Chapter 4: Weathering and Soil Formation

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

SWORD WORD

Weathering is the breaking down rocks into smaller pieces either chemically or mechanically.

Example : holes in the roads and cracks in the sidewalk

Weathering

There are 2 types of weathering, Mechanical and Chemical.

Types of 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.

Mechanical Weathering

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

Ice Wedging

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 Animal)

Gophers and Prarie Dogs – These animals burrow through the ground, these animals break rock apart.

Organic Matter (Plants and Animals)

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.

Chemical Weathering

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.

Natural Acids and Plant Acids

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.

Oxygen

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

Things that affect soil formation Parent Rock Slope of the Land ClimateTimeOrganisms

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.

Soil Formation