Here's why:

* Oxygen makes up about 46.6% of the Earth's crust by weight. It's found in many common minerals like quartz (silicon dioxide) and feldspar (aluminum silicate).

* Silicon is the second most abundant element, making up about 27.7% of the crust by weight. Silicon and oxygen combine to form the basic building block of many rocks.

Let me know if you'd like to know more about the composition of Earth's crust!