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.
