Sial – Continental Crust
The top layer of the earth is called the crust. Now, it’s important to know that there is two different kind of crusts: sial, and sima. Sima is the oceanic crust. Sial is the continental crust. The continental crust is a busy place, we all live on it! Sial is primarily made of the mineral silica as well as aluminum ore. In fact, the word sial is a word derived from the names silica and aluminum.
Scientists observe the movements of the continents with satellites in space!
Sial vs. Sima
How is sial different from sima? Besides the obvious that one is under the ocean and the other is not, there are lots of differences! It is much lighter than the sima beneath it. It is also thicker, 22 miles thick on average and not as dense as sima.
Because it is less dense, the sial “floats” on top of the sima. We call this floating ‘isostasy‘. You can see it when you notice that all the continents can fit together in a certain place. Before they started drifting around, they once all formed together to create Pangaea. But that is another story. Sial is more complex in its structure than sima. It is made at subduction zones. New sial also occurs around continental margins. There, sediment from a nearby trench adds to the continent. These margins are marked by volcanoes.
The deepest part of the crust is underneath mountains, where it can be up to 43 miles thick.
How Old is our Land?
Sial resists subductions. This means that the age of the continental crust is almost 2 billion years.
Oceanic crust being subducted under continental crust. Continental crust is not usually subducted.
Sial is more than 30 times the age of the average bit of oceanic crust, sima. This is why sial is that perfect spot to gather information about the Earth’s geological history. But understanding what we find is another task. It requires an understanding of the ocean basins from which this crust was born.
There are many things about our crust that scientists still don’t understand. Mostly because it’s not around. Either the crust sunk back into the earth due to plate tectonic action. Or it’s been moved around by activity on the Earth’s surface to make new rocks. Scientists have dated the oldest rock on earth to be 4.02 billion years old. This rock reveals that Earth used to be covered with a crust like the oceanic crust, rather than the continental crust. This rock gives us a small window into how early continents were formed so many billions of years ago.
For more information:
Could I Dig a Hole Through the Earth?: