adapted to HTML from lecture notes of Prof. Stephen A. Nelson Tulane
Plate Tectonics is a theory developed in the late 1960s, to explain how
the outer layers of the Earth move and deform. The theory has caused a
revolution in the way we think about the Earth. Since the development of
the plate tectonics theory, geologists have had to reexamine almost every
aspect of Geology. Plate tectonics has proven to be so useful that it can
predict geologic events and explain almost all aspects of what we see on
the Earth. Although we have discussed plate tectonics throughout the
course, in this chapter and lecture we look at how the theory came to be
discovered and some of the implications for the evolution of the Earth.
Tectonic theories attempt to explain why mountains, earthquakes, and
volcanoes occur where they do, the ages of deformational events, and the
ages and shapes of continents and ocean basins.
Late 19th Century theories
Contraction of the Earth due to cooling. This is analogous to what
happens to the skin of an apple as the interior shrinks as it
dehydrates. This could explain compressional features, like
fold/thrust mountain belts, but could not explain extensional
features, such as rift valleys and ocean basins. Nor could it explain
the shapes and positions of the continents.
Expansion of the Earth due to heating. This was suggested after
radioactivity was discovered. This could explain why the continents
are broken up, and could easily explain extensional features, but did
not do very well at explaining compressional features.
Wegner's Theory of Continental Drift
Alfred Wegner was a German Meteorologist who studied ancient climates
in the early 1900s. Like most people the jigsaw puzzle appearance of
the Atlantic continental margins, caught his attention. He put
together the evidence of ancient glaciations and the distribution of
fossil to formulate a theory wherein the continents moved over the
surface of the Earth, sometimes forming large supercontinents and
other times forming separate continental masses. He proposed that
prior to about 200 million years ago all of the continents formed one
large land mass that he called Pangea .
The weakness of Wegner's theory, and the reason it was not readily
accepted by geologists was that he proposed that the continents slide
over ocean floor. Geophysicists disagreed, stating the ocean floor did
not have enough strength to hold the continents and too much
frictional resistance would be encountered.
In 1950s and 1960s, studies of the Earth's magnetic field and how it
varied through time (paleomagnetism) provided new evidence that would
prove that the continents do indeed drift. In order to understand
these developments, we must first discuss the Earth's magnetic field
and the study of Paleomagnetism.
Magnetic Field and Paleomagnetism
The Earth has a magnetic field that causes a compass needle to always
point toward the North magnetic pole, currently located near the rotation
pole. The Earth's magnetic field is what would be expected if there were a
large bar magnet located at the center of the Earth (we now know that this
is not what causes the magnetic field, but the analogy is still good). The
magnetic field is composed of lines of force as shown in the diagram here.
A compass needle or a magnetic weight suspended from a string, points
along these lines of force. Note that the lines of force intersect the
surface of the Earth at various angles that depend on position on the
Earth's surface. This angle is called the magnetic inclination. The
inclination is 0o at the magnetic equator and 90o at the magnetic poles.
Thus, by measuring the inclination and the angle to the magnetic pole, one
can tell position on the Earth relative to the magnetic poles.
In the 1950s it was discovered that when magnetic minerals cool below a
temperature called the Curie Temperature domains within the magnetic
mineral take on an orientation parallel to any external magnetic field
present at the time they cool below this temperature.At temperatures above
the Curie Temperature, permanent magnetization of materials is not
possible. Since the magnetic minerals take on the orientation of the
magnetic field present during cooling, we can determine the orientation of
the magnetic field present at the time the rock containing the mineral
cooled below the Curie Temperature, and thus, be able to determine the
position of the magnetic pole at that time. This made possible the study
of Paleomagnetism (the history of the Earth's magnetic field). Magnetite
is the most common magnetic mineral in the Earth's crust and has a Curie
Temperature of 580oC
Initial studies of the how the position of the Earth's magnetic pole
varied with time were conducted in Europe. These studies showed that the
magnetic pole had apparently moved through time. When similar measurements
were made on rocks of various ages in North America, however, a different
path of the magnetic pole was found.This either suggested that (1) the
Earth has had more than one magnetic pole at various times in the past
(not likely), or (2) that the different continents have moved relative to
each other over time. Studies of ancient pole positions for other
continents confirmed the latter hypothesis, and seemed to confirm the
theory of Continental Drift.
During World War II, geologists employed by the military carried out
studies of the sea floor, a part of the Earth that had received little
scientific study. The purpose of these studies was to understand the
topography of the sea floor to find hiding places for both Allied and
enemy submarines. The topographic studies involved measuring the depth to
the sea floor. These studies revealed the presence of two important
topographic features of the ocean floor:
Oceanic Ridges - long sinuous ridges that occupy the middle of the
Atlantic Ocean and the eastern part of the Pacific Ocean.
Oceanic Trenches - deep trenches along the margins of continents,
particularly surrounding the Pacific Ocean.
Another type of study involved towing a magnetometer (for measuring
magnetic materials) behind ships to detect submarines. The records from
the magnetometers, however, revealed that there were magnetic anomalies on
the sea floor, with magnetic high areas running along the oceanic ridges,
and parallel bands of alternating high and low magnetism on either side of
the oceanic ridges. Before these features can be understood, we need to
first discuss another development in the field of Paleomagnetism - the
discovery of reversals of the Earth's magnetic field and the magnetic time
Reversals of the Earth's Magnetic Field.
Studying piles of lava flows on the continents geophysicists found that
over short time scales the Earth's magnetic field undergoes polarity
reversals (The north magnetic pole becomes the south magnetic pole) By
dating the rocks by radiometric techniques and correlating the reversals
throughout the world they were able to establish the magnetic time scale
Vine, Matthews, and Morely put this information together with the bands of
magnetic stripes on the sea floor and postulated that the bands represents
oppositely polarized rocks on either side of the oceanic ridges, and that
new oceanic crust and lithosphere was created at the oceanic ridge by
eruption and intrusion of magma. As this magma cooled it took on the
magnetism of the magnetic field at the time. When the polarity of the
field changed new crust and lithosphere created at the ridge would take on
the different polarity. This hypothesis led to the theory of sea floor
If new oceanic crust and lithosphere is continually being created at the
oceanic ridges, the oceans should be expanding indefinitely, unless there
were a mechanism to destroy the oceanic lithosphere. Benioff zones and the
oceanic trenches provided the answer: Oceanic lithosphere returns to the
mantle by sliding downward at the oceanic trenches (subducting). Because
oceanic lithosphere is cold and brittle, it fractures as it descends back
into the mantle. As it fractures it produces earthquakes that get
By combining the sea floor spreading theory with continental drift and
earthquake information the new theory of Plate Tectonics became a coherent
theory to explain crustal movements.
Plates are composed of lithosphere, about 100 km thick, that "float" on
the ductile asthenosphere. While the continents do indeed appear to drift,
they do so only because they are part of larger plates that float and move
horizontally on the upper mantle asthenosphere. The plates behave as rigid
bodies with some ability to flex, but deformation occurs only where along
the boundaries between plates.
Types of Plate Boundaries
Divergent Plate Boundaries
These are oceanic ridges where new oceanic lithosphere is created by
upwelling mantle that melts, resulting in basaltic magmas which intrude
and erupt at the oceanic ridge to create new oceanic lithosphere and
crust. As new oceanic crust is created it is pushed aside in two
directions. Thus, the age of the oceanic crust becomes progressively
greater in both directions away from the ridge. Because oceanic
lithosphere may get subducted, the age of the ocean basins is relatively
young. The oldest oceanic crust occurs farthest away from a ridge. In the
Atlantic Ocean, the oldest oceanic crust occurs next to the North American
and African continents and is about 180 million years old (Jurassic) . In
the Pacific Ocean, the oldest crust is also Jurassic in age, and occurs
near the coast of Japan.
Because the oceanic ridges are areas of young crust, there is very little
sediment accumulation on the ridges. Sediment thickness increases in both
directions away of the ridge, and is thickest where the oceanic crust is
Knowing the age of the crust and the distance from the ridge, the relative
velocity of the plates can be determined. (Absolute velocity requires
further information to be discussed later).
Relative plate velocities vary both for individual plates and for
Variations in individual plate velocities occur because spreading
of the sea floor takes place on a spherical surface rather than on a
flat surface. Velocities are greatest at large distances away from the
Different plates have different velocities depending on the amount
of continental lithosphere within the plate. Plates with continental
lithosphere have lower relative velocities than plates with only
Sea floor topography is controlled by the age of the oceanic
lithosphere and the rate of spreading.
If the spreading rate (relative velocity) is high, magma must be rising
rapidly and the lithosphere is relatively hot beneath the ridge. Thus for
fast spreading centers the ridge stands at higher elevations than for slow
spreading centers. The rift valley at fast spreading centers is narrower
than at slow spreading centers.
As oceanic lithosphere moves away from the ridge, it cools and sinks
deeper into the asthenosphere. Thus, the depth to the sea floor increases
with increasing age away from the ridge.
Convergent Plate Boundaries
When a plate of dense oceanic lithosphere moving in one direction collides
with a plate moving in the opposite direction, one of the plates subducts
beneath the other. Where this occurs an oceanic trench forms on the sea
floor and the sinking plate becomes a subduction zone. The Benioff zone
identifies subduction zones. The earthquakes may extend down to depths of
700 km before the subducting plate heats up and loses its ability to
deform in a brittle fashion.
As the oceanic plate subducts, it begins to heat up and metamorphose. As
it does so, dehydration reactions release water into the overlying mantle
asthenosphere, causing a reduction in the melting temperature and the
production of andesitic magmas. These magmas rise to the surface and
create a magmatic arc parallel to the trench.
If the subduction occurs beneath oceanic lithosphere, an island arc is
produced at the surface (such as the Japanese islands, the Aleutian
Islands, the Philippine islands, or the Caribbean islands