Describe the Earth's mantle and crust, including their composition, density, and rock types. Also, state the proportion of continental crust on the Earth's surface.

Here are the answers to your questions:

1. The mantle is primarily made of silicate rocks rich in iron and magnesium. The main minerals are olivine and pyroxene.

2. Here is the differentiation between oceanic and continental crust:

   • Oceanic Crust: Thinner ($5-10 \text{ km}$), denser (average density of $3.0 \text{ g/cm}^3$), and primarily composed of basalt and gabbro.
   • Continental Crust: Thicker ($30-70 \text{ km}$), less dense (average density of $2.7 \text{ g/cm}^3$), and primarily composed of granite and other felsic rocks.

3. Approximately 40% of the Earth's surface is made of continental crust.

4. Here is one example of rocks found in each:

   • a) Oceanic Crust: Basalt
   • b) Continental Crust: Granite