Maharashtra Board Class 11 Geography Chapter 2 Weathering and Mass Wasting PDF Download

Weathering And Mass Wasting

Let's Recall

Study the diagram given in figure 2.1 and answer the following questions:

1) Identify the types of rocks shown in the diagram.

2) Arrange the rocks according to their chronology of origin.

3) Explain how sedimentary rocks are formed.

4) Think of all the factors which may break the rocks into smaller pieces.

5) Which type of rock will break easily as compared to others? Why?

Geographical Explanation

Igneous, sedimentary and metamorphic rocks are the three types of rocks found on the Earth. Igneous rocks are formed from lava which has come from eruptions on the Earth's surface. Igneous rocks are, therefore, the first rocks formed on the Earth's surface. As we have seen in fig. 2.1, when rocks break into smaller pieces, they are carried away from one place to another. In these sediments, organic materials also gets mixed. When layers of sediments deposit on each other, immense pressure acts on them. As a result and also because of the cementing material, sedimentary rocks are formed. Metamorphic rocks are formed when igneous and sedimentary rocks are subjected to immense heat and pressure.

How can rocks break? Rocks which are exposed are affected by the climate of that region. Rock can break because of water, pressure, heat, etc. in various different ways. Those rocks which have joints or layers break easily than rocks which are homogeneous. Generally, sedimentary rocks break easily than igneous rocks.

Teacher's Note

Rocks break down slowly due to water and heat in nature. Just like a clay pot breaks when water keeps hitting it, rocks also break down over many years.

Exam Trick

Remember: Sedimentary rocks break easily. Think of sand - it is loose and breaks apart quickly. Igneous rocks like granite are hard and strong.

Points to Remember

There are three types of rocks: igneous, sedimentary, and metamorphic.
Rocks break into smaller pieces because of water, heat, and pressure.
Sedimentary rocks break more easily than igneous rocks.
Rocks with cracks break faster than solid rocks.
Breaking of rocks is the first step in soil formation.

Weathering

When rocks break, the grains in the rocks get disintegrated. This means, they get weakened and over a period of time, they get eroded and break. Weathering is the physical or chemical breakdown due to weather conditions. It occurs mainly through the action of water and temperature on rocks and the reaction of rocks to these processes. As very little or no motion of materials takes place in weathering, it is called in-situ process. Weathering often results in rounding and smoothening of sharp edges of rocks.

Types Of Weathering

The rocks are weathered physically or chemically. So, basically, there are two types of weathering: physical and chemical. Factors such as water, heat and pressure which affect weathering can cause both the types of weathering chemical and physical. Effect of such factors can be varying on different rocks in different climates.

Let us have a look at these factors that affect weathering:

1) Water

Water is a very common factor which plays an important role in weathering. The availability of water largely depends on the climate of the place. It enters the small pores, cracks and fissures inside the rocks.

In areas, where diurnal range of temperature is high, water inside the cracks freezes during nights. During daytime, it again becomes water. When water freezes, its volume increases. It starts exerting pressure on the walls of the rock and widens the cracks. Eventually, the rocks break down. This process is called freeze-and-thaw weathering. It is particularly effective in high altitudes in mid and low latitudes, high-latitudes as well as mountainous regions. The sedimentary rocks such as sandstone, grit and conglomerate get easily disintegrated due to water.

Water also breaks down the rocks chemically. This can happen in two ways. The water molecules react with minerals present in the rocks. This process is called hydrolysis. If the rock is made up of those minerals which react easily with water, the compounds of those elements will form and break the rock. This process can happen with those igneous rocks which have silicate minerals. The resultant compounds will break the rock by changing the chemical composition of the minerals in the rock after hydrolysis. For physical and chemical weathering to happen, the water present as moisture in the soil or air is also enough to cause weathering.

Another way by which water can cause chemical weathering is by solution. When some minerals in the rock react with water in the rock or moisture in the air and get dissolved, the process is called solution. They form acids, get leached from the main rock and lead to decomposition of the rock. Minerals like calcium, magnesium, nitrates, etc. dissolve in water. For example, the calcium present in the limestone reacts with the water and air to form carbonic acid. The minerals dissolve in water and get carried away in the water.

Teacher's Note

Water is the most important cause of rock breakdown in India. The Western Ghats get heavy rain, and rocks there break down quickly into soil.

Exam Trick

Remember: Freeze-thaw = water freezes and thaws. Think of ice breaking a bottle because ice takes more space than water.

Points to Remember

Water enters cracks in rocks and makes them wider.
When water freezes, it expands and breaks rocks.
Water dissolves minerals in rocks and carries them away.
Hydrolysis is when water reacts with minerals in rocks.
Water is the main factor of weathering in wet climates.

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