Chapter 4: Weathering and Soil Formation

SWORD WORD Weathering – is a mechanical or chemical process that causes rocks to change by breaking them down into small pieces.

Weathering Weathering is the breaking down rocks into smaller pieces either chemically or mechanically. Example : holes in the roads and cracks in the sidewalk

Types of Weathering There are 2 types of weathering, Mechanical and Chemical.

Mechanical Weathering Mechanical Weathering is when rocks are broken down into smaller pieces without changing them chemically. These pieces are the SAME as the original rock. Two main causes are: Ice Wedging and Living Organisms.

Ice Wedging Ice Wedging is Water seeps into the cracks of rocks Cold Temperature cause the water to freeze Freezing water puts pressure on the rocks Temperatures increase causing thawing – which melts the water and the rocks break apart. Ice Wedging occurs when temperatures are constantly increasing and decreasing

Organic Matter (Plants and Animal) Plants can grow in unlikely places. Sometimes roots grow deep into cracks of rocks where water collects. This puts pressure on the rocks and slowly pulls them apart.

Organic Matter (Plants and Animals) Gophers and Prarie Dogs – These animals burrow through the ground, these animals break rock apart.

Chemical Weathering Chemical Weathering occurs when the chemical composition of rocks change. MOST COMMON in tropical regions where its mostly warm. Two important causes: Natural Acids and Oxygen.

Natural Acids and Plant Acids Natural Acids – when water mixes with carbon dioxide you get Carbonic Acid which can change the chemical composition of minerals in rocks. Plant Acids – Many plants produce a substance called tannin. When water and tannin mix, it forms tannic acid. Tannic acid can dissolve rocks and break the remains into smaller pieces.

Oxygen When you see a rusty car, red soil, or red stains on rocks you are seeing Oxidation. Oxidation – is the effect of chemical changes causes by oxygen. This involves iron containing minerals and oxygen, leaving it weak and it can break.

Soil What is Soil??? Soil is a mix of weathered rock, organic matter, water and air that supports growth of plant life. Organic Matter means rotted leaves, twigs and roots

Soil Formation Things that affect soil formation Parent Rock Slope of the Land Climate Time Organisms

Soil Formation Parent Rock – The type of rock originally weathered Slope of the Land – The topography, or surface features of the land also influence the types of soil that forms. Steep Hills – soil has little chance of forming. Why? Rock fragments are always moving downhill.

Soil Formation Climate – If rock weathers rapidly, deep soils can form quickly. Climate also affects the amount of organic material in soil. Dead plants and animals eventually create humus, which helps soil hold water and provide nutrients for plants

Soil Formation Time – As soil develops, they become less like the rock from which they are formed. Organisms – Lichen are small organisms that consist of alga and a fungus. They grow directly on the rock. As they grow, they take nutrients from the rock that they are breaking down and form thin soil.