Today plate tectonics is a mature theory but it is remarkably recent
theories go. Most of the key elements of the theory have been
within the last 30 years but the idea that the continents had moved to
their present positions from somewhere else is a very old idea.
- The Continental Drift Hypothesis--as early as ca. 1600,
when most of the Earth had been explored and reasonably well mapped, at
least with respect to the positions and shapes of the continents, it
was pointed out that Africa, South America, North America, and Europe
appeared to fit together like pieces of a jigsaw puzzle
and then had drifted apart.
- A more fully elucidated hypothesis
was put forth by Alfred Wegener
in 1915. He cited additional evidence
for this breakup--notably the similarity
of rocks, geologic
structures, and fossils
on opposite sides of the
p. 78-79, Figs. 4.3, 4.4). He
called the assembled supercontinent Pangaea (CP&M p. 77, Fig.
The hypothesis languished for 40 years or so until the late 1950's and
early 1960's when a
number of important discoveries were made that began to revive the
continental drift hypothesis and radically alter
our view of the Earth.
B. Mapping the sea-floor and paleomagnetism (note: Chapter 3
of CP&M was not assigned as reading for this topic but forms good
background for some of the discussion of plate tectonics and some
references to figures refer you to chapter 3)
- Mapping of the Ocean Floor and Magnetic Stripes--at the
of the Second World War geologists began to develop the first detailed
maps of the topography of the ocean
floor. The development of
depth-sounding techniques (or sonar--used to look for German U-boats)
allowed these maps to be constructed. Today we still study the
oceans using this method and many other more 'cutting-edge' techniques (CP&M p. 52, Figs.
3.1, 3.2, 3.3, 3.4). One of the first major features
to be mapped was the Mid-Atlantic
Ridge, which looked like a
big seam that split the Atlantic ocean floor down the middle. An
more startling discovery was made by looking at the magnetism
of the ocean floor, which showed that as one moved perpendicularly away
from the ridge axis, the magnetic
ocean floor rocks (a type of igneous
rock called basalt)
alternated and that this
pattern was symmetric on either side of the ridge, producing a series
of magnetic stripes (CP&M p. 77, Fig. 4.14).
- What do we mean by magnetic polarity? In the 1950's
geophysicists developed a method for measuring the
magnetic polarity of a rock. Natural magnets are made from magnetite
(an iron oxide mineral) and magnetite is found in small quantities in
almost all igneous rocks (and many sedimentary and metamorphic rocks as
- The magnetite
minerals in an igneous rock will become polarized
in the direction of the Earth's ambient (look it up if you don't know
what this word means!) magnetic field (a magnetic "dipole"--like a big
magnet in the Earth, CP&M p. 41, Figs.
once the rock solidifies and cools
through a temperature of about 580°C (the Curie
Temperature or Curie
Point) As long as the rock is not reheated
above this temperature, it will record the polarity of the magnetic
field of the Earth at
the time it cooled through the Curie point.
looking at the polarity of rocks magnetized at different times in
Earth's history (especially lava flows that have cooled to form solid
have determined that the Earth's magnetic pole
has flipped back and
forth so that at
certain times in the past, a magnetic
would have pointed S instead of N (CP&M p. 41, Fig.
That is, the magnetic force lines of
the Earth, which point north today, would point south instead and so a
needle would have pointed to the south (CP&M p. 41, Figs.
2.21). The rocks record the earth's paleomagnetic
field. What causes these polarity reversals is not
known--we only know it happens and has happened many times in the past.
The rocks record these flip-flops (CP&M p. 41, Figs. 2.22, 2.23).
are magnetized in the same direction as the present-day Earth's
magnetic field are said to have normal
polarity, while those aligned
oppositely are said have reverse polarity.
- By determining the age
the rocks together with the magnetic polarity determinations it was
possible to construct a magnetic polarity time scale.
- Wandering Magnetic Poles--in addition to
polarity in the rocks, geophysicists also were able to determine the
position of the Earth's magnetic pole by examining the inclination (or
'magnetic dip') recorded inside the tiny magnetite grains (CP&M p. 81, Fig.
4.8). They found that ancient rocks (cooled lava
flows) of varying ages on different continents gave pole positions that
were very different from each other. These pole positions are linked
together to form an apparent polar wander path (or APW path as it
called). This seemed odd because the
present-day magnetic pole is aligned fairly close to the spin axis of
the Earth, and why
would two continents have such different magnetic
poles in the past? The geophysicists working on APW paths
the radical proposition that it was
not the poles that had wandered (they actually do wander a little bit)
but rather the
continents that had
wandered (i.e. Continental Drift).
At one time the continents
were together, but because the ancient lava rocks in which the magnetic
poles were measured have moved from the place where they were
originally erupted, the pole that is measured relative to the present
position of the rocks appears to be very different from the present
magnetic pole (CP&M p. 81, Figs.
- Sea-Floor Spreading--A Princeton geologist named Harry
Hess proposed an explanation for the topography of the
suggested that the sea-floor separates laterally along the rifts in the
ocean and that new magma from the mantle rises up to fill the seam. The
was called sea-floor
support for Hess's hypothesis, the geophysicists who mapped the
magnetic stripes suggested that the symmetric pattern of magnetic
reversals was like
a magnetic tape recorder recording the divergence of the
ocean crust (Vine-Matthews
hypothesis). As new magma arose to
fill the crack, as the crust spread away, it would become magnetized
with whatever the polarity of the Earth's magnetic field was at the
time (CP&M p. 85-87, Figs.
4.14, 4.15, 4.16, 4.17, 4.18).
This was very strong evidence that Hess was probably right.
- The other
major piece of evidence came from geologic age
dating. Most of the ocean floor is covered with a veneer
of sediment that is of biologic origin (primarily from tiny organisms
living in the ocean water). When they die the tiny shells of
these organisms (microfossils) fall to the bottom of the ocean floor
and accumulate. Ocean drilling research in the 1960's
recovered the microfossil sediments overlying the ocean crust from the
ocean floor (CP&M
p. 67, Fig. 3.23). Paleontologists
dated the sediments using fossil dating. The age-dating confirmed that
the ocean floor near the mid-ocean ridges was indeed very young because
only a thin veneer of very young sediment was present right at the
ridge crest. Moving away to either side of the ridge the sediment
was thicker (CP&M
p. 67, Fig. 3.24) and ages
determined from the fossils were progressively older in sediments
recovered from deeper in the drill hole.
- Thus the relatively young
age of the ocean floor compared to the great antiquity of the
continents and the symmetric pattern of magnetic reversals that
recorded the same polarity reversals that were recorded in rocks on
land was very powerful evidence that supported the sea-floor spreading
hypothesis. An amazing realization--approximately
70% of the
earth's surface has been created within the last 140 million or so
years of Earth's history (!), even though the Earth itself is
over 30 times
that old (4.55 billion years).
- Oceanic Transform Faults--It
was realized from ocean floor mapping that the oceanic ridges were
segmented into discrete pieces of ridge, and offset from each other
across deep linear cracks in the ocean floor (called oceanic fracture zones;
61, Fig. 3.16).
A Canadian geologist (J. Tuzo Wilson)
proposed that these cracks were
in fact a previously unknown type of geologic fault (a fracture in a
mass of rock across which movement has occurred). Wilson called
faults and he suggested that these faults were a type of
boundary dividing two moving pieces of ocean floor.
- Geologists had known for many years that large masses of the
Earth's crust could slide past each other along vertical fractures
because of the many years of study of large faults on land like the San
Andreas fault of California.
- Wilson accepted that spreading of the sea-floor occured at
ridges, but if so then it predicted that the earthquakes should be
concentrated almost entirely between the two offset ridge segments
along the oceanic fracture zone because this is the only part of
the fracture zone along which movement was actually taking place, if
ocean floor was spreading apart at the ridges (CP&M p. 61, Fig. 3.20
& p. 88, Fig. 4.19).
- The most common transform faults are those that occur between
offset segments of the mid-ocean ridges (CP&M p. 93, Fig. 4.26A). Wilson
also showed that transform faults can connect two subduction zone
p. 93, Fig. 4.26C) or even a ridge
and a subduction zone trench (CP&M p. 93, Fig. 4.26B).
This last situation is basically what happens along the San Andreas
fault where the San Andreas links a small oceanic ridge segment in the
Gulf of California with a zone of subduction that begins off the coast
of northern California (CP&M p. 93, Fig. 4.26D).
- The name transform fault was given to this new type of fault by
the faults 'transform' one type of plate motion into another type of
motion (i.e. along the fault the plates slide laterally past each other
but where the fault terminates the motion between the two plates
C. Great faults and deep earthquakes in the Pacific
- Wadati-Benioff Zones and the origin of very deep earthquakes--the
sea-floor spreading hypothesis
began to gain many adherents but it left a problem unresolved--where
did all the older ocean crust go? We have on land preserved evidence of
ocean-dwelling organisms that had lived many hundreds of millions of
years prior to the oldest known ocean crust.
- A Japanese earthquake scientist (seismologist) named Kiyoo Wadati,
and an American seismologist Hugo Benioff
independently studied deep earthquakes that were
associated with Pacific volcanic chains (called volcanic arcs)
like Japan and
the Aleutian Islands of Alaska, and also associated with the deep ocean trenches
in front of the arcs (CP&M p. 61, Fig. 3.15).
These two seismologists mapped out the
depth of origin of earthquakes in the vicinity of these volcanic arc
found a curious aspect to them--they found that they could map out a
plane of earthquakes that was inclined
toward the volcanic arcs and
extended down from near the surface to depths of approximately 700 km (CP&M p. 177, Fig. 7.23).
within the Earth, earthquakes at such great
depths are not observed or recorded.
- The existence of Wadati-Benioff zones
(as they are now called) was very odd, because the rock materials must
be capable of breaking or failing in a catastrophic way or moving
suddenly to cause an earthquake when subjected to stresses. At
such great depths, rocks should generally be so hot (based on measured
and calculated geothermal gradients in the Earth) that rock materials
should not deform in this way (i.e. they are too hot to store up the
necessary 'elastic' strain to cause an earthquake). Benioff
proposed that a very deep fault, along which these earthquakes occured,
extended from the oceanic trenches to well down beneath the volcanic
arcs, and that cold material that was capable of releasing energy
suddenly in an earthquake must be penetrating the mantle to these great
- We now know that these are subducted slabs of
lithosphere. Because they have spent many millions of years at
near-surface conditions, and the heat they emit has been absorbed by
the cold sewater above them over this long time period, the rock
materials that constitute the slab are indeed cold by the time they
reach the deep ocean trench where they bend down into the upper mantle
and are overidden by the other plate (on which the volcanic arc forms)
D. A Mature Theory (a
'Paradigm') and Driving Forces
In 1967 two geophysicists at Cambridge University in the U.K.
(MacKenzie and Morgan)
the theory of plate tectonics:
the Earth is broken into rigid plates
which move away from each other at mid-ocean ridges (as Hess proposed),
slide past each other along transform faults (as Wilson proposed), and
into the Earth's mantle at the trenches, where Wadati and Benioff
had mapped the deep, inclined earthquake fault zones that we now call
Within the span of about ten
years, continental drift had gone from
hypothesis to a theory with predictive power about the large-scale
dynamics of the
Earth. The principal conceptual objection to continental
drift was overcome. The continents did not plow through the ocean basin
but were instead just more buoyant (less dense) lithosphere that was
attached to the more dense oceanic
lithosphere, forming large plates of lithosphere that were riding on
mantle asthenosphere. Today we recognize about a dozen discrete
p. 75, Fig. 4.1) bounded by the
three types of plate boundaries (divergent,
transform) that we have discussed.
We accept that this is our working theory of
the dynamics of the outer Earth and plate tectonics contributes to and
has implications for many other phenomena and processes occurring on
and within the earth including:
Such an overarching framework within science
sometimes called a 'paradigm'.
Much work since the development of plate tectonic theory has been a
filling in many of
the details of plate tectonics and in trying to undertstand how plate
processes are involved in many other geologic processes, including
trying to understand the forces that
drive the plates.
- The formation of magma and the location of most of the world's
construction and location of major mountain belts
- where strategically important ores and minerals form
- the formation of
basins of sediment where energy resources (like oil and gas) are found
- the locations of most earthquakes
- long-term biological evolution
- long-term climate
- The most basic question
that must be answered is: why do plates
diverge and then sometimes sink back into the mantle? The
force(s) must also be compatible with the following 3 observations:
- Ridges are hot
elevated well above the average depth of the
- Oceanic trenches are cold and much
deeper than the average
depth of the sea-floor
- the 'leading edges'
of plates (meaning the edge that
is at the side opposite a plate's divergent edge) can
lithosphere or oceanic lithosphere, but only those with a
that is composed of oceanic lithosphere are capable of subducting.
- Several possible forces are proposed to act on the plates, of
which probably the most important are: "ridge push", "slab
pull", and "trench suction" as they are commonly called.
- The ridge area is elevated because it is hot and therefore less
dense (more buoyant).
As the lithosphere diverges it cools, thickens, and
becomes more dense and sinks. Ridge push
refers to the gravitational 'sliding' of the lithosphere on the
from the ridge (i.e. from higher elevation at the
ridge to progressively lower elevations moving perpendicular away from
the ridge; CP&M p. 101, Fig.
lithosphere is dense, partly because it is older and
colder than new lithosphere created at the ridge, but also
the lithosphere in a subduction zone sinks back into the asthenosphere,
chemical reactions occur in the subducting lithospshere to change the
minerals in the subducting plate into a new assemblage of
minerals that is much more dense than the
original assemblage of minerals when the plate was still at the
trench (this change from one set of
minerals to a new set of minerals is a type of metamorphism
that occurs under the very high pressures that the slab feels as its
pull is the pulling force that the lithosphere closer to the
surface feels at is being
'pulled on' by the very dense subducting lithosphere (CP&M p. 102, Fig 4.39).
- In addition to feeling this pull of the subducting slab, the
plate also will sink or
tend to fall down
under its own weight at a steeper angle than the dip angle of
the plate when it enters the subduction zone at the trench,
particularly if it is old
enough and dense
enough. As it does, this will cause the trench to migrate in an
oceanward direction and overriding
plate will therefore get 'pulled' horizontally in an oceanward
p. 96, Fig. 4.31B).
This force is called trench suction
- The ridge push
itself is probably not strong enough to move a continental
plate, especially a very big one. The slab pull force is a strong
force but it cannot
be exerted on a plate if the leading edge is a continent because
continental lithosphere cannot subduct. Thus, trench suction combined
with ridge push must be the main forces that drive plates whose
leading edges are continents and can help to explain why plates with
continental leading edges can diverge from another plate.