Geography Write short note

There are three main types of material which come out in volcanic eruptions namely liquid, solid and gaseous form.
Liquid material:

1. It is the molten rock material. When the molten rock material is below the earth’s surface, it is called ‘Magma’.
2. When it appears on the surface it is called ‘lava’. On the basic of percentage of silica, it is classified as:

Acidic lava:

1. It contains higher percentage of silica.
2. It has high melting point.
3. It is thick, fluid and moves slowly.

Basic lava:

1. It contains less percentage of silica.
2. It has low melting point.
3. It’s thicker, fluid and can flow over longer distance.

Solid material:

1. It consists of dust particles and rock fragments.
2. When the material is very fine, it is called volcanic dust.
3. The small sized solid particles are called ash.
4. The solid angular fragments are known breccias.
5. Sometimes, the lava material thrown into the air solidifies into small fragments before falling on the earth’s surface; it is called volcanic bombs.

Gaseous material:

1. At the time of volcanic eruption, a dark cloud of smoke can be seen over the crater.
2. On the basis of shape, cloud is called cauliflower cloud.
3. Various inflammable gases are found in these clouds.
4. These gases produce flames.

A number of landforms are formed due to cooling and solidification of magma. Some important landforms are as follows :
Lava Domes :

1. Domes are developed when magma comes out and solidifies around its mouth.
2. The shape of the dome depends upon the fluidity of lava.
3. High dome with steep slope is developed by acidic lava.
4. Due to basic lava, broad-based low domes are developed.

Lava Plateaus:

1. Due to spread of lava in huge quantity from fissure volcano, it covers large areas and plateaus are formed.
2. The Deccan Trap in India has developed from volcanic eruptions millions of years ago.

Caldera:

1. At times, the eruption of volcano brings about large quantity of material and relieves lot of pressure.
2. After the eruption, a large and deep depression remains in that area. This large depression is called caldera.
3. These can be around 10 km wide and hundreds of metres deep.
4. They may later turn into lakes. Smaller calderas are known as craters.

Crater Lake:

1. When the funnel shaped crater of an extinct volcano gets filled with rain water, it forms a crater lake.

Volcanic Plug:

1. It is formed when the lava solidifies in the volcanic neck.
2. The surrounding rock material is removed by different agents of erosion; the vent stands predominantly. It is called as volcanic plug.
3. The diameter of a plug varies between 300 and 600 metres.
4. The Devil’s Tower in Wyoming, United State of America is the best example.

Cinder Cone:

1. Solid material is ejected in large quantity.
2. This material consists of ash, cinder and breccias.
3. Cinders are half burnt pieces of solid material.
4. The solid material is deposited around the mouth until a conical hill with steep slopes is formed.
5. For example, cone of Mt. Nuovo in Italy.

Composite Cone:

1. Composite cones are built up of alternate layers of lava and cinder.
2. This cone is composed of two materials and therefore it is called a composite cone.
3. It is symmetrical in shape.
4. For example, Mt. St. Helens, USA.

Folding occurs when compressional forces are applied to rocks, that are ductile or flexible.
The different types of folds are:
Symmetrical folds:

1. The axial plane is vertical.
2. The limbs are inclined at the same angle.

Asymmetrical folds:

1. The axial plane is inclined.
2. The limbs are inclined at different angles.

Overturned fold:

1. One limb lies above the other limb.
2. Limbs slope unequally in the same direction.

Recumbent fold:

1. Axial plane is almost horizontal.
2. One limb lies over the other in horizontal direction.

Isoclinal fold:

1. The limbs slope in the same direction with same amount.
2. The axial plane may be vertical, inclined or horizontal.
3. Slope of some portion of limbs is near vertical.

A fault is a fracture in the crustal rock, formed when the crustal rocks are displaced due to tensional movement caused by endogenic forces. The displacement of rock can occur in upward, downward different or horizontal direction.
The types of faults determined by the direction of motion are:

1. Normal fault: It results when portion of land slides down along the fault plane and when the exposed portion of the plane faces the sky.
2. Reverse fault: It results when a portion of the land is thrown upward relative to other side of the land. In such situation, the fault plane faces the ground.
3. Tear fault : At times, the rock strata on either side of the fault plane do not have vertical displacement. Instead, movement occurs along the plane in horizontal direction.
4. Thrust fault: When the portion of the land on one side of the fault plane gets detached and moves over the land on the other side. The angle of fault plane is generally less than 45°.