How thick is the asthenosphere?

What is the Asthenosphere?
Asthenosphere is a layer deep inside Earth, found below the hard outer shell called the lithosphere. This layer is made of solid rock, but it is so hot and under such high pressure that it flows very, very slowly—almost like warm caramel under a chocolate shell. While it holds its shape like a solid, it can move and bend over thousands of years.

Where is the Asthenosphere?
The asthenosphere is located about 50 to 120 miles (80 to 200 kilometers) beneath Earth’s surface, and it stretches down to about 200 to 440 miles (350 to 700 kilometers) deep. Its thickness can range from 100 to 200 miles (150 to 350 kilometers), depending on where you look inside the Earth.

Why Does the Asthenosphere Flow?
The rock in the asthenosphere flows because it is extremely hot—between 1,800°F and 4,000°F (1,000°C to 2,200°C)! The heat softens the rock and makes it flexible, but the pressure from all the rock above keeps it from melting completely. This means the asthenosphere acts like a super-slow-moving liquid, taking thousands of years to flow even a short distance.

Why Does Its Thickness Change?
The asthenosphere is thinner under oceans, where it is hotter and not as deep. It is thicker under continents, which are cooler and heavier. Scientists use earthquake waves to measure these differences because the waves slow down in the softer asthenosphere.

Why is the Asthenosphere Important?
The asthenosphere matters because tectonic plates—the giant slabs that make up Earth’s surface—float and move on top of it. The slow flow of the asthenosphere helps drive the movement of these plates, which causes earthquakes, volcanoes, and even the formation of mountains. Without the asthenosphere, Earth's surface would not be able to change and grow.

How Do We Know It Exists?
Scientists discovered the asthenosphere by studying earthquake waves. These waves travel slower through the asthenosphere because it is softer than the layers above and below. This region is called the “low-velocity zone.” The name asthenosphere comes from the Greek word “asthenos,” which means “weak”—it is the weak, soft layer that lets plates move.

Interesting Fact:
Some volcanoes and earthquakes happen because the asthenosphere’s slow flow pushes and pulls on the plates above!