In the picture of the babies floating in the pool, the pool tubes are helping them stay above the water. What do you think would happen if an adult tried to sit on that same tube?
If you guessed that the tube would sink deeper, you were right! But why would that happen?
When something floats in a pool, gravity pulls down on it, trying to pull it to the bottom of the pool. But the water also pushes up on it, which causes it to float.
If you add more weight like if an adult sat on the tube, gravity pulls down more. This makes the object sink down. Depending on how much weight you add, it may not sink all the way to the bottom.
It will only sink until the water is pushing up on the object the same amount as gravity is pulling down! When these two forces are equal, the object floats in place. This is called equilibrium.
Earth’s Floating Lithosphere
Heavier areas of lithosphere push down on the asthenosphere. The asthenosphere flows to the side of the heavy lithosphere, towards an area where it is lighter. This means that heavier areas of the lithosphere sink and lighter areas get lifted up!
The lithosphere can be heavy in places where it is thicker or denser. Denser areas contain more rock in the same amount of space.
Most of the time, the earth’s lithosphere is in isostatic equilibrium. This means that the lithosphere stays at a height where the forces acting on it are equal. These two forces are gravity pulling down on it and the asthenosphere pushing up!
When this happens, the force of gravity pulling down on the lithosphere is not equal to the force of the asthenosphere pushing up on it. This is where isostasy comes in! The lithosphere will move higher or lower until it reaches equilibrium again.
There are some places on Earth that are not in isostatic equilibrium. In these places, there are other forces acting on the lithosphere.
Want to learn more? Watch this video on isostasy.
Other Great Resources
Earth Rocks! Video on Earth’s Layers and Isostasy:
Isostasy – Kids.Net.Au: http://encyclopedia.kids.net.au/page/is/Isostasy
Isostasy – Encyclopaedia Brittanica: https://www.britannica.com/science/isostasy-geology