6.3 Natural processes break down rocks 6. Minerals and Rocks 6.1 Minerals are all around us 6.2 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 Ice Wedging Pressure Release Plant Growth 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 Materials: Parts I&II Procedure: Part I 1. Make a data table (p. 205) Follow steps 2-5. Part II 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 Record your procedure in your science notebook Create a data table to organize the data you will collect Measure and record the mass or volume of each rock sample Carry out your procedure… Rock before Rock during Rock after Mass of rock