---
title: "Resources from the Earth"
book: "Primary & Middle School Earth and Space Sciences"
subject: earth
language: en
chapter: 30
exercises: 12
source: https://one-course.com/books/earth/1/en/chapter/30-resources-from-the-earth
license: CC-BY-NC-SA-4.0
credit: "One Earth and Space Sciences Book, One Course (one-course.com)"
---

# Chapter 30 — Resources from the Earth

A smartphone fits in a pocket, yet it holds dozens of chemical elements: copper for its wires, neodymium for the magnets of its speaker, gallium for the light of its screen, indium for the thin coating of its screen that carries electricity, and many more. Every one of them was dug out of the ground somewhere on the Earth. A house holds far more: an average family house needs about a hundred tonnes of [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand), [gravel](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) and crushed stone, and its share of the streets and schools around it about three times as much. Where in the Earth are these materials found, and why there?

**You already know.**

In chemistry, you learn that the materials we take from nature are *resources*; an *ore* is a [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) from which a metal can be extracted; a *renewable* resource is renewed by nature as fast as we use it; coal, oil and natural gas are *[fossil](https://one-course.com/books/earth/1/en/chapter/21-sediments-and-fossils#def-g7-sediments-and-fossils-fossil) fuels*; and oxygen, silicon, aluminium and iron are the most abundant elements of the [crust](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers). [Groundwater](https://one-course.com/books/earth/1/en/chapter/17-water-underground#def-g6-water-underground-groundwater) fills the [pores](https://one-course.com/books/earth/1/en/chapter/17-water-underground#def-g6-water-underground-porosity) of an [aquifer](https://one-course.com/books/earth/1/en/chapter/17-water-underground#def-g6-water-underground-aquifer), and [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) refills it ([Chapter 17](https://one-course.com/books/earth/1/en/chapter/17-water-underground#ch-g6-water-underground)). [Sediments](https://one-course.com/books/earth/1/en/chapter/21-sediments-and-fossils#def-g7-sediments-and-fossils-sediment) pile up in layers and become [sedimentary rocks](https://one-course.com/books/earth/1/en/chapter/15-rocks-and-minerals#def-g6-rocks-and-minerals-rock-families) ([Chapter 21](https://one-course.com/books/earth/1/en/chapter/21-sediments-and-fossils#ch-g7-sediments-and-fossils)), over times counted in millions of years ([Chapter 22](https://one-course.com/books/earth/1/en/chapter/22-the-ages-of-the-earth#ch-g7-geological-time)).

## 30.1 Why a metal has to be concentrated

Copper is everywhere: an ordinary [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) holds a little of it, a few grams in every hundred kilograms. But to get one kilogram of copper out of such a [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock), you would have to dig, crush and treat tonnes of it. Mining is worth the cost only where nature has gathered the metal into a much richer [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock).

**Definition 30.1 (Ore deposit and concentration factor).**

An *ore deposit* is a body of [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) in which natural processes have concentrated a metal or a [mineral](https://one-course.com/books/earth/1/en/chapter/15-rocks-and-minerals#def-g6-rocks-and-minerals-mineral) enough for it to be extracted at a profit. The *concentration factor* of a metal is the number of times its content in the ore is greater than its average content in the [crust](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers):

$$
\text{concentration factor} = \frac{\text{content in the ore}}{\text{average content in the crust}} .
$$

The common metals of the [crust](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers), aluminium and iron, need to be concentrated only about four times. Copper needs about 80 times, zinc about 600 times, and the rarest metals of the chart, gold, tin and chromium, thousands of times. The rarer a metal in the [crust](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers), the more nature has to work to make an ore of it, and the fewer places where it has done so.

**Example 30.2 (Copper).**

The [crust](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers) holds on average $0.006\,\%$ of copper; a copper ore can be mined from about $0.5\,\%$. The [concentration factor](#def-g9-earth-resources-ore-deposit) is $0.5 \div 0.006
\approx 83$, about 80, or close to $10^2$. Another textbook takes $40\,\mathrm{ppm}$ ($0.004\,\%$) for ordinary [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) and $1\,\%$ for the ore: $1 \div
0.004 = 250$. The two numbers differ, but both say the same thing: a copper ore is about a hundred times richer than ordinary [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock).

![How many times nature has to concentrate seven metals to make an ore worth mining: the minimum grade of the ore divided by the average content of the crust. Each step of the scale is ten times the one before. Data: D. Perkins and others, Mineralogy, and, for gold, S. Earle, Environmental Geology.](https://one-course.com/images/onecourse/chapters/earth-1/g9-earth-resources/fig-44cc58615791.svg)

*How many times nature has to concentrate seven metals to make an ore worth mining: the minimum grade of the ore divided by the average content of the [crust](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers). Each step of the scale is ten times the one before. Data: D. Perkins and others, *Mineralogy*, and, for gold, S. Earle, *Environmental Geology*.*

**Method 30.3 (Computing a concentration factor).**

1. Write the content of the metal in the ore and in the average [crust](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers) in the same unit (percent, or grams per tonne).
2. Divide the content in the ore by the content in the [crust](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers) .
3. Round the result: what matters is its size, about 10, 100 or 1000, which you can write as a power of ten.

## 30.2 How ore deposits form

Nature concentrates metals in several ways. Three of them are shown below.

![Three ways to concentrate a metal (a sketch, not to scale). (1) In a body of magma, heavy minerals rich in nickel or copper sink and gather at the bottom. (2) Groundwater heated by the magma, and water given off by the magma itself, dissolve metals, rise through cracks and drop them where the rock is cooler, filling the cracks with veins. (3) Erosion frees gold from the veins; the river carries it, and the heavy grains settle among the gravels, down to the bedrock.](https://one-course.com/images/onecourse/chapters/earth-1/g9-earth-resources/fig-fc127f259760.svg)

*Three ways to concentrate a metal (a sketch, not to scale). (1) In a body of [magma](https://one-course.com/books/earth/1/en/chapter/19-volcanoes#def-g7-volcanoes-magma), heavy [minerals](https://one-course.com/books/earth/1/en/chapter/15-rocks-and-minerals#def-g6-rocks-and-minerals-mineral) rich in nickel or copper sink and gather at the bottom. (2) [Groundwater](https://one-course.com/books/earth/1/en/chapter/17-water-underground#def-g6-water-underground-groundwater) heated by the [magma](https://one-course.com/books/earth/1/en/chapter/19-volcanoes#def-g7-volcanoes-magma), and water given off by the [magma](https://one-course.com/books/earth/1/en/chapter/19-volcanoes#def-g7-volcanoes-magma) itself, dissolve metals, rise through cracks and drop them where the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) is cooler, filling the cracks with veins. (3) [Erosion](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-erosion) frees gold from the veins; the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) carries it, and the heavy grains settle among the [gravels](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble), down to the [bedrock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock).*

**In the magma.** When a body of [magma](https://one-course.com/books/earth/1/en/chapter/19-volcanoes#def-g7-volcanoes-magma) cools, the [minerals](https://one-course.com/books/earth/1/en/chapter/15-rocks-and-minerals#def-g6-rocks-and-minerals-mineral) that crystallize first are not always the lightest. In [magmas](https://one-course.com/books/earth/1/en/chapter/19-volcanoes#def-g7-volcanoes-magma) from the [mantle](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers), heavy [minerals](https://one-course.com/books/earth/1/en/chapter/15-rocks-and-minerals#def-g6-rocks-and-minerals-mineral) rich in nickel and copper sink to the bottom and gather into a rich layer. Most of the nickel mined in the world comes from deposits of this kind.

**In cracks, from hot water.** [Groundwater](https://one-course.com/books/earth/1/en/chapter/17-water-underground#def-g6-water-underground-groundwater) that flows near a body of [magma](https://one-course.com/books/earth/1/en/chapter/19-volcanoes#def-g7-volcanoes-magma), a few kilometres below the surface, is heated and rises through the cracks of the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock). Hot water dissolves metals from the [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) it crosses. When it reaches cooler [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) nearer the surface, the metals come out of the water as [minerals](https://one-course.com/books/earth/1/en/chapter/15-rocks-and-minerals#def-g6-rocks-and-minerals-mineral) and fill the cracks: these are *veins*. Many gold deposits are veins of this kind. Around some bodies of [magma](https://one-course.com/books/earth/1/en/chapter/19-volcanoes#def-g7-volcanoes-magma), hot water has crossed a whole mass of broken [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) and left copper in it, spread out but over a huge volume: these are the largest copper deposits in the world.

![The Chuquicamata copper mine, in the Atacama Desert of Chile, seen by the ASTER instrument on a NASA satellite in 2007 (the image is about 12\, km across). The oval pit in the middle, worked for over a century, is 4.3\, km long, 2.7\, km wide and 1.1\, km deep. NASA/METI/AIST/Japan Space Systems and the U.S./Japan ASTER Science Team (public domain, via Wikimedia Commons).](https://one-course.com/images/onecourse/chapters/earth-1/g9-earth-resources/img-cb02c406ee6c.jpg)

*The Chuquicamata copper mine, in the Atacama [Desert](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-desert) of Chile, seen by the ASTER instrument on a NASA satellite in 2007 (the image is about $12\,\mathrm{km}$ across). The oval pit in the middle, worked for over a century, is $4.3\,\mathrm{km}$ long, $2.7\,\mathrm{km}$ wide and $1.1\,\mathrm{km}$ deep. NASA/METI/AIST/Japan Space Systems and the U.S./Japan ASTER Science Team (public domain, via Wikimedia Commons).*

**In river gravels.** Gold does not rust and hardly weathers. When [erosion](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-erosion) wears down a gold vein, the gold is freed as dust, flakes, grains and sometimes nuggets, and [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) carry it downstream. Gold is very dense: about $19\,\mathrm{g}/\mathrm{cm}^{3}$ pure, against about $2.5\,\mathrm{g}/\mathrm{cm}^{3}$ for ordinary [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock). In [floods](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood), when the whole bed of [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and [gravel](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) moves, the heavy grains of gold work their way down and collect on the [bedrock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock). Geologists call such deposits *placers*.

## 30.3 Coal, oil and gas

Coal, oil and natural gas are the remains of living things, buried before they could rot. Almost all the organic matter of a dead plant or animal is turned back into carbon dioxide within weeks to decades. Only the part cut off from the oxygen of the air, at the bottom of a stagnant swamp or of a deep [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), can be buried and kept for millions of years.

![A seam of coal, the dark layer across the middle, between sedimentary rocks, beside a road in Ohio, United States. The seam lies flat, as the swamp did. Photograph by James St. John (CC BY 2.0, via Wikimedia Commons).](https://one-course.com/images/onecourse/chapters/earth-1/g9-earth-resources/img-6fb6c15759ef.jpg)

*A seam of coal, the dark layer across the middle, between [sedimentary rocks](https://one-course.com/books/earth/1/en/chapter/15-rocks-and-minerals#def-g6-rocks-and-minerals-rock-families), beside a road in Ohio, United States. The seam lies flat, as the swamp did. Photograph by James St. John (CC BY 2.0, via Wikimedia Commons).*

**Coal.** Coal starts in swamps beside [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) and [deltas](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-delta), in warm and wet [climates](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-climate), where plants grow fast. Dead trunks and leaves fall into still water that holds little oxygen, and do not rot. The swamp has to last for centuries to millennia to build a thick layer. One day a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) changes course or [floods](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood), and covers the swamp with [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand). As more [sediments](https://one-course.com/books/earth/1/en/chapter/21-sediments-and-fossils#def-g7-sediments-and-fossils-sediment) pile up, the plant layer is pressed and heated, and turns into coal: first a soft, brown coal with about a third of carbon, some hundreds of metres down; a hard black coal, with up to 86 % of carbon, a few kilometres down. The coal mined today in the United States, for instance, is mostly 100 to 300 million years old.

![From a swamp to a coal seam (a sketch, not to scale). As the layer of plants sinks deeper, it is pressed and heated: water and gases leave it, it gets thinner, and its share of carbon rises.](https://one-course.com/images/onecourse/chapters/earth-1/g9-earth-resources/fig-83b13fb751fb.svg)

*From a swamp to a coal seam (a sketch, not to scale). As the layer of plants sinks deeper, it is pressed and heated: water and gases leave it, it gets thinner, and its share of carbon rises.*

**Oil and gas.** Oil and most natural gas come from the tiny plants and animals that live in the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), such as diatoms. Their remains sink with the [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) to the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) floor, and some are buried before they decay. Millions of years and kilometres of [sediment](https://one-course.com/books/earth/1/en/chapter/21-sediments-and-fossils#def-g7-sediments-and-fossils-sediment) later, heat and pressure change them: below about $2\,\mathrm{km}$, between about 60 and $120{}^{\circ}\mathrm{C}$, into oil; hotter still, into gas. Oil and gas are lighter than water. They creep slowly upwards through the [pores](https://one-course.com/books/earth/1/en/chapter/17-water-underground#def-g6-water-underground-porosity) of [permeable](https://one-course.com/books/earth/1/en/chapter/17-water-underground#def-g6-water-underground-permeability) [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock), and many reach the surface and are lost. The ones we find were stopped on their way by a layer of [impermeable](https://one-course.com/books/earth/1/en/chapter/17-water-underground#def-g6-water-underground-permeability) [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock), often at the top of an upfold, where they gather: gas at the top, oil below it, and water underneath.

![Oil and gas stopped under an impermeable layer at the top of an upfold (a sketch, not to scale; the water that fills the rest of the sandstone is not shown). Below the fold, they creep up through the permeable sandstone (arrows) and gather under the crest: gas on top, oil below.](https://one-course.com/images/onecourse/chapters/earth-1/g9-earth-resources/fig-b048b7252a78.svg)

*Oil and gas stopped under an [impermeable](https://one-course.com/books/earth/1/en/chapter/17-water-underground#def-g6-water-underground-permeability) layer at the top of an upfold (a sketch, not to scale; the water that fills the rest of the sandstone is not shown). Below the fold, they creep up through the [permeable](https://one-course.com/books/earth/1/en/chapter/17-water-underground#def-g6-water-underground-permeability) sandstone (arrows) and gather under the crest: gas on top, oil below.*

## 30.4 Water from the ground

[Groundwater](https://one-course.com/books/earth/1/en/chapter/17-water-underground#def-g6-water-underground-groundwater) is also a resource from the Earth, the one we use every day. Unlike a metal or coal, it is renewed: [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) refills the [aquifers](https://one-course.com/books/earth/1/en/chapter/17-water-underground#def-g6-water-underground-aquifer). But it is renewed slowly, and only as fast as [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) soaks in. An [aquifer](https://one-course.com/books/earth/1/en/chapter/17-water-underground#def-g6-water-underground-aquifer) pumped faster than it is refilled sees its [water table](https://one-course.com/books/earth/1/en/chapter/17-water-underground#def-g6-water-underground-water-table) fall, and [wells](https://one-course.com/books/earth/1/en/chapter/17-water-underground#def-g6-water-underground-spring) run dry ([Chapter 17](https://one-course.com/books/earth/1/en/chapter/17-water-underground#ch-g6-water-underground)). [Groundwater](https://one-course.com/books/earth/1/en/chapter/17-water-underground#def-g6-water-underground-groundwater) is a renewable resource only as long as it is not drawn faster than it is renewed.

## 30.5 Sand and gravel

The solid material that people dig out of the ground in the largest amounts is neither coal nor a metal ore: it is [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and [gravel](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble).

**Definition 30.4 (Aggregate).**

*Aggregates* are the [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand), the [gravel](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) and the crushed stone used in building: in concrete, under roads and railways, and to fill the ground.

Most [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) goes into concrete, a mix of [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand), [gravel](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble), water and cement. In 2019 the world needed about 50 billion tonnes of [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and [gravel](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) a year, that is $5 \times 10^{10}$ tonnes, or about $18\,\mathrm{kg}$ for every person on the Earth, every day. [Sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and [gravel](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) are dug from pits on land, from riverbeds and, more and more, from the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) floor. Taken from a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), they leave less [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) to reach the [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), and beaches downstream wear away.

![Aggregates at work: heaps of sand and gravel beside a concrete mixer truck pouring the floor of a new building (AI-generated illustration of a generic scene).](https://one-course.com/images/onecourse/chapters/earth-1/g9-earth-resources/img-653f1284339d.jpg)

*[Aggregates](#def-g9-earth-resources-aggregate) at work: heaps of [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and [gravel](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) beside a concrete mixer truck pouring the floor of a new building (AI-generated illustration of a generic scene).*

## 30.6 Renewable or finite?

A resource that nature renews within a human lifetime, like the water of an [aquifer](https://one-course.com/books/earth/1/en/chapter/17-water-underground#def-g6-water-underground-aquifer) that is not overpumped, can be used for ever. An [ore deposit](#def-g9-earth-resources-ore-deposit), a coal seam or an oil field took millions of years to form. We use it in a few decades: on our time scale, it is not renewed at all.

![Times of making and times of using, on a scale where each step is ten times the one before. The coal is about a million times older than the time a mine takes to empty a deposit.](https://one-course.com/images/onecourse/chapters/earth-1/g9-earth-resources/fig-5247fca7ac6f.svg)

*Times of making and times of using, on a scale where each step is ten times the one before. The coal is about a million times older than the time a mine takes to empty a deposit.*

How long will a resource last? Geologists count only what is known and can be extracted.

**Definition 30.5 (Reserve).**

The *reserve* of a [mineral](https://one-course.com/books/earth/1/en/chapter/15-rocks-and-minerals#def-g6-rocks-and-minerals-mineral) resource is the part of the known deposits that can be extracted at a profit, with the techniques and the prices of the day.

A [reserve](#def-g9-earth-resources-reserve) is not everything that is in the ground. It grows when new deposits are found, when techniques improve or when prices rise, and it shrinks as the deposits are mined. In 1970 the world’s copper [reserves](#def-g9-earth-resources-reserve) were estimated at 280 million tonnes. Since then, about 700 million tonnes of copper have been mined, two and a half times that [reserve](#def-g9-earth-resources-reserve), and yet the [reserves](#def-g9-earth-resources-reserve) of 2024 were estimated at 980 million tonnes.

**Figures as of 2025.**

In 2024 the mines of the world produced about 23 million tonnes of copper. At that rate, the [reserves](#def-g9-earth-resources-reserve) of 980 million tonnes would last $980 \div 23
\approx 43$ years. Such a number is not a date when copper will run out: it is a measure of the copper that is known and worth extracting today.

A metal, unlike coal or oil, is not destroyed when it is used: the recycling of materials, met in the chemistry books, gives it back. The copper of an old cable, of a scrapped engine or of the offcuts of a factory can be melted and refined again. In the United States in 2024, copper recovered from scrap made up about $35\,\%$ of all the copper used.

![Two roads to copper metal: from an ore, through tonnes of rock dug and crushed, or from scrap, which is melted and refined again.](https://one-course.com/images/onecourse/chapters/earth-1/g9-earth-resources/fig-fe399df8b30f.svg)

*Two roads to copper metal: from an ore, through tonnes of [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) dug and crushed, or from scrap, which is melted and refined again.*

**Example 30.6 (The rock that recycling saves).**

An ore with $0.5\,\%$ of copper holds $5\,\mathrm{kg}$ of copper in each tonne of [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock). To make one tonne of copper, about $1000 \div 5 = 200$ tonnes of ore have to be dug, crushed and treated, and almost all of it ends as waste [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock). Each tonne of copper made from scrap leaves those 200 tonnes of [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) in the ground — and the deposit lasts longer.

## 30.7 Exercises

**Exercise 30.1 ★.**

Complete with the right word ([ore deposit](#def-g9-earth-resources-ore-deposit), [concentration factor](#def-g9-earth-resources-ore-deposit), [aggregate](#def-g9-earth-resources-aggregate), [reserve](#def-g9-earth-resources-reserve)). (a) [Sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) used in concrete is an … (b) The part of the known copper that can be extracted at a profit today is the copper … (c) A [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) where a metal is rich enough to be mined is an … (d) The number of times a metal has been concentrated is its …

**Solution of Exercise 30.1.**

(a) [Aggregate](#def-g9-earth-resources-aggregate). (b) [Reserve](#def-g9-earth-resources-reserve). (c) [Ore deposit](#def-g9-earth-resources-ore-deposit). (d) [Concentration factor](#def-g9-earth-resources-ore-deposit).

**Exercise 30.2 ★.**

Which way of concentrating a metal does each sentence describe: in the [magma](https://one-course.com/books/earth/1/en/chapter/19-volcanoes#def-g7-volcanoes-magma), in cracks from hot water, or in [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) [gravels](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble)? (a) Gold nuggets lie on the [bedrock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) under a stream’s [gravel](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble). (b) Hot water fills a crack with quartz and gold. (c) A layer of heavy nickel [minerals](https://one-course.com/books/earth/1/en/chapter/15-rocks-and-minerals#def-g6-rocks-and-minerals-mineral) lies at the bottom of an old body of [magma](https://one-course.com/books/earth/1/en/chapter/19-volcanoes#def-g7-volcanoes-magma). (d) Gold is freed from a vein by [erosion](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-erosion).

**Solution of Exercise 30.2.**

(a) In [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) [gravels](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble). (b) In cracks, from hot water. (c) In the [magma](https://one-course.com/books/earth/1/en/chapter/19-volcanoes#def-g7-volcanoes-magma). (d) This is the first step of a [river-gravel](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) deposit: the freed gold is then carried by the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water).

**Exercise 30.3 ★.**

Put these steps in order: (a) the plant layer is buried under [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand); (b) trees grow in a swamp; (c) a hard black coal forms kilometres down; (d) dead plants pile up under still water; (e) a soft brown coal forms.

**Solution of Exercise 30.3.**

(b), (d), (a), (e), (c).

**Exercise 30.4 ★.**

On the scale of a human life, is each resource renewable or not? Explain in one sentence. (a) Coal. (b) The water of an [aquifer](https://one-course.com/books/earth/1/en/chapter/17-water-underground#def-g6-water-underground-aquifer) pumped less than the [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) refills it. (c) Copper ore. (d) The same [aquifer](https://one-course.com/books/earth/1/en/chapter/17-water-underground#def-g6-water-underground-aquifer) pumped twice as fast as it is refilled.

**Solution of Exercise 30.4.**

(a) Not renewable: it takes millions of years to form. (b) Renewable: the [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) refills what is taken. (c) Not renewable: an [ore deposit](#def-g9-earth-resources-ore-deposit) forms over very long times. (d) Not renewable at that rate: the [water table](https://one-course.com/books/earth/1/en/chapter/17-water-underground#def-g6-water-underground-water-table) falls and the [wells](https://one-course.com/books/earth/1/en/chapter/17-water-underground#def-g6-water-underground-spring) end up dry.

**Exercise 30.5 ★★.**

Compute the [concentration factor](#def-g9-earth-resources-ore-deposit), then write it as a power of ten, rounded: (a) zinc, $0.007\,\%$ in the [crust](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers), ore from $4\,\%$; (b) tin, $0.0002\,\%$ in the [crust](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers), ore from $0.5\,\%$; (c) iron, $5.6\,\%$ in the [crust](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers), ore from $20\,\%$. (d) Why does iron need so little concentration?

**Solution of Exercise 30.5.**

(a) $4 \div 0.007 \approx 571$, about $10^3$ (between $10^2$ and $10^3$, closer to $10^3$). (b) $0.5 \div 0.0002 = 2500$, about $10^3$. (c) $20
\div 5.6 \approx 3.6$, about 4: between $10^0$ and $10^1$. (d) Iron is already very abundant in the [crust](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers): an ore only a few times richer than ordinary [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) is enough.

**Exercise 30.6 ★★.**

Pure gold has a density of about $19.3\,\mathrm{g}/\mathrm{cm}^{3}$; ordinary [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) about $2.5\,\mathrm{g}/\mathrm{cm}^{3}$. (a) How many times denser is gold? (b) What is the mass of a grain of gold of $0.01\,\mathrm{cm}^{3}$, and of a grain of [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) of the same size? (c) Explain why, in a [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood), the gold ends up at the bottom of the [gravel](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble), on the [bedrock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock).

**Solution of Exercise 30.6.**

(a) $19.3 \div 2.5 \approx 7.7$ times. (b) Gold: $19.3 \times 0.01 =
0.193\,\mathrm{g}$; [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock): $2.5 \times 0.01 = 0.025\,\mathrm{g}$. (c) When the whole bed moves, the heavy gold grains sink between the lighter [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and [pebbles](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble), down to the solid [bedrock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock), where they stop.

**Exercise 30.7 ★★.**

Use the chart of [concentration factors](#def-g9-earth-resources-ore-deposit). (a) Which metals need a factor of more than $10^3$? (b) Between which powers of ten does copper lie? (c) How many times more does chromium need to be concentrated than aluminium? (d) Which of the seven metals would you expect to find mined in the fewest places? Explain.

**Solution of Exercise 30.7.**

(a) Gold, tin and chromium. (b) Between $10^1$ and $10^2$, close to $10^2$. (c) $3000 \div 3.7 \approx 800$ times. (d) Chromium (or tin, or gold): the higher the [concentration factor](#def-g9-earth-resources-ore-deposit), the rarer the places where nature has concentrated the metal enough.

**Exercise 30.8 ★★.**

The world needs about $18\,\mathrm{kg}$ of [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and [gravel](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) per person and per day. (a) How much is that per person in one year, in tonnes? (b) Write 50 billion tonnes in scientific notation. (c) A truck carries $25\,\mathrm{t}$. How many days does it take for one person’s share to fill it? (d) Why can taking [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) from a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) make a beach downstream shrink?

**Solution of Exercise 30.8.**

(a) $18 \times 365 = 6570\,\mathrm{kg}$, about $6.6\,\mathrm{t}$. (b) $5 \times
10^{10}$ tonnes. (c) $25000 \div 18 \approx 1389$ days, almost four years. (d) The [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) dug from the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) no longer travels downstream to the [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water); the waves keep carrying [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) away from the beach, but less arrives to replace it.

**Exercise 30.9 ★★.**

(a) From what living things do most oil and gas come? (b) Below about $2\,\mathrm{km}$, at what temperatures does oil form? (c) Why do oil and gas move upwards through the [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock)? (d) What kind of [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) stops them, and where do they gather?

**Solution of Exercise 30.9.**

(a) From tiny plants and animals of the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), such as diatoms, buried in the [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) of the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) floor. (b) Between about 60 and $120{}^{\circ}\mathrm{C}$. (c) They are lighter than water, which fills the [pores](https://one-course.com/books/earth/1/en/chapter/17-water-underground#def-g6-water-underground-porosity) of the [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock). (d) An [impermeable](https://one-course.com/books/earth/1/en/chapter/17-water-underground#def-g6-water-underground-permeability) layer, such as shale; they gather under it, often at the top of an upfold, gas above oil.

**Exercise 30.10 ★★.**

The Chuquicamata mine has produced over 29 million tonnes of copper in over 100 years. (a) Write 29 million tonnes in scientific notation. (b) Roughly what is the average production per year? (c) If the ore held $1\,\%$ of copper, how many tonnes of ore were dug to get 29 million tonnes of copper? Use scientific notation. (d) Why does a mine of such a poor ore have to be so large?

**Solution of Exercise 30.10.**

(a) $2.9 \times 10^7$ tonnes. (b) $2.9 \times 10^7 \div 100 = 2.9 \times
10^5$ tonnes a year, roughly. (c) The ore is 100 times heavier than the copper it holds: $2.9 \times 10^7 \times 100 = 2.9 \times 10^9$ tonnes. (d) Every tonne of ore gives only $10\,\mathrm{kg}$ of copper, so billions of tonnes of [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) have to be dug.

**Exercise 30.11 ★★★.**

In 1970, the world’s copper [reserves](#def-g9-earth-resources-reserve) were estimated at 280 million tonnes. From 1970 to 2024, about 697 million tonnes were mined, and the [reserves](#def-g9-earth-resources-reserve) of 2024 were 980 million tonnes, with 23 million tonnes mined in 2024. (a) If the 1970 [reserve](#def-g9-earth-resources-reserve) had been all the copper there was, when would it have run out? Compare with what was mined. (b) How many times larger is the 2024 [reserve](#def-g9-earth-resources-reserve) than the 1970 one? (c) Compute how many years the 2024 [reserve](#def-g9-earth-resources-reserve) would last at the 2024 rate. (d) Give two reasons why a [reserve](#def-g9-earth-resources-reserve) can grow even while it is being mined.

**Solution of Exercise 30.11.**

(a) At 23 million tonnes a year it would have lasted about $280 \div 23
\approx 12$ years, until the early 1980s; yet 697 million tonnes were mined, much more than that [reserve](#def-g9-earth-resources-reserve). (b) $980 \div 280 = 3.5$ times. (c) $980 \div
23 \approx 43$ years. (d) For example: new deposits are discovered; better techniques or higher prices make poorer deposits worth mining.

**Exercise 30.12 ★★★.**

A coal seam is $3 \times 10^8$ years old. Its swamp lasted $2 \times 10^3$ years, and the seam is mined out in 50 years. (a) Write 50 years in scientific notation. (b) How many times longer than the mining did the swamp last? (c) How many times older than the time of mining is the seam? (d) Use these numbers to explain why coal is called a non-renewable resource.

**Solution of Exercise 30.12.**

(a) $5 \times 10^1$ years. (b) $2 \times 10^3 \div 50 = 40$ times. (c) $3
\times 10^8 \div (5 \times 10^1) = 6 \times 10^6$ times. (d) The coal took millions of times longer to form than we take to use it: nature does not renew it on our time scale.

## 30.8 Problem: Concrete for a new town

**Problem 30.1.**

Weekend problem — a new town is built with the sand of a local pit; the final answer is the number of years the pit’s sand reserve would last

A new town of $2000$ homes is to be built in ten years, near an old [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) pit dug in the [gravels](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) of a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) terrace. Counting its share of the streets, the schools and the shops, each home needs about $300\,\mathrm{t}$ of [aggregates](#def-g9-earth-resources-aggregate), of which $40\,\%$ is [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand). The pit’s [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) layer is $1.2\,\mathrm{km}$ long, $500\,\mathrm{m}$ wide and $5\,\mathrm{m}$ thick; one cubic metre of its [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) has a mass of $1.6\,\mathrm{t}$. The pit also sells $136\,000$ tonnes of [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) a year to the rest of the region.

**Part I — How much sand?**

1. A family house alone needs about $100\,\mathrm{t}$ of [aggregates](#def-g9-earth-resources-aggregate) . Write this mass in kilograms, in scientific notation.
2. How many tonnes of [aggregates](#def-g9-earth-resources-aggregate) will the whole town need?
3. How many tonnes is that per year, over the ten years?
4. How many tonnes of [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) per year does the town need?

**Part II — The reserve.**

5. Compute the volume of the [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) layer, in cubic metres, in scientific notation.
6. A third of the layer lies under a village and a road and will not be dug. What volume can be extracted?
7. What is the mass of this [reserve](#def-g9-earth-resources-reserve) , in tonnes?
8. How many tonnes of [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) will the pit sell each year, to the town and to the region together?

**Part III — Renewable or not?**

9. Each year, the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) brings about $2000$ tonnes of new [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) to the area. How many times more [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) does the pit sell?
10. On the scale of the town’s life, is this [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) a renewable resource? Explain.
11. If the town replaced a quarter of its [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) with crushed old concrete, how many tonnes of [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) would it save each year?
12. Without that recycling, how many years would the pit’s [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) [reserve](#def-g9-earth-resources-reserve) last?

**Solution of Problem 30.1.**

**1.** $100 \times 1000 = 100\,000$ kg $= 1 \times 10^5$ kg.

**2.** $2000 \times 300 = 600\,000$ t $= 6 \times 10^5$ t.

**3.** $6 \times 10^5 \div 10 = 6 \times 10^4$ t per year.

**4.** $0.40 \times 6 \times 10^4 = 2.4 \times 10^4$ t of [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) per year.

**5.** $1200 \times 500 \times 5 = 3 \times 10^6$ m$^3$.

**6.** Two thirds of it: $2 \times 10^6$ m$^3$.

**7.** $2 \times 10^6 \times 1.6 = 3.2 \times 10^6$ t.

**8.** $24\,000 + 136\,000 = 160\,000$ t $= 1.6 \times
10^5$ t per year.

**9.** $160\,000 \div 2000 = 80$ times more.

**10.** No: the pit is emptied 80 times faster than the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) renews the [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand). On the scale of the town, this [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) is a finite resource.

**11.** $24\,000 \div 4 = 6000$ t of [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) a year.

**12.** $3.2 \times 10^6 \div (1.6 \times 10^5) = 20$: **20 years**.