Cool image, mildly interactive at the WSJ site, that lists "years of minerals in reserve, at current production rates."
Story prompted by recent media coverage over proposals to mine asteroids, the basic argument being that the seabed reserves are vast and far easier to mine - assuming the continuing advance of technology (a very good bet).
BHP Billiton, a giant in the extractive industry, says it feels there are "10,000 more years of minerals left for civilization" - a wonderfully expansive statement. We can probably agree that the estimates on the left are too small and "10,000 years" is probably too optimistic.
My sense is that we won't run out of useable/manipulatable stuff here on Earth prior to taking to space in a big way, but yeah, we'll use that as a more practical driver than sheer exploration. So eventually we'll reach a point where - cost-wise - it seems a better deal to go off-planet than continue going deeper in-planet.
In the meantime, industry execs will tell you that there is a "crustal abundance" layer in the Earth that has been truly explored and mined only to about the first half-mile. The entire "abundance" measures between 3 and 30 miles - on average around the planet. So clearly we've got a ways to go - even on land.
Still, it's always good to learn how to ride a motorcycle, stock up on leather clothes and Spam cans, and expect the Armaggeddeon. Heck, I know one religion that mandates it (at least the food part).