Plate Tectonics
Early Ideas
Introduction
• Plate Tectonics is an ideal example of how a new scientific theory
replaces a well-established older one
• Plate Tectonics is now the accepted paradigm in earth science
– Merger of earlier ideas of continental drift and sea-floor spreading
– Accepted only since the late 1960’s
– Replaced geosynclinal theory
Geosynclinal Theory
• The concept was developed by James Hall
– New York paleontologist
– Based on his observations of rocks in the Appalachians
• Noted that folded layers of rocks in the Appalachians were thicker than sediments are
nearby continent
• Deduced that sediment accumulated in trough-like depressions
• Hall never explained why these troughs became uplifted into mountains, but believed
that the amount of uplift was related to the amount of sediment
• “Theory for the origin of mountains with the origin of mountains left out.”—James Dana
Importance of Geosynclinal Theory
•
“The geosynclinal theory is one of the great unifying principles in geology. In many ways its
role in geology is similar to that of the theory of evolution, which serves to integrate the many
branches of the biological sciences. The geosynclinal theory is of fundamental importance to
sedimentation, petrology, geomorphology, ore deposits, structural geology, geophysics, and in
fact all branches of geological science. It is a generalization concerning the genetic
relationship between troughlike basinal areas of the earth’s crust which accumulate great
thicknesses of sediment and are called geosynclines, and major mountain ranges.” (Clark,
Thomas H., And Stearn, Colin W., 1968, Geological Evolution of North America: New York,
The Ronald Press, p. 83)
•
Geosyncline—a belt in the earth’s crust that subsides for long periods so that it acts as a trap
or collecting basin for sediments eroded from adjacent uplifted areas of the crust and for lavas
and ash erupted from fissures and volcanoes. When the geosyncline becomes unstable, its
sedimentary filling is deformed, metamorphosed, granitized, intruded, and eventually uplifted
into mountains.” (Clark, Thomas H., and Stearn, Colin W., 1968, Geological Evolution of
North America: New York, The Ronald Press, p. 88)
Geosyncline Defined
The Appalachian Geosyncline
Ordovician North America
The Geosynclinal Cycle
Summary
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The idea was that troughs developed adjacent to continents or between continents; the cause of the
initial subsidence was never determined
Troughs began to fill with sediment but continued to subside
– Troughs were subdivided into two parts
• Nearer the continent was the miogeosyncline which had limey sediments and clean quartz sands; not much clay or volcanics
• Farther from the continent was the eugeosyncline which had volcanics and clay-rich sands
•
Evenutually, the trough became unstable, was folded, intruded, and uplifted
– Mechanism was not explained
– Sediments depressed to great depth where temperatures increased and become unstable—The earth burped
– Clark and Stearn (1968) and others argued that horizontal compression was important although they never had a
good source
•
Tends to a model for continental accretion that adds younger rocks to the margins of continents
Some Consequences of Geosynclines
• Rocks become younger from the centers to the margins of
continents
• Continents and oceans have always been located at the same
place
• Seems to suggest that the oceans should shrink over time
• Suggests rocks on the seafloor should date from the origin of the
earth
The Transition
• People had talked about other ideas for years
• We started finding things that did not fit with geosynclinal theory
– Ocean basins should have filled up
– Oceanic rocks were young, not old
– Rocks on the continents did not always get younger toward the continental
margins
– We started accumulating data suggesting that there were large-scale,
horizontal movements of large sections of the earth
Roots of Continental Drift
• Similarity in shape of South America and Africa noted as early as
1620 by Francis Bacon
• Antonio Snider-Pellegrini (1858) published Creation and Its
Mysteries Revealed
– Suggested that all the continents were linked together in the Pennsylvanian
and later split apart
– Produced maps showing the continents joined
Roots of Continental Drift
• Edward Suess
– Noted similarities between late Paleozoic plant fossils in India, Australia,
Africa, and Antarctica
– Noted late Paleozoic glacial deposits on these same continents
– Thought the distribution of plant fossils was due to extensive land bridges
connecting the continents that later sank beneath the sea
– Proposed the name Gondwanaland for the southern landmass
Roots of Continental Drift
• Frank Taylor
– Formation of mountains due to lateral movements of the continents
– Thought present-day continents were parts of a larger polar continent that
broke apart and migrated toward the equator
– Migration was due to tidal forces generated when Earth captured the moon
about 100 million years ago
Roots of Continental Drift
• Alfred Wegener
– German meteorologist
– Origin of Continents and Oceans (1915)
– Father of continental drift
Alfred Wegener
Wegener’s Evidence—Continental Fit
• Fit of the continents
– Pangaea (Greek for “all earth”)
– Wegener suggested use of edge of continental shelf
– This is the fit of Bullard, Everett, and Smith (1965)
Continental Fit—Details
Wegener’s Evidence—Fossils
• Similarity of plant fossils
– They couldn’t swim across oceans
– Glossopteris
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Primitive land plants
Named for tonguelike leaves of seed fern
Occur in South America, South Africa, India, Australia, and Antarctica
Too uniform to have evolved on separate continents
Wegener’s Evidence—Similar Rocks
• Similar rock sequences in India, South Africa, South America,
Australia, Antarctica, Malagasy, and Falkland Islands
– Gondwana Sequence—Late Paleozoic & Mesozoic
• Early Mesozoic lavas
• Sedimentary rocks with coal beds and Glossopteris
• Late Paleozoic Tillites—glacial till
– Rocks younger than the Gondwana Sequence are quite different on these
continents
Distribution of Gondwana Sequence
Wegener’s Evidence—Glaciers
• Paleozoic Glaciation
– Tillites imply glaciation in unexpected places
– Deposits in India, South Africa, South America, Australia, Antarctica, and Falkland Islands
– Deposits lie on striated surfaces
• Striations in all directions today
Wegener’s Evidence—Glaciers
Look at what happens when the continents are pushed back together
Wegener’s Evidence—Climate Data
Distribution of Earth’s Climate Zones
Wegener’s Evidence
– Evaporites
• Imply hot, arid climate
• Today, these are 30° north or south of equator
– Wind Deposits
• Wind-deposited sandstones
• Dune sands
– Coal and Coral
• Warm, tropical water
• Locate ancient equator
– Glacial Deposits
• Locate poles
Wegener’s Evidence—Climates
Wegener’s Evidence
• Summary
– Ancient climate zones did not match well with the present position of the
poles
– Either the poles had moved or the continents had moved
– Wegener argued that the poles did not move
– More recent evidence allows us to rule out major polar wandering
Objections
• Winds or ocean currents could have distributed plants
• Polar wander could explain climate distribution
• There was no mechanism for motion
– Wegener’s continents plowing through the ocean will not work
– Tidal forces were also suggested and ruled out
– There really still is no good mechanism, but there wasn’t for older theories
either
– Mantle convection is the current favorite
Newer Evidence
• Fossils of freshwater organisms found in rocks of the same age on
different continents in the southern hemisphere
– These creatures did not swim across the ocean
– Mesosaurus
• Toothed, early reptile
• Lived in fresh water
• Brazil, South Africa, and Antarctica
– Lystrosaurus
• 1-meter long reptile living in fresh water
• Found in South Africa and Antarctica
• Could not swim ocean
Freshwater Reptiles
Newer Evidence
• Fit of margins at edge of continental shelf is even better than
Wegener thought
• Ancient mountain chains striking into edge of continents
– Cape Mounts of South Africa are thought to be an extension of mountains
in Argentina and Australia
– Appalachians extend through Newfoundland into the Caledonides of the
British Isles
Rock Correlations Across Continents