Ask GeoMan...

Why are oxygen and silicon so important to the earth's crust?

As far as we know, all matter is made up of protons, neutrons, and electrons in varying amounts. Because of the way things seem to work, there are only 92 different types of elements that nature can make. All of the elements on the periodic chart above Atomic Number 92 are synthetic and increasingly unstable (which is probably why nature doesn't make them - they tend to annihilate themselves rather quickly). But, the 92 that the earth does make are enough to make some pretty amazing things - minerals, rocks, baby ducks, bean farts, and so forth.

As fortune would have it, of these 92 elements only eight (8) of them really matter in the crust insofar as quantity is concerned. Together, these 8 elements account for approximately 98.5% of the earth's crust. There are two way to describe quantity in nature: by mass (how much actual matter is involved) or by volume (how much space it occupies).

Either way you look at it, oxygen is the big dog in town - it accounts for 46.6% of the earth's crust by mass (and 94% of it by volume!). This is an incredible fact, and worth saying a second time (please read the last sentence again). If you then include silicon at 27.7% of the crust by mass, we come up with the astounding realization that nearly 75% of the earth's crust is composed of silicon and oxygen. Wow!

The other six (6) elements are:

Aluminum . . . . . 8.1% Iron . . . . . . . 5.0% Calcium. . . . . . 3.6% Sodium . . . . . . 2.8% Potassiun. . . . . 2.6% Magnesium. . . . . 2.1%