In previous videos we learned about plate tectonics, how the Earth’s surface is covered
by thick plates of various sizes, and how these plates move.
We saw how plates are constantly and very slowly moving, sometimes towards or away from
each other and sometimes just sliding past each other, a bit like what happens with students
in crowded school hallways between classes.
We also learned how these slow, continuous movements cause earthquakes and volcanic eruptions
and make mountains and rift valleys.
Earth’s jostling plates have torn continents apart and put new ones together at the same
time making new oceans appear and old ones disappear.
These related processes are called continental drift and seafloor spreading and these ideas
led to the Theory of Plate Tectonics.
In this video, we’ll learn how scientists pieced together how continental drift and
seafloor spreading shape the solid surface of our world, leading to the development of
Plate Tectonic Theory.
Scientists saw evidence for continental drift first.
Since the 16th century, mapmakers noticed the jigsaw-puzzle fit of the coasts of Africa
and South America.
This suggested to some that these two continents were once together and had since drifted apart.
All through the 1900’s scientists began to understand that the Earth had been around
for a very long time, although it wasn’t until the middle of the 20th century that
we learned it was more than four and a half billion years old.
It’s much easier to imagine the continents moving great distances from each other even
if they move very slowly if there is enough time.
In the late 1800s, a Swiss geologist named Eduard Suess was amazed by the similarity
of rocks and fossils of different southern continents and tried to explain this.
He proposed that all the southern continents, not just Africa and South America but also
Antarctica, Australia.
and the subcontinent of India was combined into a supercontinent he called Gondwana.
Suess named the supercontinent after the Gond people of India.
At the beginning of the 20th century, a German scientist named Alfred Wegener built on these
ideas about supercontinents breaking up to argue that a supercontinent even bigger than
Gondwana existed.
He argued that all of the continents were together in a supercontinent he called Pangea,
a name that in Greek means “All Earth.
He suggested that Pangea broke up during the Mesozoic Era, or “middle-life” era, which
is also called the age of dinosaurs.
Wegner’s theory was challenged because scientists didn’t yet understand how it was possible
for the continents to move.
They couldn’t plow through Earth’s crust and upper mantle like a ship moves through
water, so how could they move to make supercontinents like Gondwana and Pangea and then move apart
to where they are today?
The answer lay at the bottom of the sea, but we knew almost nothing about the rocks on
the seafloor.
The Second World War in the early 1940s led to two important advances in technology designed
to find enemy submarines: magnetometers and SONAR.
Magnetometers measure small changes in Earth’s magnetic field, such as might be caused by
a nearby submarine, and SONAR sends out pulses of sound and listens for reflections due to
a nearby sub.
After the war was over, scientists began using these two tools to map seafloor depth and
magnetization.
Scientists like Marie Tharp generated the first maps of seafloor depths and discovered
a great underwater mountain range that could be followed down the middle of the Atlantic
Ocean and called it the Mid-Atlantic Ridge.
Marie Tharp also discovered that there was a deep rift valley that could be traced all
along the Mid-Atlantic Ridge.
Scientists also discovered that the seafloor around the Mid-Atlantic Ridge was magnetized
into a strange pattern of parallel bands of weaker and stronger magnetic stripes.
In 1961, two scientists Robert Dietz and Harry Hess used the new information about the seafloor
to propose that “sea floor spreading” was occurring along Mid-Ocean Ridges.
They argued that seafloor spreading created new oceanic crust by volcanic activity along
the Ridge, making the ocean a little wider every year, as we saw in episode 4.
Seafloor spreading helped explain continental drift.
The continents move away from each other as the ocean widens by seafloor spreading between
them.
The theory of seafloor spreading was confirmed in 1966 by discovery of the symmetric magnetic
patterns on either side of the mid-ocean ridges.
Remember we mentioned in a previous episode that oceanic plates are rich in iron?
The abundance of iron makes the oceanic crust like a tape recorder that can remembers the
history of Earth’s magnetic field.
Another important discovery in the 1960s was the discovery of transform faults.
Ah-ha, now you probably recall episode 5 on transform plate boundaries.
One last mystery had to be solved before the discoveries of continental drift and seafloor
spreading could emerge as the Theory of Plate Tectonics: what happens to old seafloor?
If the Atlantic Ocean is getting wider and wider, either the Earth is increasing in size
or old seafloor must disappear somewhere else.
The answer was found in the Pacific Ocean.
The Pacific Ocean is very different from Atlantic Ocean and the biggest differences are seen
around the margins of the two oceans.
In episode 3, we talked about convergent plate margins, where oceanic lithosphere is subducted
back into Earth’s mantle.
We identified three things that made it easy to identify subduction zones: deep oceanic
trenches, zones of earthquakes that can be traced deep into the mantle, and chains of
volcanoes.
The Pacific Ocean is surrounded by these three things.
It didn’t take long after the seafloor spreading idea was accepted for scientists to figure
out that the Earth is not expanding.
Instead that old seafloor is being destroyed by sinking in Pacific trenches at the same
rate that new seafloor is produced by seafloor spreading.
A few more details were added by the time the modern Theory of Plate Tectonics was clear
and widely accepted by 1970, including how the moving fragments of lithosphere are the
tectonic plates.
This story teaches us some valuable lessons about science.
Scientists are always trying to explain the world around them but often make mistakes
or don’t have a complete picture, like Wegner, who couldn’t figure out what made the continents
move.
Scientists are often stubborn in refusing to consider new ideas, like those who opposed
Wegner’s ideas of continental drift.
In spite of this, science is constantly giving us better and better ways to see the world
and the universe more clearly.
Don’t ever listen to anyone who tells you that all the big scientific problems have
been figured out already.
The story of continental drift, seafloor spreading, and plate tectonics is just one example of
how the universe can surprise us.
There is always more to discover and figure out!