EARTH SCIENCE CHAPTER SEVEN

WEATHERING, SOIL, AND EROSION

7.1 WEATHERING

Weathering is the breakup of a rock due to exposure to processes that occur at Earth’s surface

Two main types: Mechanical weathering (disintegration) occurs

when rocks are broken down into smaller pieces but do not change in composition.

Chemical weathering (decomposition) occurs when the rock’s minerals are changed into different substances

MECHANICAL WEATHERING

Several processes: Frost wedging Wetting and drying Abrasion by rock materials Plant and animal activity Exfoliation due to upward expansion

FROST WEDGING

Water occupies 10 % more space when it freezes than it does as a liquid.

When water that is trapped inside the cracks and pores rocks freezes, it can break the rock as it expands.

This is called frost wedging or ice wedging.

WETTING AND DRYING

Liquid water also plays a role in mechanical weathering.

Some rocks that are made heavily of clay and silt (like shale), swell when they are wet and shrink as they dry.

This repeated action can break the rock over time.

ABRASION

Occurs when moving sand, rocks, or pebbles grind and scrape against one another.

The sand we walk on at the beach was once rock that has been repeatedly broken down and carried by the waves.

PLANT AND ANIMAL ACTIVITY

Lichens, plants, and mosses grown on rocks and wedge into their pores—roots can eventually split the rocks.

Even trees have been known to grow through boulders.—pic pg 259

Many animals (ants, earthworms, rabbits, etc) make holes in the ground that expose rocks to agents like wind, ice, etc.

UPWARD EXPANSION

Upward expansion occurs when granite beneath the earth’s surface is pushed upward by forces below.

This extreme force causes rocks on top of the granite sheets to break apart and erode.

The granite sheets are then exposed and begin to break and erode also.

This is called exfoliation-- The peeling away of surface layers of exposed

bedrock

CHEMICAL WEATHERING

Can occur anywhere that water or water vapor is present

Can occur by acids Can occur by oxygen

WATER AND CHEMICAL WEATHERING

The reaction of water with other substances is called hydrolysis.

Water readily reacts with some minerals (like feldspar, hornblende, and augite) to dissolve into ions that slowly react with water to form different types of clay minerals.

ACIDS AND CHEMICAL WEATHERING

Carbonic acid, H2CO3, is an acid commonly found in water. It is formed when CO2 in the atmosphere dissolves in rainwater.

This acidic mixture of carbonic acid and water breaks down even more minerals to form clay.

Sulfur dioxide, nitrogen compounds, and carbon dioxide from pollution forms acid rain.

This acid rain can very quickly decompose many types of structures.

OXYGEN AND CHEMIMCAL WEATHERING

Oxidation is the chemical reaction of oxygen with other substances.

Oxidation is very effective in weathering rocks that contain iron. Oxidized iron is called rust.

RATES OF WEATHERING

Under average conditions, weathering is a slow process.

The process is influenced by three major factors: Surface exposure—all weathering happens at the

surface, so more surface area=faster process Composition of the rock—certain types of rock are more

susceptible to oxidation and acid reactions, thus they would weather faster than other rocks in the same area

Climate—in warmer, wetter climates—both mechanical and chemical weathering play about the same role. However, in dryer, colder climates, mechanical weathering plays a greater role than chemical weathering

7.2 EROSION and DEPOSTION

Erosion is the removal and transport of materials by natural agents such as wind and running water. Is affected by climate Is affected by the composition of the

rock The overall effect depends on how the

process of erosion compares with the ongoing process of uplift

Deposition is when the materials are dropped in another location.

AGENTS OF EROSION

Gravity Running Water Glaciers Wind Living things To be discussed more in Chapter 8

7.3 SOIL

Soil is made of loose, weathered rock and organic material in which plants with roots can grow

Without the weathering process we studies in Section 1, there would be no soil.

Without soil, what would we eat?

HOW SOIL FORMS—PARENT MATERIAL

The material from which soil forms is called the parent material. If the parent material is the bedrock

beneath the soil, the soil is called a residual soil.

If the parent material is not from the underlying bedrock but from weathered particles that were carried there by wind, waves, etc., the soil is called a transported soil.

HOW SOIL FORMS—THE PROCESS

Forms as parent material is weathered away.

Rate varies by location. Climate plays a role. Organic materials (decaying plant

and animal remains) are added.

SOIL PROFILE

A cross-section of the earth exposed by digging

Allows us to study the “layers of soil” Mature soils have three distinct

zones or soil horizons A Horizon B Horizon C Horizon

A-HORIZON

Called topsoil Generally gray to black in color Contains humus—organic material

that forms from decayed plant and animal materials

Tends to be sandy

B-HORIZON

Called subsoil Usually red or brown from oxides that

formed in topsoil and then got washed down into the subsoil

Often contains clay and soluble minerals like calcium and potassium

C-HORIZON

Made of slightly weathered parent material

Often has rock fragments Lies just above the bedrock or parent

material

SOIL COMPOSITION

Several factors affect soil composition: Parent material Time Plants and animals Topography Climate

SOIL AS A RESOURCE

The world’s population is increasing at an alarming rate.

All of these people need to eat!!!!!! However, less than 25 % of the Earth’s

land can be used to grow crops. Soil fertility is the ability of soil to grow

plants. Certain types of soil are best suited for certain

crops. Many things currently threaten soil fertility.

SOIL DEPLETION

Soil depletion occurs when the soil gradually becomes so lacking in nutrients that it can no longer be used to grow a usable crop.

Farmers can do some things to prevent this. Allow the soil to rest by rotating crops

SALINIZATION

Irrigation is often used to make soils fertile.

Irrigation water often brings in dissolved minerals.

These dissolved minerals are often salts.

Over time, the buildup of salt in the soil is called salinization. Often renders the soil completely

useless

SOIL CONSERVATION

Several methods exist: Windbreaks—also known as shelter belts—lines of trees

planted along the edges of fields Contour farming—farmers plant crops parallel to land

contours to prevent water from running rapidly down a slope

Terraces—flattens a slope into separate terraces to slow the speed of runoff

Strip cropping—alternate rows of crops to minimize soil erosion

No-till—method of farming where everything is done at once (plowing, planting, fertilizing, weed control) and the soil is left completely alone until harvest time