Continental Drift- The Theory

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Continental Drift-The Theory
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The Earths crust is broken into about 12 rigid
plates, which slide over a semi-molten plastic
layer of the mantle.
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Tectonic Plates on Modern Earth
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There are three types of plate boundaries

• 1- Convergent
• 2- Divergent
• 3- Transform

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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
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The material making up the plates can be
divided in two types i. Oceanic crust ii.
Continental crust
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The driving force for plate tectonics is slow
moving convection currents in the underlying
plastic mantle material.
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• lava lamp!

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How does convection work? No one knowsbut they
arent afraid to propose models!
Whole-mantle convection
Two mantle convection cells
Complex convection
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The History of the Theory of Continental Drift
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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.
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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.
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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 balanced
planet. 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.
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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).

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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.
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Earth 200 million years ago
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Day 2
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Continental Drift Evidence

• Geographic fit of South America and Africa
• Fossils match across oceans
• Rock types and structures match across oceans
• Ancient glacial features
• Earthquake Patterns
• Volcano Patterns

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Continental Drift Evidence
Tight fit of the continents, especially
using continental shelves.
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Continental Drift Evidence
Fossil critters and plants
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Continental Drift Evidence
Correlation of mountains with nearly identical
rocks and structures
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Continental Drift Evidence
Glacial features of the same age restore to
a tight polar distribution.
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The Rise of Plate Tectonics
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 ridgessubmerged mountain
ranges fracture zonescracks perpendicular to
ridges trenchesnarrow, deep gashes abyssal
plainsvast flat areas seamountsdrowned
undersea islands
Dredged rocks of the seafloor included only
basalt, gabbro, and serpentiniteno continental
materials.
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The Rise of Plate Tectonics
Hypothesis Stripes indicate periodic reversal
of the direction of Earths 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...
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The Rise of Plate Tectonics
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.
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The Rise of Plate Tectonics
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.
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Bands of seismicitychiefly at trenches and
oceanic ridges
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Divergent boundaries Chiefly at oceanic
ridges (aka spreading centers)
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How magnetic reversals form at a spreading center
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Divergent boundaries also can rip apart (rift)
continents
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How rifting of a continent could lead to
formation of oceanic lithosphere.
e.g., East Africa Rift
e.g., Red Sea
e.g., Atlantic Ocean
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Presumably, Pangea was ripped apart by such
continental rifting drifting.
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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
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Another subduction zonethis one with oceanic
material on both sides.
Modern example Japan
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Earthquake depth indicates subduction zones
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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.
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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.
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Hotspots, such as the one under Hawaii, have
validated plate tectonic theory.