What is a Rock?

• Naturally-occurring mixtures of minerals, glass or organic matter.

What is a Rock?

• Rocks are divided into 3 groups based on how they were formed:

•IGNEOUS•SEDIMENTARY•METAMORPHIC

What is the difference between a rock and a

mineral?• Rocks are made up of ONE or MORE minerals.

Once a rock is formed, does it stay the same rock forever?• NO!

• Rocks are continually changed by many processes, such as weathering, erosion, compaction, cementation, melting, and cooling

• Rocks can change to and from the three types

The Rock Cycle—earth materials change back and forth among the different types

of rocks

• “Ignis” = Latin for “fire”

• Formed from the cooling of either magma or lava

• The most abundant type of rock

• Classified according to their origin and composition

ORIGIN— Where rocks are formed

• Below ground = from magma (intrusive igneous rock)

• Usually have LARGE crystal grains (they cooled slowly)

Some have large & small crystals

• Above ground = from lava (extrusive igneous rock)

• Usually have SMALL or NO crystals (they cooled too quickly)

Intrusive or Extrusive? Why?

Granite: Intrusive or Extrusive? Why?

Intrusive or Extrusive? Why?

Intrusive or Extrusive? Why?

COMPOSITION— What kind of substances the rocks are made of

Diorite

Basalt

Granite

• Formed from sediments (rock fragments, mineral grains, animal & plant remains) that are pressed or cemented together or when sediments precipitate out of a solution.

• These sediments are moved by wind, water, ice or gravity.

• Sedimentary rocks represent 7% of the Earth’s crust, but they cover 70% of the Earth’s surface.

• Sedimentary rocks are fossil-carrying rocks.

What turns solid rock into sediments?

• Water or wind breaks down and deposits sediment (erosion & deposition)

• The heavy sediments press down on the layers beneath (compaction)

• Dissolved minerals flow between the particles and cement (glue) them together (cementation)

How can sedimentary layers help us understand the age of fossils?

• As sedimentary rocks are deposited, they form horizontal layers

• Scientists know that the layers on top (and the fossils in the top layer) are YOUNGER than the fossils in lower layers.

3 Types of Sedimentary Rocks:

• Some are made of broken pieces of other rocks

Organic—remains of plants and animals are deposited in thick layers

• Examples• Fossil rich limestone is made from the

shells of ocean animals; used to make chalk

3 Types of Sedimentary Rocks:

• Chemical—minerals dissolved in lakes, seas, or underground water

3 Types of Sedimentary Rocks:

Mineral crystals are made as the shallow water that has flooded the bottom of Death Valley evaporates. Click on image for full size (66K JPG)

Courtesy of Martin Miller, University of Oregon

Examples• Limestone

made when calcite mineral precipitates from sea water

• Rock Salt—made from evaporation of sea waters

• Rocks that have changed due to intense temperature and pressure

• “Meta” means “change” and morphosis means “form” in Greek

• Igneous, sedimentary and other metamorphic rocks can change to become metamorphic rocks

What occurs in the Earth to change these rocks?

• Pressure from overlying rock layers• High heat, but not enough to melt the rock• Rocks may be flattened or bent or atoms may be exchanged to form new minerals.

Properties of metamorphic rocks

• Usually resistant to weathering and erosion and are therefore very hard-wearing.

• Can be polished to give a smooth shiny surface.

• Example: Marble, slate.• Uses:

Examples of rocks becoming metamorphic

• Example: marble formed from limestone

Where do metamorphic rocks usually form?

• Where magma intrudes relatively cool rock

• Near colliding plates (near mountain ranges)

• Places that are covered miles thick with other rock causing pressure

• When hot water flows between rocks