What is a rock?

WHAT IS A ROCK?

A rock is a mixture of minerals and other materials.

FORMATION OF ROCKS

Rocks are categorized by how they were formed.

Igneous: about 150 rocks

Sedimentary –about 50 rocks

Metamorphic – about 300 rocks

CLASSIFYING ROCKS

Using these properties, Geologists can classify a rock according to its origin – or how the rock formed.

Color TextureMineral

Composition

TEXTURE

Texture – Rocks are made of particles of other minerals called grains.

Geologists classify rocks into 3 major groups: Igneous rock: from cooled magma/lavaSedimentary rock: from weathered sediment compressed

Metamorphic rock: existing rock changed from heat/pressure (forms underground)

When studying a rock sample, geologists observe the rock’s:Mineral composition: can be 1 or many more that make up a rock

Color: lightness/darknessTexture: Grain size, grain shape, grain pattern

IN SUMMARY…

Forms from magma or lava

IGNEOUS ROCKS

They are either intrusive (formed beneath surface) or extrusive (formed above Earth’s surface)

CLASSIFYING IGNEOUS ROCKSBY ORIGIN

Basalt rock

CLASSIFYING IGNEOUS ROCKSBY TEXTURE

Fine grained Course-grained

Rapidly cooling lava Slowly cooling magmaSmall crystals Large crystalsExtrusive rocks Intrusive rocks

Granite - IntrusiveRhyolite - extrusive

CLASSIFYING IGNEOUS ROCKSBY MINERAL COMPOSITION

Mineral composition of Granite

Uses of Igneous Rocks• Granite - For buildings & monuments – Egyptians used for

statues over 3,500 years ago.• Basalt – crushed for gravel in construction.• Pumice – used for polishing.• Obsidian – used for ancient tools.

MINERAL COMPOSITION

Feldspar – 63%Quartz – 27%Mica & Hornblende – 10%

Igneous rocks ALL were formed from magma or lava

They are classified by origin, texture and mineral composition

They may form ON or BENEATH Earth’s surfaceExtrusive rock: From lava on the surface Intrusive rock: From magma that hardened under the surface

Since they are hard, dense and durable they have been used for tools and building materials.

IN SUMMARY…

SEDIMENTARY ROCKS

Sedimentary rocks are formed through a series of processes: erosion, deposition, compaction, and cementation.

TYPES OF SEDIMENTARY ROCKSClastic – particles are squeezed together.

Organic – remains of plants and animals.

Chemical – solutions evaporating

Shale

Conglomerate

Sandstone

coal limestone

Rock salt

Sediment is small, solid pieces of material that come from rocks or living things

Series of processes: Erosion (water/wind), deposition (sediment lands somewhere), compaction (presses sediment together with weight/pressure), cementation (dissolved minerals crystallize and glue particles of sediment together)

Clastic rocks: formed when rock fragments squeezed together (sandstone)

Organic rocks: remain of plants/animals deposited in thick layers. (coal)

Chemical rocks: minerals dissolved in a solution crystallize (limestone)

IN SUMMARY…

METAMORPHIC ROCKS

Heat and pressure deep beneath Earth’s surface can change any rock into metamorphic rock.

FOLIATED VS. NONFOLIATED

Two categories foliated: grains arranged in parallel layers or bands non-foliated: mineral grains arranged randomly.

Heat and pressure DEEP beneath Earth’s surface can change ANY rock to a metamorphic rock.

When changed into metamorphic rock it changes its appearance, texture, crystal structure and mineral content

High temperatures/pressure can change minerals into other minerals!

Metamorphic rock classified by the arrangement of grains that make up the rock Parallel layers : foliated Random grains: nonfoliated

Used for building and sculpture ( marble/slate) Marble formed from limestone subjected to heat/pressure below

Earth’s surface

IMPORTANT CONCEPTS

THE ROCK CYCLE

Energy is continuously used and recycled as rocks go through the rock cycle.

Rocks are continuously built, destroyed and changed in the crust.

Example: Igneous rock granite formed beneath surface Forces of mountain building push granite upward (forming

mountain) Slowly, water/wind wear away granite Granite particles become sand, carried by streams to the

ocean Over millions of years sandy layers pile up on ocean floor Slowly, sediment changes to sandstone (sedimentary rock) Over time, sandstone is buried Heat/pressure change rock’s texture Sandstone changed into metamorphic rock : quartzite

IMPORTANT CONCEPTS