Geology is the study of the Earth — what it's made of, and how it changes over enormous spans of time. A geologist reads rocks the way you'd read the pages of a very old book.

Two facts are worth locking in before anything else. First, the Earth is astonishingly old: about 4.5 billion years. Second, it's layered, like an onion, and it gets hotter the deeper you go.

The Earth's layers, from the surface down

• Crust — the thin, solid, rocky skin we live on. Next to the whole planet it's thinner than the skin on an apple. It comes in two kinds: thick, lighter continental crust (the land) and thinner, denser oceanic crust (under the seas).
• Mantle — a thick layer of hot rock below the crust. It's mostly solid, but over millions of years it slowly flows like extremely stiff putty. That slow churning drives a great deal of geology.
• Outer core — a layer of liquid metal, mostly iron and nickel. Its swirling motion creates Earth's magnetic field.
• Inner core — a ball of solid metal at the very centre. It's even hotter than the outer core, but it stays solid because the crushing pressure squeezes it that way.

One more word you'll meet later: the lithosphere is the rigid outer shell — the crust plus the very top of the mantle — and it's broken into the giant moving pieces we'll call "plates" in Lesson 5.