Lithosphere and Asthenosphere: The Earth Beneath Your Feet

The lithosphere and the asthenosphere are the two layers of the Earth closest to the surface. Wait, but wasn’t the crust the layer that’s closest to the surface? Well… there’s more than one way to look at the Earth’s layers.

In essence, the lithosphere is the Earth’s hard and rigid shell that encompasses the outermost rocky shell of the Earth (and other rocky bodies as well). The asthenosphere is a softer, more malleable layer that allows for the dynamic movement of the tectonic plates.

Here’s why this matters a lot to geologists — and should also matter to you.

The lithosphere, asthenosphere, crust, and mantle

We all know (hopefully) that the Earth’s layers are the thin crust, the thick mantle, and the core (which is usually considered as having two sides: the outer and inner core). But where do the lithosphere and asthenosphere come in?

Well, there are two ways of looking at the layers of the Earth. The first is based on the layers’ chemical composition, or what they’re made from. The other is on their mechanical classification, so how they behave physically. The crust and mantle are composition-based, while the lithosphere and asthenosphere are a mechanical classification.

• Composition-based classification (crust and mantle) helps us understand the Earth’s internal makeup and the materials present at different depths.
• Mechanical classification (lithosphere and asthenosphere) aids in understanding how different parts of the Earth behave under stress and how they interact, which is crucial for the study of plate tectonics and geodynamics.

It’s important to keep in mind that the lithosphere includes the crust and the top (rigid) part of the mantle, while the asthenosphere includes the ductile part of the mantle. But the lower mantle is included in neither the lithosphere nor the asthenosphere.

Crust and mantle vs Lithosphere and asthenosphere

So the terms “crust,” “mantle,” “lithosphere,” and “asthenosphere” do overlap a bit, but they refer to different aspects of Earth’s structure. Here’s how they differ, for the most part

1. Crust and Mantle (Composition-based classification):
   • These terms classify Earth’s layers based on their chemical composition.
   • The crust is the outermost layer, differentiated into oceanic and continental based on its composition and thickness. It consists of rocks like granite (continental) and basalt (oceanic).
   • The mantle lies beneath the crust and is composed mainly of silicate rocks that are rich in iron and magnesium.
2. Lithosphere and Asthenosphere (Mechanical or Rheological classification):
   • These terms classify parts of the Earth based on their mechanical properties or behavior, especially how they respond to stress.
   • The lithosphere is the rigid outer layer, encompassing the crust and the uppermost part of the mantle. It behaves elastically over short time scales, meaning it’s brittle and can break, leading to phenomena like earthquakes.
   • The asthenosphere, on the other hand, is ductile and can flow over geological time scales. While it’s solid, it behaves plastically, allowing the lithosphere to move atop it due to its malleable nature.

The Earth’s layers: a recap

We have a full article on the layers of the Earth but let’s just do a brief recap of the main layers of our planet — compositionally.

1. Crust: This is the Earth’s outermost layer and the one we live on. It’s relatively thin compared to the other layers. There are two types of crust:
   • Continental Crust: Found under continents and some major islands. It’s thicker and mostly composed of light-colored rocks like granite.
   • Oceanic Crust: Found under the ocean floors. It’s thinner and mostly made up of dark-colored rocks like basalt.
2. Mantle: Right beneath the crust, there’s the mantle. It’s much thicker than the crust, it’s the thickest layer of the Earth. The mantle is solid but can flow slowly over long periods. Think of it as having the consistency of hot asphalt. The mantle is primarily made of silicate rocks, which are rich in iron and magnesium.
3. Outer Core: Deeper still, we find the outer core. This layer is unique because it’s in a liquid state. Composed mainly of iron and nickel, the outer core is responsible for creating Earth’s magnetic field through its movement. The temperatures here can soar, melting the components of the inner core.
4. Inner Core: At the very center of our Earth is the inner core. Despite the even higher temperatures here—reaching up to temperatures close to the Sun—this layer is solid. The immense pressure at this depth keeps the iron and nickel in a solid state. The inner core is constantly spinning, and its rotation plays a crucial role in influencing the Earth’s magnetic field.

So everything we see on this surface is a part of the crust (or the lithosphere, depending on how you consider it). But this is just a thin part of the Earth. To scale, it’s less than the skin of an apple.

Why the lithosphere and asthenosphere matter

So why go to all this trouble with a different classification?

Well, scientists love to be precise, especially when they’re interested in multiple things. Geologists are interested in the chemistry of the Earth, but they’re also very interested in the movement of the planet’s surface. The lithosphere and asthenosphere especially help geologists in the context of plate tectonics.

A Dance of Plates

The lithosphere isn’t one continuous shell. Think of it more as a jigsaw puzzle. These pieces, called tectonic plates, float on the asthenosphere’s flowing river. Yes, you heard that right. The ground beneath you is floating!

When these plates move, they can collide, pull apart, or slide against each other. And when they do? It results in natural phenomena like earthquakes, volcanic eruptions, and the creation of mountain ranges.

Just below the lithosphere is where the real magic happens. Enter the asthenosphere. It’s not solid like the lithosphere. Instead, picture a slowly flowing river of molten rock. This isn’t lava, though. The asthenosphere consists of semi-molten rock, which can flow and move, albeit very, very slowly.

Ever wondered why the continents shift or why earthquakes shake the ground? The asthenosphere plays a vital role. The moving and shifting in this layer influence the world you experience every day.

Asthenosphere: The unsung hero of Earth’s movements

The asthenosphere, with its flowing characteristics, acts like a conveyor belt for these tectonic plates. While the lithosphere gets the credit for the dramatic events, it’s the asthenosphere that’s the real power player behind the scenes.

It’s in the asthenosphere that convection currents happen. As heat from the Earth’s core rises, it makes the semi-molten rock in the asthenosphere move. This movement pushes and pulls the lithosphere, making continents drift apart or come together. Think of the asthenosphere as the engine driving the motion of the tectonic plates.

Significance in Our Daily Lives

You might be wondering, “How does this affect me?” Well, think about it. The fruits you enjoy from volcanic soils, the gold or diamonds adorning your jewelry, even the threat of natural disasters in some regions—all can be traced back to the activities in the asthenosphere and the movement of the lithosphere.

Moreover, understanding the asthenosphere can help us predict certain events. While we can’t stop earthquakes or volcanic eruptions, insights into the asthenosphere’s workings can offer a heads-up.

The Future: A lot of science

As you read this, the asthenosphere continues its subtle dance, moving and shaping our world. This layer’s understanding, while still evolving, promises opportunities. From tapping into geothermal energy to disaster mitigation, geological research in the lithosphere and asthenosphere holds a lot of promise.

In time, as technology and research progress, we might be able to delve even deeper, understanding the mysteries of this magnificent layer better. The asthenosphere, often overlooked, could hold answers to some of our planet’s most pressing questions.

In the meantime, if there’s only one lesson you take from the lithosphere and asthenosphere, let it be this. Our planet is a wonderfully complex system and there’s more than one way to look at it.