Chapter 3

Rocks

Rocks

• Rock – any solid mass of mineral or mineral-like matter that occurs naturally as part of our planet– Usually solid mixtures

of minerals– Some composed of

just one mineral– Each mineral retains

its properties in mixture

The Rock Cycle

• Three Major Types– Igneous– Sedimentary– Metamorphic

• Interactions among three things cause rocks to change from one type to another:– Water– Air– Land

• Rock cycle – continuous processes of rocks changing

The Rock Cycle – Igneous Rocks

• Magma – molten material that forms beneath Earth’s surface

• Lava – magma that reaches the surface

• Igneous rocks– Form when magma

cools and hardens beneath surface -OR-

– Volcanic eruption

The Rock Cycle – Sedimentary Rocks

• Weathering – rocks are physically and chemically broken down by water, air, living things.

• Sediments – weathered pieces of earth materials– Moved by water, gravity, glaciers, wind

• Sedimentary rocks formed by:– Sediments being compacted and cemented

The Rock Cycle – Metamorphic Rock

• Sedimentary rock – buried deep within Earth– Exposed to increased pressure and temp

• Metamorphic rock formed:– Sedimentary rock exposed to extreme

pressure and temp

• When metamorphic rocks exposed to additional pressure or higher temps:– Melt to form magma -> eventually crystallizes

to form igneous rock again

Alternate Paths

• Igneous Rock: – Some remains deeply buried: exposed to

strong forces & increased temp – becomes metamorphic rock

– If pressure & temp high enough – could melt and reform igneous

• Metamorphic and sedimentary:– Could weather to become sediments again– Then become sedimentary rocks again

Energy

• Igneous & metamorphic rocks– Both driven by heat from Earth’s interior

• Sedimentary rocks– Weathering and movement of weathered

materials– External processes powered by energy from

the sun

Dynamic Rock Cycle

Igneous Rocks

• Different kinds of igneous rock form when magma and lava cool and harden– Intrusive– Extrusive

Intrusive Igneous Rocks

• Form when magma hardens beneath Earth’s surface– Intrude into existing

rocks– Common rock: granite

Extrusive Igneous Rocks

• Form when lava hardens– They are extruded

onto the surface– Common: rhyolite

Classification of Igneous Rocks

• Two main characteristics:– Texture

• Size• Shape• Interlocking crystals

– Composition• light vs. dark minerals

Igneous Rocks – Texture

1. Coarse-grained – Slow cooling results

in formation of large crystals

2. Fine-grained– Rapid cooling results

in small, interconnected mineral grains

Igneous Rocks – Texture

3. Glassy– Formed when ions in

lava do not have enough time to arrange themselves in network of crystals

– Ex. – obsidian, pumice

4. Porphyritic – (large crystals surrounded by fine-grained minerals)

– Formed when minerals that crystallize from magma do not form at same rate or same time

Igneous Rocks – Composition

1. Granitic Composition• Light-colored silicate minerals• Major rocks of continental crust – 70% silica• Ex – rhyolite: extrusive granitic rock

2. Basaltic Composition• Dark silicate minerals & plagioclase feldspar• Rich in iron and magnesium• Darker and denser than granitic rocks• Ex – basalt, gabbro

Igneous Rocks - Composition

3. Other Compositional Groups• Andesitic composition –

between granitic and basaltic rocks

• common volcanic rock – andesite• 25% dark silicate minerals

• Peridotite – much of upper mantle

• Ultramafic – composed mostly of dark minerals

• Rare at Earth’s surface

Sedimentary Rocks

• Form when existing rocks broken down into sediments

1. Weathering - any process that breaks rocks into sediment

2. Erosion – water, wind, ice, gravity3. Deposition – loses energy, drops

sediment• Sediments deposited according to size• Largest deposited first – smallest deposited

last

Sedimentary Rocks

• Processes that change sediments into sedimentary rocks:

1. Compaction – squeezes, or compacts sediments

• Much of the water is driven out

2. Cementation – dissolved minerals deposited in tiny spaces among sediments

Classification of Sedimentary Rocks

• Classified into two groups according to the way they form:

1. Clastic sedimentary rocks – rocks made of weathered bits of rocks and

minerals– Conglomerate – gravel-sized or larger particles make up

most of rock– Breccia – angular particles– Sandstone – sand-size grains

Classification of Sedimentary Rocks

2. Chemical and biochemical sedimentary rocks– Dissolved minerals precipitate from water

solutions– Occurs when water evaporates or boils off, leaving

a solid product– Limestones, rock salt, chert, flint, rock gypsum

90 % of limestones formed from biochemical sediments Shells and skeletal remains of organisms on ocean floor

Features – Sedimentary Rocks

• Can give clues to how, when, and where rocks formed

1. Each layer: records a period of sediment deposition• Oldest layers found at bottom

2. Ripple marks: rock formed along beach or stream bed

3. Mud cracks: record of a dry environment

4. Fossils: answer questions about rocks– Did rock form on land or ocean?– Was climate hot or cold?– Match rocks from different places (compare age)

Metamorphic Rocks

• Existing rocks are changed by heat and pressure– Conditions are a few

km below surface and extend into the upper mantle

Two Types of Metamorphism

1. Contact metamorphism: hot magma moves into rock

• Low-grade metamorphism

• Marble – forms from limestone

Two Types of Metamorphism

2. Regional Metamorphism• During mountain

building, large areas of rock subjected to extreme pressures and temps

• High-grade metamorphism

Agents of Metamorphism

1. Heat – most important agent• Comes from two sources: magma and change in

temp with depth• Provides energy to drive chemical reactions• Minerals are stable at different temps

2. Pressure (Stress)• Increases with depth• Pressure on rocks from within Earth comes from all

directions• Causes rocks to flow, not fracture

• Minerals flatten and elongate

Agents of Metamorphism

3. Hydrothermal Solutions

• When solutions increase in temp, reactions among substances occur at a faster rate

• Promote recrystallization by dissolving original minerals and depositing new ones

Classification of Metamorphic Rocks

1. Foliated• Some minerals

recrystallize at right angles to the direction of force

• Gives rock a layered or banded appearance

• Ex: shale – slate – schist – gneiss

Foliated MetamorphicShale Slate

Schist Gneiss

Classification of Metamorphic Rocks

2. Nonfoliated:• Does not have

banded texture• Most only contain

one mineral• Limestone (made

of calcite) - marble

limestone

marble