6. Minerals and Rocks

6.1 Minerals are all around us6.2 Rocks form in different ways

6.3 Natural processes break down rocks

6.4 Geologic maps show Earth’s surface features

6.3 Natural processes break down rocks

• Before, you learned: – Minerals make up almost all rocks– Different minerals have different properties– Rocks are broken down to form sediments

• Now, you will learn:– About the relationship between weathering and the

geological cycle– How mechanical weathering breaks down rocks– How chemical weathering changes rocks– What factors affect the rate at which weathering occurs

Warm-up Questions

• The set of natural processes that form, change, and re-form rocks is called the rock cycle (t/f)

• True• A rock is usually made up of only one type of

mineral• A rock is usually made up of more than one type

of mineral• Igneous rock develops from layers of sediment• Sedimentary rock develops from layers of

sediment

Weathering helps change Earth’s features over time

• Water, ice, and wind can weather mountains down into small pieces of sediment– Weathering: set of natural process that break down rocks

• Physical or Chemical

• The sediment can be changed back into large rocks

• New mountains can be pushed up (tectonic plate movement)

• These are processes in the geologic cycle

– Includes all the processes by which Earth’s features are worn down and built up

– Rock cycle?

– Weathering?

Geologic Cycle

Mechanical weathering produces physical changes in rocks

• Does smashing a rock with a hammer change into a new substance?

• Mechanical weathering – the breaking up of rocks by physical forces

1. Ice Wedging2. Pressure Release3. Plant Growth4. Abrasion

Mechanical Weathering: 1. Ice Wedging

• When water freezes, it expands– Can freeze in the cracks and pores of rocks– The expansion can split the rocks apart

• Common in places where temperatures rise above and fall below the freezing point of water (0oC or 32oF)

link

Mechanical Weathering: 2. Pressure Release

• Rocks deep within Earth are under great pressure

– Processes within Earth may push the rock up to the surface, or the rock above may wear away

– The pressure inside the rock (the internal pressure) is still high (it was balancing the external pressure from the surrounding rock)

– When the external pressure decreases:

• External pressure < internal pressure

• Causing the rock to expand – it cracks, leading to exfoliation

– Exfoliation: a process in which layers or sheets of rock gradually break off (onion-skin weathering)

Mechanical Weathering: 3. Plant Root Growth

• Trees, bushes, and other plants may take root in cracks in rocks

• The roots grow and wedge open the cracks – splitting rocks

Mechanical Weathering: 4. Abrasion

• Water can wear down rocks on riverbeds and along shorelines– Worn down by friction (rubbing of one object

or surface against another)– The water can wear down rocks or can cause

a rock to tumble, wearing down other rocks

Chemical weathering changes the mineral composition of rocks• Ex: nail rusting: the steel in the nail

contains iron; oxygen in air and water react with the iron to form rust!

• Chemical weathering – the breakdown of rocks by chemical reactions that change the rocks’ composition

• Minerals in contact with air and water may dissolve or change into different minerals

Chemical Weathering - Dissolving

• Will water cause chemical weathering?

• It is the main cause of it!– Water can become acidic from the carbon

dioxide in the atmosphere and also soils• The acids can dissolve rocks, and also make them

more likely to break apart

– Air pollution can make water more acidic

Chemical Weathering - Rusting

• Oxygen in the air will rust minerals that contain iron– Iron “oxidizes” (combines with oxygen) and

produces “iron oxides”, or rust

Weathering occurs at different rates

• Mostly over long periods of time– Hundreds or thousands of years for a very

hard rock to wear down a few millimeters (1000 mm = 1m !)

• Factors affect the rate of weathering:– Surface area (amount of the surface exposed

to the weathering element)– Rock composition– climate

Weathering Rates

• Surface area – the more surface exposed to air/water, the faster the rock will break down– (think about crushed or cube ice – which melts faster?)

• Rock Composition – granite breaks down much more slowly than limestone

• Climate – chemical weathering speeds up in hot, wet regions vs cold, dry regions– Mechanical weathering caused by freezing/thawing may

occur more in cold regions

How are rocks classified?

• By how they form – Rocks change, typically over thousands to

millions of years – they break down and re-form

• Rock types:– Igneous Rock– Sedimentary Rock– Metamorphic Rock

Math in Science

• Each mineral makes up a certain proportion, or fraction, of a granite sample– You can compare mineral amounts by expressing

each mineral’s fraction as a percentage• To change a fraction to a percentage, you must find an

equivalent fraction with 100 as the denominator– 1/5 to percent?

• First, divide 100 by the denominator 5 = 20• Then multiply the numerator and denominator by

20 = 20/100 = 20%

Rocks in the Crust

• Igneous, sedimentary and metamorphic rocks are all found in Earth’s crust

• 95% is igneous and metamorphic rock• 5% is sedimentary, a thin covering on Earth’s surface

– Surface of Crust: 75% sedimentary, 25% Ign & Met.– Entire Crust: 5% sedimentary, 95% Ign & Met.

• Sedimentary is most common at surface because formed by processes that occur at the surface– Igneous and metamorphic are formed by process that

occur deeper within Earth

Chapter Investigation: Weathering (p.204-205)

• Overview and Purpose: Mechanical and chemical weathering break down rocks. Different types of rocks react in various ways to different weathering processes.

• In this investigation you will:– Observe conditions that allow rusting, a form of

chemical weathering, to occur– Design a procedure to model the effects of mechanical

and chemical weathering on different types of rocks

Lab: Weathering

Lab: Weathering

• Materials: Parts I&II• Procedure:

– Part I• 1. Make a data table (p. 205)• Follow steps 2-5.

– Part II

1.Design a procedure to model the effects of different weathering processes on several rock samples. You might use some or all of the materials available. Your procedure should enable you to collect data, determine if the rocks changed, and describe how they changed

2.Record your procedure in your science notebook

3.Create a data table to organize the data you will collect

4.Measure and record the mass or volume of each rock sample

5.Carry out your procedure…

Rock before

Rock during

Rock after

Mass of rock