Question

Which of the following statements is/are correct regarding Igneous rocks?

1. Intrusive igneous rocks are formed by the cooling of lava on the earth’s surface.

2. Extrusive igneous rocks are formed when magma solidifies below the earth’s surface.

1. Only 1
2. Only 2
3. Both 1 and 2
4. Neither 1 nor 2

Answer 1

Correct Answer - Option 4 : Neither 1 nor 2

The correct answer is Neither 1 nor 2.

• Igneous Rocks are formed by crystallization from a liquid or magma.
• There are two types of Igneous rocks based on their mode of occurrence: 

1. Volcanic or extrusive igneous rocks that form when the magma cools and crystallizes on the surface of the Earth
2. Intrusive or plutonic igneous rocks wherein the magma crystallizes at depth in the Earth.

• Magma is a mixture of liquid rock, crystals, and gas with a wide range of chemical compositions, temperatures, and properties of a liquid.
• Extrusive igneous rocks
  • They are formed by the cooling of lava on the earth’s surface. Hence, Statement 2 is incorrect.
  • As lava cools very rapidly on coming out of the hot interior of the earth, the mineral crystals forming these rocks are very fine.
  • These rocks are also called volcanic rocks. Basalt is a very common example of such rocks.
  • These rocks are found in volcanic areas.
  • Deccan plateau’s regur soil in India is a formation of lava.
• Intrusive igneous rocks
  • They are formed when magma solidifies below the earth’s surface. Hence, Statement 1 is incorrect.
  • The rate of cooling below the earth’s surface is very slow, which gives rise to the formation of large crystals in the rocks.
  • Deep-seated intrusive rocks are termed plutonic rocks and shallow depth intrusive rocks are termed as hypabyssal. Granite is a common example of intrusive rocks.