Plate Tectonics Part1

PLATE TECTONICS

Geology 105

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

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


Kuhn’s (1962) ideas have been controversial, but do

in fact apply to most of the sciences

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


• 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

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

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

ALFRED WEGENERLed one of the first major meteorological expeditions to Greenland in 1930

ALFRED WEGENERDied on Oct. 30, 1930, returning from a supply drop in bad weather

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

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

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

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)

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

2. PERMIAN PANGEA GLACIATIONS

Even some of the ancient glacial scratches appear to line up as if they crossed the Atlantic

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)

4. IDENTICAL PERMIAN DEPOSITS

Identical sequence of Permian glacial deposits and redbeds with lavas on most Gondwana continents

5. MATCHING BEDROCKEven the ancient bedrock trends match across the Atlantic

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


Tongue-shaped leaves of the extinct seed fern

Glossopteris, found on all the Permian Gondwana

continents


Small aquatic reptile Mesosaurus, found in

lake beds in Brazil and South Africa, but too small to have swum

across the modern Atlantic


Small herbivorous synapsid (formerly known

as “mammal-like reptiles”) Lystrosaurus


Bear-sized predatory synapsid Cynognathus


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

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

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

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

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

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

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

1. PALEOMAGNETISMIn 1956, Cambridge paleomagnetists Keith Runcorn and Ted Irving both showed that apparent polar wander curves were better explained by movement of continents

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

COX, DOELL, AND DALRYMPLE

Allan Cox drilling paleomagnetic cores

Brent Dalrymple (Oxy’59) doing K-Ar dates

Cox, Doell, and Runcorn receive

Vetlesen Prize

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

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

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

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

“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

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

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

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

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)

VINE & MATTHEWSAt Cambridge in 1963, Fred Vine (left) and Drummond Matthews (right) first provided evidence for seafloor spreading

5. SEAFLOOR MAGNETICSThe symmetrical “zebra stripes” of positive and negative magnetic anomalies can be matched with the Cox, Doell, and Dalrymple reversal timescale

5. SEAFLOOR MAGNETICSAnomalies are mirror-image symmetrical over center

One profile

Same profile flipped

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

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

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

3 PLATE MARGIN TYPES

1. Divergent = spreading = passive2. Convergent = subduction = active3. Transform: neither subducting nor

spreading, but sliding on huge strike-slip faults

3 PLATE MARGIN TYPES

CRUSTAL CONSERVATION

Unless earth is expanding,

the rate of production of

new crust must balance

the rate of destruction of

old crust

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

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

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Plate Tectonics Part1