Answers to coursebook questions 5: The fragile crust Unit 5.1 1 a False: Triassic reptiles could not have swum the distances required to populate different continents. b True: There are similar mountain ranges in the USA and Europe, and also in Africa and South America. c False: Many of the continents that do not have glaciers now were once cold enough to have them. d True: Coal deposits above the Arctic Circle suggest the land has floated there from warmer climates. e False: The rock of the ocean floor is much younger than that of the continents. f False: Continental rock is less dense than the rock of the ocean floor and floats on it. g True: Magnetic stripes on the ocean floor suggest that new rock is made along mid-ocean ridges. 2 Six pieces of evidence that suggest the continents were once joined are: the shape of the continents; fossil remains of the same fern-like plants and Triassic reptiles; rock formations in mountains on different continents; ancient glaciers; coal above the Arctic Circle; and magnetism in ancient rocks. 3 a Gondwana: Australia, Antarctica, South America, Africa, India b Laurasia: North America, Europe, most of Asia 4 The evidence that Australia once was far colder than it is now: valleys made by glaciers; the remains of a small dinosaur called Leaellynasaura found near Apollo Bay which had larger-than-normal optic lobes allowing it to see in the 24-hour darkness of an Antarctic winter. 5 Five surprising facts about the ocean floor when it was first mapped were: huge volcanic mountain ranges exist down the centre of the oceans; the rock of the ocean floor is much younger than that of the continents; deep ocean trenches exist; the rock of the continents is less dense than that of the ocean floor; the ocean floor has magnetic stripes that indicate that the rock is of different ages. 6 The longest mountain range in the world is the Mid-Atlantic Ridge, being 65 000 km long. 7 The ocean floor often has magnetic stripes because as the lava flows out of the midocean ridge it cools and solidifies; it adopts the magnetism of the Earth at that moment; the magnetic field of Earth changes every now and then, forming ‘stripes’ of magnetism. 8 The rock of the ocean floor is like a conveyor belt in that it forms new rock at the mid-ocean ridge and then transports it over many years into the trench and subduction zone formed where the ocean plate hits and dives under the continental plate. © Pearson Education Australia (a division of Pearson Australia Group Pty Ltd) 2006. This page from the Science Dimensions 3 Teacher’s Edition CD may be photocopied for classroom use. Answers to coursebook questions 5: The fragile crust 9 a Tectonic plate: sheet of crust, partly molten bottom and top—layer of mantle that all shifts as one b Mantle: molten rock under intense pressure and temperature—the layer under the crust c Crust: Thin layer of solid rock that we live on—the ‘skin’ of the Earth 10 The crust is 8 to 64 km thick. 11 The oldest rocks on the ocean floor are those closest to the trenches and the youngest are next to the ridges. 12 Convection currents are caused by hot material rising and cooler material dropping. 13 Convection currents can take millions of years to cycle around the mantle. 14 The mantle is kept hot because the crust traps heat like a blanket; patches are continually being heated by radioactive decay of uranium, thorium and potassium. 15 The theory of plate tectonics suggests that the Earth’s crust is made from a series of rock plates that shift around on convection currents in the mantle. 16 The temperatures along the ridges are higher than elsewhere in the ocean because this is where magma emerges and cools to become solid rock. This hot lava will heat the water around it. 17 It would be impossible for coal to form in the Arctic or Antarctic now because it is too cold there for the plants that make up coal to grow. 18 Diagrammatic answer required 19 Smoke rises up a chimney on a rising, hot, convection current of air warmed by the fire. 20 If the mantle cooled and became solid, the plates would stop moving. 21 Another possible reason why Australia doesn’t have any glaciers now could be global climate change raising the temperature of the whole planet. Hence, Australia could still be in the same position, but the climate would be too warm for glaciers to exist. 22 If an average lifetime is 80 to 100 years, then the plate on which Australia sits will move northward at between 5 × 80 = 400 cm to 5 × 100 = 500 cm per year. This is equivalent to 4 to 5 metres. Unit 5.2 1 a transform or scraping b spreading c collision 2 a conservative: transform or scraping (a) b destructive: collision (c) c constructive: spreading (b) © Pearson Education Australia (a division of Pearson Australia Group Pty Ltd) 2006. This page from the Science Dimensions 3 Teacher’s Edition CD may be photocopied for classroom use. Answers to coursebook questions 5: The fragile crust 3 A fault is a weakness in the Earth’s crust. 4 A rift valley is the crack or valley caused by a fault. 5 Examples of rift valleys: East African Rift Valley, the Jordan Rift Valley holding the Sea of Galilee, Dead Sea and Gulf of Aqaba. 6 The mid-ocean ridge constantly tries to heal itself, forming a ‘rock scab’ that is the ridge itself. Magma keeps breaking through the scab, however, oozing out and forcing the repair to happen all over again. 7 a faster b heavier c ocean plate 8 A subduction zone is where one plate (usually the ocean plate) dives another (continental) plate. Friction causes earthquakes along it and melts the rock. The molten rock may have enough pressure to break the surface to form volcanoes. 9 The deepest underwater trench is the Mariana Trench, located in the western Pacific Ocean. Its depth is 11 033 m. 10 Island chains created by the collision of two ocean plates: the islands of Japan, Indonesia, the Philippines, the Caribbean and the Aleutians. 12 The Andes Mountains lie parallel to the Peru-Chile trench. 13 At a scraping or transform boundary, plates scrape along each other. These don’t make mountains or volcanoes but do produce lots of earthquakes, some very strong. Although most of these boundaries are underwater, some are on land. 14 A major transform boundary is the San Andreas fault, which runs 1300 km through California, USA, directly under San Francisco and close to Los Angeles (often called LA). 15 a rock L b rock E c rock J 16 a b c d e f g collision spreading collision collision collision scraping collision 17 a b c d e spreading transform spreading collision spreading © Pearson Education Australia (a division of Pearson Australia Group Pty Ltd) 2006. This page from the Science Dimensions 3 Teacher’s Edition CD may be photocopied for classroom use. Answers to coursebook questions 5: The fragile crust f spreading 18 Diagrammatic answer required 19 a b c d e f g h Himalayas: Indo-Australian plate with Eurasian plate Andes: South American plate with Nazca plate Mid-Atlantic Ridge: African plate with South- and North-American plates Caribbean islands: North American plate with Caribbean plate Japan: Pacific plate with Eurasian plate Mariana Trench: Philippine plate with Pacific plate San Andreas fault: Pacific plate with North-American plate Dead Sea: Indo-Australian plate with African plate 20 Assuming an average lifetime of between 70 and 90 years, the Himalayas will grow between 70 and 90 centimetres. If you reach 100, they will have grown 1 metre. 21 a A further 10 m will take 1000 years. b A further 100 m will take 10 000 years. c A further 1 km (1000 m) will take 100 000 years. Mediterranean Sea: the Red Sea needs to widen another 260 km (500 – 240 km). 260 km = 26 000 000 cm. So the time taken would be: 26 000 000/20 = 1 300 000 years = 1.3 million years. b Atlantic Ocean (6100 km): time = 29 300 000 years = 29.3 million years c Pacific Ocean (14 000 km): time = 68 800 000 years = 68.8 million years 22 a Unit 5.3 1 The friction between the plates is normally enough to stop movement of the plates for a while. The plates are still pushing, however, and the pressure will build until it overcomes the friction. That’s when the plates will move, suddenly. 2 The deepest the focus can be below the surface is 200 km. 3 A seismometer detects an earthquake. The graph it produces is called a seismograph. 4 Body: Primary (P) and secondary (S) Surface waves: Rayleigh (R) and Love (L) 5 Refraction is what happens to waves as they change speed on entering a new material. 6 Different densities and temperatures of the rock below the surface cause changes in speeds of P and S waves and cause them to be bent or refracted. 7 S waves apparently do not pass through the Earth’s core since there is always a ‘shadow’ opposite the epicentre. S waves cannot pass through liquid, indicating that the outer core must be liquid. 8 A single seismograph tells us how far away a quake is but gives no information about its direction. © Pearson Education Australia (a division of Pearson Australia Group Pty Ltd) 2006. This page from the Science Dimensions 3 Teacher’s Edition CD may be photocopied for classroom use. Answers to coursebook questions 5: The fragile crust 9 Scientists can predict how far an earthquake is away from them by measuring the time difference between the arrival of the P and the S waves from an earthquake. 10 The two surface seismic waves are Rayleigh (R) and Love (L) waves. 11 Diagrammatic answer required 12 a Body waves travel through the body of the Earth, while surface waves travel the longer distance across the surface. b The focus is the point underground where the quakes starts. It is the point of slippage. The epicentre is the place on the surface directly above it. c A longitudinal wave is a ‘push-pull’ wave, and moves particles back and forth in the direction of the movement of the wave. A transverse wave is an ‘up-down’ wave that moves particles at right angles, or sideways, to the direction of the movement. 13 a P waves, sound b S and L waves, water waves, light 14 Diagrammatic answer required 15 a b c d e f g h i L (and sometimes R) S and L P P and S P L R R S and L 16 The branch bends and stores a lot of energy. It gets to a point where it suddenly breaks, releasing all of this stored energy suddenly. Earthquakes move and bend rock and store energy as they do. They release all that energy when the quake occurs, just like the broken branch. 17 All of Australia sits on the Indo-Australian plate. There are no major boundaries running through it. Papua New Guinea and New Zealand both straddle the IndoAustralian and Pacific plates and sit on a boundary where earthquakes can be expected. 18 a b c d 2500 km 1300 km 2150 km 6350 km 19 If P and S waves arrive at the same time then you must be at the epicentre of the quake (actually, you must be at the focus itself!). 20 a 7.8 min or 7 min 48 s b 2.6 min or 2 min 36 s © Pearson Education Australia (a division of Pearson Australia Group Pty Ltd) 2006. This page from the Science Dimensions 3 Teacher’s Edition CD may be photocopied for classroom use. Answers to coursebook questions 5: The fragile crust c 5 min d 1.6 min or 1 min 36 s 21 Arrival time of P waves (h:min:s) 10:24:00 Arrival time of S waves (h:min:s) 10:32:00 Time difference (min:s) 8:00 Time (min) Distance of epicentre (km) 8.0 6200 04:48:20 4:52:50 4:30 30 ÷ 60 = 0.5 so time is 4.5 min 2900 2:55:21 p.m. 3:01:21 p.m. 6:00 6.0 4150 7:37:03 p.m. 7:42:33 p.m. 5:30 5.5 3700 14:08:34 14:11:46 3:12 3.2 1950 20:21:02 20:25:50 4:48 4.8 3150 05:45:10 05:50:10 5:00 5.0 3300 11:28:00 11:34:30 6:30 6.5 4650 08:08:56 08:12:56 4:00 4.0 2500 15:21:04 15:28:40 7:36 7.6 5800 22 a The tectonic plates involved in the earthquake that triggered the 2004 Boxing Day tsunami were Eurasian and Indo-Australian plates. b The earthquake occurred at a collision boundary. Unit 5.4 1 Three types of faults are normal, revere and transcurrent. 2 Diagrammatic answers required 3 If the rock that makes the fault scarp is hard, it will weather slowly. If soft, it will weather quickly and will be carried away by erosion, leaving a rounded rise instead. 4 Australian examples of horst and graben are the Spencer and St Vincent gulfs in South Australia. 5 Loch Ness is a Scottish lake that has filled part of a transcurrent fault. 6 A substance shows plastic behaviour if it can bend and fold without breaking. 7 Rock can act in a plastic way if it is under extreme pressure and temperatures, typically found in folding. 8 Diagrammatic answer required 9 It is evident that the Himalayas were once below the sea because fossilised seashells have been found high on Mt Everest. © Pearson Education Australia (a division of Pearson Australia Group Pty Ltd) 2006. This page from the Science Dimensions 3 Teacher’s Edition CD may be photocopied for classroom use. Answers to coursebook questions 5: The fragile crust 10 Diagrammatic answer required 11 An igneous intrusion is solid rock formed when magma has cooled and solidified below ground. 12 Hot spots are located under the Hawaiian islands (Pacific Ocean), Yellowstone National Park (USA), Reunion Island (Indian Ocean), the Azores, the Canary Islands, Cape Verde, Ascension Island, St Helena, Tristan de Cunha, Gough Island and Bouvet Island (all in the Atlantic Ocean) and under Bass Strait. 13 The Hawaiian Islands in order from youngest to oldest: Hawaii, Maui, Oahu, Kauai 14 If the magma at a hot spot boils underground water, it can then force its way to the surface as geysers, steaming lakes and mud pools. 15 Examples of fossils fuels: oil, petrol, kerosene, natural gas, coal 16 Intense heat and pressure is needed to convert kerogen into hydrocarbons. Weak spots can provide these conditions. The other idea is that oil and gas would be squeezed into the more porous rock that weak spots would provide. 17 Kerogen is a tar-like substance made from decomposed plant and animal matter. A hydrocarbon is the actual chemical energy store in fossil fuel and is thought to have come from ‘cooking’ kerogen. 18 Shaking a can of soft drink builds up pressure in it. When its top is popped the pressure is suddenly relieved by spraying liquid out from its top. A composite volcano does something similar. Pressure builds until it ‘pops’ its top, spraying lava and ash everywhere. 19 Diagrammatic answer required 20 The mountains and volcanoes of New Zealand are both caused by the collision of the Pacific with the Indo-Australian plates. Mountains have buckled up and volcanoes have formed from the subduction zone. 21 Evidence that the Hawaiian Islands are moving westwards: the oldest island Kauai is in the far west; the youngest Hawaii is in the far east; a new underwater volcano, Loihi, is forming east of Hawaii; volcanic activity is only under Hawaii and Loihi. 22 The natural gas reserves of Bass Strait would be expected to be about longitude 40S due to the weakness caused by a hot spot. Because the Indo-Australian plate is moving north the reserves may be a little north of 40S. 23 a Diagrammatic answer required b Layer K was laid down first, followed by J, I, H, G, F, E and D on top. All were laid flat. Pressure folded the layers upwards, forming an upward fold or anticline. Erosion removed the top of the fold, until D and E were nearly worn away. The erosion left the surface flat once more. Sediment laid new layers: C first, then B and A on top. © Pearson Education Australia (a division of Pearson Australia Group Pty Ltd) 2006. This page from the Science Dimensions 3 Teacher’s Edition CD may be photocopied for classroom use. Answers to coursebook questions 5: The fragile crust Chapter review 1 A map of the world in the future will be different from what it is now because all of the plates and their continents are shifting. Some will join, others will part, some will slide along each other. 2 The mysteries of the ocean floor were discovered only in the 20th century with the invention of sonar and the need for good ocean-floor maps in World War II. 3 a The north poles of ancient rocks that are magnetic all point in different directions. When the continents are pieced together the north piles all point in the same direction, suggesting the continents have shifted and twisted. b Magnetic stripes exist parallel to the mid-ocean ridges. The stripes closest to the ridges are the newest and the ones further out are older, suggesting that they are moving away from the ridges and towards the trenches. 4 Convection currents push the rock of the mantle around. The bottoms of the plates are partially molten or ‘soggy’ and will be carried with the mantle as it moves underneath them. 5 Magma is molten rock full of gas (mainly steam). Its density is less than the surrounding rock and so it will push upwards through the covering tectonic plate. 6 The ocean floor is like a conveyer belt as it carries the newly created rock from the mid-ocean ridges across the ocean towards the trenches. 7 The longest mountain ridge (about 65 000 km long) is down the middle of the Atlantic. The highest is the Himalayas on the border of India and China/Tibet. 8 Dense materials sink and lighter materials float. The rock of the continental plates is less dense than the rock of the ocean plates. The continents thus will ‘float’ on the ocean floor, and the ocean plate will sink under the continent. 9 Plate boundaries are where plates separate, collide or scrape over each other. Friction will occur and will stop movement until the pressure is sufficient to overcome the friction. When it does, the plate slips and an earthquake results. 10 The subduction zone is completely molten 200 km below the surface. 11 Primary, secondary, Raleigh and Love waves are all detected by the seismometer. They are in the order P first, S next and R and L basically together and last. 12 Diagrammatic answer required 13 a S and L b P c R 14 Diagrammatic answer required 15 Three different ways mountain ranges can form are: continent/continent collision, forming folded mountains; volcanic action at plate boundaries or hot spots; normal faults create horst and graben which can erode into mountain ranges and basins. © Pearson Education Australia (a division of Pearson Australia Group Pty Ltd) 2006. This page from the Science Dimensions 3 Teacher’s Edition CD may be photocopied for classroom use. Answers to coursebook questions 5: The fragile crust 16 A fault is a weak spot or break in the crust along which the crust can move in an earthquake. There is no break in the crust when folded. The crust buckles instead of breaking and shifting. 17 A cinder cone has steeper sides than a shield volcano because it is simply a pile of rock. The pile will build until the rock begins to tumble further down the slopes. A shield volcano is made from cooled lava. Being molten it will spread further than solid rock. 18 The mantle is solid but still able to move due to the extreme pressures and temperatures there. Other ‘solid’ substances that can ‘move’ are plasticine, clay and mud. 19 The Earth is like toast on soup in that both have slabs of moving solid crust floating on a hot, thick liquid. 20 All the current continents were part of Pangaea. Hence, it is literally all the lands. Its ‘babies’ are Gondwana and Laurasia. 21 The theory of continental drift assumes that only the continents are shifting. The theory of plate tectonics involves much larger slabs of rock (which also carry the shifting continents). 22 The temperature near the ceiling of a room is always hotter than at floor level because of convection currents. The warmer air rises and the cooler air drops: the temperature will increase as you go higher in the room. 23 One easy way of remembering what P, S, R and L waves do is P = push/pull, S = shake, R = roll, L = leftovers! © Pearson Education Australia (a division of Pearson Australia Group Pty Ltd) 2006. This page from the Science Dimensions 3 Teacher’s Edition CD may be photocopied for classroom use.