EAS 2200
The Earth System
Spring 2012
Lecture 7
From Continental Drift to Plate Tectonics
Plate Tectonics in a Nutshell
Basic idea is that surface of the Earth consists of rigid lithospheric plates that move relative
to one and other.
Oceanic lithosphere is formed at mid-ocean ridges, moves away and eventually subducts
back into the mantle. Continental lithosphere is more or less permanent.
All this is driven by convection in the mantle.
Plate Tectonics - the Surface Expression of Mantle
Convection
Ever notice how the continents seem to fit together?
You’re not the first!
Belgian map-maker Abraham Ortelius (1527-1598), English philosopher Francis Bacon
(1561-1621), Texan Antonio Snider-Pellegrini (1802-1885).
Plate tectonics is how the Earth cools off.
Hot mantle rises beneath mid-ocean ridges, forming new lithosphere that moves horizontally,
cooling as it goes. When it has cooled, it sinks back into the mantle.
Alfred Wegener’s Crazy Idea
German meteorologist Alfred Wegener (1880-1930) proposed the idea of Continental
Drift in 1915.
Wegener not only proposed the idea, but set out to prove it.
Wegener’s evidence
Fit of the continents
Matching geologic features
Paleobiogeography
Paleoclimatology
Paleogeography
Geologic features “matched” across the Atlantic
Paleozoic (300Ma) glaciers appear to flow from the sea; not if continents were together
How do we know which way glaciers flowed? – Striations
Matching orogenic belts*
Paleobiogeography
Distribution of fossils made sense if continents were together, but not if they were
separate.
Wandering Magnetic Poles
By the 1950’s people realized the remnant magnetism in rocks preserved the relative
position of the pole at the time the rock formed.
These pole positions appeared to “wander” through time.
Poles determined from different continents “wandered” through different paths.
What was wandering – the poles or the continents?
Different continents produced different polar wander paths, but not if the continents have
moved rather than the poles.
Distribution of Earthquakes & Volcanoes
*
An orogeny is an episode of rock deformation, metamorphism and mountain-building.
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EAS 2200
The Earth System
Spring 2012
Lecture 7
Earthquakes not randomly distributed, but occur in narrow belts we now recognize as plate
boundaries.
Deep earthquakes found only in a few regions we now recognize as subduction zones.
Volcanoes also limited to narrow belts – regions we now recognize and divergent and
convergent plate boundaries.
The New Age of Discovery
WW II had a dramatic effect on science, most particularly oceanography, because:
Wartime technology could be applied to exploration of the ocean (sonar, magnetometers,
etc.)
Realizing the value of science, governments began, for the first time, funding peace-time
science.
New Mysteries from the Ocean Floor
Fit of the continents improved if continental margins considered
Global Mid-Ocean Ridge system
Young volcanism everywhere on this system
Deep Trenches and Island Arcs
Exploration of the oceans also revealed deep gashes or trenches, as deep as 11 km, on the
ocean floor.
Sometimes they are located near continents
Almost always closely associated with an “arc” of volcanoes.
Magnetic Anomaly Patterns
Survey’s with magnetometers (originally developed to locate submarines) revealed striking
patterns of magnetic anomalies (regions of more intense or less intense magnetic field) on
the ocean floor.
Vine & Matthews: The Tape Recorder Analogy
Chains of Islands and Seamounts - all nearly parallel
J. Tuzo Wilson’s Hotspot model for generation of seamount & island chains
Fracture Zones and their earthquakes
Heat Flow: decreases with the square root of age of lithosphere and distance from midocean ridges
Testing the Theory Sediment Age and Thickness
Prediction of classical geologic theory: Sediment thickness constant in ocean basins;
deepest sediment should be very old
Prediction of plate tectonics: Sediment age and thickness increases away from mid-ocean
ridges
The test was to drill through the sediment in Deep Sea Drilling Project. The results,
increasing age and thickness away from mid-ocean ridges, were just what plate tectonics
predicted.
Implications of Plate Tectonics:
Because plates are rigid, most (but not all) tectonic activity occurs at plate boundaries.
(Tectonics refers to deformation, including faulting, earthquakes, folding, mountainbuilding, etc.)
Most (but not all) volcanism also occurs at plate boundaries.
Three kinds of Boundaries
Divergent boundaries: plates moving away from each other; rifts & mid-ocean ridges
Transform boundaries: plates slide past each other
Convergent boundaries: one plate pushes under, or subducts beneath the other.
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