Rocks, Minerals and the Rock Cycle
Learn about the three types of rocks, how they form, the continuous rock cycle that transforms them, and the basics of plate tectonics.
Minerals vs Rocks
Mineral
A naturally occurring, inorganic solid with a definite chemical composition and an ordered crystal structure.
Examples: Quartz (SiO2), feldspar, mica, calcite, diamond (pure carbon)
Rock
A solid mixture of one or more minerals (or mineral-like substances). Rocks are classified by how they formed.
Examples: Granite (igneous), sandstone (sedimentary), marble (metamorphic)
Key distinction: A mineral is a single substance with a specific chemical formula. A rock is made up of a combination of minerals. For example, granite contains the minerals quartz, feldspar, and mica.
The Three Rock Types
Rocks are classified into three main types based on their formation process. Each type has distinctive features that geologists use for identification.
1. Igneous Rocks
Formed when molten rock (magma or lava) cools and solidifies. As the liquid rock cools, mineral crystals form. The speed of cooling determines the crystal size.
Intrusive (slow cooling)
Cools slowly underground. Forms large crystals visible to the naked eye.
Example: Granite
Extrusive (fast cooling)
Cools quickly on the surface (from lava). Forms small crystals or glassy texture.
Example: Basalt, obsidian
Key features: Interlocking crystals, no layers, no fossils, hard
2. Sedimentary Rocks
Formed when sediments (fragments of rock, shells, organic matter) are deposited in layers, compacted by the weight of layers above, and cemented together over millions of years. This process is called lithification.
Formation process: Weathering & Erosion → Transport → Deposition → Compaction → Cementation
Key features: Visible layers (strata), may contain fossils, often softer, can be crumbly, rounded grains
Examples: Sandstone, limestone, shale, mudstone, chalk
3. Metamorphic Rocks
Formed when existing rocks (igneous, sedimentary, or other metamorphic) are changed by intense heat and/or pressure deep within the Earth, without melting. The minerals recrystallise into new arrangements.
Parent rock → Metamorphic rock:
Limestone → Marble | Sandstone → Quartzite | Shale → Slate | Granite → Gneiss
Key features: Hard, may have bands or layers (foliation), crystals often aligned, no fossils (destroyed by heat/pressure)
Examples: Marble, slate, quartzite, gneiss, schist
Comparing Rock Types
| Feature | Igneous | Sedimentary | Metamorphic |
|---|---|---|---|
| How formed | Cooling of magma/lava | Compaction & cementation | Heat & pressure |
| Crystals | Interlocking | Rounded grains | Aligned / banded |
| Layers | No | Yes (strata) | May have foliation |
| Fossils | Never | Often | Rarely (destroyed) |
| Examples | Granite, basalt | Sandstone, limestone | Marble, slate |
The Rock Cycle
The rock cycle is the continuous process by which rocks are created, transformed, broken down, and reformed over millions of years. Any rock type can become any other rock type through the right conditions. There is no fixed start or end.
Rock Cycle Diagram
Important: The rock cycle shows that any rock type can transform into any other rock type. The process takes millions of years and is driven by plate tectonics, weathering, erosion, and Earth's internal heat.
Plate Tectonics Basics
Earth's outer layer (the lithosphere) is broken into large pieces called tectonic plates. These plates float on the semi-molten mantle beneath and move very slowly (a few centimetres per year) due to convection currents in the mantle.
Plate tectonics drives the rock cycle by causing volcanic eruptions (forming igneous rocks), building mountains (creating heat and pressure for metamorphic rocks), and causing uplift and weathering (producing sediments).
Divergent Boundary
Plates move apart. Magma rises to fill the gap, forming new crust. Creates mid-ocean ridges and rift valleys.
Convergent Boundary
Plates move together. One plate may subduct under the other, causing volcanoes and mountain building. Creates trenches.
Transform Boundary
Plates slide past each other horizontally. Friction builds up and is released as earthquakes. No rock is created or destroyed.
Key Vocabulary
Weathering
The breakdown of rocks in situ (in place) by physical, chemical, or biological processes.
Erosion
The transport of weathered rock fragments by water, wind, ice, or gravity.
Lithification
The process of turning loose sediments into solid sedimentary rock through compaction and cementation.
Magma / Lava
Magma is molten rock below the surface; lava is molten rock that has reached the surface through a volcano.
Foliation
Layered or banded appearance in metamorphic rocks caused by the alignment of mineral crystals under pressure.
Strata
Visible horizontal layers in sedimentary rocks. Each layer represents a different period of deposition.
Worked Examples
A rock has large, interlocking crystals and no visible layers or fossils. What type of rock is it?
Step 1: Large crystals suggest slow cooling, which means it formed underground from magma.
Step 2: No layers rules out sedimentary. No fossils is consistent with igneous (or metamorphic).
Step 3: Interlocking crystals (not aligned/banded) point to igneous rather than metamorphic.
Answer: This is an intrusive igneous rock (e.g. granite). It cooled slowly underground, allowing large crystals to form.
Explain how a sedimentary rock can become an igneous rock.
Step 1: The sedimentary rock is buried deep in the Earth or subducted at a plate boundary.
Step 2: Under extreme heat, the rock melts and becomes magma.
Step 3: The magma then cools and solidifies, either underground (intrusive) or on the surface as lava (extrusive).
Answer: Sedimentary rock is melted into magma by intense heat deep underground, then the magma cools and crystallises to form an igneous rock.
What happens to limestone when it is subjected to great heat and pressure?
Step 1: Limestone is a sedimentary rock made mainly of calcium carbonate (calcite).
Step 2: When subjected to heat and pressure (without melting), it undergoes metamorphism.
Step 3: The calcite crystals recrystallise into a harder, more interlocking structure.
Answer: Limestone transforms into marble, a metamorphic rock.
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
Which type of rock is formed by the cooling and solidifying of magma or lava?
Question 2
A rock contains visible layers and fossils. It is most likely:
Question 3
What metamorphic rock does limestone become when subjected to heat and pressure?
Question 4
An igneous rock with very small crystals (fine-grained) most likely formed by:
Question 5
At a convergent plate boundary, plates:
Key Concepts Summary
- ●Minerals are single inorganic substances; rocks are mixtures of minerals.
- ●Igneous: formed from cooling magma/lava. Sedimentary: formed by compaction/cementation of sediments. Metamorphic: formed by heat and pressure.
- ●The rock cycle continuously transforms rocks from one type to another over millions of years.
- ●Crystal size in igneous rocks depends on cooling rate: slow = large crystals, fast = small crystals.
- ●Plate tectonics drives the rock cycle through volcanic activity, mountain building, and weathering/erosion.