Now that we understand the composition of the Earth, it’s time to get a closer look
at some important geological processes, starting with plate tectonics.
Plate tectonics may sound like a fancy food in a gastropub, but it’s actually a thoroughly
corroborated theory describing how Earth’s outermost layer, the lithosphere, slides around
on top of the ductile portion of the upper mantle, the asthenosphere.
Plate tectonics is responsible for everything from the striking peaks of the Himalayas to
the underwater smokers along mid-ocean ridges.
So how did this theory come to be?
The story begins with an astronomer named Alfred Wegener.
Dr. Wegener earned his PhD in astronomy at the University of Berlin in 1904.
In an odd turn of events, he spent most of his career studying the weather of very cold
places, like Greenland.
In 1911, Dr. Wegener came across a paper on the presence of similar Permian-aged fossils
in disparate places like South America and Africa, or Antarctica and Australia.
He concluded that the only way these organisms could have existed together would be if all
the continents had been joined together into one huge supercontinent, which he named Pangea.
Unfortunately, Dr. Wegener died of overexertion during one of his Greenland expeditions before
he could substantiate Pangea’s existence.
In fact, the idea of Pangea and the theory of plate tectonics that it relied upon was
resoundingly rejected throughout much of the 20th century.
Further evidence for plate tectonics came in the 1950s during the study of magnetic
minerals in rocks.
When an igneous rock crystallizes, tiny needles of the mineral magnetite align themselves
parallel to Earth’s magnetic field lines, and studying the orientation of these “magnetic
fossils”, so to speak, makes it possible to reconstruct Earth’s magnetic field over
time.
Now, Earth’s magnetic field does somewhat drift around the poles, but the study of these
magnetic rocks seemed to indicate that over the past 500 million years, Earth’s south
magnetic pole drifted from the northern hemisphere, through the equator, and to its present location
over the geographic south pole.
But the magnetic poles must be located near the geographic poles because the magnetic
field is strongly related to Earth’s rotation.
Therefore, this observation could only be explained by the movement of the continents
themselves over a semi-stationary field.
Throughout World War II, geologists used sonar to map the ocean floor and were shocked by
the topography they discovered.
The ocean floor wasn’t a flat plain, but rather was full of mountain ridges with perpendicular
fractures, deep trenches, and long chains of underwater volcanoes.
Based on this data, Harold Hess, a geologist from Princeton University and former World
War II submarine sonar operator, proposed a theory for how continents might move.
Hess postulated that the linear volcanic ridges at the centers of oceans were caused by areas
of convection-driven, hot, rising mantle that solidified and spread out laterally to create
new oceanic crust.
Evidence for this included low seismic velocities, the presence of volcanoes, and high heat flow
from the mantle at these mid-ocean ridges.
Once formed, oceanic crust moves away from the ridge, cooling and becoming more dense
with age until it eventually sinks back into the mantle at deep sea trenches.
According to Hess’s theory, the simultaneous creation of oceanic crust at mid ocean ridges,
and its destruction at subduction zones, would limit the age of the world’s ocean basins
to a few hundred million years, which agreed with the relatively thin layer of pelagic
sediment on the seafloor.
Though the evidence described so far may seem convincing, it was still only circumstantial
at the time, but not for long.
The smoking gun would eventually come from the discovery of magnetic striping of the
seafloor.
Prior to this discovery, paleomagnetism had been extensively studied in terrestrial rocks
and it was well known that the Earth’s magnetic field periodically reversed.
The current configuration of the magnetic field is said to be “normal” polarity
and the flipped polarity is called “reverse”.
Paradoxically, the magnetic pole located at the north pole today is actually the magnetic
south pole, since magnetic field lines always point from the north to the south pole of
a magnet.
At any rate, if Hess was right, the magnetic striping along the seafloor would be mirrored
across the ridge and correspond to the already known record of magnetic reversals, and in
1963, Fred Vine and his PhD advisor Drummond Matthews provided the evidence, leading to
the wide acceptance of the theory of plate tectonics.
While Dr. Hess’s work explained the formation of mid-ocean ridges and trenches, there were
still two pesky questions left unexplained.
First, how were seamounts formed, like the Hawaiian Islands, and second, what created
long perpendicular fractures along mid ocean ridges, as exemplified along the South Atlantic
Ridge?
In 1963 Canadian geologist J. Tuzo Wilson figured out the answer.
He proposed that seamounts were formed due to volcanism induced by a cylindrical, stationary
area of hot, rising mantle called hotspots, or more specifically, mantle plumes.
Using the Hawaiian Islands and the Emperor seamounts as an example, Dr. Wilson observed
that the alignment of the island chains followed the motion of the Pacific plate, which led
him to develop the following understanding of seamounts.
At some point in time, a mantle plume forms beneath an area of the crust which leads to
the initiation of volcanism.
Then over millions of years, the plate moves steadily over the stationary mantle plume,
leaving a long trail of inactive volcanic islands behind a front of active volcanism,
which is where the islands are created.
As an area of the crust moves off the hotspot, volcanism shuts off and the seamount begins
to be slowly eroded by the sea.
When looking at a satellite image of the Pacific Ocean, you can see a chain of increasingly
eroded seamounts extending all the way from Hawaii, which is currently beneath the plume,
to the Aleutians.
This is a precise recording of the Pacific plate’s motion over the past 65 million
years, called a hotspot track.
Interestingly, it is not linear, but is shaped like a hockey stick, which beautifully illustrates
a sudden change in plate motion that occurred around 40 million years ago.
Two years after proposing mantle plumes, Dr. Wilson suggested an explanation for the perpendicular
faults along mid ocean ridges.
Mid-ocean ridges don’t form in a single continuous line, but are made up of hundreds
of ridge segments.
Each ridge segment is connected to its neighbor by a perpendicular feature called a transform
fault, which is just a type of strike-slip fault that connects two plate boundaries.
When new crust is formed along a mid-ocean ridge, transform faults form to accommodate
changes in the spreading rate along the ridge.
With these two final questions answered, Dr. Wilson proposed a unified theory of plate
tectonics, combing Hess’s ideas with his own.
Today, the theory of plate tectonics is accepted by any serious geologist and has been expanded
upon to explain the periodic creation and destruction of single-continent land masses
called supercontinents.
That cycle, the Wilson cycle, is what we will tackle next, so let’s move forward and learn
more about tectonic plates.