We are all moving, all the time. Even as we sleep, we move; as we sit in class, we move.
We are all passengers on top of a tectonic plate that is slowly traveling across Earth's surface.
In this lesson, we will describe what we mean when we use the term "tectonic plate", and
we will complete an exercise so you can quickly sketch your own map of the world's major plates.
Having that map in your head will make it easier to understand the concept of plate
tectonics, perhaps the most important idea in all of geology.
Earth can be divided into three major compositional layers. The crust, mantle and core. The crust
is a thin skin, underlain by a much thicker mantle layer. The mantle surrounds a two-part
core composed of a solid metallic inner core and an outer core with a molten mix of metals.
A tectonic plate is composed of both the outer layer of crust and another layer in the uppermost
mantle. Together, these two parts are relatively rigid and form a mobile slab of lithosphere
up to a few hundreds of kilometers thick.
These plates move over an underlying layer in the upper mantle known as the asthenosphere.
This layer can deform due to the presence of a small percentage of molten minerals.
This allows the rocks in the asthenosphere to flow very, very, very slowly and the rigid
plates of the lithosphere ride above this layer.
Keep in mind that the mantle is 2900 kilometers thick, so we are just talking about rocks
in the very uppermost part of the mantle. Most of the mantle, about 90%, is not even
involved in the lithosphere and asthenosphere.
So, the lithosphere makes up a shell that surrounds the Earth that is a few hundred
kilometers thick. However, it's not one complete piece. Imagine an orange where we divided
up the skin into a series of separate pieces that fit together. Earth is much like that,
with the pieces of orange representing different tectonic plates.
These tectonic plates move around relative to each other, generating earthquakes, volcanoes,
and mountain belts at their boundaries. We can look for these zones of tectonic activity
to identify the locations of plate boundaries on Earth's surface.
Before we sketch the locations of the plate boundaries on our map, let's first look at
the distribution of some recent moderate to large earthquakes. This map shows the location
of more than 1600 earthquakes of magnitude 5 or greater that have occurred in one year.
Now, before we go any further, get yourself a blank map of the world. Like this one, or
this one, or this one. If you don't have one handy, pause the lesson and go and ask the
Google to find one for you.
So, now that you are armed with your map, let's find some plate boundaries using these
earthquake locations. We will start in the Atlantic Ocean. We can connect the dots to
identify the approximate location of the plate boundary along here.
Now, you try doing something similar, starting about half-way between South Africa and Antarctica.
Moving east, where is the plate boundary?
You might have drawn it like this, and then wondered where to go from there. You could
either continue northward,
or you could follow the line of earthquakes to the southeast.
Both lines represent plate boundaries.
Now we have two boundaries. One that separates Africa and South America or North America
and Europe. And another that separates Africa and Antarctica or Australia and Antarctica.
Now let's try something similar for the Pacific Ocean. How many plates can we separate out
using this data.
We can begin here near New Zealand, and continue clockwise around the rim of the Pacific until
we get to South America.
Now we have some options. We could do something like this.
And then we could use our inside knowledge to complete a few small plates.
Such as here, and here
and again, down here.
We have a few other gaps to fill where the data is more limited.
Now what's left, can you identify the location of a plate in the western Pacific Ocean?
We have some additional earthquakes along the northern margin of the Indian Ocean and
through parts of Asia. Can you finish off the location of those plates?
Go ahead and pause the lesson and give it a try.
Now let's see what you came up with. Here is a map we cooked up earlier. These are the
major plate boundaries. Most plates are named for their most important geographic feature,
such as a continent or ocean. We have the Pacific,
North American
South American
African
the Antarctica plate, the Eurasian plate, the Indian-Australian plate, and the Nazca
plate.
These are the eight biggies.
We can identify a few other smaller plates as well. Such as the Cocos,
and the Caribbean
the Scotia
the Philippine
and then the Arabian plate.
That brings us to the end of this lesson. How confident are you that you could successfully
complete tasks associated with the two learning objectives.
Ok, that's it for us. We're off to practice making more plate boundary maps.