Minerals and Rocks

Every rock on Earth is igneous, sedimentary, or metamorphic, and the three types transform endlessly in the rock cycle. Igneous rocks cool from magma, sedimentary rocks form from deposited sediment, and metamorphic rocks change under heat and pressure. Rocks are classified by cooling rate, mineral makeup, and degree of metamorphism, and minerals by properties such as hardness and cleavage. Here you can use the process slider to follow the cycle steps among igneous, sedimentary, and metamorphic rocks.

Three faces of stones

🪨 Rock types

①All rocks on Earth fall into one of three types

②Igneous (cooled magma), sedimentary (deposited and cemented), metamorphic (transformed by heat/pressure)

③These three transform endlessly into each other → the rock cycle

Rock cycle

Select process0

💡 Key observations

①0: Igneous → weathering/erosion → sediments → sedimentary

②1: Sedimentary → heat & pressure → metamorphic

③2: Metamorphic → melt → magma → cool → igneous again

Rock classification

Three types of rocks

Properties of minerals

Definition of mineral

natural + inorganic + solid + crystalline + definite chemical composition

all 5 conditions must be met to be a mineral

Mineral identification

ListMethods to identify minerals

Hardness

talc (1) ~ diamond (10)

Mohs scale 1~10

Streak

may differ from mineral color

color of powder on streak plate

Cleavage / fracture

mica: cleaves in one direction

cleavage = breaks along certain planes

Luster

how the surface reflects light

metallic / non-metallic

Worked Examples and Exam Practice

Example 1

What is an igneous rock called when magma cools slowly deep underground, forming large crystals, and what is a typical example?

Magma cooling slowly deep underground grows large crystals → plutonic (intrusive) rock.

A typical plutonic rock is granite. Cooling fast at the surface instead gives fine-grained volcanic rock (basalt).

▸ Plutonic rock (e.g., granite)

Cooling rate controls crystal size: the slower the cooling, the larger the crystals.

Example 2

On the Mohs scale, diamond is 10 and quartz is 7. If you scratch quartz with diamond, which one gets scratched?

The Mohs scale is a relative hardness ranking based on which mineral scratches which.

A harder mineral scratches a softer one, so diamond (10) scratches quartz (7).

▸ The quartz gets scratched

A harder mineral scratches a softer one. Mohs is a relative scale from 1 (talc) to 10 (diamond).

CSAT-style

Which statement about minerals and rocks is correct?

①Plutonic rock cools fast at the surface, giving small crystals

②Metamorphic rock forms under heat and pressure

③Coal and limestone are igneous rocks

④The higher the Mohs hardness, the softer the mineral

⑤Sedimentary rock forms from solidified magma

▸ ② Metamorphic rock forms under heat and pressure

Metamorphic rock is existing rock whose minerals and texture have changed under high heat and pressure (gneiss, marble).

Plutonic rock cools slowly with large crystals (①), coal and limestone are sedimentary (③), higher Mohs means harder (④), and solidified magma gives igneous rock (⑤).

Summary

Minerals & Rocks core

rock cycle: igneous ⇌ sedimentary ⇌ metamorphic (cycling through magma)

rocks classified by cooling rate (igneous texture), constituent minerals, and degree of metamorphism

🎯 Exam Points

①Igneous: plutonic (slow, large crystals) vs volcanic (fast, small crystals)

②Sedimentary features: bedding (layers), may contain fossils, clastic/chemical/organic

③Metamorphic: heat + pressure → foliation (banding) (schist, gneiss)

④Bowen reaction series: order of crystallization during cooling magma (olivine → quartz)

⑤Mohs hardness: talc (1) < gypsum (2) < calcite (3) ... < diamond (10)

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Geological Time & Environment

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