To understand plate tectonics, we need to understand what a ‘Plate’ is. First,
we should be clear that the term ‘Plate’ is short for ‘tectonic plate’ or ‘lithospheric plate’.
In other words, plate, tectonic plate, and lithospheric plate indicate the same thing:
the cracked pieces of Earth’s outermost rigid layer, the lithosphere. You can imagine a cracked
shell of a hardboiled egg. Unlike an eggshell, however, what makes the plates is how the rocks
behave (mechanism) rather than what they’re made of (composition). You may remember that Earth has
three major compositional layers: the core, the mantle, and the crust. One of the most confusing
things about plate tectonics is the difference between the Earth’s crust and the tectonic plates.
Many people confuse Earth’s lithosphere with its crust. The crust is part of the lithosphere.
What you may not know is that most of the lithosphere is actually made of the uppermost
part of the mantle that is cooler and more brittle than the hot solid rock underneath.
Crust and mantle are made of different kinds of rock, so have different chemical compositions, but
together make the lithospheric plate. In short, a plate is a cracked piece of Earth’s lithosphere
and is composed of the crust and some of the upper mantle below it.
In order to understand plate tectonics, it’s also important to know that Earth has two
kinds of crust: continental and oceanic. Continental crust is thick and less dense,
so it rises higher and mostly is dry land above sea level.
Oceanic crust is thinner and more dense, and so it is lower and usually hidden under the oceans.
Because of these are two very different kinds of crust, there are two different types of
lithosphere: continental and oceanic. Oceanic lithosphere is more dense than continental
lithosphere because its crust contains minerals with more heavy elements like iron. A single plate
can also be made of partly oceanic and partly continental lithosphere,
such as the African or the North American plates.
A plate can also be entirely oceanic, such as the biggest one: The Pacific plate.
Oceanic lithosphere and oceanic crust are created by seafloor spreading at divergent
plate boundaries. Divergent plate boundaries are where two plates move away from each other,
like the Mid-Atlantic Ridge or the East Pacific Rise. Typically, oceanic lithosphere is about
50 to 100 km thick and can be even thinner at the ocean ridges where the temperatures are higher.
Only the top 6 or 7 km of the oceanic lithosphere is oceanic crust. The oldest oceanic lithosphere
on Earth is about 180 million years old and formed when dinosaurs roamed the continents.
Oceanic lithosphere is dense enough to sink into the underlying mantle
at convergent plate boundaries, where two plates move towards each other.
Continental lithosphere is typically thicker and older than oceanic lithosphere.
Some parts of the continental crust are billions of years old!
So, now you know that a plate is just a piece of Earth’s lithosphere,
what the difference is between crust and lithosphere,
and what the difference is between oceanic and continental lithosphere.
Now, you may be wondering how they interact to make plate tectonics. Or, what evidence we look
for on Earth’s surface to identify plate boundaries. Check out our next episode!