Chapter 5- Weathering and Soil Earth Science 6th Grade

Bell work Ch5 L.1 Weathering: the mechanical and chemical processes that change objects on Earth’s surface over time Mechanical weathering: When physical processes naturally breaks rocks into smaller pieces Chemical weathering: Changes the materials that are part of a rock into new materials Oxidation: Combines the element oxygen with other elements or molecules

Lesson 1- Weathering How does weathering break down or change rock? How do mechanical processes break rocks into smaller pieces? How do chemical processes change rocks?

Launch lab (work in pairs) Each pair gets 12 “rocks” (chocolate candies), 3 plastic cups and a jar with a lid. NO, YOU MAY NOT EAT THE CANDY. Place four rocks in cup and the other eight in the jar. Fasten lid tightly Shake jar vigorously 300x Remove half of the pieces and put them in second plastic cup Replace lid and shake another 300x Put remaining pieces in third plastic cup ANSWER: compare and contrast rocks from each cup. What do you think caused the rocks to change? TURN IT IN

Weathering and its effects Weathering: the mechanical and chemical processes that change objects on Earth’s surface over time Also changes Earth’s surface Break, wear, abrade and chemically alter rocks and rock surfaces Weathering breaks rocks into smaller and smaller pieces= sediment Sand, silt and clay (largest=sand, smallest=clay) Can change chemical makeup of a rock, sometimes makes it easier to break

Mechanical Weathering When physical processes naturally breaks rocks into smaller pieces Chemical makeup is NOT changed, just change the size

Mechanical Weathering Examples of mechanical weathering Intense heat (like from forest fire) causes nearby rocks to expand and crack Ice wedging= water enters cracks in rocks and when the water freezes (at 0C), it expands and that widens the crack. Abrasion= grinding away of rock by friction of impact. A strong current in a stream can carry rock fragments downstream. Fragments hit each other and grind each other down into smaller and smaller pieces.

Mechanical Weathering Examples of mechanical weathering Plants= Can grow into the cracks of rocks, roots absorb minerals making the rock weaker. The stems and roots get wider and pushes on the cracks making the rock break. Animals= animals that live in the soil create holes where water an go and weather rocks. They can also help break down rocks as they dig.

Mechanical Weathering Surface area: increases when something is broken in smaller pieces Amount of space on the outside of an object Rate of weathering depends on surface area exposed to environment (for chemical weathering too) Clay is 100x smaller than sand, so it has an increased surface area Means that weathering has greater effect. Soil with small particles is affected more than soil with larger particles.

Chemical weathering Changes the materials that are part of a rock into new materials Changes the composition and the size

Chemical weathering Water and chemical weathering Most substances dissolve in water Minerals in rocks dissolve slowly in water Dissolving happens when minerals in a rock break into smaller parts in a solution Example: salt (sodium chloride) breaks down into individual sodium and chloride ions when dissolved in water

Chemical weathering Dissolving by acid= increases rate of chemical weathering more than rain or water. Attracts atoms away from rock minerals and dissolves them in acid Enter air naturally (volcanoes) or form pollutants (burning coal)

Chemical weathering Use pH to tell if a solution is acidic (0-7), basic (7-14) or neutral (7). Vinegar= 2-3 so it’s an acid Rain water= 5.6 so it’s slightly acidic. Reacts with carbon dioxide to form weak acid, can dissolve rocks. Acid rain= 4.5 so it’s more acidic than normal rain. Happens when burning coal produces sulfur oxide which reacts with rain water.

Oxidation Combines the element oxygen with other elements or molecules Usually oxygen comes from the air Addition of oxygen produces an oxide Iron oxide is a common oxide= hematite Aluminum’s oxide= bauxite

Oxidation rate Not all parts of the iron-containing rock oxidize at the same rate Part exposed to oxygen oxidizes faster than part not exposed to oxygen Example: Iron oxide- oxidized part turns red (minerals are different than minerals in the center of the rock that are not oxidized) Right side not oxidized, left side oxidized

What affects weathering rate? Similar rocks can weather at different rates. Why? Environment helps determine weathering rate Both types of weathering depend on temperature and water

What affects weathering rate? Mechanical weathering Occurs fastest in environments with lots of temperature changes Cycles of freezing and thawing or wetting and drying Chemical weathering Occurs fastest in warm, wet places. Fastest in regions near equator

What affects weathering rate? Rock type Made of many minerals. Most easily weathered mineral in a rock determines the rate (only as strong as the weakest link) Low hardness= easier to undergo mechanical weathering More mechanical weathering= more surface area exposed More surface area exposed= more chemical weathering Holes in a rock= easier to weather

HOMEWORK Ch5 L.1 Vocabulary words on flash cards Memorize for quiz Lesson review questions p.155 #1-10 Outline lesson 1 Quiz!

Bell Work Ch5 L.2 Soil: a mixture of weathered rocks, rock fragments, decayed organic matter, water and air Organic matter: the remains of something that was once alive Pore: Small holes and spaces in the soil Decomposition: the process of changing, once-living material into dark-colored organic matter Parent material: Starting material of soil Climate: Average weather of an area Topography: Shape and steepness of the landscape Biota: All of the organisms that live in the region Horizon: Layers of soil formed from the movement of the products of weathering

Lesson 2- Soil How is soil created? What are soil horizons? Which soil properties can be observed and measured? How are soils and soil conditions related to life?

Launch lab p.158 work in pairs Place a cup of soil in a jar with a lid Add a few drops of liquid soap Add water until almost full Firmly attach lid Shake for one minute Observe contents of jar after 2 minutes, after 5 minutes. ANSWER: How many different layers did your sample form? What do you think makes up each layer? TURN IT IN

What is soil? Soil is a mixture of weathered rocks, rock fragments, decayed organic matter, water and air. Half the volume is solid materials Life on Earth depends on soil

What is soil? Fragments seen in Ch5. L1 don’t become good soil until animal and plants live in them. Plants and animals add organic matter to the rock fragments Organic matter= the remains of something that was once alive

Pores Small holes and spaces in the soil Enable water to flow into/out of the soil Size depends on particles that make up the soil As particle size increases= pore size increases Can have water and air in them, the amount varies.

The organic part of soil Organic part of soil= solid part that was once part of an organism. Pieces of leaves, dead insects, waste products Roots die, leaves and twigs fall to the ground and organisms in the soil decompose these materials for food

Organic part of soil Decomposition: the process of changing, once-living material into dark-colored organic matter. Organic matter gives soil important properties Dark color of soil absorbs light Organic matter holds water and provides plant nutrients Organic material holds minerals together in clusters= keeps pores open for movement of water and air

Inorganic part of soil Inorganic= materials that have never been alive. Mechanical/chemical weathering of rocks into fragments forms inorganic matter in soil Classify soil according to size of fragments

Inorganic part of soil Rock fragment sizes= boulders, cobbles, gravel, sand, silt or clay. Sand- feels rough Silt- feels smooth Clay- feels sticky Large particles have large pores, they drain rapidly. Small pores retain more water in the soil.

MiniLab p.159 Observe soil sample, record sizes of particles. Add a little bit of water to moisten the soil Rub the moist soil between your fingers Record observation Determine if sample is mostly sand, mostly silt or mostly clay based on observations. ANSWER: what texture does your soil have (when wet/dry)? What other properties did you observe?

Formation of soil Many kinds of soil that form depend on five factors= factors of soil formation Parent material Climate Topography Biota Time

Parent material Starting material of soil Rock/sediment that weathers and forms soil Bedrock= rock that weathers to form soil in the same location where rock was formed. Soil can be made from rocks that were carried by wind/water to another location Particle size and type of parent material can determine properties of soil

Climate Average weather of an area Precipitation and daily and average temperatures are measures of climate Warm, wet climates can form soil quickly Also places where lots of freezing/thawing occurs make soil quickly

Topography Shape and steepness of the landscape. Determines what happens to the water that reaches the soil surface Flat landscape= most water is absorbed into soil Steep landscape= most water runs downhill (can carry soil with it)

Biota All of the organisms that live in the region Help speed up the process of soil formation Some form passages for water to move through Most soil organisms involved in decomposition of material that form organic matter

Time Weathering is constantly acting on rock and sediment Soil formation is constant, slow process. Soil is still young after 1,000 years Mature soil develops layers New soil forms on top of old soil Each layer has its own characteristics

Horizons Layers of soil formed from the movement of the products of weathering Each layer has characteristics based on the kind of material it contains Three horizons common to most soils A-horizon, B-horizon, C-horizon O-horizon= top organic layer R-horizon= unweathered bedrock layer

Three common horizons A-horizon B-horizon C-horizon What you see when you dig a shallow hole Darkest usually because contains most organic matter B-horizon Mostly clay deposited from upper layer by water seeping through A layer. C-horizon Layer of weathered parent material (rock or sediments)

Soil properties and uses Observing and measuring soil properties Feeling the soil allows you to estimate how much sand, silt and clay is present Color is easily observed and shows how much organic matter the soil contains In the lab: Determine pH, nutrient content to see if the soil is suitable for farming

Soil properties and uses Soil properties that support life Nutrients in soil come from decaying and weathering How well plants grow can tell you about the nutrient content If add fertilizer, then it doesn’t really matter Soil formation takes thousands of years so replenishing nutrients takes a long time

Soil types and locations The amount of horizons and amount of weathering depends on location where soil is forming Not all soil is alike

HOMEWORK Ch5 L.2 Vocabulary words on flash cards Memorize for quiz Lesson review questions p.165 #1-10 Outline lesson 2 Study for quiz OPTIONAL Extra Credit p. 169-173 all. You MUST copy down the entire question and the answer. Only answers will not be accepted.

Soil lab p.166-167 Compare types of rock, climate and topography of Minnesota, Colorado and Florida. Record similarities/differences. Examine soil profile from each location. Record similarities/differences. Draw sample profiles and label A,B,C horizons How does each horizon relate to the factors of soil formation? Choose one location and model the profile using materials provided. Label location and horizons. How are soil profiles affected by soil-forming factors? Were any profiles missing from any locations? Why might one be missing? Was one of the horizons thicker in any of the profiles? Why? How does a soil profile relate to soil-forming factors? TURN THIS IN