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Have you ever walked up a hill or swam in the ocean? Then you’ve probably noticed that the Earth’s surface isn’t flat! Instead, the ground rises up into mountains and sinks to the deepest part of the ocean. Isostasy explains why the Earth’s surface has such interesting topography.
Map of the Earth and its continents.

This topographic map of Earth uses colors to show how high the ground is!

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?
Babies floating in rings.

These babies are excited to help you learn about isostasy!

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

In most places on Earth, the lithosphere is in an equilibrium, much like the tube floating in the pool. The lithosphere, which is made of the crust and upper mantle, floats on top of the asthenosphere.
The asthenosphere is made of rock. When we see rocks above the ground, they usually aren’t liquid like water is. But it is so hot in the asthenosphere that the rock can act like a liquid! We call this semi-melted rock magma.’
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.
Image of beach.

The ocean floor is lower than the land because it is made of denser rock.

An example of this is the ocean floor. The oceanic crust is made of a dense igneous rock called basalt. This is why the ocean crust is lower than the continental crust!

Isostatic Equilibrium

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!
But sometimes the weight of the lithosphere, or things on top of the lithosphere, might change. Erosion could wear away at the crust, or a glacier could melt or move away. It might even be that a mountain formed!
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.
Image of the Himalayan Mountains.

The Himalayan mountains are slowly getting taller!

One example is the Himalayan mountains. These mountains are growing taller, even though they are not getting lighter. This is because two of Earth’s tectonic plates are colliding in that spot!

Isostasy Video

Want to learn more? Watch this video on isostasy.

Other Great Resources

Earth Rocks! Video on Earth’s Layers and Isostasy: 

Isostasy – Encyclopaedia Brittanica: https://www.britannica.com/science/isostasy-geology
Written by: Alexa Wnorowski.