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The fragilecrust
C H A P T E R 5
Stimulus questions1 Australia doesn’t get many earthquakes but New Zealand does. Why?2 Los Angeles and San Francisco are 1200 km apart, yet could be
suburbs of each other some day. How?3 Can volcanoes move about? If so, how?4 What is the biggest mountain on Earth and why is it in the Pacific
Ocean?5 What are plastic rocks?6 Seashells have been found near the top of the Himalayas. How did
they get there?
OutcomesEarth and space
sciences
6.1Explain the implications of crustalmovements of the Earth.
6.2Describe the extraction, processingand use of geological resourcesand associated environmental andsocial issues.
SCI3 100
Fig 5.1.3 The Appalachian mountains in North America are very similar tothose in northen Europe. Other rocks are similar, too.
If you look at a map of the world, it appearsthat some coastlines could fit neatlytogether. Francis Bacon first noticed it in1620: the eastern coast of America had justbeen mapped and seemed to fitthe coasts of Africa and Europelike a jigsaw.
Continents thatmove!n
In 1915 Alfred Wegener proposed a theorythat there was once a supercontinentcalled Pangaea, which split to form thecontinents. These then drifted into theircurrent positions.
Some pieces of evidence he used were:• Two hundred million year old fossils of
the same fern-like plant and Triassicreptiles were found across the southerncontinents. The reptiles could not swimthe distance required and ferns can’t
Unit 5.1 Plate tectonics
Fig 5.1.1 Wegener had lots of evidence to support his ideas aboutdrifting continents. Geologists did not take his work seriously,however, since Wegener was not a geologist but an astronomerand meteorologist.
swim. They all seem to have started on a singlecontinent, then were taken around the world when itsplit and the pieces drifted apart.
• The rock and formations of mountains in the easternUSA are similar to those of mountains acrossnorthern Europe. Likewise, Africa seems to be
Fig 5.1.2 The migration of Triassic reptiles could have happened only if the continentswere joined.The spores (seeds) of ancient ferns could not survive in the sea.
Australia
Antarctica
IndiaAfrica
South America
Mesosaurus
Glossopteris
LystrosaurusCynognathus
NorthAmerica
Atlantic ocean Europe
Iceland
Greenland
101 Chapter 5 The fragile crust
U N I T 5 . 1matched with South America.
• Ancient glaciers have leftvalleys and debris acrossmany continents, includingsome now too warm toproduce glaciers. Thesecontinents seem to havemoved from a colderclimate.
• Coal has been found abovethe Arctic Circle. Coalcomes from decomposedplants and it is far too coldfor plants to grow there now.These regions seem to haveshifted from one with awarmer climate whereplants could grow.
• As lava from a volcanocools, it adopts themagnetism of the Earth atthat moment. Threehundred million year oldmagnetic rocks in South and North America havebeen found with their north poles pointed indifferent directions. The continents seem tohave shifted so that these mini-magnetspoint in mismatched directions. If thecontinents are put together however, theyall point in the same direction.
It was later suggested that Pangaea split first into twosmaller supercontinents, Gondwana (comprisingAustralia, Antarctica, South America, Africa and India)and Laurasia (North America, Europe and the most ofAsia) before breaking again. The idea didn’t catch on atthe time however, since it was generally thought thatthe Earth was solid rock. How could continents moveacross solid rock, and what pushed them?
Evidence from below!nDuring World War II, the military needed accuratemaps of the seabed and they also needed to findunderwater reserves of fossil fuels to assist the wareffort. Using the newly developed technology of sonar,some surprising results were found:• Huge underwater volcanic mountain ranges run
down the centre of most oceans, the longest beingthe Mid-Atlantic Ridge with a length of 65 000 km.
• The ages of the rocks of the ocean floor vary frombrand new to 200 million years old, far youngerthan the rock of the continents.
• Incredibly deep ocean trenches exist, the deepestbeing over 11 km deep.
• The rock of the continents is less dense than that ofthe ocean floor and seems to ‘float’ on it.
• The rocks of the ocean floor have magnetic ‘stripes’,parallel with the underwater ridges. The magnetic
The manfrom Snowy
River‘He hails from Snowy River,up by Kosciusko’s side,where the hills are twice assteep and twice as rough.’The Snowy River valley inVictoria/New South Waleswas partly formed by a15 km long glacier. Had the‘man’ lived in the Ice Agehe would have needed asnowboard instead of ahorse! The closest we getto glaciers in Australia noware patches of snow onMt Twynam in NSW. Thesepatches are 30 m deep andare heavy enough tocompact to a density ofabout 80% that of ice.
Sciencesnippet
Fig 5.1.4 Ancient magnetic rocks have their north poles scattered.If the continents are joined, the poles all point to the same spot.
Prac 1p. 104
Dinosaurs near Apollo Bay!
About 100 million years ago, when they were part of
Gondwana, Australia was separated from Antarctica
only by a wide and deep rift valley. Victoria was much
colder than it is now, and would have had 24 hours of
darkness each day in winter. At that time, a small
dinosaur called Leaellynasaura roamed in the area of
the Otway Ranges, eating insects and plants. It was
very small, its skull only 5 cm long. It had larger than
normal eye sockets, however, suggesting it could see
well in the dark. A specimen on display at the
Melbourne Museum has one bad leg, 1 cm shorter than
the other: this dino limped! Other Victorian dinosaur
fossils exist from around this time: Inverloch and San
Remo had a 5 m long ‘cool crocodile’, Koolasuchas.
Science snippet
common pole(roughly where Hawaiiis now)
N
N
N
N
field of the Earth has changed many times in itshistory, with the North Pole becoming South andvice versa. The stripes show this reversal and indicatethat the youngest rock is next to the ridges and theoldest next to the trenches.All this evidence suggests that the rock under the
ocean floor is molten and is emerging along the mid-ocean ridges. The molten rock cools as it hits the water,building new mountains and pushing the old onesaside. The ocean floor acts like a conveyer belt, carryingeverything towards the trenches.
Plates floatingnon soupn
You should remember from SCI 1 that the Earth ismade of layers. We live on the crust, which varies inthickness from about 8 km under the ocean to amaximum of 64 km under the continents.
Next is the 2800 km thick mantle. The rock here isunder extreme heat and pressure and behaves like asludgy slow-moving liquid. The crust above keeps theheat in and patches of mantle are heated further by thedecay of radioactive elements like uranium, thorium andpotassium.
Hot air and liquids rise; rock over a hot patch will,too. Likewise, ‘cool’ rock will drop. This cycle of movingrock is called a convection current and can takemillions of years to complete. Slabs of the crust float onthese convection currents, like pieces of toast floating onvery thick, hot soup. The toast moves if the soup isstirred. The bottom of the toast also gets a little soggy.The slabs of crust, soggy bottom and top layer of themantle move as one and are called tectonic plates. Onthe plates sit our continents. If the plates move, then sodo they. The idea of moving plates is called the Theoryof Plate Tectonics and was first developed in 1962 byAmerican Harry Hess.
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Fig 5.1.6 The ocean floor is a conveyor belt dragging new rock from mid-ocean and into the trenches.
Fig 5.1.7 The inner structure of the Earth (not to scale)
Fig 5.1.5 Magnetic stripes show that rocks ofthe ocean floor varies in age.
Prac 3p. 106
Prac 2p. 105
Key:
rocks have
rocks have
platemovement
Antarctica
Australia
TasmanSea
N
Noldest ocean rock
oldest ocean rock
new rock
new rockmid-ocean ridge
continentalplate
continentalcrust
sedimentoceantrench
direction ofmovementof magma
mantle
oceaniccrust
risingmagma
risingmagma
volcanic activity(island arc)
melting
oceanicridge
crust
6400 km deep
inner core
outer core
mantle
2800 km
8–64 kmdeep
5100 km
103 Chapter 5 The fragile crust
U N I T 5 . 1
1 Copy the following, correcting any incorrect statementsso they become true:a Triassic reptiles could have swum the distances
required to populate different continents.b There are similar mountain ranges in USA and
Africa, and also in Europe and South America.c Continents that do not have glaciers now have
always been too warm to have them.d Coal deposits above the Arctic Circle suggest the
land has floated there from warmer climates.e The rock of the ocean floor and that of the continents
are the same age.f Continental rock is denser than the rock of the ocean
floor.g Magnetic stripes on the ocean floor suggest that new
rock is made along mid-ocean ridges.2 What six pieces of evidence suggest that the continents
were once joined?
3 What are the following?a Tectonic plateb Mantlec Crust
4 How thick is the crust?5 What five surprising facts were found when the ocean
floor was first mapped?6 What landmasses are thought to have made up:
a Gondwana?b Laurasia?
7 The temperatures along the ridges are higher thanelsewhere in the ocean. Suggest why.
8 Where are the oldest and youngest rocks on the oceanfloor?
9 What causes convection currents?10 How long do convection currents take to cycle around
the mantle?11 Draw the probable convection currents in an oven.12 Why does smoke rise up a chimney?13 What is the Theory of Plate Tectonics?14 What keeps the mantle from cooling down and
becoming solid?
U n i t 5 . 1 Questions
Fig 5.1.8 Hot patches and cool patches create the convection currents that push the plates around.
high low
ridge
convection currentsconvection currents
hot plume
hot spot
Red Sea
African continent India
Mantle temperature
15 If the mantle cooled and became solid, what wouldhappen to the plates?
16 Australia once had glaciers but doesn’t have any now.Plate tectonics explain why but what could be anotherpossible reason?
17 The plate on which Australia sits is moving northward atabout 5 cm per year. How far will it move in anaverage lifetime?
1 Find out what these people did in developing thetheories of continental drift and plate tectonics:Abraham OrteliusAntonio Snider-PellegriniAlfred WegenerEduard SeussFrank TaylorAlexander Du ToitArthur Holmes
2 Another theory about Earth is that it was once a lotsmaller. It expanded, the continents cracked andseparated. Find out more about this theory and theworks of Hilgenberg, Carey and Owen.
3 What is a continental shelf? Draw a map of Australiaand its shelf.
4 Explore www.pbs.org/wgbh/nova/ice/continents tofind ways scientists measure the shifts of continents.
5 Find out about Pangaea and its parent supercontinentRodini at www.scotese.com.
6 What is sonar and how does it measure depth?
Unit 5.1 Prac 1The planet Splatter
Splattonians think that the continents on their planet moveand were once the supercontinent Squidgewana. Evidence
comes from shape, magnetic fields and fossil remains of thegolden splattered slug and the squidgian tinea fern.
You will needA4 photocopy of the map in Fig 5.1.9.
SCI3 104
U n i t 5 . 1 Research /Extension
U n i t 5 . 1 Practicalactivities
Key
N direction of north poleof ancient magnetic rocks
fossil remains of goldensplatted slug
fossil remains ofsquidgian tinea fern
N
N
N
N
N
Map ofthePlanet Splatter
Fig 5.1.9 Map of the planet Splatter
WS 5.1
105 Chapter 5 The fragile crust
U N I T 5 . 1What to do1 Cut out the continents and arrange them to rebuild
Squidgewana.2 Stick the map in your workbook.
Questions1 Explain why each piece of evidence
suggests that a supercontinent once existed.2 Are there any other ways the supercontinent
could be arranged?3 Design your own planet. Include at least five
continents that once formed either one ortwo supercontinents.
Unit 5.1 Prac 2Convection currents
You will needLarge (500 mL or 1000 mL) beaker, potassiumpermanganate, tweezers, plastic bag, ice, safetyglasses, hot plate or Bunsen burner, bench mat,tripod and gauze mat
What to doPart A Hot convection1 Three-quarter fill the beaker with cold water.2 With the tweezers drop a single crystal into the centre of
the beaker.3 Gently heat the beaker on the hot plate or over the
Bunsen burner.4 Carefully observe and draw the motion of the purple
stain.Part B Cold convection1 Three-quarter fill the beaker with cold water.2 Put some ice in the plastic bag.3 Use the tweezers to hold a crystal at the top of the
beaker, just below the bag of ice.4 Draw what happens.
Questions1 What are convection currents caused by?2 What does a ‘hot’ current do?3 Which direction does a ‘cold’ current go?
beaker of cold water
Part Abeaker
hot plate
crystal potassium permanganate
Part B
plastic bag of ice
tweezers to
hold crystal
Fig 5.1.10 Hot and cold convection currents
Unit 5.1 Prac 3Future Earth
You will needMap of tectonic plates, A4 sheet of paper, 6 overheadtransparencies, 1 sheet of tracing paper, overheadtransparency pen or marker, sticky tape
What to do1 There are seven main tectonic plates. Trace the African
plate and Africa onto a piece of A4 paper and tape itto the desk. Trace the other plates and continents ontooverhead transparencies, one plate per sheet.
2 On each sheet draw a large arrow pointing in thedirection the plate is moving.
3 Lay all the sheets on top of each other to reproduce thecurrent map of the world.
4 Move each transparency 1 cm in the directions of thearrows.
5 Trace the new arrangement of the continents. Transferthe diagram to your workbook.
6 Move the transparencies 1 cm more and re-trace.7 Do this three more times, so that you have a series of
‘maps’.8 Mountain chains will form when a continent hits another.
Mark them and give them a name.9 Name any new seas or oceans formed.
Questions1 In this prac each plate moved at the same speed. Is this
so in reality?2 What is the distant future of these bodies of water?
a The Mediterranean Seab Atlantic Oceanc Red Sea
3 Try to locate the positions of these cities in the distantfuture:a Darwinb Tokyoc Hong Kongd Los Angelese Rome
4 What would you expect Victoria’s climate to be like inthe future?
5 Compare your maps with those atwww.scotese.com/earth.htm
SCI3 106
overhead transparencieswith plates, continents
shift the sheets 1 cm in the directions of the arrows
A4 sheet with tracing of Africa
Fig 5.1.11 Predicting a new world
107 Chapter 5 The fragile crust
Tectonic plates shift at about the rate at whichour fingernails grow. Action at their edges(called boundaries) creates the basic landscapeof our world and causes earthquakes.There are three types of plate boundaries:1 Spreading boundaries are where plates move apart.
They are also known as constructive boundaries sincenew rock is being made on the ocean floor.
2 Collision boundaries are where one plate collides withanother. These are destructive boundaries since rock ismelted here and is returned to the mantle for recycling.
3 Transform or scraping boundaries are where platesscrape along each other. They are conservativeboundaries since they conserve rock. They do not createnor destroy it.
A broken scab:nspreading platesn
Mapping of the ocean floor shows that some platesare moving apart at a rate of up to 20 cm a year. Aweakened line (called a fault) in the crust causes ahuge crack or rift valley to form and hot liquidmagma forces its way up from the mantle to fill it.The magma cools and solidifies as it hits the water,and creates mountain ridges like those observed inthe Atlantic and Pacific Oceans. As more magmasqueezes out, the older rocks crack and aresqueezed out of the way. New magma then fills thecrack and the process repeats itself. It’s like awound: a scab begins to repair the wound but anystress cracks it, allowing blood to ooze again. Thescab then needs to reform.
Unit 5.2 At the edges
Philippineplate
Pacific plate
Caribbeanplate
Nazca plate
North American plateEurasian plate
Cocosplate
SouthAmerican
plate
African plate
Indian–Australianplate
main movement directionsmovement not knowntransform boundaries
collision boundariesspreading boundaries
N
S
EW
Fig 5.2.1 Tectonic plates and their boundaries: split, bang and scrape
Most rift valleys are underwater but a few are on thesurface. The largest is the East African Rift Valley,which is filled in part with lakes (e.g. Lake Victoria) andin other parts with huge volcanoes like Mt Kilimanjaro.Other rift valleys pass through the Dead Sea (its surfaceis 400 m below sea level, making it the lowest point onEarth not under an ocean) and the Sea of Galilee (209 m below sea level). This rift valley continues intothe Red Sea, indicating that it will widen and becomean ocean in the future. Another rift valley is graduallysplitting Iceland in two.
SCI3 108
Fire and ice!Although a relatively small island, Iceland produces more than one-fifth of the total lava output from all volcanoes around the Earth!
Science snippet
Fig 5.2.3 The Helgafjell volcano erupting in 1973
Prac 1p. 111
Fig 5.2.2 The Sea of Galilee, Dead Sea and Gulf of Aqaba all lie in a giant rift valley that joins with the Red Sea.
Sea of Galilee
Dead Sea
The Red Sea
109 Chapter 5 The fragile crust
U N I T 5 . 2
Going under:nsubduction zonesn
Ocean plates don’t just separate but cancollide too, with other ocean plates or withcontinental plates. Continental plates can alsohit other continental plates. Each collisioncreates something different.
The rock of the ocean plates is denser thanthe plates that the continents sit on. If they hit,the heavier ocean plate is forced under thecontinental plate at an angle of 20° to 60° tothe surface. This angled dive is called asubduction zone. Meanwhile the upper plategets crushed, thickens and forms foldedmountains along its edge.
The two plates do not slide easily over eachother due to friction between them. When theydo slip, it’s sudden and an earthquake results.
Friction also generates heat, which melts therock as it submerges. By the time the oceanplate has reached a depth of 200 km it hasmelted away and become part of the mantle once more.
Some water gets in through the fault and becomessteam. The steam is carried down into the subductionzone and makes the rock very gassy and light. The gas-filled molten rock (magma) will try to force its way backto the surface, perhaps to burst out as a ridge ofvolcanoes. The Andes Mountains in South Americawere formed in this way.
Parallel to them is the Peru–Chile trench. Ocean trenches form where theocean plate drops below thecontinental plate. Although someof these trenches are filled withsediment, many are incrediblydeep.
Going up:nisland chainsn
Densities are the same if ocean plate hitsocean plate: here the fastest plate sinks inthe collision.
Once again a subduction zone iscreated. The upper plate gets thicker andvolcanoes form, some of which push out ofthe water to form islands and islandchains. Examples are the islands of Japan,Indonesia, the Philippines, the Caribbeanand the Aleutians.
Really big mountainsnThe Himalayas are the tallest mountains on Earth withMt Everest being the biggest at 8850 m. They formedwhen the plate that carries India collided head-on withthe plate that carries the bulk of Asia. Both plates havesimilar densities and neither can push the otherunderneath. Instead the plates crumple and fold.
Intense heat from the collision also meltssome rock and forms a solid ‘mountain root’that resists weathering.
Fig 5.2.4 Volcanoes and trenches come from collisions between an ocean plate and acontinental plate.
Really deep!The Mariana Trench in
the western Pacific was
discovered in 1951 by
the British survey ship
Challenger. With a
depth of 11 033 m it is
more than six times
deeper than the Grand
Canyon in the USA. Mt
Everest could easily sit
in it, leaving plenty of
room for Mt Kosciusko
to fit in as well! In
January 1960 DrJacques Piccard of
Switzerland and Lt
Donald Walsh of the
USA took the US Navy
bathyscaphe Trieste to a
depth of 10 915 m in
the trench.
Sciencesnippet
Prac 2p. 112
Climb Mt Everest now!No dinosaurs ever climbed Mt Everest since it did not exist whenthey did. The collision of plates is still happening and is buildingthe Himalayas at about 1 cm a year. Current mountaineers nowneed to climb half a metre more than the first successful climbers,Sir Edmund Hillary and Tenzing Norgay, in 1953. Every year thepeak gets taller, so don’t wait: do it now!
Science snippet
earthquakessubduction
zonecontinental
plate
ocean plate
volcanoocean trench
magma
WS 5.2
Fig 5.2.7 The San Andreas Fault has nearly separated Point Reyes fromthe rest of California. It is an example of a transform fault.
Plates that scrapenPlates scrape along each other along a transformboundary. These don’t make mountains or volcanoesbut do produce lots of earthquakes, some very strong.Although most of these boundaries are underwater,some are on land. The most important of these is theSan Andreas fault which runs 1300 km throughCalifornia USA, directly under San Francisco andclose to Los Angeles (LA). The coastline ofCalifornia slips 5 cm along it every year, moving LAnorth and closer to San Francisco.
SCI3 110
Fig 5.2.5 When ocean plate meets ocean plate, a chain of islands forms.
Fig 5.2.6 Massive folded mountains form when continent collides with continent.
1 What are the three types of plate boundaries?2 Which is more likely to sink when they hit each
other:a the fast or slow plate?b the heavy or light one?c the continental plate or the ocean plate?
3 Which boundaries:a are conservative?b are destructive?c are constructive?d have subduction zones?e form rift valleys?f dive into the mantle?g cause trenches?h cause huge folded mountains?i have only sideways movement?j form island chains?k form mountains?
4 On what type of boundaries are these placessituated?a Icelandb The San Andreas faultc Mt Everestd Mt Kilimanjaroe Lake Victoriaf The Dead Sea
5 Draw or trace a labelled diagram of thefollowing:a Spreading plate boundariesb Ocean plate meets continental platec Ocean plate meets ocean plate
U n i t 5 . 2 Questions
island arc
ocean plate
ocean trench
subductionzone
active volcano
oceanplate
melting
earthquakes
rising magma
continental plate
mountains
continental plate
111 Chapter 5 The fragile crust
U N I T 5 . 24 On a map find the island chains that make up Japan,
Indonesia, the Philippines, the Caribbean and theAleutians.
5 Explore www.scotese.com/newspage13.htm orwww.pbs.org/wgbh/nova/everest/earth/birth.html tofind animations of colliding plates.
6 ‘Black smokers’ are found on mid-ocean trenches. Whatare they?
Unit 5.2 Prac 1Plates that separate
You will needA3 sheet of paper, coloured pencils or highlighters, scissors,sticky tape, pegs
What to do1 Push two desks together.2 Cut the sheet of paper lengthwise and tape the pieces
together to make a long strip.3 Fold and push both free ends of the paper through the
gap between the desks.4 Attach a peg to the leading edge of each sheet of paper
as it emerges from the gap.
d Continental plate meets continental platee Transform plate boundaries
6 Draw four-frame cartoons to show the development of the:a Himalayasb Indonesian islandsc Mid-Pacific Ridged Andes
7 Name the two plates that created the:a Himalayasb Andesc Mid-Atlantic Ridged Caribbean islandse Japanf Mariana Trenchg San Andreas faulth Dead Sea
8 How is the mid-ocean ridge like a scab?9 What happens in a subduction zone?
10 A plate often gets thicker when another plate is forcedunder it. Explain why.
11 The Himalayas have a ‘mountain root’. What does thismean and how did it form?
12 How far will the Himalayas grow in an average lifetime?13 How long will it take for them to grow a further:
a 10 m?b 100 m?c 1 km?
14 Currently the Red Sea is about 240 km wide andwidening at about 20 cm per year. Calculate the time itwill take for it to become the same width as the:a Mediterranean Sea (about 500 km)b Atlantic Ocean (6100 km)c Pacific Ocean (14 000 km)
1 Locate the East African Rift and the geological featuresthat lie on it on a map.
2 Find the Jordan Rift Valley on a map. What lies along it?What connects it with the East African Rift?
3 Find information about Iceland, its eruptions andearthquakes, and how Icelanders make use of its volcanicnature.
U n i t 5 . 2 Research /Extension
U n i t 5 . 2 Practicalactivities
C R E A T I V E W R I T I N GTell them where to stick it!
A new type of power station has beendeveloped: only ten are needed to supply allthe electricity needs for the whole world! Butthey are safe only if they are not disturbed. TheUnited Nations will build them but it needsyour advice about where to put them. No morethan two can be located on any one continent,and they must be close to large populationcentres. You must be convincing sinceCalifornia, Japan, Iceland, Indonesia and NewZealand all want them. Give reasons for yourten choices.
SCI3 112
5 As each 5 cm emerges, colour or decorate each newstrip of paper.
Questions1 How does this activity relate to the spreading at the mid-
ocean rifts?2 What is each of the following represented by in your
model?a Ocean floor or plateb The waterc The lava flowd Gravitye Mid-ocean riftf The magnetic strips found in rocks parallel to the
mid-ocean ridges3 What did you notice about the height of the paper
as it emerged from the gap compared to thepaper further out?
4 Which of the strips that you coloured would bethe ‘oldest’ rock and the ‘youngest’ rock?
5 Which of these strips would be the first to be‘swallowed’ by an ocean trench?
Unit 5.2 Prac 2Colliding plates
You will needA stack of about 30 A4 pages (recycle: use scrappaper!), textbook
What to do1 Split the stack of A4 paper into two smaller
stacks of about fifteen sheets each.2 Place each on the desk and slide them slowly
into one another.3 Observe what happens to the layers as they
collide. Repeat four times to confirm yourobservations. Sketch what usually happened.
4 Now hold the end of one stack and hold it still. Push the otherstack into it. Observe which layer climbs on top of the other.Repeat for confirmation and draw what happened.
5 Finally, place a textbook on the desk and push a stack of thirtysheets into it. Observe which goes under.
Questions1 How are the above tests similar to plate tectonics?2 The stack of paper had obvious layers. Does rock have layers
and if so how do they form?3 Which tests simulated the following collisions?
a A continental plate with another continental plateb An ocean plate with another ocean platec An ocean plate with a continental plate
4 Give an example of where on Earth each of these collisionstakes place.
colour each 5 cm stripas it emerges
push paper up
pegpeg
tape
Fig 5.2.8 Modelling spreading plates.
Test 1
keep this stack still
Test 3
Test 2
Fig 5.2.9 Modelling plate collisions
113 Chapter 5 The fragile crust
Fig 5.3.1 The focus is where the earthquake starts.Seismic waves spread from here to theepicentre and beyond.
Tectonic plates separate, collide and scrapeover each other. None of this movement issmooth since the plates must build pressureto overcome the incredible friction forcesthat are causing them to ‘stick’. When theydo slip, movement is sudden, explosive andoften catastrophic. The plates are constantlytrying to move, so the release of pressure isfrequent—more than one millionearthquakes occur per year, although mostare so small or so remote that they are notnoticed and cause little damage.
Focus and epicentrenThe focus of an earthquake is the point where it begins.It is where the plates slip and is always on a fault line atthe plate edges. A focus can be shallow or deep butdeeper quakes are not necessarily stronger than shallowones—some of the most devastating are shallow onesthat occur in transform boundaries. Collision
Unit 5.3 Earthquakes
Fig 5.3.2 A typical seismograph, showing P, S and surface waves
boundaries make both shallowand deep quakes. Quakeshappen along this subductionline anywhere from the surfaceto where the ocean plate hascompletely melted away (about200 km below the surface).
The epicentre is the pointon the Earth’s surface that isdirectly above the focus andwill suffer the most damage.
Seismic wavesnEarthquakes generate different types of waves (calledseismic waves) that roll through and around Earth. Aninstrument called a seismometer detects all these waves.Each wave produces a characteristic squiggle on thegraph (a seismograph) that the seismometer makes.
Seismic waves can be split into two categories:1 Body waves travel through the body of Earth which
can be either primary (P) or secondary (S)2 Surface waves travel around the globe on its surface
and are either Rayleigh (R) or Love, sometimescalled long (L)
Fleas causeearthquakes!In the earthquake-prone
Kamchatka peninsula in
Siberia, an old tale has
the god Tuli riding with
the Earth on a sled being
pulled by flea-ridden dogs.
When they stop and
scratch, the Earth shakes!
Sciencesnippet
epicentre R and L waves(surface waves)
P and S waves (body waves)
focus
surfaceof earth
P wavesbeginarriving
time P and S and surface waves
surfacewavesarrive
P and Swaves S waves
arrive
WS 5.3
Body wavesnPrimary waves move fastest and are the first to berecorded. They are an example of longitudinal orcompression waves that push and pull and do notbehave in the up-down motion of water waves that weare used to. Sound is another example of a longitudinalwave.
P waves can travel through both solid and liquidrock. They hit the surface with an up-and-down or push-pull motion.
S waves are slower than P and are thenext to be recorded. They are transversewaves and have an up-down movement justlike water waves. S waves travel only throughsolid rock. Molten rock blocks them. Swaves hit the surface with a shaking or side-to-sidemotion.
Earthquake shadowsnThe speeds of P and S waves depend on what they aretravelling through. The denser the rock, the faster theygo. The waves change speed as they pass into rock ofdifferent density, and also change direction. This iscalled refraction and happens to all types of waves as
they change speed on passing from one material toanother. Water waves do it and so do light and soundwaves.
When a quake occurs seismometers around theworld record all the waves that reach them. When all ofthe seismographs are analysed, a pattern showing thespread of waves is produced.
Note that in Fig 5.3.4:• There is a ‘shadow’ directly opposite the epicentre
of a quake where S waves do not go. Theouter core is blocking the S waves. Why?It must be liquid rock since S cannottravel in liquid. It must also be very hot.
• The paths of S and P waves are notstraight. They are bent (refracted): therock changes in density and temperature.
SCI3 114
Fig 5.3.3 Body waves: P and S
Prac 1p. 117 Fig 5.3.4 Refraction of P and S waves and ‘shadows’ where they do not arrive
Deliberately creating quakesGeologists searching for minerals and fossil fuels oftencreate ‘mini-quakes’ with explosives. A seismographshows the shock waves produced and gives informationon thickness, density and position of different rocklayers. This will hopefully pinpoint materials worthmining.
Science snippet
A transverse wave
The Earth’ssurface is shaken.
rockvibration
movementof secondarywave
movementof
primarywave
A longitudinal or compression wave
The Earth’s surface ispushed and pulled.
rockvibration
both P and Swaves arrive
both P and Swaves arrive
epicentreof quake
shadow zone—no P or S waves
P waves only—no S waves
shadow zone—no P or S waves
Fig 5.3.7 Rayleigh and Love waves are very dangerous and travel acrossthe surface.
Fig 5.3.6 Locating the epicentre: Newcastle is hit.
Fig 5.3.5 The different arrival times of P and S waves tell us how faraway the quake is.
8000
7000
6000
5000
4000
3000
2000
1000
00 2 4 6 8 10Difference in arrival times of P and S waves (minutes)D
ista
nce
from
ear
thqu
ake
(kilo
met
res)
8.6 minutes
Melbourne0 500 km
Adelaide
Newcastle
Sydney
Brisbane
10:32:09 10:34:15Adelaide
P S10:31:45 10:32:09
Melbourne
Brisbane
10:32:16
10:31:06
h min s
Rayleigh wave
Love wave
115 Chapter 5 The fragile crust
U N I T 5 . 3
Finding the epicentrenP and S waves arrive at seismometers at different times.Their speeds are well known and the distance from theepicentre can be calculated from the difference in timebetween their arrivals.
If the S wave arrives 8.6 minutes after theP wave, the graph in Fig 5.3.5 indicates thatthe epicentre is 7000 km away.
For an exact location a reading needs to betaken in three different places. Distances from eachplace to the epicentre can be calculated and circlesdrawn around them to show where the quake could be.The epicentre is where the three circles intersect.
Surface wavesnRayleigh and Love waves travel around the Earth andnot through it. They have further to go than P and Swaves and arrive after them. They are more dangerousthan P and S waves however, since their effect on thesurface is more severe.
A cheesy moon?
An old tale has the moon made from green cheese. Moon rock
brought back from the Apollo missions of the 1970s looks like rock
but it does have some cheese-like properties! Speeds of seismic
waves in moon rock range from 1.2 km/s to 1.84 km/s, comparing
well with those found in cheese: Muenster cheese has the lowest
seismic speed at 1.57 km/s and Swiss cheese has the highest
at 2.12 km/s. In contrast Earth rocks have speeds of 4.9 km/s to
5.9 km/s.
Science snippetPrac 2p. 118
1 Why does friction exist at plates boundaries?2 Use friction to explain what causes an earthquake.3 Australia has few earthquakes, yet our neighbours
Papua New Guinea and New Zealand have lots. Usethe Theory of Plate Tectonics to explain why.
4 What is the difference between:a body and surface waves?b the focus and the epicentre of a quake?c a longitudinal and a transverse wave?
5 Give examples of a:a longitudinal waveb transverse wave
6 Sketch what happens to a slinky spring if the followingwaves move down it:a Longitudinal waveb Transverse wave
7 Which of the seismic waves, P, S, R or L:a are the most dangerous?b are up-down waves?c are compression waves?d pass through the earth?e are the fastest?f are the last to arrive?g are like surf?h are often only 1 cm high?i cannot travel through liquid?
8 Draw how L waves can crush objects on the surface.9 What is refraction?
10 What causes refraction of P and S waves?11 What evidence exists that the Earth’s core is liquid?12 What is the deepest the focus can be below the surface?13 What distance is the epicentre away if the time between
the P and S waves arriving is:a 4.0 minutes?b 2.2 minutes?
c 3.5 minutes?d 8.1 minutes?
14 Where would you need to be for the P and S waves toarrive at the same time?
15 What does a single seismograph tell us about a quake?16 How long would it be between the arrival of the P and
the S waves if the epicentre is this far away?a 6000 kmb 1500 kmc 3300 kmd 900 km
17 Use a diagram to explain how S waves change an up-down movement in the rock into a side-to-side movementof the surface.
18 Information about a number of quakes is given in thetable below. Complete it. To calculate column 4 convertthe seconds into a decimal by dividing by 60.
SCI3 116
U n i t 5 . 3 Questions
R waves are rolling waves, like surf at a beach. Theyare normally small (about 1 cm) but were as violent asheavy breakers in Assam in India in 1897.
L waves arrive last. They are up-down waves, just likewater waves but cannot travel through water—an ocean
will stop them. When R and L waves form a crest, thesurface may crack, swallowing anything on top. Whenthe same spot becomes a trough a few moments later,the crack is squeezed shut, crushing whatever fell in.
Arrival time Arrival time Time Time Distance ofof P waves of S waves difference (min) epicentre (km)(h:min:s) (h:min:s) (min:s)
10:24:00 10:32:00
04:48:20 04:52:50 4:30 30 ÷ 60 = 0.5so time is 4.5 min
2:55:21 p.m. 3:01:21 p.m.
7:37:03 p.m. 7.42:33 p.m.
14:08:34 14:11:46
20:21:02 20:25:50
05:45:10 5.0
11:34:30 6:30
08:12:56 2500
15:21:04 5800
WS 5.4
117 Chapter 5 The fragile crust
U N I T 5 . 3
1 The most devastating earthquake in recent times inAustralia was in Newcastle, New South Wales in 1989.Find out about it: strength, damage and injury/deathtoll.
2 Japanese traditional architecture is designed towithstand frequent earthquakes. Find out why it is idealfor an earthquake-prone country.
3 How do engineers and architects design buildings towithstand earthquakes?
4 Make a list of the damage expected at each level of the0 to 10 Richter scale.
5 Find out what you should do if an earthquake hits.6 For great pictures of earthquake zones explore
www.seismo.unr.edu/ftp/pub/louie/class/100/plate-tectonics.html
Unit 5.3 Prac 1Slinky springs
You will needSlinky spring, dense smaller diameter slinky, paper, sticky tape
What to do1 Tape a small sheet of paper onto a slinky. This will
represent the surface of the Earth.Part A Transverse waves1 Lay the slinky along the floor. Hold the spring at both
ends, stretching it lightly. One person should hold theirend still at all times.
2 Move one end sideways so that a ripple-like wavemoves down the spring.
3 Draw a diagram showing the movement of the paperand which direction it moves in.
4 Test bigger and smaller waves. Compare their speedsand ‘height’ or amplitude.
Part B Longitudinal waves1 Quickly move one end of the spring in and out about 30
cm. A compression should move down the spring. Notewhat happened to the paper.
Part C Waves in different densities1 Attach the denser, thinner slinky to the less dense,
narrower one and repeat the above experiments. Drawyour observations on speed and size when the wavetravels from big to small diameters and vice versa.
2 Repeat, but attach a piece of string instead of thesmaller diameter spring.
Questions1 Copy and complete:
a The paper moves __________ in an transverse waveand __________ in a longitudinal wave.
b The speed of a wave __________ and the size of awave __________ as it enters material of a differentdensity.
2 Describe what an ant would feel if it was standing onthe paper in part A and part B.
3 Which parts best represented a:a P wave?b S wave?c L wave?
U n i t 5 . 3 Research /Extension
U n i t 5 . 3 Practicalactivities
paper
paper
springkeep handstationary
keep handstationary
move handside to side
move handin and out
stick small sheet ofpaper to spring
waves
waves
Fig 5.3.8 Making waves
Unit 5.3 Prac 2Locating the epicentre
You will needCompass, ruler
What to do1 Calculate the difference in time between the arrival of
the P and S waves for each seismograph.2 Use the graph in Fig 5.3.5 to find the distance the
epicentre is away from each location.3 Trace the map of Australia into your workbook.4 Use a compass to draw three circles on it with radii
equivalent to these distances.5 The epicentre is the point or small area where the circles
intersect.
Questions1 Where was the epicentre of the quake?2 Where would the focus be?3 How long would it be between the arrival of the P and S
waves in Broome?4 Check an atlas to find the town most likely to be
affected by the quake.
SCI3 118
Cairns
AdelaidePerth
Hobart
Broome
CanberraSydney
Melbourne
Brisbane
0 1000 km
Alice Springs
h min s15:13:11 15:17:47
P
15:06:47
S 15:10:37 15:13:49
15:07:41
Fig 5.3.9 Find the epicentre.
119 Chapter 5 The fragile crust
The crust built up in layers. The oldest rockis the deepest and the youngest rock andsediment from erosion will be on the verytop. These layers were originally flat andhorizontal but have since been cracked andfolded by plate movement and puncturedby volcanoes. Weathering and erosionfurther shaped the landscape to form whatwe see today.Weathering was investigated in SCI 1, so here we willonly look at how the original landscape of the crustformed, before weathering and erosion took over.
Faulty landscaping!nFaults are lines of weakness on the Earth’s crust.
Faults can be:• normal • reverse • transcurrent
Normal and reverse faultsIn an earthquake, the crust shifts up or down alongnormal and reverse faults. After many quakes a fault
scarp is created. If the rock is hard and weathering slow,a cliff will form. If the rock is soft then erosion will wearit down to a gentle rise.
Sometimes two faults allow a block of rock to thrustup to form a horst. Sunken blocks form graben or riftvalleys. The Spencer and St Vincent gulfs in SouthAustralia are examples of horst and graben (in this case
Unit 5.4 Landscapingthe crust
Fig 5.4.1 Faults are lines of weakness.The arrows show likely movement.
Fig 5.4.2 Normal faults can erode into different landscapes. It willdepend on how hard the rock is.
normal fault transcurrent faultreverse fault
soft rock
hard rock
Fig 5.4.4 A transcurrent fault splits Scotland nearly in two.It is partly filled with water and includes the verydeep and mysterious lake, Loch Ness.
<Fig 5.4.4 transcurrent fault inScotland> 55 x 70mm
filled with water). Erosion sometimes moulds theminto parallel mountain ranges.
Transcurrent faultsHere plates move sideways. No mountains areformed but the movement shatters rock along thefault. Smaller rock is easier to weather than largerrock, so heavy erosion creates troughs that often fillwith water to form lakes and inlets.
FoldingnRock is under extreme pressure and high temperatureswhen continental plates collide. Under these conditionsrock acts like plasticine: it changes shape withoutbreaking (scientists call this plastic behaviour). It canbe folded to build mountain ranges or hills. As the
SCI3 120
Fig 5.4.6 Synclines, anticlines and overfolds all occur when rock bucklesunder intense pressure and high temperature.
rocks buckle, downward folds (calledsynclines), upward folds (anticlines)or overfolds form.
Erosion can wear away exposed softrock or can level the folded layers.When new sediments lay on top ofthese old and eroded folds, anunconformity is created.
The rock that makes up theHimalayas is sedimentary, a type made
Fig 5.4.3 Horsts and graben are blocks of rock with faults on two sides.
grabengraben
graben
horsthorst
weathersto
Matterhorn
SENW
African plateEurasian plate
Alps
Fig 5.4.5 The rocks of the European alps have seen intense folding due to thecollision of plates.These rocks are in the Austrian Alps.
synclineoverfoldanticline
Loch Ness
121 Chapter 5 The fragile crust
U N I T 5 . 4
Fig 5.4.7 Erosion will expose layers of folded rock and allows new layersto form.
from eroded material carried into the sea or valleys bywater. The collision of the Indian-Australianplate with the Eurasian plate has folded thisrock so much that it has emerged from the seafloor to make mountains.
The sea creatures have been pushed up too:fossils of seashells have been found high on Mt Everest!
Volcanic landscapesnVolcanoes were discussed in SCI 1, so here we will lookonly at how they affect the landscape.
Volcanic mountains can be built in three differentways. The biggest is what vulcanologists call a shieldcone. This gets bigger every time aneruption takes place, with lavacooling as it slides down its sides.The slopes are very gentle and thevolcano resembles a shield lying onthe ground. Mauna Loa on Hawaiiis an active shield volcano.
Cinder cones are very commonand relatively small. These are pilesof hot rock and cinders that spewedout of the vent only to fall backaround it.
Composite cones are made of layers of cooledlava and cinder rock, showing a history of eruption.They include the ‘typical’ volcanoes of Mt Fuji(Japan), Mt St Helens (USA) and Mt Vesuvius (Italy).
Fig 5.4.8 Paricutin is a cinder cone. It began as a hole in a farmer’s fieldin 1943. Nine years later it was 500 m high and stopped growing.
Fig 5.4.9 Mt Fuji, a composite volcano
Prac 2p. 126
Blacksmiths make lava!
In ancient Greece it was thought that the god of fire, Hephaestus,
lived under Mt Etna in Sicily. Here he made weapons for the gods.
When he hammered the red-hot iron, fire flicked out of the
volcano above. The ancient Romans had their god Vulcan living
under the island of Vulcano. The most recent volcanic action in
Australia was about 13 000 years ago in western Victoria: obviously
these gods have gone out of business!
Science snippet
On edge!Uluru (sometimes knownas Ayres Rock) is themassive tip of a foldformed when layers ofsandstone were folded300 million years ago.The folding was so severethat the layers are 80° tothe horizontal.
Sciencesnippet
Prac 1p. 125
unconformity
oldrock
newerrock
erosion of folds
Plugs and othernintrusionsn
Magma is filled with gases and will always try to force itsway up. Sometimes it breaks the surface and flowsacross it until the lava cools to form solid rock.
Often it cools before it gets to the surface, sometimesin the vent of a dying volcano. Over the years the softerrocks of the walls erode away, leaving a volcanic plugwhere the vent once stood.
If the magma cools below the surface it is called anigneous intrusion (remember SCI 2 classified volcanicrock as igneous rock). Each igneous intrusion dependson where it eventually cools. Some different types are:• dykes • batholith• sills • lacolith
Moving volcanoesnVolcanoes are usually located at the weak edgesof tectonic plates. Some are nowhere near anedge, however: these volcanoes are directly overhot spots or plumes. Although there is noobvious weakness in the plate above it, themagma has so much pressure that it can force itsway through.
The islands of Hawaii lie3200 km from the nearestplate boundary. Underwatervolcanoes formed over a hotspot, eventually rising above sealevel to form islands. All aredifferent ages. In the west is Kauai,the oldest at 5.5 million years. Theyoungest is the ‘big island’ of Hawaiiitself, which began building 700 000years ago and is still being extendedby lava flows from the continuallyerupting Mt Kilauea.
Although the hot spot neverchanges position, the plate abovedoes, carrying the islands with it. Theislands were built and then movedwest with the plate. Hawaii is directly
over the hot spot now and will eventually move on, too.An underwater volcano called Loihi is already forming alittle to its east and will become the newest island in thechain.
The Australian territories of Heard Island and LordHowe Island were once located over hot spots. An activehot spot is at longitude 40°S, crossing under Victoria,Bass Strait, Tasmania and the Tasman Sea. Luckily, itsactivity is confined to minor earthquakes with epicentreslocated east of Flinders Island.
A few hot spots are on land. An example isYellowstone National Park in the USA. Here the magma
SCI3 122
Fig 5.4.10 Hawaii’s islands are moving west with the plate.
Fig 5.4.11 Mauna Loa and Mt Kilauea are active volcanoes located over ahot spot, and are moving with the Pacific plate.The lava flowsare obvious in this photo of Mauna Loa.
Touristdrives to top
of world’stallest
mountain!Mauna Loa on the islandof Hawaii rises 9169 mdirectly from the oceanfloor and thus is a muchhigher mountain than MtEverest at 8850 m. Only4169 m of Mauna Loa isabove water, however,and it has a road all theway to the top!
Sciencesnippet
hot spot
Pacificplate moves
Hawaii(brand newto 700 000years old)
Maui(750 000 to1 millionyears old)
Oahu(2.5 to4 millionyears old)
Kauai(5 millionyears old)
123 Chapter 5 The fragile crust
U N I T 5 . 4
10 The Glasshouse Mountains in Queensland are volcanicplugs. Draw a diagram to show how they formed.
11 How can volcanoes form away from plate boundaries?12 What evidence exists that the Hawaiian Islands are
moving westwards?13 Use a diagram to explain how hot-spot islands can
move and their volcanoes die.14 Sketch how Uluru could have formed from a fold.15 New Zealand has huge mountains in its South Island
and active volcanoes in the North Island. Suggest howthey formed.
16 Victorians use the natural gas stored deep under BassStrait. Along which longitude would you expect the gasreserves to be and what weakness in the Earth’s crusthas contributed to it?
17 Give two possible reasons why fossil fuels are found atweak spots in the crust.
18 Trace the landforms shown in Fig 5.4.13 (overleaf):a on your diagrams, label all faults, anticlines,
synclines and unconformities.b Suggest the order in which the landform was
created.
1 What are the three basic types of faults?2 Use diagrams to show the differences between:
a a normal and transcurrent faultb a fault and a foldc a syncline and an anticline
3 What is the difference between:a a plug and a dyke?b sedimentary and igneous rock?c a shield and cinder cone volcano?d horst and graben?e kerogen and hydrocarbon?
4 Fault scarps are often very rounded and not sharp likecliffs. Explain why.
5 Horst and graben exist along mid-ocean trenches. Why?6 What evidence is there that the Himalayas were once
below the sea?7 What are the properties of something that shows plastic
behaviour?8 How can rock act in a plastic way?9 Draw diagrams of shield, cinder and composite
volcanoes.
Fig 5.4.12 A geyser
U n i t 5 . 4 Questions
boils underground water which then forces its way to thesurface as geysers, steaming lakes and mud pools.
Treasure from belownFossil fuels (oil, coal and natural gas) are usually foundnear weaknesses in the crust. Plate boundaries, smallerfaults, folds, hot spots and even the craters of dead (extinct)volcanoes have all been found to have stores of them. Fossilfuels are actually plant and animal matter that decomposedto form a tar-like compound called kerogen. This needs tobe ‘cooked’ for some millions of years to form othercompounds called hydrocarbons, which are the actualenergy store in the fuel. The intense pressures and heat atthe weak spots may provide just the right ‘cooking’conditions to do the job. Weak spots may also provide amore porous or sponge-like rock for oil to be squeezed into.
WS 5.5
WS 5.6
1 Hot spots can produce basalt plains instead ofmountains. How do they form and where are theylocated?
2 Some ocean hot spots are Reunion Island (in the IndianOcean), the Azores, the Canary Islands, Cape Verde,Ascension Island, St Helena, Tristan de Cunha, GoughIsland and Bouvet Island in the Atlantic. Find theirlocations on a map.
3 Find these transcurrent faults on a map. Trace the areaaround them and indicate the probable movement oneach side: Great Glen Fault through Loch Ness(Scotland), Cabot fault through Newfoundland(Canada), San Andreas Fault through California (USA).
4 Trace a map of Hawaii. On each island, locate thevolcanoes and label them as active or dead. Indicatewhere Loihi is likely to be and where future islands mayform.
5 Where is the nearest fault to Yellowstone National Park?6 Find out more about the natural gas reserves of Bass
Strait. What else is found there, how is it tapped andwhat processing is needed?
SCI3 124
a A
B
CD
A
B
C
D
EFG
H
I
J K
b
Fig 5.4.13
U n i t 5 . 4 Research /Extension
C R E A T I V E W R I T I N G Darwin, an outer suburb
of Hong Kong!All the tectonic plates are moving faster than everbefore! What would have taken millions of years isnow happening in a single day! Australia’s north-westcoast is heading for the underbelly of China: Darwinand Hong Kong are to join!
You are the information officer for the StateEmergency Services and have the job of providinginformation to the citizens of Darwin.
You must:• describe why the plates are shifting• describe what the climate will do• describe what events can be expected before the
collision• describe what will happen once the continents
collide• supply a list of relatively safe cities in Australia and
overseas to evacuate to.Write it as information sheets, a Powerpoint
presentation or a script for a TV announcement. Beimaginative, but base all descriptions on what youknow about tectonic plates.
125 Chapter 5 The fragile crust
U N I T 5 . 43 Copy the following table into your workbook:
Geological What to build Before After Aerial feature erosion erosion view
aftererosion
Syncline(downwardfold)
Anticline(upwardsfold)
Overfold
Normal fault
Transcurrentfault
Horst andgraben
4 Model each feature shown in the table. To make faults,cut the cake with the hacksaw or hold the wire tightlywith two hands and saw down through it.
5 Once again use the hacksaw blade or wire to cut in thedirection shown by the arrows in each diagram. This isour ‘erosion’.
6 For each feature, draw a cross-section or side view after‘erosion’. Colour the layers appropriately.
7 Look down on the feature as if you are travelling over itin an aircraft. Draw what you see.
Question1 Identify which geological feature created these layers
(Fig 5.4.15) when seen from the air.
Unit 5.4 Prac 1Modelling faults and folds
You will needPlasticine in 4 colours, a rolling pin, fine wire or a hacksawblade
What to do1 Roll the plasticine flat into 1 cm layers.2 Make a layered ‘cake’ with the plasticine.
U n i t 5 . 4 Practicalactivities
a
b
c
d
Fig 5.4.14 Making faults, folds and erosionFig 5.4.15
Unit 5.4 Prac 2Shaping volcanoes
When material piles up into a cone, the angle the sidemakes with the horizontal is called the angle of repose.
You will needFunnel, retort stand, bosshead and clamp, 1 sheet of graphpaper, ruler with millimetre markings, materials such as finesand, coarse sand, flour, fine blue metal screenings, candles(old stumps will do), matches, protractor, access to ascientific calculator
What to doPart A1 Set up the funnel as shown.2 Place your finger over the end of the funnel and fill with
fine dry sand. Remove your finger quickly and let thesand run out.
3 Measure the height of the mound. Use the graph paperto estimate the diameter of its base. Calculate its radius.
4 Draw a scale diagram of the side-view (cross-section) ofthe mound.
5 Use a protractor to measure the angle that the sandmakes with the horizontal.
6 Another way is to use trigonometry. Follow these steps:a On your calculator, divide the height by its radius.
The answer should be less than 1.b Push the tan–1 button (push the inverse or shift button
first if necessary!)c The answer is the angle of repose of the sand.
7 Put the angle on your diagram.8 Repeat for each of the other materials.9 Make bigger ‘volcanoes’ using two funnel-loads of
material and calculate the angles of repose for each.Part B1 Repeat the experiment but make the cone with melted
candle wax. Drip a small amount of wax onto the graphpaper and allow it to cool.
2 Drip more and more wax, allowing it to cool each time.3 Calculate the angle of repose of the wax.
Questions1 What type of volcano did you build in parts A and B?
Explain.2 Arrange the materials in order from smallest to largest
angle of repose.
SCI3 126
3 Why might the angles differ between materials?4 Did bigger ‘volcanoes’ have different angles to smaller
‘volcanoes’ of the same material?
5 Design and test a model for a composite volcano.
candle
graph paper
graph paperretort stand
angle of repose
radius
height
Fig 5.4.16 Making faults, folds and erosion
127 Chapter 5 The fragile crust
U N I T 5 . 4
Unit 5.1 Plate tectonics
Word Clue
1 s___ ___ er ___ ___ ___tin ___ ___ ___ A continent that split into others.
2 ___ ___ ___ gaea Wegener thought this split into the continents of today.
3 ___ ___ ___ ___ ___ana Contained Australia, Antarctica, South America, Africa and India.
4 ___ ___ ___ ___ asia Contained North America, Europe and Asia.
5 ___ ___ ust Thin layer of rock that we live on.
6 man ___ ___ ___ The second layer of Earth.
7 ___ ___ ___ ___ ___ ___ tion current The movement of hot and cold rock, liquid or air.
8 ___ ___ ___ ___ onic plates A moving layer of crust.
1 Why is the Earth like toast on soup?2 Pangaea is Greek for ‘all the lands’. Why is this a good
name for the original supercontinent and what are thenames of its ‘babies’?
3 How does magnetism in rocks suggest:a that the continents were once joined?b that the ocean floor is spreading?
4 What is the difference between the Theory ofContinental Drift and the Theory of Plate Tectonics?
5 Why weren’t the mysteries of the ocean floor discovereduntil the twentieth century?
6 A map of the world in the future will be different to theone we know now. Explain how.
7 How can the mantle be solid but still be able to move?What other substances are like this?
8 How does convection push tectonic plates around?9 Explain why the temperature near the ceiling of a room
is always hotter than that at floor level.10 Why does magma want to rise?11 The ocean floor has been likened to a conveyer belt.
Why?
12 Where on Earth is the longest mountain ridge and thehighest?
13 How does density affect which plate sinks?14 Why do earthquakes happen at plate boundaries?15 At what depth is the subduction zone completely molten?16 Which waves will be detected by a seismometer and in
what order will they be detected?17 Draw a transverse wave.18 What is an easy way of remembering what P, S, R and L
waves do?19 Which waves produce these motions at the Earth’s
surface?a Side-to-sideb Up-downc Rolling
20 P and S waves are refracted. Draw what this means.21 Describe three different ways mountain ranges can form.22 What is the difference between a fault and a fold?23 Why are the sides of a cinder cone steeper than those
of a shield volcano?
Chapter review questions
Sci-words
SCI3 128
Unit 5.2 At the edges
Word Clue
1 bo ___ ___ ___ ary Edge of a plate.
2 sp ___ ___ ___ ___ ing boundaries Plates are separating.
3 ___ ___ ___ ___ ision boundaries Subduction zones occur here.
4 ___ ___ ansf ___ ___ ___ boundaries Scraping boundaries.
5 con ___ ___ ___ ___ ___ tive New rock is made.
6 ___ ___ ___ truc ___ ___ ___ ___ Rock is melted and returned to the mantle.
7 c ___ ___serv ___ ___ ___ ___ ___ Boundary at which rock is neither destroyed or made.
8 ___ ___ ___ ___ valley Formed at spreading boundaries.
9 m___ ___ ___ a Molten gassy rock.
10 sub ___ ___ ___ ___ ion zone Angled dive where plates collide.
Unit 5.3 Earthquakes
Word Clue
1 ___ ocu ___ Where a quake starts.
2 s___ ___ ___ mic waves P, S, R and L waves.
3 ___ ___ dy waves Travel through the Earth.
4 ___ urf ___ ___ ___ waves Travel around the Earth.
5 ___ ___ ___ mary waves The first waves to arrive from a quake.
6 sec ___ ___ ___ ary waves The second waves to arrive.
7 ___ ___ ___ leigh waves Rolling waves.
8 ___ ___ ve waves Often the most dangerous wave from a quake.
9 ___ ___ ___ ___ itud ___ ___ ___ ___ waves Compression waves.
10 ___ ___ ___ ___ ___ verse waves Up-and-down waves.
11 ref ___ ___ ___ ___ ion Bending of waves as their speed changes.
Unit 5.4 Landscaping the crust
Word Clue
1 ___ ___ ___ ___ al fault Nearly vertical fault.
2 tr ___ ___ ___ curr ___ ___ ___ fault A sideways fault.
3 fault s ___ ___ ___ p A cliff formed at a normal fault.
4 h ___ ___ ___ ___ An upthrust block.
5 ___ ___ ___ ben Rift valley.
6___ las ___ ___ ___ behaviour Can be folded without breaking.
7 ___ ___ ___ cline Downfold.
8 ___ ___ ___ ___ cline Upfold.
9 un ___ ___ ___ form ___ ___ ___ Erosion flattens a fold but new material is laid on top.
129 Chapter 5 The fragile crust
10 sed ___ ___ ___ ___ tary Rock formed from eroded material.
11 ___ ___ ___ eld cone Flattish volcano formed from lava.
12 cin ___ ___ ___ cone Sharper volcano made from falling rock.
13 ___ omp ___ ___ ___ te cone Volcano made from lava and falling rock.
14 volcanic ___ ___ ___ ___ Magma cooled in the vent of a volcano.
15 ___ ___ ___ ___ous rock Rock made by a volcano.
16 ign ___ ___ ___ ___ intrusion Magma that cooled before reaching the surface.
17 ___ ___ ___ spot Weak point nowhere near a plate boundary.
18 ___ ___ ___ ous Sponge-like.
19 k ___ ___ ___ gen Tar-like substance.
20 ___ ___ ___ ___ ocar ___ ___ ___ Actual energy source in fossil fuel.
