Explanation

 Silicon is the second most abundant element in the Earth's crust,

Making up a large portion of most rocks and minerals, particularly silicates, which are composed of silicon and oxygen. 

Earth's Crust Elements

1 Oxygen (O) 46.1%

2 Silicon (Si) 28.2%

3 Aluminum (Al) 8.2%

4 Iron (Fe) 5.6%