Cambridge IGCSE Marine Science

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1 The Earth and its oceans

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Chapter 1

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The Earth and its oceans

IN THIS CHAPTER YOU WILL:

describe the motion of the Earth, Moon and Sun relative to each other and state what keeps them in regular motion

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describe the internal structure of the Earth and explain how this structure creates features that shape our oceans

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identify the five oceans and describe how water flows around and between them

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describe and explain the regular changes in tides

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describe tsunamis and rip tides and explain how they form.

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GETTING STARTED Spend two minutes thinking about each of these questions before comparing notes with another learner for a further two minutes, adding to or correcting your own work. Be prepared to share your thoughts with the class.

Why do the Sun and Moon rise and fall in the sky each day?

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What causes earthquakes and volcanoes?

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How are oceans different to seas?

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What is the difference between tides and currents?

MARINE SCIENCE IN CONTEXT

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Do migrating turtles have a compass?

The internal structure of the Earth offers clues to changes that have taken place through time. The Earth’s magnetic field is created by the Earth’s core, and this magnetic field has changed throughout the history of the Earth. Some of those changes in the magnetic field have been measured. The changes in the magnetic field affect the location of turtle nesting sites along beaches.

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Each year sea turtles around the world return to the beaches they hatched on to lay their eggs. How do they know how to get there? Scientists have known for a long time that turtles, like many other animals, find their way using the invisible lines of Earth’s magnetic field. Turtles contain a magnetic material called magnetite in their heads, and scientists think they use this as an internal compass to find their way using Earth’s magnetic field. But how are the turtles able to find their way to the exact beach where they hatched?

and use this magnetic field to find their way home.

However, this magnetic field is not as fixed and permanent as we might think.

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Figure 1.1: Olive Ridley sea turtles returning to a beach in Costa Rica.

We now know that each coastline has its own unique magnetic field based on the rocks and the shape of the coast. This helps the turtles, who remember

Discussion questions 1

If the Earth acts as a big magnet, what might be happening to the magnetic material inside the Earth to cause the magnetic field to change direction?

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What might be the advantages to turtles of returning to the same beach to lay their eggs?

KEY WORDS magnetic field: a region of space around the Earth in which a magnet experiences (feels) a force core: the very hot, dense centre of Earth

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1 The Earth and its oceans

1.1 Structure of the Earth

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The Earth is one of eight planets orbiting the Sun in the solar system. The planets can be seen in Figure 1.2. The Sun is the largest object in the solar system and it has the greatest gravitational pull (gravity) on other objects, particularly planets, in the solar system. Gravitational pull keeps the Earth in orbit around the Sun. Earth’s position as the third planet from the Sun helps to create the ideal temperatures for life to exist on Earth.

over the Tropic of Capricorn in the Southern Hemisphere, causing this to experience the most intense light. Light from the Sun only reaches the southern limit of the Arctic Circle, resulting in no light on any of the Earth’s land or ocean north of this line. The Antarctic Circle near to the South Pole is completely exposed to light from the Sun. This results in daylight that lasts for weeks or months, but the intensity of this light is very low compared to near the Equator. At the poles, there is very limited heating due to light from the Sun. This means that the polar regions are the coldest on Earth.

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23.5º

Earth’s axis

Arctic Circle

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Tropics of Cancer

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Earth

the Equator

Earth’s rotation

Tropics of Capricorn Antarctic Circle

Figure 1.3: How the Earth’s axis is tilted at 23.5 ° causing uneven heating in the Northern and Southern hemispheres.

KEY WORDS solar system: eight planets and their moons in orbit around the Sun gravity / gravitational pull: the force that exists between any two objects with mass orbit: the path of an object as it moves around a larger object

The Earth orbits the Sun but the Earth is also spinning on its axis every 24 hours (one day). Figure 1.3 shows that the Earth spins on its axis at an angle of 23.5° to the direction of the Sun’s rays. This tilt of the Earth’s axis means for most of the year there is unequal exposure to light and heat from the Sun. In Figure 1.3 the Southern Hemisphere is exposed to more heat and light than the Northern Hemisphere.

axis: the imaginary line between the Earth’s North and South Poles

The intensity of the Sun is greatest near to the Equator and between the Tropics of Cancer and Capricorn.

The more intense heating in these regions continues throughout the year. In Figure 1.3 the Sun is directly

tropics: the region between the Tropics of Cancer and Capricorn, between which the Sun moves will be directly overhead at some point during a year

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Figure 1.2: The Sun and the eight planets we now know orbit the Sun.

hemisphere: half of a sphere; the Earth can be considered to be made of two hemispheres divided by the Equator Equator: an imaginary line drawn round the Earth halfway between the North Pole and the South Pole

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month

southern hemisphere

northern hemisphere

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position of Earth

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season

winter

March

autumn (fall)

spring

June

winter

summer

September

spring

autumn (fall)

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summer

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December

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Figure 1.4: How the Earth experiences seasons as it orbits the Sun. 1

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Earth takes 3654 days to complete one full orbit of the Sun. As the Earth moves around the Sun, the Earth’s axis continues to point in the same direction. This causes the Northern and Southern Hemispheres to experience seasons as the length of the day changes during the year. Longer days and greater exposure to more heat and energy from the Sun causes more warming of the Earth’s surface in summer months.

Shorter days with less exposure to heat and energy from the Sun causes cooler weather in winter months.

This can be seen in Figure 1.4.

The Moon, shown in Figure 1.5, is a natural satellite that orbits the Earth every 27.5 days. The closeness of the

Moon to the Earth, and relatively large size of the Moon compared to the Earth, causes a strong gravitational pull between them. This keeps the Moon in orbit around the Earth. This gravitational pull between the Earth and Moon also has a large effect on the water in seas and oceans, causing tides (see Section 1.4).

KEY WORDS moon: a natural satellite of a planet satellite: an object that orbits a planet

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1 The Earth and its oceans

CONTINUED Shine the lamp towards the ‘Earth’. The axis of the ‘Earth’ needs to be at an angle to the vertical.

Turn the ‘Earth’ anticlockwise (looking down over the North Pole) on its axis. This demonstrates why we experience day and night.

ACTIVITY 1.1

You will need: a lamp

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a globe of the Earth.

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Earth is a rocky planet, but the rock is only solid at or near to the surface. Figure 1.7 shows a cross-section of the Earth, showing how the solid rocks at the surface form a thin layer called the crust. The crust beneath the ocean has a greater density and varies in thickness from 3 km to 10 km thick. Continental crust has a lower density and usually varies in thickness from 35 km to 40 km thick, though continental crust can be as much as 100 km thick.

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Modelling day, night and seasons

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Figure 1.5: The Earth and Moon with the Sun in the distance.

Next move the ‘Earth’ slowly around the ‘Sun’.

Observe how the light reaches the North or South Poles only when that pole is pointing towards the Sun. Investigate the difference in seasons in both hemispheres as the Earth moves around the Sun. Discuss how the length of day changes near to the poles. Also discuss the intensity of light near the poles compared the intensity of light near the Equator.

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For this whole-class activity, use a lamp to represent the Sun. If you have a globe, use that to represent the Earth; however, an orange with a pencil sticking through it could be used instead.

This activity will work best in a darkened room.

Figure 1.6: Modelling changes each day, month and year.

Beneath the crust is a much thicker layer (the mantle) made of viscous molten rock called magma. Beneath the mantle is the core, made almost entirely of metal, particularly iron. The core is solid in the middle and molten around this.

KEY WORDS crust: outermost solid layer of the Earth density: expresses the relationship between the mass of a substance and the volume it occupies: density = mass ÷ volume mantle: the region of Earth found between the crust and the core viscous: describes a liquid that flows slowly magma: hot semi-liquid rock found below or within the crust iron: a metallic element with magnetic properties

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CAMBRIDGE IGCSE MARINE SCIENCE: COURSEBOOK

crust

0–100 km

land

continental crust oceanic crust

mantle outer core

2900 km

inner core

5100 km

ocean

6370 km

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Figure 1.7: Cross-section of Earth showing relative depths of crust, including oceanic and continental, mantle and core.

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KEY WORD

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migrate: movement of animals from one region to another, usually in response to changes in the seasons

Figure 1.8: Earth’s magnetic field.

Questions 1

Name the force that keeps Earth in orbit around the Sun. Draw a cross-section of Earth and label the layers found inside.

What produces a magnetic field around the Earth?

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An important effect of the iron in the Earth’s core is that this creates a magnetic field around the Earth, shown in Figure 1.8. We use the Earth’s magnetic field for navigation. A compass uses a magnet that can freely rotate to align with the Earth’s magnetic field to point to the magnetic North Pole. The Earth’s magnetic field is important in helping some species to navigate when they migrate.

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1.2 Plate tectonics Volcanoes and major earthquakes occur in limited regions of the Earth. These regions often form patterns around the Earth, as shown in Figure 1.9.

It was only in the 1960s that the theory of plate tectonics was developed to help explain the distribution of earthquakes and volcanoes, and why these features exist. The Case Study Project at the end of this chapter looks at the development of this theory in more detail, from its initial suggestion in 1912 to its acceptance in the 1960s.

In Section 1.1 we saw the Earth’s structure consisting of a core, surrounded by semi-molten rock called

KEY WORDS theory of plate tectonics: a theory that helps explain the formation of some of the important features on the Earth’s surface and how the continents move

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1 The Earth and its oceans

Figure 1.9: Earthquakes and volcanoes around the Earth.

Mid-oceanic ridge

Trench

Subduction

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Oceanic crust

Ocean

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Continental crust

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Convection current

Oceanic crust Continental crust Convection current

Outer core

Mantle

Inner core

Figure 1.10: Convection currents in the Earth’s mantle causing the crust to move.

mantle, covered with a relatively thin solid layer of rock called crust.

Tectonic plate theory describes how the Earth’s core heats the mantle, creating convection currents in the mantle. This is shown in Figure 1.10. The convection

KEY WORDS convection currents: the transfer of thermal energy by the motion of a fluid

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currents create tension in the crust as the mantle moves in different directions beneath the crust, causing the crust to break into large pieces called tectonic plates, which are shown in Figure 1.11.

KEY WORDS tectonic plates: large sections of crust

60º

Juan Do Fuca plate

30º

Eurasian Plate

North American Plate Caribbean Plate

Pacific plate

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Earth’s Tectonic Plates

Cocos plate

African Plate

0º

30º

South American plate

Arabian Plate

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Nazca plate

Philippine Plate Pacific Plate

Indo-australian Plate

Caroline Plate Fiji Plate Bismarck Plate

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Scotia plate

60º

150°

120°

90°

60°

30°

Subduction zones Divergent boundaries Plate movement

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30°

60°

90°

120°

150°

Antarctica Plate

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Figure 1.11: Main tectonic plates on Earth and their movement.

ACTIVITY 1.2

Modelling convection currents You will need:

tripod and gauze

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Bunsen burner

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glass beaker

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water

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small crystal of potassium manganate(VII).

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Use the equipment shown in Figure 1.12 to see convection currents occur. If you don’t have this equipment, you could use a different container of water (e.g. a saucepan) over a different heat source (e.g. cooker hob) and any soluble solid with a dark colour (e.g. instant coffee).

Safety: take care when heating and when using flames – hot liquids and equipment can cause burns.

Follow all safety instructions from your teacher.

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1 The Earth and its oceans

CONTINUED

Figure 1.12: Equipment to demonstrate convection currents.

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Potassium manganate(VII)

When the water is heated observe the direction in which the colour moves from the crystals, both in the water and at the surface. What happens if you add small floating objects to the surface in the middle of the water? Compare what you have seen with what you have learned about convection currents in the Earth’s mantle. Discuss what you observe with the rest of the class.

KEY WORDS

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The theory of plate tectonics describes how this movement of tectonic plates has occurred over hundreds of millions of years. Using computer models, we can trace the movements of the plates back in time.

Figure 1.13 shows how all the continents were once joined to form a giant supercontinent called Pangea.

As you can see from the diagrams in Figure 1.13 most of the continents have slowly drifted apart from each other over time.

KEY WORDS

supercontinent: a large landmass thought to have broken into the current continents

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Pangea: a large landmass thought to have broken into all of the current continents

Plate boundaries

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Where two tectonic plates meet, one of three type of plate boundary can occur: •

divergent plate boundaries: plates move away from each other

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convergent plate boundaries: plates move towards each other

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transform plate boundaries: plates slide against each other.

divergent plate boundary: where two tectonic plates move apart, forming new land between them convergent plate boundary: where two tectonic plates move towards each other transform plate boundary: where two tectonic plates try to slide past each other

Divergent plate boundaries When two tectonic plates move away from each other, a gap is created between the two plates. As the layer beneath the crust is molten, this causes magma from the mantle to rise and fill the gap. The molten rock is called lava once it gets to the upper surface of the crust.

The lava cools when it is exposed to the cool air or water above, and solidifies to produce new solid rock.

The movement of the tectonic plates is not smooth and constant. The plates often move suddenly, causing earthquakes, and can suddenly releases molten lava to

KEY WORDS lava: molten rock that erupts from the Earth’s crust earthquake: a sudden shaking of the ground, usually caused when two tectonic plates suddenly slip against each other

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CONTINENTAL DRIFT

Laurasia

Pangea

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Gondwana

Permian period 225 million years ago

Triassic period 200 million years ago

Jurassic period 150 million years ago

Europe

North America

Asia

Africa

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South America

Australia

Antarctica

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Cretaceous period 65 million years ago

Present days

Figure 1.13: How the continents have moved over time by continental drift.

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the surface that cools to form new rock. As a result, the new rock forms uneven mountain ranges either side of the divergent plate boundary, which slowly move apart over time.

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Most divergent boundaries occur in the oceans between continents. Figure 1.14 shows the mid-Atlantic ridge running from Iceland in the North Atlantic, down the Atlantic Ocean to the Southern Ocean near Antarctica.

This forms one of the longest mountain ranges in the world. Most of this mountain range is underwater. Some parts do rise above the ocean to form Iceland and other islands in the Atlantic, including the Azores, a group of volcanic islands in the Atlantic.

KEY WORDS

volcanic islands: islands formed when a volcanic eruption result in the formation of an island or group of islands

Figure 1.14: The mid-Atlantic ridge.

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1 The Earth and its oceans

Features associated with divergent plate boundaries include: earthquakes, due to sudden movements of the tectonic plates

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volcanoes, both underwater and on land, formed as the plates move apart allowing magma to rise up and escape as lava

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mid-ocean ridges, extensive mountain ranges formed deep in the ocean that extend for hundreds of miles either side of the parting plates.

KEY WORDS volcanoes: part of the Earth’s crust from which lava erupts

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ocean trenches, deep underwater valleys running along the plate boundary – the Mariana Trench is the deepest part of the world’s oceans and is between Japan and Papua New Guinea.

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mid-ocean ridge: extensive mountain ranges formed deep in the ocean that extend for hundreds of miles either side of the parting tectonic plates

Figure 1.15: Fossilised shell found on a mountain.

Convergent plate boundaries

KEY WORDS

When two tectonic plates move towards each other, the plates collide. What happens at the boundary depends on what type of crust is colliding.

subduction: the downwards movement of one tectonic plate beneath another

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If both sections of crust converging are continental crust, this results in fold mountains. While fold mountains are not directly linked to the oceans, they do provide evidence supporting the theory of plate tectonics. The Himalaya mountain range is an example of fold mountains created by the convergence of the Indo-Australian plate into the Eurasian plate. Fossils of sea creatures can be found on mountains in the Himalaya, such as the shell shown in Figure 1.15. These fossils support the idea of plate movement and uplifting of land over time as the Himalaya are far too high to ever have been covered by ocean.

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If a section of continental crust converges with a section of oceanic crust, the denser oceanic crust subducts (moves) beneath the continental crust, as shown in Figure 1.16.

Features associated with convergent plate boundaries include: •

earthquakes, due to sudden movements of the tectonic plates

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tsunamis, caused by sudden violent earthquakes

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volcanoes – as the plates converge, the oceanic crust subducts and melts, allowing magma to rise through the continental crust and escape as lava

tsunami: a fast-moving wave created by ocean floor displacement or landslide ocean trench: a deep underwater valley formed when oceanic crust slides below continental crust

Transform plate boundaries When two tectonic plates slide against each other, the movement of the plates is not smooth and constant.

The plates build up tension until the forces on them become so great that they slip suddenly, causing earthquakes. These earthquakes are less likely than earthquakes at a convergent boundary to cause a tsunami as the plates do not usually cause a large displacement of seawater.

Most transform boundaries beneath the ocean occur near to mid-ocean ridges linking sections of divergent plates, as shown in Figure 1.17.

The features most associated with transform plate boundaries are earthquakes, due to sudden movements of the tectonic plates.

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Ocean trench Volcanoes

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Continental crust

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Melting

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m fault

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Figure 1.16: Subduction of oceanic crust at a convergent boundary with continental crust.

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Mid-ocean ridges

Separate two tectonic plates

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Oceanic crust

Plate direction

Mantle

Figure 1.17: Transform fault beneath the ocean.

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Tsunamis

Tsunamis occur when an extremely large volume of seawater is displaced, such as when the tectonic plates move suddenly. Figure 1.18 shows how a tsunami can be created by plate movements at a convergent boundary.

When a tsunami forms, the wave travels very quickly through deep ocean – this can be up to 800 km per hour, allowing tsunamis to cross oceans in less than a day.

A tsunami wave may be as little as 30 cm high in the middle of the ocean. However, as a tsunami wave nears the coast

on the continental shelf, the ocean is much less deep and this causes the wave to increase in height. The shallower depth also causes the wave to slow, causing the wave to gain even more height. As a tsunami wave reaches the coast it may have slowed to as little as 30 km per hour. The wave can be as high as 30 m, resulting in the wave flooding the coast and spreading inland as far as 16 km.

Tsunamis can also be caused by other events that displace huge volumes of seawater, such as a huge landslide underwater, a powerful volcanic eruption, or the impact of a large meteorite hitting an ocean. Whichever way they form, a tsunami wave always travels in the same way.

When a tsunami occurs, a huge amount of energy is transferred through the moving water in the tsunami wave. This energy moves rocks and sediments on the sea floor, destroying habitats and organisms on the sea bed. The tsunami off the coast of Japan in March 2011 moved ocean sediments and deposited them on other habitats creating huge sea-floor sand dunes.

KEY WORDS continental shelf: part of the continental plate extending from the coast/shore in shallow water with little or no slope sediments: small fragments of rock, such as gravel, sand and silt

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1 The Earth and its oceans

Subduc ting p late

a) The continental plate gets stuck over the subducting oceanic plate.

Earthquake starts tsunami

b) The continental plate distorts as tension continues to build.

Tsunami waves spread

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Stuck plate slips, releasing energy in an earthquake

Slow distortion

Overriding plate

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Stuck

c) The plates suddenly slip, causing an earthquake and displacing a huge volume of water in the ocean above.

d) The displaced water sinks back down and spread, creating a tsunami.

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Figure 1.18: How Earthquakes at Convergent boundaries can cause a tsunami.

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Coral reefs act as natural breakwaters and coastal defences against tidal action and tsunamis.

These important marine habitats face the full force of tsunamis and can suffer great damage that can take many years to recover from. Other important marine habitats affected by tsunamis include coastal sea-grass beds and mangrove forests.

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Due to the great height of tsunami waves and their energy, tsunamis can do great damage to human communities along coastlines. The speed with which a tsunami wave floods a shore gives very little time for people to evacuate. The tsunami in the Indian Ocean on 26 December 2004 killed over 200 000 people in 14 countries around the Indian Ocean.

In addition to loss of life, the impact on coastal economies can be devastating, with fishing boats and gear destroyed and tourism disrupted. Farmland may be flooded with saltwater, leading to the loss of crops and difficulties in farming the flooded land for many years.

KEY WORD breakwater: a barrier which reduces the energy in waves before waves reach the coast

Questions 4 5

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Describe what is meant by the theory of plate tectonics.

Describe the plate movement for each of these boundaries: a convergent b divergent c transform.

State the types of plate boundary the following features occur at, and describe how they form: a volcanoes b earthquakes.

What is a tsunami and how does a tsunami form?

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from the Equator the latitude increases, from zero at the Equator to 90°N at the North Pole or 90°S at the South Pole.

1.3 Oceans and seas Oceans and seas cover 71% of the Earth’s surface, with areas of land only covering 29%. The oceans are interconnected to encircle the Earth as a World Ocean, meaning that water flows between the oceans and seas through currents. This flow is described in detail in Section 1.4.

The vertical reference line is the prime meridian, which runs from the North Pole to the South Pole through the British Royal Observatory in Greenwich, England.

As you move east or west from this line the longitude increases, from zero at the prime meridian to 180° on the opposite side of the globe.

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Most of the water on Earth (97%) is found in oceans and seas as salt water, with just 3% of the world’s water being fresh water.

KEY WORDS

Oceans separate most of the continents and are generally much deeper than seas. Seas are smaller areas of water.

Seas are sometimes found within oceans, often partially enclosed by land. The Earth’s five oceans and many of the seas are shown in Figure 1.19. The Pacific Ocean is the largest ocean, with an area of over 160 million km2 and a volume of 660 million km3.

World Ocean: all the oceans, which are interconnected to encircle the world

coordinates: a pair of numbers used to identify a point

latitude: the angular distance from the Equator to the North Pole or South Pole

We use a system of coordinates to form a grid enclosing the Earth. These coordinates help us identify and locate places on Earth. Just like axes on a graph, this system has reference lines which act as zero for the coordinate scales (Figure 1.20).

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prime meridian: a line from the North Pole to the South Pole through the Royal Greenwich Observatory in London

longitude: the angular distance east or west from the prime meridian

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The horizontal reference line is the Equator, around the middle of the Earth. As you move north or south Lincoin Sea

Chukchi Sea

Arctic Circle

Baffin Bay

Norwegian Sea

Gulf of Alaska

Sargasso Sea

Tropic of Cancer

Caribbean Sea

Atlantic Ocean

Black Sea

Caspian Sea

Sea of Japan

Arabian Sea

Bay of Bengal

Amundsen Sea

East China South Sea China Sea

Timor Sea

Tropic of Capricorn

Southern Ocean

East Siberian Sea

Sea of Okhotsk

Gulf of Guinea

Chilean Sea

Laptev Sea

Kara Sea

Barents Sea

Mediterranean Sea

Pacific Ocean

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Arctic Ocean

North Sea

Labrador Sea

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Bering Sea

Beaufort Sea

Greenland Sea

Indian Ocean

Arafura Sea

Pacific Ocean

Coral Sea

Tasman Sea

Scotia Sea

Weddell Sea

Antarctic Circle

Ross Sea

Figure 1.19: Earth’s oceans and seas.

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1 The Earth and its oceans

Longitude

Latitude Arctic Circle 90 60

North (+)

90

150

60

180

150 120

120 30 Tropic of Cancer

30

0

Tropic of Capricorn 30

30

60 Antarctic Circle 90

90

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90 Equator

West (–)

East (+)

60

60 South (–)

90

30

Tropic of Capricorn

0 Prime meridian

30

Tropic of Cancer

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Figure 1.20: Latitude and Longitude.

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Equator. This is the warmest region of Earth, resulting in warmer oceans and seas.

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Polar zone

Ar

c tic

Circle (66.5°

N)

Temperate Zone

Tropic o f Cancer (23.5° N)

Tropical Zone

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Equator (0° Latitude)

Tropical Zone

Tropic of Capricorn (23.5° S)

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Antartic Circle (66.5° S)

Temperate Zone

Polar zone

Figure 1.21: Tropical, temperate and polar zones.

In Section 1.1 we learned that the Earth’s axis is tilted and how this tilt causes uneven heating of the Earth’s surface. The tropical zones are the areas of Earth’s surface between the Topic of Cancer at 23.5°N of the Equator and the Tropic of Capricorn at 23.5°S of the

The Arctic Circle is at a latitude of 66.5°N and the Antarctic Circle is at a latitude of 66.5°S. These polar zones are the coldest on Earth, resulting in water often freezing at the surface of seas and oceans in these areas.

The zones between the tropics and Arctic or Antarctic Circles are described as temperate zones. These areas experience more variation in temperatures during a year with warmer temperatures in summer and colder temperatures in winter. Figure 1.21 summarises the different zones around the Earth.

In Section 1.2 we learned how plate tectonics has shaped the Earth, including the oceans, and how movements of plates create different features in and around the oceans.

This link between geological processes and the physical

KEY WORDS tropical zones / tropics: the area between the Tropic of Cancer and the Tropic of Capricorn polar zones: the area north of the Arctic Circle or south of the Antarctic Circle temperate zones: the areas between the Tropic of Cancer and the Arctic Circle and between the Tropic of Capricorn and the Antarctic Circle

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features of the Earth’s surface (both on land and below the seas and oceans) is referred to as geomorphology.

Continental slope

Figure 1.22 shows a simple cross-section of the ocean to illustrate some of the key features that shape the ocean floor. A summary of each of these features and how they form can be seen in Table 1.1.

Volcanic islands

Continental shelf

Continental slope

LE

Sea mount

Abyssal plain

P

Abyssal plain

Continental shelf

Mid-ocean ridge

Ocean trench

Figure 1.22: Geomorphology of the oceans.

Description

How feature forms

continental shelf

part of the continental plate extending from the coast/shore in shallow waters with little or no slope; generally no more than 100–200 m deep

Continental crust that is below sea level is often shallower due to sediments accumulating due to erosion from the coast.

continental slope

the steeper slope from the continental shelf to near the abyssal plain

Continental slopes occur at the edge of continental crust where this meets oceanic crust; oceanic crust is much thinner, causing steep slopes as the depth of crust and therefore depth of ocean changes.

A

M

Feature

mid-ocean ridge

underwater mountain ranges found either side of diverging tectonic plates

Sea-floor spreading causes tectonic plates to move apart. As magma rises to fill the gap this magma cools and solidifies. New rock is formed, creating ridges in the form of vast mountain ranges under the ocean.

S

Table 1.1: A summary of geomorphological features found in oceans.

KEY WORDS

geomorphology: the study of the physical features of the surface of the Earth and their relation to its geological structures and processes

continental slope: the steeper slope from the continental shelf to near the abyssal plain

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1 The Earth and its oceans

Description

How feature forms

abyssal plain

generally flat ocean floor usually at depths of 3000–6000 m

Sea-floor spreading causes tectonic plates to move apart. The peaks of ridges created by new rock gradually erode and the gaps fill with sediment to create large flat areas.

ocean trench

very long deep depressions in the ocean floor; an example is the Mariana Trench, which is the deepest point in the oceans, with a maximum depth of about 11 000 m

Ocean trenches occur at convergent plate boundaries where oceanic crust is subducted beneath continental crust, creating a deep trench along the plate boundary.

sea mount

large landform rising from the sea floor but not Volcanic activity causes lava to erupt and breaking through to the surface; often active or solidify, gradually adding more height to the extinct underwater volcanoes sea floor. This process repeats many times to create sea mounts.

volcanic islands

volcanoes that continued to erupt and so end up above sea level and create islands

LE

Feature

KEY WORDS

P

Table 1.1: Continued.

Similar to sea mounts, volcanic activity causes lava to erupt and solidify, gradually adding more height until they create islands above sea level.

abyssal plain: generally flat ocean floor usually at depths of between 3000 m and 6000 m

M

sea mount: large landforms rising from the sea floor but not reaching the surface

Questions 8

S

A

Figure 1.23 shows a photograph of Earth taken from space. a The Arctic Ocean is mostly covered in ice at the top of the image. Explain why. b Which ocean is shown in the middle of the photo?

9 Why is it difficult to describe where one ocean ends and another starts?

10 Which ocean is: a the deepest b the largest?

11 Explain the differences between a mid-ocean ridge and an ocean trench.

Figure 1.23: The Earth seen from space.

1.4 Tides and currents Tides Throughout history, humans have observed the regular rising and falling of sea level along the coastline.

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CAMBRIDGE IGCSE MARINE SCIENCE: COURSEBOOK The first record of a suggested explanation was given by the Greek astronomer Pytheas who sailed to the British Isles in about 325 bce and observed that the tides experienced there were greater than in his native Greece.

He noticed that the tides matched the movement of the Moon around the Earth, and also that the height of the tides depended on the phase of the Moon.

0 hours

EARTH

P

M

A

S

18 hours

EARTH

But why does the height of the tides change during each month? The answer is that the Sun also has a gravitational pull on the water in the oceans. Although the Sun is much larger than the Moon, the gravitational pull of the Sun on the oceans is much less than the gravitational pull from the Moon. This is due to the distance between the Sun and Earth being much greater than the distance between the Moon and Earth. The Sun therefore causes a second set of bulges in the ocean.

These are much smaller than the bulges caused by the Moon, but they increase or decrease the height of the tides as the Moon orbits the Earth during each month.

12 hours

EARTH

The bulge of water in the oceans caused by the gravitational pull of the Moon on the Earth moves around the oceans as Earth rotates once on its axis every 24 hours.

This causes the height of the sea level at any location on Earth to rise and fall as it moves into and out of these bulges. This regular rising and falling of the sea level each day is called the tides. The time between each high tide and the following low tide is actually a little over six hours due to the Moon slowly orbiting around the Earth. This adds about 25 minutes to each change in tide, giving a total of 12 hours 50 minutes between each high tide.

6 hours

EARTH

LE

We learned in Section 1.1 how gravity keeps the Earth in orbit around the Sun and also that gravity keeps the Moon in orbit around the Earth. The gravitational force of the Moon pulls water in the oceans towards the Moon. Water is a liquid and is able to flow, unlike solid land. This creates a bulge in oceans on the side of the Earth nearest to the Moon. The centrifugal force caused by the Earth spinning also creates a slightly smaller bulge in oceans on the opposite side of the Earth from the Moon. This can be seen in Figure 1.24.

TIDES OVER A 24 HOUR CYCLE

24 hours

EARTH

KEY WORDS

tides: the rising and falling of the sea caused in regular cycles, caused by the gravitational attraction between the Earth, Moon and Sun phases of the Moon: the changes in the observed shape of the Moon caused by changes in the amount of the visible surface of the Moon

Figure 1.24: How the Moon causes tides each day.

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1 The Earth and its oceans

Tidal amplitude is exactly half of the tidal range.

Spring tides are when the tidal range and tidal amplitude are greatest. Neap tides are when the tidal range and tidal amplitude are lowest.

KEY WORDS spring tides: tides experienced during a new moon and full moon, with higher high tides and lower low tides than usual

LE

When the Moon, Earth and Sun are lined up with each other in a straight line, the gravitational forces of the Moon and Sun combine to create higher high tides and lower low tides. These are called spring tides. The spring tides are greatest when the Moon is between the Sun and the Earth (around the new moon), with the gravitational pull of both the Sun and Moon acting in the same direction. When the Sun and Moon are on opposite sides of the Earth (around the full moon) the tides are still spring tides, but they are not quite as high or low.

When the Moon, Earth and Sun are at right-angles to each other, the gravitational pull of the Moon and the Sun on the Earth are in different directions.

This causes lower high tides and higher low tides.

These are called neap tides. These effects are shown in Figure 1.25.

neap tides: tides experienced during the first and last quarters of the Moon, with lower high tides and higher low tides than usual

Two important terms relating to measuring tides are tidal range and tidal amplitude. The tidal range is the difference between the height of the sea at high tide and the height at the following low tide.

tidal amplitude: half the difference in height between high tide and the following low tide

P

tidal range: difference in height between high tide and the following low tide

LUNAR TIDES

New Moon Spring Tide

Full Moon

Third Quarter

Neap Tide

Spring Tide

Neap Tide

M

SUN

First Quarter

MOON

A

Solar Tide

Lunar Tide

S

Lunar Tide

Solar Tide

Solar Tide

Lunar Tide

Lunar Tide

High tide

Low tide

Solar Tide

High tide Spring Tide

Low tide

Neap Tide

Figure 1.25: How the alignment of the Sun, Earth and Moon affect tides.

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CAMBRIDGE IGCSE MARINE SCIENCE: COURSEBOOK

ACTIVITY 1.3 Measuring tidal amplitude

Example: The next high tide on the Maldives is 0.89 m The following low tide is 0.15 m The tidal range is 0.89 m – 0.15 m = 0.74 m The tidal amplitude is 0.74 m ÷ 2 = 0.37 m

M

P

Calculate the tidal amplitude at the same location for the next two tides and compare this to the first value you calculated. Discuss why the tidal amplitude has increased or decreased.

Currents

A

An oceanic or sea current is a continuous flow of seawater in a particular direction. Currents at the surface are generally caused by the direction of prevailing winds.

For example, at the Equator the prevailing winds blow from east to west. This movement of the air above the sea surface pushes water at the surface in the same direction as the wind, from east to west. Since the winds are often blowing in the same direction for much of the time this creates currents at the surface of the ocean moving in the same direction.

As these surface currents flow towards land, the direction of the current changes to move along the coast away from the Equator. This, combined with the spinning of the Earth on its axis, creates currents which flow in opposite directions in the Northern Hemisphere compared to the Southern Hemisphere. In the Northern Hemisphere currents move in a clockwise direction.

In the Southern Hemisphere currents move in an anticlockwise direction. Figure 1.26 shows the main surface currents in the oceans.

S

Tides can also cause currents, which reverse direction with each change in tide. Tidal currents are most often seen in areas where the coastline is funnel shaped. As the sea level rises with a rising tide, the water moves up a narrowing funnel-shaped channel creating strong currents. These currents also create much higher tides further up the enclosed channel. When the sea level starts to decrease again after high tide the current reverses direction and flows out of the channel. This creates much lower tides at the narrowest part of the bay. As a result of these tidal currents the tidal range in funnel-shaped channels can be much greater than in other parts of the world. The tidal range in the Bay of Fundy in Canada can be over 16 m.

LE

Working on your own, look up some tide information for a location near to you – this information can be found by searching for ‘tide times’ on the internet. Identify the height of the next high tide and the following low tide.

Calculate the difference between these two heights – this is the tidal range. To find the tidal amplitude, divide the tidal range by two.

Surface currents circulating in the oceans form gyres in most of the oceans. These gyres, shown in Figure 1.27, circulate clockwise in the Northern Hemisphere (in the North Atlantic and North Pacific). Gyres circulate anticlockwise in the Southern Hemisphere (in the South Atlantic, South Pacific and Indian Ocean).

Some currents cause seawater to move vertically (upwards and downwards) in the ocean. These currents are caused by changes in density of the seawater (see Section 2.4). Cooler or saltier water is denser than warmer or less salty water. When seawater cools it sinks deeper into the ocean, creating downward currents.

These currents move down into the deep ocean and then flow along the ocean floor at much slower speeds than surface currents. Eventually the currents are pushed up to the surface again, often many thousands of miles from where the water went down into the deep ocean. Winds can also cause water to move to the surface from the deep ocean. This is called an upwelling (see Section 2.6).

KEY WORDS (sea or ocean) current: continuous flow of seawater in a particular direction prevailing winds: the direction winds usually blow at a particular location gyre: a large system of rotating ocean currents upwelling: the movement of cold, nutrient-rich water from deep in the ocean to the surface

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1 The Earth and its oceans

Pacific Ocean

LE

Pacific Ocean

Atlantic Ocean

P

Indian Ocean

M

Figure 1.26: Major surface currents around the world.

A

North Pacific Gyre

South Pacific Gyre

South Atlantic Gyre

S

Indian Ocean Gyre

North Atlantic Gyre

Figure 1.27: The five ocean gyres.

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CAMBRIDGE IGCSE MARINE SCIENCE: COURSEBOOK

LE

Atlantic Ocean

Pacific Ocean

Indian Ocean

P

warm surface currents

cold deep-water currents

Figure 1.28: Global conveyor belt of surface and deep ocean currents.

A

M

Many surface currents are linked to vertical currents and the slow currents moving in the deep ocean. This linkage creates a ‘global conveyor belt’ of moving water around the entire globe (Figure 1.28). The water in the global conveyor belt takes around 1000 years to complete one circulation. This global circulation, combined with the movement of water in ocean gyres, help to circulate water around the World Ocean.

Breaking waves Rip current

Beach Sandbar

Rip currents

S

A rip current is a dangerous narrow current that can form on beaches; unfortunately, many people get into difficulties in these currents each year. Usually, when waves push water against a beach, the water moves back to the ocean evenly across the width of the beach.

This does not normally cause rip currents. Sometimes sand or sediment in the sea near to the beach creates a sand bar. Waves moving water towards the beach flow over the bar easily. As water from the wave begins to flow back out to sea the water level over the sand bar falls, and water tries to get through where the sand bar is already lowest. Over time these lower sections form gaps or channels which get deeper as more water flows back to the ocean through them, as shown in Figure 1.29. This creates powerful rip currents flowing out to

Figure 1.29: Formation of rip currents.

sea from the beach, which can extend over 500 m into the ocean. Swimmers caught in rip currents are pulled far out to sea and then struggle to swim back to shore.

Any swimmer who finds themselves in a rip current should remember that rip currents are very narrow.

KEY WORDS rip current: a narrow powerful current moving from a beach out to the ocean

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1 The Earth and its oceans

If swimmers try to swim at right angles to the current, they may escape the current and be able to swim back to the shore more easily.

ACTIVITY 1.4 Danger – rip currents

Peer assessment

Rate the poster of another learner according to the following scheme for each of the points listed: if they did it really well

if they did not try to do it or did not succeed.

The poster should:

KEY WORDS

Global Positioning System (GPS): a network of artificial satellites forming a navigation and location system

P

i f they made a good attempt at it and partly succeeded

Measurements of currents involve two separate pieces of information: speed and direction. We can measure the speed of currents in several ways. The simplest involves floating an object on the surface of a current and timing the distance it moves over a period of time. This can be carried out alongside a boat with any floating object, using a compass to measure the direction the object moves in. Modern equipment often uses Global Positioning System (GPS) sensors to record speeds using locations and times. These devices may contain a compass to record the direction the sensor moves in the current, although the direction can also be determined from the GPS data.

LE

Every year many swimmers get into trouble after being caught in rip currents. Working in a pair or small group, make a poster to warn swimmers at a beach. Your poster should clearly explain what rip currents are and describe how swimmers can escape from them.

Measuring currents

identify what a rip current is

•

describe the danger to swimmers caused by rip currents

•

explain how to swim out of a rip current

•

have a clear layout that is quick and easy to understand.

12 Describe the difference between spring tides and neap tides.

13 State what is meant by an ocean current.

14 State four factors that can cause ocean currents.

15 Compare the similarities and differences between tides, ocean currents and ocean gyres.

S

A

M

•

Questions

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CAMBRIDGE IGCSE MARINE SCIENCE: COURSEBOOK

SUMMARY The Earth is a planet and the Moon is a natural satellite of Earth. Gravity keeps the Earth in orbit around the Sun and the Moon in orbit around the Earth.

The Earth is a rocky planet, with a thin solid outer crust and semi-molten mantle beneath. The Earth has a core made mostly of iron, which creates a magnetic field around the Earth.

LE

The Earth’s crust is broken into large tectonic plates that float and move around the Earth slowly. This movement is caused by convection currents in the mantle.

The movement of tectonic plates caused giant supercontinents to break up and separate over hundreds of millions of years.

Tectonic plates move against each other in different directions causing convergent, divergent and transform plate boundaries. These can cause earthquakes and create features that shape the Earth and oceans, including volcanoes, mid-ocean ridges and ocean trenches.

Tsunamis are caused by a sudden displacement of very large volumes of seawater. Tsunamis can have a devastating effect on marine life and human coastal communities.

The Earth has five distinct oceans that are interconnected to form a World Ocean. Oceans and seas contain most of the world’s water and cover more than two thirds of the planet.

P

The gravitational effects of the Moon and Sun on water in oceans and seas create tides. The tidal amplitude changes as the Moon orbits the Earth, causing spring tides and neap tides.

Oceanic currents are the continuous flow of seawater in a particular direction. These currents are caused by prevailing winds, the spinning of the Earth, tides and changes in the density of seawater.

M

Gyres are large systems of circular oceanic currents. There are five main gyres found in some of the largest oceans. Ocean currents and gyres circulate water around the World Ocean.

Rip currents can form along beaches and coastlines. These currents can be very dangerous to swimmers.

CASE STUDY PROJECT

Evidence for the theory of plate tectonics

A

Our understanding of plate tectonics has developed quite recently. We have only begun to understand how plates move around the Earth since the 1960s.

But what is the evidence to support the theory?

S

In 1912 the German geophysicist and meteorologist Alfred Wegener developed a theory called continental drift. He noticed that fossilised remains of several species were found on different continents.

He also saw that the coastlines of some continents appeared to fit together and that regions where fossils were found on each continent also matched, as shown in Figure 1.30. However, the theory of continental drift was not accepted by most scientists for nearly 50 years as there was no explanation of how the plates were able to move.

In a pair or small group, produce a five-minute presentation to show to other learners in the class showing evidence to support the theory of plate tectonics.

Try to include a range of evidence, which could include: •

what you have learned about convection currents

•

types of plate boundary and the features they create

•

where fossilised remains have been found and how this helps us to know that there used to be supercontinents in the past.

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1 The Earth and its oceans

CONTINUED

Fossilised remains of Lystrosaurus, a reptile that lived on land.

LE

AFRICA

INDIA

SOUTH AMERICA

AUSTRALIA

M

Fossilised remains of Cynognathus, a reptile that lived on land.

P

ANTARTICA

Fossilised remains of Mesosaurus, a reptile that lived in fresh water.

Fossilised remains of Glossopteris, a fern that lived on land across many of the continents.

Figure 1.30: Distribution of fossilised remains of species across continents.

A

You could even research to see if the Earth’s magnetic field has changed over time and how this provides further evidence to support the theory of plate tectonics.

You should include diagrams to support your explanations. Try to include as many relevant key words from the topic as you can.

S

Peer assessment

Give feedback to another group. Look at their evidence: •

how much evidence is included?

•

how clearly is the evidence presented?

•

have they made good use of diagrams and key words?

Provide written or verbal feedback in the form of ‘two stars and a wish’. Describe two things about the work that you liked and give one suggestion for improvement: ‘It could be even better if …’

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CAMBRIDGE IGCSE MARINE SCIENCE: COURSEBOOK

REFLECTION What do you think helps you to understand theories such as the theory of plate tectonics?

•

•

Do demonstrations and models help?

•

What about diagrams?

Discuss with a partner how using different methods of learning and reviewing information can help to improve your understanding.

LE

Has it helped you to try and explain the theory to others by creating your presentation?

EXAM-STYLE QUESTIONS

COMMAND WORD

[1]

P

1 The Moon orbits the Earth every 27.5 days. a State the name of the force that keeps the Moon in orbit around the Moon.

M

Figure 1

S

A

b Figure 1 shows the Moon at one point in its orbit around the Earth. i Describe the alignment of the Moon with the Earth and Sun causing the image shown in Figure 1.

[1] ii Explain how this alignment affects tidal amplitude.

[2] 2 An earthquake occurs at a convergent plate boundary and creates a tsunami. a Describe how an earthquake forms at a convergent plate boundary.

[3] b Describe a tsunami.

[3] c Explain how a tsunami is formed.

[2] d Describe the impacts of tsunamis on human coastal communities.

[2] 3 Figure 2 shows some features on the ocean floor.

state: express in clear terms

COMMAND WORDS describe: state the points of a topic / give characteristics and main features explain: set out purposes or reasons / make the relationships between things evident / provide why and / or how and support with relevant evidence

C

A

B Seamount

Figure 2

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1 The Earth and its oceans

CONTINUED

COMMAND WORD

SELF-EVALUATION CHECKLIST

Needs more work

I can

[3] [3]

identify: name / select / recognise

[5]

LE

a Identify features A, B and C. b Describe how the sea mount shown may one day form an island.

4 The Atlantic Ocean contains two ocean gyres. Describe how oceanic currents form to create ocean gyres.

Getting there

Confident to move on

See Section 1.1

state the force that keeps Earth and the Moon in their orbits

1.1

describe the layers and types of material that make up the structure of the Earth

1.1

state why Earth has a magnetic field

1.1

P

describe the Earth relative to the Sun and the Moon relative to the Earth

1.2

describe the three types of plate boundary

1.2

explain how plate boundaries give rise to earthquakes and volcanoes

1.2

describe how tsunamis form and outline their effects on marine ecosystems and human coastal communities

1.2

identify and describe the location of Earth’s oceans

1.3

describe the extent of the oceans and explain how they are interconnected

1.3

describe seas in comparison to oceans

1.3

describe and identify the geomorphology of the oceans

1.3

outline how the gravitational effects of the Sun and Moon cause tides

1.4

describe how to measure tidal amplitude

1.4

describe ocean currents, and how to measure their speed and direction

1.4

state reasons why ocean currents form

1.4

describe gyres and identify the locations of the five main oceanic gyres

1.4

state how water circulates in the World Ocean

1.4

describe how rip currents form and why they are dangerous to swimmers

1.4

S

A

M

explain what tectonic plates are, and outline tectonic plate theory and how supercontinents once existed

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