1.1 Formation of Rocks

IGCSE Environmental Management (0680)

2026 📋 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Describe and interpret the rock cycle — explain how rocks are constantly changing from one type to another through a series of natural processes.
  2. State and explain the formation and characteristics of named igneous, sedimentary, and metamorphic rocks — know how each type forms and what it looks like.
  3. Named rock examples to know:
    • Igneous: granite and basalt
    • Sedimentary: limestone, sandstone, and shale
    • Metamorphic: marble and slate

📖 Main Content

https://cambridgeassistant.com

Section 1: What Are Rocks and Minerals?

Before we look at the three rock types, it helps to understand what rocks actually are.

  • A mineral is a naturally occurring, non-living substance with a specific chemical make-up. Think of minerals like the ingredients in a recipe — for example, quartz, feldspar, and mica are all minerals.
  • A rock is a combination of one or more minerals all joined together. Think of a rock as the finished dish made from those ingredients.

The Earth's crust (the thin, solid outer layer of our planet) is made almost entirely of rock. When the Earth first formed, all of its rocks were one single type — igneous. Over billions of years, these rocks have been breaking down, reforming, and changing into new types through a never-ending process called the rock cycle.

Section 2: The Three Types of Rock

There are three main rock types. Each one forms in a completely different way.

Rock TypeHow It Forms
IgneousMolten rock cools and hardens
SedimentaryTiny particles build up in layers and get squashed together
MetamorphicExisting rocks change due to heat and/or pressure

Section 3: Igneous Rocks

How Do Igneous Rocks Form?

Deep inside the Earth, the temperature is so high that rock melts into a liquid. This liquid rock is called magma (molten rock below the Earth's surface). When magma cools down, it hardens and becomes igneous rock.

The key thing to understand about igneous rocks is that the speed at which the magma cools determines what the rock looks like — specifically, how big its crystals (the small, shiny grains you can see in rock) are.

Here's how it works step by step:

  1. Magma is made up of dissolved minerals floating around as tiny particles called ions (atoms that carry an electrical charge).
  2. As magma cools, these ions slow down and start sticking together in organised patterns — this is how crystals form.
  3. If cooling is slow, the ions have plenty of time to join together and build large crystals.
  4. If cooling is fast, the ions freeze in place almost immediately and only small crystals can form.

Where Does Cooling Happen?

  • Underground (slow cooling): If magma rises up through the Earth's crust but never reaches the surface, it cools slowly surrounded by other rock. This produces large crystals and a coarse-grained (rough, grainy) texture.
  • At the surface (fast cooling): When a volcano erupts, magma is released onto the Earth's surface as lava. In contact with the cool air or ocean water, it cools very quickly. This produces small crystals and a fine-grained (smooth) texture.

Named Igneous Rocks

🪨 Granite

  • Forms underground when magma cools slowly.
  • Has large, visible crystals — you can clearly see grains of different colours (often pink, white, and black/dark).
  • This is described as a coarse-grained texture.
  • Granite is a very hard and durable rock.

🪨 Basalt

  • Forms at the surface when lava from a volcano cools quickly.
  • Has very small, tiny crystals that you can barely see with the naked eye.
  • This is described as a fine-grained texture.
  • Basalt is dark in colour (usually dark grey or black) and dense (heavy for its size).

💡 Quick memory tip: Granite = Giant crystals (underground, slow). Basalt = Bit smaller crystals (surface, fast).

Sign in to view full notes