Everything we use comes from Earth: water from rivers and aquifers, metals from ores, energy from sunlight, wind and fuels, and stone and sand from the rock cycle. Resources fall into two groups by how fast nature replaces them.
| Renewable: replaced within a human lifetime | Nonrenewable: took millions of years to form |
|---|---|
| Sunlight, wind, flowing water | Coal, oil and natural gas (fossil fuels) |
| Trees and crops (if regrown) | Metal ores, such as copper and iron |
| Water in rivers and shallow aquifers (if not overused) | Water in some deep aquifers that refill only over thousands of years |
Key idea
Renewable resources are replaced by nature within a human lifetime; nonrenewable ones took millions of years to form. Both are spread unevenly, because they are found only where past geologic processes put them.
Why it works
Geology decides where resources are. Oil and natural gas formed from tiny sea creatures buried in sediment and slowly cooked by Earth's heat over millions of years, so they lie in thick piles of sedimentary rock. Much coal comes from swamp plants buried about 300 million years ago. Many metal ores were left by hot water flowing through cracks near cooling magma. These processes are far too slow to refill what we use in a lifetime.
Worked example: How long will it last?
A region has an estimated 600 million tonnes of a mineral it can mine, and it mines 20 million tonnes a year. At that rate, how many years will it last?
- Years = amount available ÷ amount used each year.
- $600 \div 20 = 30$ years.
Answer: About 30 years
Worked example: An aquifer's budget
An aquifer gains about 2 million m³ of water a year from rain soaking in, and wells pump out 5 million m³ a year. What happens to the water stored in it each year?
- Change = water in − water out = $2 - 5 = -3$ million m³.
- It loses about 3 million m³ a year, and the water table drops.
Answer: It shrinks by about 3 million m³ a year
Common mistake
Thinking resources are spread evenly, or that groundwater always refills quickly. Ores, oil and good aquifers lie only where past geologic processes put them, and some aquifers, like parts of the High Plains aquifer under the Great Plains, are being pumped much faster than they refill. Check: ask how long nature takes to replace a resource, and compare it with how fast we use it.