Most of the earth's crust, or outer layer, is made up of eight elements. Oxygen tops the list in terms of abundance, with the planet's shell being composed of almost 47% of this nonmetallic chemical element. Silicon posts a second-place ranking at nearly 28% by weight. Add these two elements together, and you get silicates. The majority of rocks scattered across the landscape of our terrestrial planet are silicate minerals, such as quartz and feldspar. The crust of the earth is much like a skin covering deeper layers of the third planet from the sun. Under the continents, the crust may measure as much as 25 miles deep. At the bottom of the oceans, which cover more than 70% of the earth's surface, the crust reaches depths of 3 to 6 miles. Compare that apple-peel-thin coating to the earth's mantle, which dips 1,800 miles toward the earth's core.