1. Extrusive rocks generally have small crystals because in the formation of these rocks, the solidification of magma takes place at a slower rate. 

2. Silicates are the most common rock forming minerals because they are most common minerals in the Earth’s crust. 

3. Rocks are of great economic significance Some of the main benefits of rocks are as follows : 

(a) After disintergration, they turn into valuable soils. 

(b) They are the store-houses of a large number of minerals. 

(c) They are the source of precious metals like gold, silver, platinum etc. 

(d) They are source of fuel-minerals. 

(f) They provide different types of stones. 

4. The composite zone inhabited by most living creatures is called biosphere. It includes a part of the atmosphere, the hydrosphere and a part of the lithosphere. All three layers are essential for life to exist. As lithosphere is only a small part of the biosphere, therefore, man’s habitat is the biosphere and not the lithosphere in the true sense.