Introduction

The first question this chapter asks is a simple one: what do you actually picture when you think of the earth? Most of us imagine a solid ball, something like a cricket ball. But is that accurate? Or is the earth more like a hollow ball wrapped in a thick outer cover of rock — that cover being the lithosphere? The chapter opens with these images because they force you to confront how little direct knowledge we have of what lies beneath our feet.

You have probably seen television footage of a volcanic eruption. Think about what that shows: hot molten lava, dust, smoke, fire, and magma pouring out of the crater. That eruption is one of the few moments when the interior of the earth literally surfaces. Yet even that dramatic event only gives us a glimpse. The truth is that the interior of the earth can be understood only through indirect evidence. No one has ever reached it, and no one ever can.

Why does this matter? Because the surface of the earth is largely a product of what happens inside it. The landscape you see — mountains, plains, valleys — is constantly being shaped by two kinds of forces:

• Exogenic processes — those operating on the surface, like weathering and erosion.
• Endogenic processes — those operating from within the earth, like volcanic activity and plate movements.

If you ignore the endogenic forces, your understanding of a region's physiography remains incomplete. And since human life is deeply influenced by the physiography of a region — where people settle, how they farm, what hazards they face — it becomes necessary to understand the forces that shape the landscape. To answer questions like why the earth shakes during an earthquake, or how a tsunami wave is generated, you need to know certain details about the earth's interior.

In the previous chapter, you noted that earth-forming materials are arranged in layers, from the crust down to the core. The interesting part is how scientists gathered information about these layers without ever seeing them, and what the characteristics of each layer are. That is exactly what this chapter sets out to explain.