Mount Everest, Victoria Falls, the Grand Canyon - it's safe to say Earth is home to some pretty
dramatic landscapes. But the most extreme Landscapes on Earth aren't really landscapes
at all. Some of our planet's most stunning geographical features are actually found under
the sea, like the Mariana Trench. At nearly seven miles below the ocean's surface, the pitch black,
ice cold waters of the Mariana Trench are the deepest place on Earth. In fact, it's so deep
that if you submerged the world's tallest mountain into the Mariana Trench, it would still be nearly
a mile and a half under water. And the ocean floor isn't only home to deep sea trenches, either.
You can also find volcanic activity, mountain ranges, and brand new pieces of the Earth's crust.
So what's going on under the sea that creates all these different geological features,
and how is it different from the processes that shape the land?
Today we'll be diving into the depths of plate tectonics to answer these questions,
and more. In this lesson we're going to explain how plate tectonics create
mid-ocean ridges and deep-ocean trenches, explain the processes of seafloor spreading
and subduction, and summarize the evidence for seafloor spreading. Let's get started!
Before we jump in, let's review a bit about plate tectonics. The theory of plate tectonics states
that the Earth's crust is broken up into about 20 segments, called tectonic plates,
that slowly drift around the planet's surface, moving the continents with them.
However, not all of the plates that cover Earth's surface are the same.
There are two kinds of tectonic plates. Take a look at this map. What do you think the two
kinds of plates could be? Pause the video here, and write your answer in your guided notes.
Tectonic plates beneath the ocean are called oceanic plates,
while plates beneath land masses are called continental plates. Continental plates are much,
much thicker than oceanic plates. But oceanic plates are made out of denser, heavier rocks.
When a continental plate and an oceanic plate crash into each other at a convergent boundary,
the denser oceanic plate slips underneath the continental plate. This makes the sea
floor buckle and change shape, often forming deep oceanic trenches as it's pulled down,
like the Mariana Trench. When an oceanic plate is pushed down underneath a continental plate,
it's called subduction. During this process, the plate melts back into the Earth's mantle. Because
of subduction, the tectonic plates that carry Earth's oceans are constantly being destroyed.
So you might be wondering, what's going to happen to the sea floor if it's slowly
being absorbed back into the planet? If you dive deep enough, can you reach the mantle? What will
happen to the ocean when all of the oceanic plates slip beneath the continental plates?
To answer these questions, well, we need to leave the subduction zone at this convergent
boundary and head to a divergent boundary between two oceanic plates, instead. These boundaries are
places where the ocean floor is pulling apart. But even as the plates separate, it doesn't
just create a gap in the crust where you could swim down to the mantle. That's because these
divergent boundaries between oceanic plates are the site of a process called seafloor spreading.
As the two oceanic plates pull apart from each other, magma from the planet's mantle
rises up to fill the empty space. When this magma cools and hardens into igneous rock,
it bridges the gap between the plates. This creates new oceanic plates to replace the ones
being melted down during subduction at convergent boundaries. At the same time, the process of
seafloor spreading is why subduction happens in the first place. Oceanic plates are denser than
continental plates because they're made of igneous rock. And they're made of igneous rock because of
the magma that wells up during seafloor spreading. See how the processes that shape the Earth are all
interconnected? Even the Earth recycles its own crust. As this magma cools and hardens, it does
more than just create new oceanic plates, though. These new rocks tend to really pile up along the
edges of the plates, creating entire undersea mountain ranges known as mid-ocean ridges.
Now, seafloor spreading might seem like a wild idea, but geologists have found lots of evidence
that supports it. Firstly, these mid-ocean ridges are made of basalt, an igneous rock. This basalt
also tends to be much younger than the ocean floor around it, telling us that it formed recently,
in geological terms. They've also discovered that the temperature of the water around
mid-ocean ridges tends to be warmer. What do you think makes this a convincing piece of evidence?
Pause the video here, and write your answer in your guided notes.
Remember, even the upper parts of the mantle are nearly
2,000 degrees fahrenheit in temperature. The magma that wells up from the planet's interior
during seafloor spreading is hot enough to raise the temperature of the ocean nearby. Geologists
agree that the evidence for seafloor spreading is pretty convincing. It helps us understand
yet another way that interconnected geological processes are constantly shaping our planet.
So, let's go over what we learned today.
Now you know that mid-ocean ridges form along divergent boundaries, while deep-ocean trenches
form along convergent boundaries. Oceanic plates are melted down during subduction, then recreated
during seafloor spreading. And scientists have found evidence that supports seafloor spreading,
including the makeup of mid-ocean ridges and higher than normal water temperatures nearby.
Be sure to check out the engaging games and thought-provoking extension activities
that go with this lesson. And remember - in Earth Science, as in life, you rock! See you next time!