Continental Drift- The Theory. The Earth’s crust is broken into about 12 rigid plates, which slide over a semi-molten plastic layer of the mantle. Tectonic Plates on Modern Earth. There are three types of plate boundaries:. 1- Convergent 2- Divergent 3- Transform.
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The Earth’s crust is broken into about 12 rigid plates, which slide over a semi-molten plastic layer of the mantle.
Tectonic Plates on Modern Earth
1. Convergent (aka subduction zone) boundaries – where one plate overrides another plate. Each plate is bounded by some combination of these three plate boundary types.
2. Divergent (aka spreading) boundaries – where plates are moving apart
3. Transform fault boundaries – where plates are moving past each other
The material making up the plates can be divided in two types:
i. Oceanic crust
ii. Continental crust
The driving force for plate tectonics is slow moving convection currents in the underlying
plastic mantle material.
How does convection work? No one knows—but they aren’t afraid to propose models!
Two mantle convection cells
1600 - development of a comprehensive world map (missing details of some continents such as
Australia and Antarctica).
1620 - Francis Bacon noted the similarity of the shape of the continents on either side of the
Southern Atlantic – Africa and South America.
1666 - Francois Placet suggested that prior to the Great Flood the land was undivided by
oceans and that the Atlantic formed when Atlantis sank.
1858 - Antonio Snider developed a theory that when the Earth cooled from a molten mass
continents formed only on one side. This created instability causing the Americas to be
pulled away from the rest of the continents.
1879 – Sir George Darwin said that the Pacific Ocean was the scar left behind when the Moon
pulled away from the Earth. After this happened, the continents moved to create a
1890s- Edward Suess suggested that at one stage all the continents were joined as one massive
continent, which he called Gondwana. His evidence for this was the location of
mountain ranges and common fossils.
1924 - Alfred Wegener developed the theory of Continental Drift to explain the similarities of
rocks, fossils and other geological structures on either side of the Atlantic. At this time it
was accepted that the continents sat like icebergs on the mantle and as the continents
eroded they rose out of the mantle. Wegener suggested as well as moving up and down
in the mantle, continents could move sideways in the mantle.
-Wegener was a meteorologist and his theory was not well accepted. (He died on an
expedition in Greenland collecting ice samples).
1928 - Arthur Holmes suggested that convection currents in the mantle as the driving force of
continental drift. He had no evidence to support his theory.
1950s- Extensive mapping of the ocean floor, especially the Mid-Atlantic Ridge.
1960s- Harry Hess suggested that sea floor separates at the mid-oceans ridges and new sea floor
is created by up welling of the mantle.
1970s- Theory of plate tectonics well accepted.
Earth ~200 million years ago
Tight fit of
the continents, especially using
Continental Drift: Evidence
Fossil critters and plants
Continental Drift: Evidence
Correlation of mountains with nearly
identical rocks and structures
of the same age
restore to a
WW II and the Cold War: Military Spending
U.S. Navy mapped seafloor with echo sounding (sonar) to find and hide submarines. Generalized maps showed:
oceanic ridges—submerged mountain ranges
fracture zones—cracks perpendicular to ridges
trenches—narrow, deep gashes
abyssal plains—vast flat areas
seamounts—drowned undersea islands
Dredged rocks of the seafloor included only basalt, gabbro, and serpentinite—no continental materials.
The Rise of Plate Tectonics
Hypothesis: Stripes indicate periodic reversal of the direction of Earth’s magnetic field.
To test this hypothesis, scientists determined the eruptive ages AND the polarity of young basalts using the newly developed technique of K-Ar radiometric dating.
The study validated the reversal hypothesis...
The Rise of
And then (1962-1963) geologists realized that the patterns are SYMMETRICAL across oceanic ridges.
The K-Ar dates
show the youngest
rocks at the ridge.
Meanwhile, U.S. military developed new, advanced seismometers to monitor Soviet nuclear tests.
By the late 1950s, seismometers had been deployed in over 40 allied countries and was recording 24 hrs/day, 365 days/year.
Besides the occasional nuclear test, it recorded every moderate to large earthquake on the planet. With these high-precision data, seismologists found that activity happens in narrow bands.
Bands of seismicity—chiefly at trenches and oceanic ridges
Divergent boundaries: Chiefly at oceanic ridges
(aka spreading centers)
How magnetic reversals form at a spreading center
Divergent boundaries also can rip apart (“rift”) continents
How rifting of a
continent could lead to formation of
e.g., East Africa Rift
e.g., Red Sea
e.g., Atlantic Ocean
Pangea was ripped apart by such continental
rifting & drifting.
Subduction zones form at convergent boundaries if at least one side has oceanic (denser) material.
Modern examples: Andes, Cascades
Major features: trench, biggest EQs, explosive volcanoes
Another subduction zone—this one with
oceanic material on both sides.
Modern example: Japan
Earthquake depth indicates subduction zones
Collison zones form where both sides of a convergent boundary consist of continental (buoyant) material.
Modern example: Himalayas
This probably used to be a subduction zone,
but all the oceanic material was subducted.
Most transform boundaries
are in the oceans.
Some, like the one in California, cut continents.
The PAC-NA plate boundary is MUCH more complex than this diagram shows.
Hotspots, such as the one under Hawaii,
have validated plate tectonic theory.