Way back in 1912 a scientist by the
name of Alfred Wegener came up with a crazy idea. He noticed that all
of the continents seemed to fit together like the pieces of a giant
puzzle. He thought, "Maybe they were once all joined together in
a single, giant landmass that broke up and drifted apart over time?".
He decided to give this supercontinent a name and called it Pangea,
meaning, "all lands". At the time he presented his idea to
the scientific community it came to be known as continental
drift theory. Wegener was unable to find solid evidence
to support his theory, so the other scientists laughed him off as a
crackpot. One of his suggestions for the cause of continental drift
was that centrifugal force from
the rotation of the earth caused the continents to slide into each other
and move around on the surface. They all calculated that there wasn't
enough force generated by the earth's rotation to cause shifting of
the crust and nobody took him seriously. They were all convinced the
earth was rock-solid and immovable.
But then in 1929, along came a scientist
named Arthur Holmes who didn't think that Wegener's theory of continental
drift was too farfetched. "Now wait just a minute. Maybe he's got
something here", he told them. He mentioned one of Wegener's other
theories about the source of continental drift; the idea that the molten
mantle beneath the earth's crust experiences
thermal convection and that the movement of these convection
currents in the mantle could cause an upwelling beneath
the crust, forcing it to break apart and move. Now, that sounded like
a semi-reasonable explanation for the movement of the earth's crust.
As a matter of fact, if you looked closely at this idea it explained
a lot of things, not just the continental puzzle idea. It also explained
how mountain ranges were formed - by continents crashing into each other
and 'rumpling up rock'. Still, the other scientists just nodded and
said, 'Yeah. Fine. Whatever'. And the theory was neatly tucked away
and ignored.
Scientists are trained to be skeptical.
They were all waiting for a preponderance of evidence that backed up
this harebrained theory.
See the largest crystals in the world - HUGE!
Over the next thirty years a lot of
new and surprise discoveries were made as new technologies were developed
for exploring the ocean floor . The discovery of volcanic activity on
the ocean floor in the middle of the Antlantic that turned out to be
part of a long, unbroken mountain chain of undersea volcanoes was the
most ground-breaking discovery that supported the theory of continental
drift. Scientists developed instruments for measuring earthquake activity
around the world and began plotting the locations of earthquakes. They
all got together and started drawing
a new map of the world that showed volcanic and seismic (earthquake)
activity was concentrated along certain areas that looked like the margins
of huge crustal plates. During the 1960s several scientists published
papers that reviewed the preponderance of evidence that had been gathered
for the theory of continental drift and it soon came to be known as
the theory of plate tectonics.
The evidence that supports the theory consists of the following discoveries;
Mid-ocean Ridges
A spreading
boundary is where the tectonic plates
are separating. Some spreading boundaries are places where the crust
is sinking downward as it is stretched thin - like in the East Rift
Valley of Africa, where the Dead Sea is located.
Many of the spreading boundaries are located deep in the ocean on the
sea floor. These are places where volcanic activity is at a premium
because the crust is being torn apart. New crust is forming when magma
from the mantle deep down is forced upward out of the cracks where the
plates are coming apart. Long chains of undersea mounts (the world's
longest is the mid-Atlantic Ocean Ridge) and volcanic islands typically
characterize these type of plate margins.
Geomagnetic Anomalies
New rock formed from magma records the orientation of Earth's magnetic
field at the time the magma cooled. By collecting and measuring samples
of rock from various locations along the Mid-Atlantic Ridge, scientists
have discovered that the newest, youngest crustal rocks are located
in the center of the ridge, while the rocks get older as you move away
from the ridge center. This supports the idea that oceanic crust continues
to be pulled apart, while new crust is formed along the edges of the
plates.
Deep Sea Trenches
At the same time, some of the oldest ocean crust occurs in
deep sea trenches, which run parallel to continental mountain ranges.
A lot of very large earthquakes have been plotted along deep ocean trenches,
suggesting that these are seismically active
areas (meaning the crust is moving). Scientists put two and two together,
noting that the youngest oceanic crust was along the mid-ocean ridges
and the oldest ocean crust was along the very bottoms of deep sea trenches.
That neatly defined the edges of the tectonic plates and showed the
direction of their movement. Where the deep sea trenches were found
came to be called converging boundaries.
A converging
boundary is the opposite of a spreading
boundary. Typically you will see a converging boundary on a tectonic
plate that is on the opposite side of a spreading boundary. As a plate
moves in one direction it collides with the adjacent plate on its "front"
end in a deep sea trench, while the trailing end of the plate is being
pulled and stretched (spreading) from the plate on the other end at
a mid-ocean ridge. For example, look at the Pacific
plate. The entire plate is moving north and westward as the top edge
converges with the North American and European plates. You can
see the left side of the Pacific plate is converging
with the Indian plate. Then if you look at the bottom and right edges
of the plate you can see it's spreading apart from the Antarctic
and Nazca plates.
Sometimes you'll
see volcanic activity at converging boundaries where plates are crashing
into each other. When one plate (usually the lighter continental crust)
rides up over the top of the other it's called a subduction
zone - because one plate margin is
being subducted under the other.
A good example
of this type of plate margin is where the Nazca and South American plates
are crashing into each other. The lighter continental South American
plate is riding up over the heavier oceanic Nazca plate. Deep down where
the leading edge of the Nazca plate is diving down under the South American
plate it's making contact with the molten magma of the earth's mantle.
The long cordillera, or chord-like chain of volcanic mountains
known as the Andes, are a result of the rumpling of the South
American plate where the Nazca plate crashes into it, and the volcanoes
that have formed from the increased seismic activity on the Nazca plate
margin deep down.
In other converging
boundaries, there is no volcanic activity because the tectonic plates
are both continental plates, weighing the same. No subduction happens
along these margins, just massive deformation
of the edges of the plates. A good example of this is the Himalayan
Mountains where the European and Indian plates meet. The two plates
have continued ramming into each other, causing the crust to buckle,
wrinkle, and uplift into the highest mountain
range on earth.
The few transverse
boundaries are places where the two
plates are just sliding past each other. In many of these boundaries
there is a lot of tension and strain where the two plates are sliding
and scraping past each other. The resulting strain from the sliding
action of the plates causes cracks in the crust called faults.
As the larger plates move past each other some chunks of crust and overlying
rock are broken into fault blocks.
When there is a big enough movement along the cracks or faults in the
earth's crust we feel it in the form of earthquakes.
One of the most
famous faults is the San Andreas, which runs along the west coast of
California. It's famous for generating many of the larger quakes in
California, including the world-renowned San Francisco earthquake of
1906. Funny thing is, the 1906 earthquake itself didn't do nearly as
much damage as the fires that burned the city afterwards - all the water
mains had burst and broken during the 'quake so there was no water to
put out the fires!
Hot Spots
About 30 years ago a Geophysicist
named J. Tuzo Wilson came up with an idea to explain why there was volcanic
activity out in the middle of the Pacific Ocean, in the middle of the
huge Pacific Plate. At the time, scientists thought that volcanoes only
happened at plate boundaries, but nobody could explain why they were
happening out in the middle of a tectonic plate. Dr. Wilson said that
there are "hot spots", under the earth's crust in some places. These
are called hot spots because they
are places where a lot of heat is concentrated in a small area. The
heat causes the overlying rock to melt. Since the magma is liquid and
is lighter than the surrounding rock it "floats" to the surface and
forces its way out of fissures in the crust. once magma erupts through
the crust it is known as lava. Over time, the continual outpouring of
lava can form a sea mount or island volcano if the hot spot is under
the ocean floor, as in the case of the Hawaiian
Islands. There is just one hot spot that never moves. But the Pacific
Plate continually (and slowly) moves north over the hot spot, forming
a new volcano on the overlying plate each time.
Doing the
Science
Scientists had a lot of questions
about why there were volcanic islands way out in the middle of the Pacific
plate. It just didn't seem to fit in with their theory of plate tectonics.
Dr. Wilson's idea of hot spots helped the island volcanoes to fit into
the theory of plate tectonics. If the Pacific plate was moving over
a hot spot, then that would explain why a chain of sea mounts and volcanoes
had formed as the plate moved. If this was true, then the volcanoes
should be of different ages, from oldest to youngest in a single direction.
In order to test his theory, Dr. Wilson
took samples of volcanic rock from each of the volcanic islands in the
Hawaiian chain and tested them to see how old they were on a geologic
time scale. He found that the oldest rocks were from the northernmost
island of Kauai, which also had the most weathering of rock. He also
found that progressively younger rocks were found on the Hawaiian islands
the further south he went (see
map). The youngest rocks of all were found on the big island of
Hawaii, the southernmost island. In fact, new "rocks" are still forming
on the island of Hawaii, making it the youngest volcano in the island
chain. There is even more evidence to support his theory because there
is a new volcano forming on the sea floor south of Hawaii, called Loihi.
Right now it's just a sea mount, but if the lava continues to build
up on its slopes, someday it will be a new island.
Read about a scientist who "chases"
volcanoes
