ACT® · Science: Background Knowledge - Earth and Space Science
Earth's Resources
A resource is nonrenewable if nature replaces it far slower than people use it, and renewable if nature replaces it fast enough to keep up, but only up to some limited rate.
Earth's resources get sorted by comparing two speeds: how fast the resource forms and how fast it gets used. Coal, oil, and metal ores form over millions of years, so no matter how useful they are, any human-scale extraction rate outpaces their formation rate. That mismatch is the whole definition of nonrenewable, not the fact that there's a fixed amount sitting underground.
Extraction follows a predictable curve. Early on, people take the easiest, richest, most accessible deposits, so output rises fast. As those run out, companies have to chase harder, thinner, or deeper deposits, so the rate of extraction flattens, peaks, and then falls even though plenty of the resource may still exist underground. The falling side of the curve is about rising difficulty, not running out completely.
Renewable doesn't mean infinite. A fish stock, a forest, or groundwater recharges on a human timescale, but only at some maximum rate the system can sustain. Pull resources out faster than that replacement rate, as a fishing fleet does when it takes more tonnes per year than the stock can regrow, and the renewable resource collapses just like a nonrenewable one runs out. This connects directly to reserves: new technology or new prices can shift which part of a deposit counts as economically recoverable, moving the cutoff grade without changing the rock itself.
Key terms
- Nonrenewable resource
- A resource that re-forms far slower than the rate at which it is consumed, on a human timescale.
- Renewable resource
- A resource that replaces itself fast enough to match human use, but only up to a limited sustainable rate.
- Depletion curve
- A graph of extraction rate over time that rises as easy deposits are taken, then peaks and falls as remaining deposits get harder to reach.
- Cutoff grade
- The minimum concentration of a resource in rock that is still economically worth extracting.
- Economic reserve
- The portion of a total resource that can be extracted profitably given current technology and prices.
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