DK - WOW! The Visual Encyclopedia of Everything

ROCKS AND MINERALSRocks are mixtures of natural chemical compounds called minerals, which form crystals with distinctive shapes. There are three main types of rock. Igneous rock is formed when molten rock cools and hardens, a metamorphic rock is one that has been changed by heat or pressure, and sedimentary rock is generally made from fragments of rock cemented together.

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Slate

This dark rock is created when

sedimentary shale is put under intense pressure. It forms thin sheets that can be cut into squares and used for roofing.^2

Cockscomb barite

This whitish mineral

is often found in veins running through rocks. Its crystals form clusters that can resemble cockscombs (roosters’ head crests).^3

Schist

Like slate, schist is created by

pressure and heat, which transforms a soft sedimentary rock into a very much harder metamorphic rock.^4

Chalk

A type of limestone, chalk is

built up from the remains of tiny marine organisms, which sank to the bottom of a tropical sea during the age of dinosaurs.^5

Marble

Hard and usually pale, marble

is a metamorphic form of limestone. It can be carved and polished into statues, and decorative slabs are used in architecture.^6

Calcite

The main mineral in

limestone and marble, calcite forms

the stalactites and stalagmites seen in limestone caves.

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Limestone

All limestones are made of

chalky minerals, particularly calcite. They are easily dissolved by rainwater, creating extensive cave systems.^8

Halite

Formed by the evaporation of

salt lakes, halite is rock salt – the mineral that we use to flavour our food.^9

Biotite

Dark brown biotite is a type

of mica, a mineral found in most granites and schists. Its plate-like crystals resemble thin, flaky sheets of hard plastic.^10

Eclogite

A dense, heavy metamorphic

rock formed deep beneath Earth’s surface, eclogite contains bright green pyroxene

and glittering red garnet minerals. 11

Tremolite

Thin, transparent, fibrous-

looking crystals of tremolite form from limestones that have been subjected to intense heat deep underground.^12

Beryl

This very hard mineral

forms transparent, often greenish crystals that can be cut to create emeralds and aquamarines.

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Granite

One of the main rocks

that form continents, granite results from molten rock cooling slowly deep underground to form big quartz, feldspar, and mica crystals.^14

Breccia

This sedimentary rock is made

of broken, sharp-edged rock fragments cemented together by finer particles.^15

Obsidian

Also known as volcanic

glass, this shiny black or dark green rock is formed when molten lava cools too quickly to form crystals.^16

Gabbro

This is a coarse, dark, iron-

rich, crystalline rock that makes up much of the deep ocean floor.^17

Pumice

Gas erupting from volcanoes

often forms bubbles inside cooling lava. This can then form pumice, which has so many gas bubbles that it floats on water.

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Corundum

This dull-looking stone is

a type of corundum, the hardest mineral after diamond. Its crystals are used to make rubies and sapphires.^19

Albite

A pale, sodium-rich form of

feldspar, albite is a common ingredient of granite, visible as big, blocky crystals that glint in the sunshine.^20

Graphite

Made of pure carbon – like

diamond – graphite is a soft, metallic mineral that leaves a dark streak. It is used to make the “lead” in pencils.^21

Basalt

Heavy, dark basalt is the

fine-grained form of gabbro, created when iron-rich lava from oceanic volcanoes cools quickly, often underwater.^22

Sandstone

Sand cemented together

by other minerals forms sandstone. This red sandstone was once a desert dune.^23

Pyrite

Known as “fool’s gold”, this

yellow metallic mineral is actually made of iron and sulfur. It often forms big cubic crystals like the ones seen here.^24

Conglomerate

Very like breccia, this

rock is a solid, cemented mass of rounded

pebbles, like those found on riverbeds

and lake shores.

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