Geology

Since one of you has recommended that I put up more hints and tips on Landforms and Rocks (Chapter 7), and also requested more information on fold mountains, I'll cover some of the more important aspects of the chapter here.

• The Earth

As we all know, the Earth has several layers:

(i) Crust
(ii) Mantle
(iii) Core

• Landforms: Fold Mountains

When two crustal plates collide, the rocks at the edges of the plates are compressed, causing them to buckle and shift. This results in the layers of rock becoming folded and warped, and thrust upwards. This uplifting creates mountains known as fold mountains.

Examples of fold mountains include:

1. Alps in Europe
   
2. Himalayas in Asia
   
3. Rocky Mountains in North America
   
4. Andes in South America
   
5. Atlas Mountains in north Africa
   

• Rocks

There are 3 types of rocks:

1. Igneous rock

Igneous rocks are formed by the cooling or hardening of magma and lava beneath or on the Earth's surface.

(i) When magma cools and hardens slowly in the Earth's crust, coarse-grained igneous rocks are formed. Granite is an example of a coarse-grained igneous rock; the individual grains and crystals in the rocks can be seen by the naked eye.

(ii) When lava cools and hardens quickly on the Earth's surface, fine-grained igneous rocks are formed. Basalt is an example of a fine-grained igneous rock; the individual grains and crystals in the rocks are too small to be seen by the naked eye.

2. Sedimentary rock

Sedimentary rocks are formed from the deposition and accumulation of sediments. As the sediments pile up in layers, the weight of the layers on top compresses the layers below. The pressure cements the layers of sediments together, forming sedimentary rock.

Like igneous rocks, sedimentary rocks can be coarse-grained or fine-grained.

(i) Coarse-grained sedimentary rocks are formed from deposits of larger sediments such as sand, gravel and pebbles.

(ii) Fine-grained sedimentary rocks are formed from very fine sediments such as silt, mud and clay.

Limestone however, is formed from the fossilised remains of ancient plants and animals.

3. Metamorphic rock

Sometimes, under extreme heat and pressure, igneous and sedimentary rocks may be completely transformed into a different rock type altogether - metamorphic rock.

• Weathering & Erosion

It is important that you know the difference between weathering and erosion:

Weathering involves either the disintegration or decomposition of the rock in situ. No movement of loose rock material is involved. In other words, the rock breaks apart but is not transported anywhere. The major agents of weathering include large changes in temperature, rainwater, freezing water, and plants.

1) Large changes in temperature: Changes in temperature cause the surface of the rock to expand and contract more than the interior of the rock. This weakens the rock and causes cracks to form.

2) Rainwater: As rain falls, the rainwater combines with carbon dioxide in the air to form a very weak acid, which dissolves certain minerals in the rock. Limestone and marble are especially vulnerable to slowly dissolving away in rainwater, forming caves and other features.

3) Freezing water: Water may enter cracks in rocks. In areas where the temperature falls below the freezing point of water, the water will turn into ice when it is cold enough. Water expands as it freezes. The ice now exerts pressure on the walls of the crack, pushing them further apart and widening the crack. Over time, the crack becomes wider and the rock may break apart.

4) Plants: Plants that grow in cracks in rocks can force these cracks to widen due to the roots growing into the rock.

Erosion refers to the wearing away and removal of weathered rock. Movement of loose rock material is involved. In other words, the rock is being worn away while at the same time, loose material is being transported away. The main agents of erosion are water, waves, ice and wind.

1) Water: As water flows over rocks, it will wash away loose material. Also, as water flows, sediments being carried by the water (such as sand grains and pebbles) will scour and scrape against the rocks, eroding away at the material.

2) Waves: Strong waves carry a lot of energy, and the pounding of these waves can remove large amounts of rock. Also, sand, gravel and pebbles carried by these waves can grind and wear away at the rock.

3) Ice: Glaciers are rivers of ice found in mountainous and polar regions. They move extremely slowly, but they pick up and carry away pieces of rock. Also, just like in a river, the material carried by a glacier can scrape and erode away at the surrounding rocks.

4) Wind: In dry areas, winds are strong, and there is little vegetation to hold the loose soil together. Hence materials such as sand and soil will be picked up and carried away by the wind. Also, in strong wind, the sand and soil picked up by the wind will be blasted at rocks, smoothening and polishing these surfaces.