally noticed in the 1600s when the first accurate maps of the Atlantic suggested that Africa and South America fit together, but Wegener expanded on the idea, and suggested that all the continents once formed Pangea
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1.FIT OF THE CONTINENTSIn the 1950s,
geophysicist Sir Edward Bullard
did a more rigorous fit, and
showed the coastline match
is no accident (orange overlaps
due to later growth after
ripping apart)
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2. PERMIAN PANGEA GLACIATIONS
Glacial deposits of Permian (250-290
million years old) age only make sense if
southern continents once were joined to
form Gondwanaland. Modern distribution
of those ice sheets is otherwise nonsense
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2. PERMIAN PANGEA GLACIATIONS
Even some of the ancient glacial scratches appear to line up as if they crossed the Atlantic
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3. PERMIAN CLIMATIC BELTS
Distribution of Permian tropical coal deposits, subtropical desert deposits, etc.,
only make sense in the Pangea
configuration (impressive to a
meteorologist like Wegener, but not other geologists)
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4. IDENTICAL PERMIAN DEPOSITS
Identical sequence of Permian glacial deposits and redbeds with lavas on most Gondwana continents
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5. MATCHING BEDROCKEven the ancient bedrock trends match across the Atlantic
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6. DISTINCTIVE PERMIAN FOSSILS
• Seed fern Glossopteris• Primitive aquatic reptile Mesosaurus• Small herbivorous synapsid (“mammal-
like reptile”) Lystrosaurus• Large predatory synapsid CynognathusNONE of these could have swum or
floated across the modern Atlantic
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6. DISTINCTIVE PERMIAN FOSSILS
Tongue-shaped leaves of the extinct seed fern
Glossopteris, found on all the Permian Gondwana
continents
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6. DISTINCTIVE PERMIAN FOSSILS
Small aquatic reptile Mesosaurus, found in
lake beds in Brazil and South Africa, but too small to have swum
across the modern Atlantic
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5. DISTINCTIVE PERMIAN FOSSILS
Small herbivorous synapsid (formerly known
as “mammal-like reptiles”) Lystrosaurus
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5. DISTINCTIVE PERMIAN FOSSILS
Bear-sized predatory synapsid Cynognathus
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6. DISTINCTIVE PERMIAN FOSSILS
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WEGENER’S MODELVisualized continents as rafts or icebergs, floating on the mantle, drifting apart and colliding. But opponents could not imagine continents plowing through oceanic crust (where are the deformed crustal rocks?) and also what could drive them
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ARTHUR HOLMES (1928)Groundbreaking Scottish geologist1915: published the first radiometric dates that established the age of the earth1928: suggested a remarkably modern-looking idea of continents driven by mantle convection
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ARTHUR HOLMES Arthur Holmes (1890-1965)“Father of the Geological Time Scale”Originator of the idea of mantle convection driving continental driftReceived the Vetlesen Prize in Geology, 1964
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ALEXANDER L. DUTOIT Continental drift was
always more popular in the Southern
Hemisphere, where the evidence is abundant,
but most geologists lived in the north and never saw or thought about
this evidence
South African geologist A.L. Dutoit’s 1937 map from Our Wandering Continents
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CONTINENTAL DRIFT REJECTED
• By the 1950s, most Northern Hemisphere geologists dismissed continental drift as a crackpot, fringe idea
• They could not imagine how continents could plow through oceanic crust without leaving huge amounts of crumpled rock as evidence
• Major 1949 (published in 1952) symposium dismissed the evidence of fossils, and argued that they could be explained by rafting or dispersal
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CONTINENTAL DRIFT REBORN
• Ironically, at the same time land-based geologists continued to dismiss continental drift, new data was being gathered in the sea
• Post-WWII, major effort to understand the deep sea through seismics, magnetics (using old antisubmarine technology) and deep sea cores of sediment
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PLATE TECTONICS: NEW EVIDENCE
1. Paleomagnetism: APW curves and magnetic reversal time scale
2. Oceanic surveying: mapped seafloor for first time, discovered mid-ocean ridges, fracture zones, and deep trenches
3. Seismology: Benioff zones beneath trenches4. Gravity: Evidence that trenches have less
gravity than they should5. Sea floor magnetic anomalies
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1. PALEOMAGNETISMIn 1956, Cambridge paleomagnetists Keith Runcorn and Ted Irving both showed that apparent polar wander curves were better explained by movement of continents
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1. PALEOMAGNETISMStarting in 1959, but especially 1963-1969, Cox, Doell and Dalrymple (USGS) and Ian McDougall (Australian National University) established the magnetic reversal time scale, the Rosetta Stone for sea floor spreading
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COX, DOELL, AND DALRYMPLE
Allan Cox drilling paleomagnetic cores
Brent Dalrymple (Oxy’59) doing K-Ar dates
Cox, Doell, and Runcorn receive
Vetlesen Prize
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2. OCEANIC SURVEYINGBy the 1950s, detailed maps of the ocean floor were produced for the first time, showing the gigantic chain of mid-ocean ridges, fracture zones, and trenches
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Mid-ocean ridges: discovered in the 1950s, longest chain of mountains in world, with Grand-Canyon-sized rift valley down middle
Heezen and Tharp maps
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3. SEISMOLOGYIn the 1930s, Wadati and Benioff used seismology to show that a deep crustal slab must lie under oceanic trenches. The 1964 Alaska earthquake showed the power of subduction
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4. GRAVITYAs early as 1938, Harry Hess noticed that the gravity near trenches was much less than expected, suggesting lighter crustal material (not mantle) at depth. Heat flow was also higher than expected
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“GEOPOETRY”In 1962, Harry Hess proposed the essentially modern model of plate
tectonics, using all the data that had been gathered so far. However, without the evidence of seafloor
spreading, he called in “an exercise in geopoetry.” A year later, that evidence
was discovered. . .Harry Hess
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5. SEAFLOOR MAGNETICS
Starting in the late 1940s, most oceanographic cruises routinely towed a proton-precession magnetometer (originally developed in WWII to detect submarines) behind the ship to survey the details of seafloor magnetism over a wide area. An immense amount of data had to be collected before a pattern began to emerge
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5. SEAFLOOR MAGNETICSTo their surprise, the seafloor had a pattern of “zebra stripes” of seafloor that was anomalously stronger than present-day earth’s field (positive anomaly) or less than the field (negative anomaly). First profile across the Pacific was complex, confusing
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5. SEAFLOOR MAGNETICSThen in 1963, Fred Vine
and Drummond Matthews were working on a much
simpler, more symmetrical pattern in the
North Atlantic, and realized it could be
explained by seafloor spreading recording the
flips of the earth’s magnetic field
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5. SEAFLOOR MAGNETICS• Symmetrical “stripes” of positive and negative
gravity anomalies originally mysterious• Vine and Matthews (1963) realized that positive
anomalies result when oceanic crust is normally magnetized, and adds to modern magnetic field of earth felt by magnetometer; negative anomalies occur when submarine crust is reversely magnetized, and partially cancels the earth’s field felt by magnetometer (giving lower than average magnetic readings)
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VINE & MATTHEWSAt Cambridge in 1963, Fred Vine (left) and Drummond Matthews (right) first provided evidence for seafloor spreading
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5. SEAFLOOR MAGNETICSThe symmetrical “zebra stripes” of positive and negative magnetic anomalies can be matched with the Cox, Doell, and Dalrymple reversal timescale
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5. SEAFLOOR MAGNETICSAnomalies are mirror-image symmetrical over center
One profile
Same profile flipped
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5. SEAFLOOR MAGNETICS
• Vine and Matthews were vindicated in 1970, when the Deep Sea Drilling Project drilled both sides of the mid-Atlantic ridge, and found that the rocks got older away from the ridge crest
• Combining many different oceanic spreading records, and calibrating it with the dated magnetic sections on land, Heirtzler et al. (1968) generated a magnetic time scale for the past 100 million years
• This in turn could be used to date the age of the seafloor using magnetic anomalies
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5. SEAFLOOR MAGNETICSThe surprise is that the seafloor is very young (no older than 150 m.y.), a fraction of the age of the older continental rocks--so seafloor recycles rapidly
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HERESY TO PARADIGM• By 1970, plate tectonics was well established
among younger geologists, and second-generation studies were applying it to new fields
• Older generation (especially oil geologists) continued to resist and publish embarrassing reactionary papers; most died off or were eventually converted
• Most of the giants of plate tectonics were young when they made their discoveries and are still alive today--the most recent scientific revolution of all
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3 PLATE MARGIN TYPES
1. Divergent = spreading = passive2. Convergent = subduction = active3. Transform: neither subducting nor
spreading, but sliding on huge strike-slip faults
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3 PLATE MARGIN TYPES
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CRUSTAL CONSERVATION
Unless earth is expanding,
the rate of production of
new crust must balance
the rate of destruction of
old crust
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TRIPLE JUNCTIONSAll plate boundaries must come to an end at some other plate boundary. In some places, three plate edges come together to form a triple junction
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TRIPLE JUNCTIONS
Ridge-rift-transform triple junction in the
Afar triangleRocks include:1.Deep-sea turbidites, shales and cherts
scraped off top of oceanic plate2.Slices of ophiolite from downgoing slab3.Blueschists recycled from deeper in
trenchAll are intensely sheared and deformed
into melange