---
title: "The Oceans"
book: "Primary & Middle School Earth and Space Sciences"
subject: earth
language: en
chapter: 26
exercises: 12
source: https://one-course.com/books/earth/1/en/chapter/26-the-oceans
license: CC-BY-NC-SA-4.0
credit: "One Earth and Space Sciences Book, One Course (one-course.com)"
---

# Chapter 26 — The Oceans

In December 1872, a British navy ship, HMS *Challenger*, left England for a voyage of three and a half years around the world. Its task was new: to find out what lies at the bottom of the [oceans](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean). Again and again the ship stopped in the open [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), and the crew lowered a heavy weight on a long rope until it touched the bottom; the length of rope paid out gave the depth. When the ship came home in May 1876, its soundings had shown that the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) floor has deep [plains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) and great [mountain ranges](https://one-course.com/books/earth/1/en/chapter/9-the-physical-map-of-the-world#def-g4-world-physical-map-range).

![HMS Challenger preparing to sound the depth of the ocean, 1872. Engraving from the reports of the expedition, reproduced in W. A. Herdman, Founders of Oceanography (1923); public domain, via Wikimedia Commons.](https://one-course.com/images/onecourse/chapters/earth-1/g8-the-oceans/img-2d383c3a63ef.jpg)

*HMS *Challenger* preparing to sound the depth of the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), 1872. Engraving from the reports of the expedition, reproduced in W. A. Herdman, *Founders of Oceanography* (1923); public domain, via Wikimedia Commons.*

Today a ship [maps](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) floor with sound. A device under its hull sends a short pulse of sound downwards; the sound bounces off the bottom and comes back as an echo. The time the echo takes gives the depth, again and again, while the ship keeps sailing. This is *sonar*.

**You already know.**

[Salt water](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-salt-fresh) covers most of the Earth, in one world [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) divided into [oceans](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) and [seas](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) ([Chapter 3](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#ch-g2-water-on-earth)). [Plates](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) move apart at the [ocean ridges](#def-g8-the-oceans-seafloor) and one [plate](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) sinks under another at the trenches ([Chapter 18](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#ch-g7-plate-tectonics)). Over the [oceans](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), the winds blow in great belts: [trade winds](https://one-course.com/books/earth/1/en/chapter/24-winds-and-the-global-circulation#def-g8-winds-global-circulation-global-winds), [westerlies](https://one-course.com/books/earth/1/en/chapter/24-winds-and-the-global-circulation#def-g8-winds-global-circulation-global-winds) and [polar easterlies](https://one-course.com/books/earth/1/en/chapter/24-winds-and-the-global-circulation#def-g8-winds-global-circulation-global-winds), turned aside by the Earth’s rotation ([Chapter 24](https://one-course.com/books/earth/1/en/chapter/24-winds-and-the-global-circulation#ch-g8-winds-global-circulation)).

## 26.1 The ocean floor

If the water of the [oceans](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) could be drained away, a whole hidden [landscape](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landscape) would appear. Along the [coasts](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), the land goes on under the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) as a gentle, shallow shelf. Then the floor plunges down a steep slope to vast, flat [plains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), kilometres deep. [Mountain ranges](https://one-course.com/books/earth/1/en/chapter/9-the-physical-map-of-the-world#def-g4-world-physical-map-range) rise from these [plains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), and here and there narrow trenches go down deeper than anything else on Earth.

![The parts of the ocean floor, from a coast to the open ocean and another coast (a sketch: depths and widths are not to scale).](https://one-course.com/images/onecourse/chapters/earth-1/g8-the-oceans/fig-b0eb92eb3fa0.svg)

*The parts of the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) floor, from a [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) to the open [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) and another [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) (a sketch: depths and widths are not to scale).*

**Definition 26.1 (The parts of the ocean floor).**

- The *continental shelf* is the shallow, gently sloping edge of a [continent](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent) under the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) .
- The *continental slope* is the steep slope that leads down from the shelf to the deep [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) floor.
- An *abyssal plain* is a vast, very flat and deep part of the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) floor.
- An *ocean ridge* is a long chain of underwater [mountains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) rising from the abyssal [plains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) .
- An *ocean trench* is a long, narrow and very deep trough in the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) floor, often next to a chain of [mountains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) or volcanic islands.

The [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) is about $3700\,\mathrm{m}$ deep on average, almost half the height of the highest [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) on land. Its deepest point, the Challenger Deep in the Mariana Trench in the Pacific [Ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), is nearly $11\,000\,\mathrm{m}$ below the surface.

**Method 26.2 (Reading a profile of the ocean floor).**

1. Read the axes: the distance across, the height or depth up and down, and the vertical exaggeration.
2. Find [sea level](https://one-course.com/books/earth/1/en/chapter/9-the-physical-map-of-the-world#def-g4-world-physical-map-altitude) (height 0) and the [coasts](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) where the profile crosses it.
3. Follow the floor from a [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) : the shallow shelf, the steep slope, then the deep flat [plains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) .
4. Look for what rises from the [plains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) (a ridge) or drops below them (a trench), and read its depth.

**Example 26.3 (Across the South Atlantic).**

On the top profile, past the [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) of South America, the floor drops to more than $3000\,\mathrm{m}$ deep in less than $300\,\mathrm{km}$. [Abyssal plains](#def-g8-the-oceans-seafloor), between $3500\,\mathrm{m}$ and $5000\,\mathrm{m}$ deep, fill most of the width. Half-way across, about $4400\,\mathrm{km}$ along the profile, the Mid-Atlantic Ridge rises to about $2300\,\mathrm{m}$ below the surface: more than $2000\,\mathrm{m}$ above the [plains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), a [mountain range](https://one-course.com/books/earth/1/en/chapter/9-the-physical-map-of-the-world#def-g4-world-physical-map-range) hidden under the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean). The [plains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) deepen again to more than $5000\,\mathrm{m}$ before the floor climbs to Africa.

![Two real profiles of the ocean floor, from the ETOPO1 relief read every degree of longitude. Top: across the South Atlantic, from South America to Africa. Bottom: across the Pacific edge of South America. The heights are stretched 260 and 148 times compared with the distances (vertical exaggeration), otherwise the ocean would look as thin as a sheet of paper; and with one point every degree, the narrow trench looks shallower than it is.](https://one-course.com/images/onecourse/chapters/earth-1/g8-the-oceans/fig-ca6447d32602.svg)

![Two real profiles of the ocean floor, from the ETOPO1 relief read every degree of longitude. Top: across the South Atlantic, from South America to Africa. Bottom: across the Pacific edge of South America. The heights are stretched 260 and 148 times compared with the distances (vertical exaggeration), otherwise the ocean would look as thin as a sheet of paper; and with one point every degree, the narrow trench looks shallower than it is.](https://one-course.com/images/onecourse/chapters/earth-1/g8-the-oceans/fig-800c4472b2e4.svg)

*Two real profiles of the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) floor, from the ETOPO1 relief read every degree of [longitude](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-longitude). Top: across the South Atlantic, from South America to Africa. Bottom: across the Pacific edge of South America. The heights are stretched 260 and 148 times compared with the distances (*vertical exaggeration*), otherwise the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) would look as thin as a sheet of paper; and with one point every degree, the narrow trench looks shallower than it is.*

## 26.2 Salty water

[Sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) water tastes salty because salts are dissolved in it. Most of them come from the [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) of the [continents](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent): [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) slowly wears the [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) away and [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) carry their dissolved salts to the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean). Some come from hot springs on the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) floor.

**Definition 26.4 (Salinity).**

The *salinity* of [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) water is the mass of salts dissolved in a kilogram of [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) water. On average it is about $35\,\mathrm{g}$ per kilogram: $3.5\,\%$ of the mass of [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) water is salt.

The [salinity](#def-g8-the-oceans-salinity) is not the same everywhere. When [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) water evaporates, the water leaves but the salts stay behind: where the Sun is strong and [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) is rare, the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) becomes saltier. Where much [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) falls, or where a large [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) flows in, the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) is less salty. When [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) water freezes into [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) ice, the salt is left in the water around, which becomes saltier too. On the whole, the [salinity](#def-g8-the-oceans-salinity) is low near the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator), where it [rains](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) a lot, and near the [poles](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-poles); it is highest in the middle [latitudes](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-latitude), under the dry belts near $30{}^{\circ}$.

![What changes the salinity of the sea: evaporation and freezing leave the salts behind; rain and rivers add fresh water.](https://one-course.com/images/onecourse/chapters/earth-1/g8-the-oceans/fig-791091d32594.svg)

*What changes the [salinity](#def-g8-the-oceans-salinity) of the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean): evaporation and freezing leave the salts behind; [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) and [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) add [fresh water](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-salt-fresh).*

![A research ship lowers a frame of sampling bottles into the sea, to bring back water from the depths and measure its saltiness and its temperature (AI-generated illustration of a generic scene).](https://one-course.com/images/onecourse/chapters/earth-1/g8-the-oceans/img-0b09073ab6a8.jpg)

*A research ship lowers a frame of sampling bottles into the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), to bring back water from the depths and measure its saltiness and its temperature (AI-generated illustration of a generic scene).*

## 26.3 Currents at the surface

The [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) is never still. The winds drag on its surface and set the water moving: the steady [trade winds](https://one-course.com/books/earth/1/en/chapter/24-winds-and-the-global-circulation#def-g8-winds-global-circulation-global-winds) push it towards the west near the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator), and the [westerlies](https://one-course.com/books/earth/1/en/chapter/24-winds-and-the-global-circulation#def-g8-winds-global-circulation-global-winds) push it towards the east in the middle [latitudes](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-latitude). Like the winds, the moving water is turned aside by the Earth’s rotation: to the right in the [northern hemisphere](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator), to the left in the [southern hemisphere](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator).

**Definition 26.5 (Ocean current and gyre).**

An *ocean current* is a steady movement of [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) water in one direction, like a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) in the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean). A *gyre* is a very large loop of [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) currents that turns round an [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) basin.

There are five great [gyres](#def-g8-the-oceans-current): in the North and South Atlantic, in the North and South Pacific, and in the Indian [Ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean). Those of the [northern hemisphere](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) turn clockwise, those of the [southern hemisphere](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) anticlockwise. On the western side of each [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), the [gyre](#def-g8-the-oceans-current)’s current is narrow and fast; on the eastern side, it is broad and slow.

**In the lab — A gyre in a dish.**

Indoors, fill a large round dish with water and sprinkle a pinch of pepper on the surface. Blow gently and steadily across the dish through a straw, always in the same direction, close to one edge. Watch the pepper: the water near the surface starts to move with your breath, and soon the whole surface turns in a loop round the dish.

![The five great gyres, simplified: clockwise in the northern hemisphere, anticlockwise in the southern. Red: warm currents, coming from the tropics; blue: cold currents, coming from higher latitudes. The North Atlantic gyre is edged by the Gulf Stream on the west and the Canary Current on the east.](https://one-course.com/images/onecourse/chapters/earth-1/g8-the-oceans/fig-f73d5a6dc5a9.svg)

*The five great [gyres](#def-g8-the-oceans-current), simplified: clockwise in the [northern hemisphere](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator), anticlockwise in the southern. Red: warm currents, coming from the [tropics](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-tropics); blue: cold currents, coming from higher [latitudes](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-latitude). The North Atlantic [gyre](#def-g8-the-oceans-current) is edged by the Gulf Stream on the west and the Canary Current on the east.*

The best known of these currents is the Gulf Stream. It leaves the Gulf of Mexico, runs north along the [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) of Florida, turns east off North Carolina and crosses the Atlantic towards the north-east. Its warm water moves on average at about $6.4\,\mathrm{km}$ per hour, up to about $9\,\mathrm{km}$ per hour at the surface, and it carries more water than all the [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) of the world together. It keeps the east [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) of Florida warmer in winter and brings mild air to western Europe.

![The temperature of the sea surface off the east coast of North America, measured by NASA’s Aqua satellite on 18 April 2005. The Gulf Stream is the band of warm water (light colours, up to about 23 C) that winds from the bottom left towards the right, between colder water (dark, down to about -1 C); land is grey. NASA (public domain, via Wikimedia Commons).](https://one-course.com/images/onecourse/chapters/earth-1/g8-the-oceans/img-62222c175663.jpg)

*The temperature of the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) surface off the east [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) of North America, measured by NASA’s Aqua satellite on 18 April 2005. The Gulf Stream is the band of warm water (light colours, up to about $23{}^{\circ}\mathrm{C}$) that winds from the bottom left towards the right, between colder water (dark, down to about $-1{}^{\circ}\mathrm{C}$); land is grey. NASA (public domain, via Wikimedia Commons).*

## 26.4 Deep currents and the ocean’s heat

The winds drive the currents at the surface. Far below, in the cold darkness of the deep [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), the water moves too, much more slowly. In the far North Atlantic, the warm water brought by the Gulf Stream gives its heat to the cold air; cooled, and made saltier when [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) ice forms, it becomes denser and sinks to the bottom. This cold, salty water creeps south along the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) floor, all the way to Antarctica, then into the Indian and Pacific [Oceans](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), where it slowly rises and returns at the surface. A drop of water can take about $1000$ years to go once round this great loop, the *global [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) conveyor belt*.

![The global ocean conveyor belt, simplified to a single loop (a sketch). Red: warm water at the surface; blue, dashed: cold, salty water creeping along the deep ocean floor. The water sinks in the far North Atlantic, between Greenland and Norway, and rises in the Pacific (and, not drawn, in the Indian Ocean).](https://one-course.com/images/onecourse/chapters/earth-1/g8-the-oceans/fig-ab0490fd2220.svg)

*The global [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) conveyor belt, simplified to a single loop (a sketch). Red: warm water at the surface; blue, dashed: cold, salty water creeping along the deep [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) floor. The water sinks in the far North Atlantic, between Greenland and Norway, and rises in the Pacific (and, not drawn, in the Indian [Ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean)).*

**Example 26.6 (How slow is the deep water?).**

The loop drawn on the [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) is roughly $70\,000\,\mathrm{km}$ long. If a drop of water takes about $1000$ years to go round it, it moves on average about $70\,000 \div 1000 = 70\,\mathrm{km}$ a year, that is about $200\,\mathrm{m}$ a day. The Gulf Stream, at $6.4\,\mathrm{km}$ an hour, covers about $150\,\mathrm{km}$ in a day: it goes as far in about 11 hours as the deep water in a whole year.

Together, the surface and deep currents carry heat: warm water flows from the [tropics](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-tropics) towards the [poles](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-poles), and cold water flows back towards the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator). Like the winds, the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) evens out the uneven heating of the Earth by the Sun.

## 26.5 Exercises

**Exercise 26.1 ★.**

Put these parts of the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) floor in the order in which a submarine meets them as it leaves a [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) for the middle of the Atlantic: [abyssal plain](#def-g8-the-oceans-seafloor); [continental slope](#def-g8-the-oceans-seafloor); [ocean ridge](#def-g8-the-oceans-seafloor); [continental shelf](#def-g8-the-oceans-seafloor).

**Solution of Exercise 26.1.**

[Continental shelf](#def-g8-the-oceans-seafloor), [continental slope](#def-g8-the-oceans-seafloor), [abyssal plain](#def-g8-the-oceans-seafloor), [ocean ridge](#def-g8-the-oceans-seafloor).

**Exercise 26.2 ★.**

True or false? Correct the false sentences. (a) The [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) is about $37\,\mathrm{km}$ deep on average. (b) The [gyres](#def-g8-the-oceans-current) of the [northern hemisphere](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) turn clockwise. (c) Evaporation makes [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) water less salty. (d) The Gulf Stream is a warm current.

**Solution of Exercise 26.2.**

(a) False: about $3700\,\mathrm{m}$, that is $3.7\,\mathrm{km}$. (b) True. (c) False: evaporation removes water and leaves the salts, so the water gets saltier. (d) True.

**Exercise 26.3 ★.**

Give the word for each description: (a) a long chain of [mountains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) under the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean); (b) the mass of salt in a kilogram of [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) water; (c) a very large loop of [ocean currents](#def-g8-the-oceans-current); (d) a long, narrow and very deep trough in the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) floor.

**Solution of Exercise 26.3.**

(a) An [ocean ridge](#def-g8-the-oceans-seafloor). (b) The [salinity](#def-g8-the-oceans-salinity). (c) A [gyre](#def-g8-the-oceans-current). (d) An [ocean trench](#def-g8-the-oceans-seafloor).

**Exercise 26.4 ★.**

On the [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) of the [gyres](#def-g8-the-oceans-current): (a) which way does the [gyre](#def-g8-the-oceans-current) of the South Atlantic turn? (b) Name a warm current and a cold current of the North Atlantic. (c) Which winds push the water towards the west near the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator)?

**Solution of Exercise 26.4.**

(a) Anticlockwise. (b) Warm: the Gulf Stream; cold: the Canary Current. (c) The [trade winds](https://one-course.com/books/earth/1/en/chapter/24-winds-and-the-global-circulation#def-g8-winds-global-circulation-global-winds).

**Exercise 26.5 ★★.**

A sample of $1\,\mathrm{kg}$ of [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) water contains $35\,\mathrm{g}$ of salts. (a) What percentage of its mass is salt? (b) How many grams of salts are there in $20\,\mathrm{kg}$ of this water? (c) How many kilograms of [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) water contain $1\,\mathrm{kg}$ of salts?

**Solution of Exercise 26.5.**

(a) $35 \div 1000 \times 100 = 3.5~\%$. (b) $20 \times 35 = 700\,\mathrm{g}$. (c) $1000 \div 35 \approx 28.6$: about $29\,\mathrm{kg}$.

**Exercise 26.6 ★★.**

A ship’s sonar sends a pulse of sound straight down. The echo comes back $6\,\mathrm{s}$ later. Sound travels about $1500\,\mathrm{m}$ each second in [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) water. (a) How far does the sound travel in $6\,\mathrm{s}$? (b) Why is the depth only half of this distance? (c) What is the depth? (d) Which part of the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) floor could the ship be above?

**Solution of Exercise 26.6.**

(a) $1500 \times 6 = 9000\,\mathrm{m}$. (b) The sound goes down to the bottom and comes back up: it covers the depth twice. (c) $9000 \div 2 =
4500\,\mathrm{m}$. (d) An [abyssal plain](#def-g8-the-oceans-seafloor) (a trench would be deeper, a ridge or a shelf shallower).

**Exercise 26.7 ★★.**

Use the profile across the South Atlantic. (a) How deep are the [abyssal plains](#def-g8-the-oceans-seafloor), roughly? (b) At what distance along the profile is the crest of the Mid-Atlantic Ridge, and how deep is it? (c) By how many metres does the ridge rise above the [plains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform)? (d) Why does the profile need a vertical exaggeration?

**Solution of Exercise 26.7.**

(a) Between about $3500\,\mathrm{m}$ and $5000\,\mathrm{m}$. (b) About $4400\,\mathrm{km}$ along the profile, about $2300\,\mathrm{m}$ deep. (c) About $4500 - 2300 = 2200\,\mathrm{m}$ (between 1200 and $2700\,\mathrm{m}$, depending on the [plain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) chosen). (d) The [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) is a few kilometres deep but thousands of kilometres wide: drawn at the same scale, the whole profile would be thinner than a line.

**Exercise 26.8 ★★.**

[Sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) A lies between two [deserts](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-desert): it hardly ever [rains](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) there and no large [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) flows into it. [Sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) B, in a rainy region, receives many large [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water). Explain which of the two is saltier than the open [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), and which is less salty. Use the words *evaporation*, *[fresh water](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-salt-fresh)*, *salts stay behind*.

**Solution of Exercise 26.8.**

[Sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) A is saltier: the Sun and the dry air cause strong *evaporation*, the water leaves, the *salts stay behind*, and almost no [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) or [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) brings *[fresh water](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-salt-fresh)* back. [Sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) B is less salty: its [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) and its [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) bring a lot of *[fresh water](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-salt-fresh)*, which dilutes the salts.

**Exercise 26.9 ★★.**

A message in a bottle is thrown into the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) off the [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) of Florida. (a) Which current carries it first, and in which direction? (b) Which way does it then cross the Atlantic? (c) If it went on round the [gyre](#def-g8-the-oceans-current), along which [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) would it travel south? (d) Is this current warm or cold?

**Solution of Exercise 26.9.**

(a) The Gulf Stream, towards the north along the [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), then towards the north-east. (b) Towards the east (north-east), with the North Atlantic Current. (c) Along the [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) of north-west Africa (and Spain and Portugal), with the Canary Current. (d) Cold.

**Exercise 26.10 ★★.**

The Gulf Stream moves on average at about $6.4\,\mathrm{km}$ per hour. (a) How far does its water travel in one day? (b) In a week? (c) Compare with a person walking at $5\,\mathrm{km}$ per hour.

**Solution of Exercise 26.10.**

(a) $6.4 \times 24 = 153.6$: about $154\,\mathrm{km}$. (b) $153.6 \times 7
\approx 1075\,\mathrm{km}$. (c) The Gulf Stream is a little faster than a walker, but it never stops, day and night.

**Exercise 26.11 ★★★.**

The [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) is about $3700\,\mathrm{m}$ deep on average, and its deepest point is nearly $11\,000\,\mathrm{m}$ deep. (a) Write both depths in kilometres. (b) How many times deeper than the average is the deepest point? Round to one decimal. (c) A sonar echo from the deepest point: how many seconds does it take to come back, at $1500\,\mathrm{m}$ each second? (d) A drop of water of the conveyor belt sinks in the North Atlantic. If it takes about $1000$ years to go round, how many years is that in powers of ten?

**Solution of Exercise 26.11.**

(a) $3.7\,\mathrm{km}$ and $11\,\mathrm{km}$. (b) $11000 \div 3700 \approx 3.0$ times. (c) The sound travels $2 \times 11000 = 22\,000\,\mathrm{m}$; $22000
\div 1500 \approx 14.7$: about $15\,\mathrm{s}$. (d) $1000 = 10^3$ years.

**Exercise 26.12 ★★★.**

The great conveyor belt of the deep [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) starts in the Norwegian [Sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), off the [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) of Norway, where warm water brought by the Gulf Stream gives its heat to the cold air. (a) Using the [gyres](#def-g8-the-oceans-current) [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) and the chapter, explain step by step how this warm water travelled there from the [tropics](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-tropics), and which winds pushed it. (b) Explain why the water then sinks. (c) Where does it go next?

**Solution of Exercise 26.12.**

(a) Near the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator), the [trade winds](https://one-course.com/books/earth/1/en/chapter/24-winds-and-the-global-circulation#def-g8-winds-global-circulation-global-winds) push warm tropical water towards the west. At the western edge of the Atlantic it turns north with the Gulf Stream, a warm current, along Florida and North America. The [westerlies](https://one-course.com/books/earth/1/en/chapter/24-winds-and-the-global-circulation#def-g8-winds-global-circulation-global-winds) then push it across the Atlantic towards the north-east, as far as the [seas](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) off Norway. (b) There it gives its heat to the cold air: it cools, and it becomes saltier when [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) ice forms, because the salt is left in the water. Cold, salty water is dense, so it sinks to the bottom. (c) It creeps south along the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) floor, towards Antarctica, then into the Indian and Pacific [Oceans](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), where it slowly rises again.

## 26.6 Problem: A drifting buoy

**Problem 26.1.**

Weekend problem — a drifting buoy is launched off Florida and carried round the North Atlantic gyre; its journey is followed leg by leg, and the time it takes to go once round is worked out

Scientists launch a drifting buoy off the [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) of Florida. It floats just under the surface, moves with the currents, and sends its position to a satellite. Its batteries last about one year. The [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) shows the four legs of its journey round the North Atlantic [gyre](#def-g8-the-oceans-current).

| leg | current | length (km) | average speed (km per day) |
| --- | --- | --- | --- |
| A | Gulf Stream | 3800 | 100 |
| B | North Atlantic Current | 2100 | 20 |
| C | Canary Current | 3100 | 20 |
| D | North Equatorial Current | 6100 | 20 |

![](https://one-course.com/images/onecourse/chapters/earth-1/g8-the-oceans/fig-972da86c96c6.svg)

**Part I — The launch.**

1. In which [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) , and in which [hemisphere](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) , is the buoy launched?
2. Which way does the [gyre](#def-g8-the-oceans-current) turn? Is the buoy carried north or south at first?
3. Is the water of leg A warm or cold? And of leg C? Explain.
4. Which winds drive the currents of leg B? Of leg D?

**Part II — Around the gyre.**

5. How many days does leg A take?
6. How many days do legs B, C and D take?
7. What is the total length of the journey round the [gyre](#def-g8-the-oceans-current) ?
8. Why is leg A so much faster than the others?

**Part III — The journey’s length.**

9. How many days does the buoy need to go once round the [gyre](#def-g8-the-oceans-current) ?
10. Taking a month as 30 days, how many months is that? Round to the nearest whole month.
11. On which leg will the batteries run out, after about one year (365 days)?
12. How many more months of battery would the buoy need to send its position all the way round?

**Solution of Problem 26.1.**

**1.** In the Atlantic [Ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), in the [northern hemisphere](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator).

**2.** Clockwise; the buoy is carried north at first.

**3.** Leg A (Gulf Stream) is warm: it comes from the [tropics](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-tropics). Leg C (Canary Current) is cold: it comes from higher [latitudes](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-latitude), towards the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator).

**4.** Leg B: the [westerlies](https://one-course.com/books/earth/1/en/chapter/24-winds-and-the-global-circulation#def-g8-winds-global-circulation-global-winds). Leg D: the [trade winds](https://one-course.com/books/earth/1/en/chapter/24-winds-and-the-global-circulation#def-g8-winds-global-circulation-global-winds).

**5.** $3800 \div 100 = 38$ days.

**6.** B: $2100 \div 20 = 105$ days. C: $3100 \div 20 = 155$ days. D: $6100 \div 20 = 305$ days.

**7.** $3800 + 2100 + 3100 + 6100 = 15\,100\,\mathrm{km}$.

**8.** The Gulf Stream is the narrow, fast current on the western side of the [gyre](#def-g8-the-oceans-current); the currents on the other sides are broad and slow.

**9.** $38 + 105 + 155 + 305 = 603$ days.

**10.** $603 \div 30 = 20.1$: about **20 months**.

**11.** After legs A, B and C, $38 + 105 + 155 = 298$ days have passed; 365 days come during leg D (the North Equatorial Current), $365 -
298 = 67$ days into it.

**12.** $603 - 365 = 238$ more days, that is $238 \div 30 \approx 8$ more months.