D. Give reasons for the following:
1. Igneous rocks are also called primary rocks.
2. Extrusive igneous rocks have generally small
crystals.
3. Rocks are of great economic significance
Answers
Answer:
1. Igneous rocks rocks are referred to as the primary rocks, because they represent the rocks which directly or indirectly provided materials for the formation of other types of rocks. Magma erupts from volcanic eruptions and on reaching earths crust it is called lava and solidifies by cooling.
2. The lava cools quickly as it pours out onto the surface (Figure below). Extrusive igneous rocks cool much more rapidly than intrusive rocks. The rapid cooling time does not allow time for large crystals to form. So extrusive igneous rocks have smaller crystals than igneous intrusive rocks.
3. Rocks are of great economic significance Some of the main benefits of rocks are as follows : After disintergration, they turn into valuable soils. They are the store-houses of a large number of minerals. They are the source of precious metals like gold, silver, platinum etc.