Geology

Rocks, plate tectonics, radiometric dating, and Earth's structure

What is geology?

Geology is the study of the Earth itself — the rocks and minerals it's made of, the enormous forces that shape its surface over time, and the long history recorded in its layers.

The rock cycle

Rocks are constantly, if very slowly, being transformed from one type into another in what's called the rock cycle. Igneous rock forms when molten rock (magma) cools and solidifies. Sedimentary rock forms when small particles of eroded rock and other material settle and compress together over long periods of time. Metamorphic rock forms when existing rock is transformed by intense heat and pressure without fully melting. Each type can eventually be broken down or melted and recycled into another, on timescales of millions of years.

Plate tectonics: Earth's moving crust

Earth's outer shell is broken into several large, rigid plates that slowly move relative to each other, floating on the slowly flowing rock beneath them. This movement, called plate tectonics, is responsible for much of Earth's large-scale geological activity: mountain ranges form where plates collide and push rock upward, earthquakes occur where plates grind past or catch against each other, and volcanoes often form where plates pull apart or where one plate slides beneath another.

Reading Earth's history in layers

Sedimentary rock typically forms in distinct layers over time, with older layers buried beneath newer ones. By studying these layers and the fossils and minerals within them, geologists can reconstruct much of Earth's long history — including the sequence of past climates, sea levels, and living organisms — stretching back billions of years.

Why geology matters

Geology guides the search for essential natural resources (water, oil, metals, and building materials), helps assess earthquake and volcanic hazards to keep people safe, and provides the deep-time record needed to understand how Earth's climate and landscapes have changed and continue to change.