Forces of weathering
In this chapter:
The natural chemical or physical processes that change rocks are called weathering
Heat and water usually speed up the weathering process
There are three types of weathering: physical; chemical; and biotic
The changing of rocks by physical forces like temperature changes, heat, wind and frost is called physical weathering
Rocks can be changed by chemical reactions that involve water and the rock's minerals
Different forces of nature create different landforms. These forces can be internal or external. Internal forces are generated from inside the Earth. External natural forces are generated above the Earth's crust. This chapter looks at the external type of natural force that also plays a very important role in shaping the surface of the Earth. This chapter is about weathering.
Weather and weathering
Plate tectonics change the Earth's crust, causing different types of rocks to be formed, destroyed or mixed together. Some rocks are lifted to the surface while others are buried deep down in the Earth's crust. As soon as rocks are lifted to the surface they become exposed to wind, rain, frost, the heat of the sun or to water waves. In other words, they are exposed to different weather conditions. These weather conditions change the rocks. The natural chemical or physical processes that change rocks are called weathering. Weathering is a very slow process. Unlike earthquakes and volcanic eruptions that can change landforms in a matter of hours, changes caused by weathering take thousands or millions of years.
The speed at which weathering forces work depends a lot on the action of water and the temperature of the area. Heat and water usually speed up the weathering process by speeding up the chemical reactions that change the rocks.
Types of weathering
There are three types of weathering: physical; chemical; and biotic.
The changing of rocks by physical forces like temperature changes, heat, wind and frost is called physical weathering. For example, when water freezes it expands, which means that it takes up more space. A block of ice, for example, will take up more space than the amount of liquid from which it was formed. When water freezes it generates a very strong force that can break the hardest rocks. Physical weathering usually affects rocks near the Earth's surface.
Rocks can be changed by chemical reactions that involve water and the rocks' minerals. This sort of weathering is called chemical weathering. Chemical weathering can change the colour of rocks, break them up or cause them to form different materials which, in turn, will form new types of rocks.
The rusty, red-coloured spots on some rocks, for example, are the result of chemical weathering. These spots appear because some rocks contain iron. When iron reacts with air and water it becomes rust.
Rainwater contains different chemicals which have been absorbed from the air. Rainwater is a very powerful weathering force.
Biotic weathering is the combination of physical and chemical weathering caused by plants and animals.
A plant's roots, for example, can crumble surrounding rocks while the plant grows. Some animals can produce a type of acid that affects rocks.
Forces of erosion
Weathering usually involves different natural forces that work together. For example, flowing water, wind or ice contribute to the natural destructive process called erosion. There are two stages in the erosion process: weathering and transporting. Firstly, rocks are broken into smaller pieces by weathering. They are then transported from the higher to the lower parts of the Earth's crust. Rivers and sea waves are important rock transporters.
The force of flowing water is very strong. Rivers and streams slowly soften and mould the lines of our landscapes. Water wears away the materials in river beds and their banks and carries them from one area to another.
The minerals in river water make ocean water salty. The faster the river flows, the more rocks and minerals it can move. Water flows at different speed in different parts of a river. The water flow is slowest near the banks and at the bottom of the river.
Movements of earth down a steep slope are called landslides. Landslides happen when hills or mountains become unstable. Slopes become unstable when the pieces of rock that form the slopes are not properly supported. Human activities, water, frost and ice can cause landslides.
Sometimes, too many buildings are built on a slope which is not strong enough to support them. Cutting down forests or digging channels can also cause landslides.
Rainwater which falls during a storm loosens the soil, turning it into mud that slides down from slopes. This is called a mudslide.
Frost can push pieces of rocks up the slope. These pieces melt then settle lower down the slope, causing a very slow movement of soil, called soil creep. Frost also makes rocks split, forming big cracks along the slopes.
Glaciers are large streams of ice that flow through mountain valleys. Glaciers slowly slide lower and lower down the valley until they finally melt. The ice in glaciers is made of compressed snow lumps that are packed together.