Minerals, Rock Types, and the Rock Cycle
A mineral is a naturally occurring, solid, inorganic substance with a definite chemical composition and an ordered crystal structure, and a rock is a solid mixture of one or more minerals; the three rock types are igneous, sedimentary and metamorphic, and the rock cycle describes how each can be turned into the others over millions of years. In brief: igneous rocks form when magma or lava cools (granite, basalt), sedimentary rocks form from compacted and cemented sediments (sandstone, limestone) and metamorphic rocks form when heat and pressure change existing rock (marble, slate). The rock cycle links them through melting, weathering, deposition, burial and metamorphism.
Rocks record the history of the planet and supply building stone, metals and fuels. This guide explains minerals and how to identify them, the three rock types, the rock cycle, compares the groups in a table, works through simple calculations on density and cooling, outlines the history of the science and corrects common mistakes.
What are minerals?
Minerals are the building blocks of rocks. Quartz (SiO2), feldspar, mica, calcite (CaCO3) and olivine are common examples. Each mineral has a fixed chemical formula (or a narrow range) and a regular internal arrangement of atoms, which often shows up as crystal shapes. Silicate minerals, built from silicon and oxygen, make up most of Earth’s crust.
Minerals are identified by simple tests: hardness on the Mohs scale (from 1 for talc to 10 for diamond), luster (metallic or glassy), streak (the color of the powder), cleavage and fracture (how it breaks), color and density. A fingernail has a hardness of about 2.5, a copper coin about 3 and a steel knife about 5.5, so these can be used to place a mineral on the scale. An ore is a rock containing enough of a mineral (for example hematite, an iron oxide) to be mined at a profit.
How do igneous rocks form?
Igneous rocks form when molten rock cools and crystallizes. If it cools slowly underground, large crystals have time to grow, and the rock is called intrusive (plutonic), such as granite and gabbro. If lava cools quickly at the surface, the crystals are tiny or absent, and the rock is extrusive (volcanic), such as basalt and obsidian (volcanic glass). Pumice is full of gas holes and is so light that it can float.
The crystal size therefore tells the cooling history. Igneous rocks are usually hard, non-layered and contain no fossils, because the rock was molten.
How do sedimentary rocks form?
Sedimentary rocks form at or near the surface in several steps: weathering and erosion break rock into sediment, which is transported and deposited in layers (strata), then buried, compacted by the weight above and cemented by minerals dissolved in water. This is called lithification.
There are three groups. Clastic rocks consist of fragments: conglomerate (rounded pebbles), sandstone (sand) and shale (mud and clay). Chemical rocks form when minerals precipitate from water, for example rock salt and some limestone. Organic (biological) rocks come from the remains of living things, such as coal from plants and limestone from shells and corals. Sedimentary rocks are layered, often contain fossils and are generally softer than igneous rocks.
How do metamorphic rocks form?
Metamorphic rocks form when existing rock is changed by heat, pressure or hot fluids without melting completely. Minerals recrystallize and often align in bands, giving a foliated texture; others stay non-foliated. Common changes are shale to slate and then schist and gneiss, limestone to marble, sandstone to quartzite and granite to gneiss.
Contact metamorphism happens next to a hot intrusion of magma, while regional metamorphism affects large areas where plates collide and mountains rise. Because the rock was altered in the solid state, fossils and original layers are usually distorted or destroyed.
What is the rock cycle?
The rock cycle shows that rocks are not permanent. Any rock can become any other, following many possible paths.
- Magma cools and crystallizes into igneous rock.
- Weathering and erosion break the rock into sediment.
- Sediment is transported, deposited, buried and lithified into sedimentary rock.
- Heat and pressure from deep burial or plate collision turn it into metamorphic rock.
- With more heat, the rock melts to magma and the cycle begins again.
Shortcuts exist: igneous rock can be metamorphosed directly, and sedimentary or metamorphic rock can be uplifted and weathered without melting. The cycle is driven by Earth’s internal heat (melting, metamorphism, uplift) and by the Sun and gravity acting on the surface (weathering, erosion and deposition), and it is closely linked to plate tectonics.
Rock types compared
| Feature | Igneous | Sedimentary | Metamorphic |
|---|---|---|---|
| Origin | Cooling of magma or lava | Compaction and cementation of sediment | Heat and pressure on existing rock |
| Texture | Interlocking crystals or glass | Grains and layers | Crystals, often banded |
| Fossils | No | Often | Rarely |
| Examples | Granite, basalt, obsidian | Sandstone, limestone, shale, coal | Marble, slate, gneiss, quartzite |
Worked example: density and cooling
- Density of a sample: a rock sample has a mass of 540 g and a volume of 200 cm3. Density = mass ÷ volume = 540 ÷ 200 = 2.7 g/cm3, a value typical of granite and many crustal rocks.
- Volume by displacement: a stone is lowered into a measuring cylinder, and the water level rises from 50 mL to 70 mL. The volume is 70 − 50 = 20 mL = 20 cm3. If the mass is 56 g, the density is 56 ÷ 20 = 2.8 g/cm3.
- Floating: pumice can have a density below that of water (1.0 g/cm3), so it floats, while granite at 2.7 g/cm3 sinks.
- Crystal size: two rocks have the same composition. One has crystals 5 mm long and the other crystals under 0.1 mm. The first cooled slowly (intrusive, like granite) and the second quickly (extrusive, like rhyolite).
- Thickness of sediment: if sediment accumulates at 0.2 mm per year, 100 m (100,000 mm) takes 100,000 ÷ 0.2 = 500,000 years. Rates vary widely, and compaction shortens the final thickness.
How did we learn about rocks?
In the 18th century, James Hutton in Scotland argued that rocks are formed and destroyed by slow processes that still operate today and that Earth is very old. This idea, called uniformitarianism, was spread by Charles Lyell. In the 19th century, geologists described the order of rock layers and used fossils to correlate them. In the 20th century, radioactive dating gave absolute ages, with the oldest known Earth minerals (zircon crystals) dating to about 4.4 billion years, and plate tectonics explained where rocks form and are recycled.
Which mistakes are common?
Mistake 1: Confusing minerals and rocks
A mineral has a defined chemical composition; a rock is usually a mixture of minerals. Granite contains quartz, feldspar and mica.
Mistake 2: Thinking that all rock forms from lava
Magma that cools underground makes intrusive igneous rock without ever reaching the surface as lava.
Mistake 3: Expecting fossils in all rocks
Fossils are found mainly in sedimentary rocks. Igneous rocks form from molten material, and metamorphism usually destroys fossils.
Mistake 4: Believing that the cycle follows one order
The rock cycle has many paths and shortcuts, and a rock may skip stages or stay unchanged for hundreds of millions of years.