Do you know the longest mountain range on the Earth is underneath the sea?
Have you wondered how ocean basins were created and why they are where they are?
All these questions can be answered by plate tectonics,
especially processes associated with divergent plate boundaries.
In this episode, we are going to introduce you to divergent plate boundaries.
A divergent plate boundary is where two plates move away from each other.
Here is a map showing Earth’s topography without ocean water.
You probably have already noticed these strange black lines.
These show what we call a mid-ocean ridge, and they marked the divergent plate boundaries,
which are almost entirely hidden under ocean water. The mid-ocean ridge system can be traced
around the world like the seam of a baseball and is the longest, linear uplifted feature of the
solid Earth. The mid-ocean ridge system is about 55,000 km long. That’s almost 8 times longer than
the Andes Mountains of South America which is the longest above-water mountain range in the world.
From looking at a map, it is hard to see their height, but from the side view we
can clearly see that the mid-ocean ridge system is a chain of undersea mountains,
with tops that are commonly 2–3 km higher than the seafloor on either side of them.
The steepness of mid-ocean ridges is controlled by how fast the plates are moving apart.
Remember we told you in an earlier episode that the plates move at different speeds? The fastest
moves about how fast your hair grows and slowest moves about how fast your fingernails grow.
When the spreading rate is low, the mid-ocean ridge tends to form a middle
valley at the divergent plate boundary, which scientists call a ‘Rift Valley’.
This is what we can see in the Atlantic Ocean.
When the plates spread more rapidly, the divergent plate boundary makes a
gentle undersea mountain rise without a valley as we see in the eastern Pacific Ocean.
We haven’t known about mid-ocean ridges for very long.
The first indications that an underwater ridge runs down the middle of the Atlantic Ocean basin
came from the results of the British Challenger expedition in the nineteenth century.
It was not until after World War II, when the ocean floor was surveyed in more detail,
that the full extent of mid-ocean ridges became known. Scientists then further explored the other
oceans and seas and discovered that mid-ocean ridges are not unique to the Atlantic Ocean.
They also found that plenty of shallow earthquakes happened beneath the ridge crest.
Scientists actually traced the mid-ocean ridges using these earthquakes.
Moreover, fresh lavas were found in the rift valley, indicating recent volcanic eruptions.
They found less sediment at the ridges than on seafloor farther away,
suggesting the ridges were young. When researchers probed the crust to measure temperatures,
they found the crust was warmer at ridges than elsewhere in the ocean.
All this evidence told scientists that something unusual is happening at mid-ocean ridges.
Scientists now think what happens at mid-ocean ridges is that they mark where two plates
move away from each other at a divergent plate margin by a process called seafloor spreading.
The space between the two plates allows hot solid rock from the asthenosphere to rise up
and partially melt to form magma. Once the magma reaches the seafloor, it erupts as lava,
cools rapidly and forms an igneous rock called basalt. These basalts become the new seafloor.
This is how the lithosphere and its plates grow at divergent plate boundaries.
As you probably have already noticed, the movements of the plates towards convergent
boundaries and away from divergent boundaries are connected. Seafloor spreading at divergent
plate boundaries makes new oceanic crust and lithosphere at the same time that subduction
at the convergent boundaries destroy old oceanic crust and lithosphere. Scientists now think
that plate motions, including moving apart, or divergence, at mid-ocean ridges is caused by the
sinking of oceanic lithosphere in subduction zones, at convergent plate boundaries.
Most seafloor spreading happens under the sea, but a related process happens in the continents,
called rifting. Continental rifting splits one continent into two or more tectonic plates and
sometimes evolves into a divergent plate boundary and a mid-ocean ridge with seafloor spreading.
This process can be observed in East Africa, where the East African Rift
will eventually break Africa in two and create a new ocean basin millions of years from now.
Another example is the Rio Grande Rift Valley of New Mexico.
Continental rifting creates earthquakes, rift valleys, mountains, and volcanoes.
Now we have discussed the two types of plate boundaries that tell us where oceanic plates are
formed and where they are subducted. What do you think the third type of the plate boundaries will
be? You will find the answer in our next episode when we learn about transform plate boundaries!
Thanks for watching! See you next time!