technological advances and the history of science - oxford ......remains much to discover. advances...
TRANSCRIPT
● Understand the different models that explain the structure and composition of the Earth.
● Distinguish the main landforms found on the ocean floor and understand how they were formed.
● Recognise the evidence for sea-floor spreading and continental drift.
● Relate the dynamic internal structure of the Earth with the theory of plate tectonics.
● Appreciate that scientific knowledge is something that is constantly changing and its role in explaining natural phenomena such as volcanic eruptions, earthquakes and the formation of mountain chains.
● Carry out a research task.
1YOU WILL LEARN TO…
TECTONIC PLATES
What natural phenomena do you think have happened in the picture?
Do you think that the continents can move? Explain your answer.
The longest mountain range on Earth is the Mid-Ocean Ridge. How do you think this mountain range was formed?
What do you think the relationship between the Earth’s interior, earthquakes and volcanoes is?
How have scientists been able to discover the composition and state of the Earth’s interior?
Final task
51. Tectonic plates
Technological advances and the history of science
On 23 January 1960, Jacques Piccard and Don Walsh, on board the bathyscaphe Trieste Mariana, descended into the Mariana Trench, the deepest known point on the Earth. Since then, only one more person has gone down to these extreme depths, the film director, James Cameron, in 2012.
Until only a few decades ago, the sea floor and the Earth’s interior, were unexplored areas of our planet. Since then, our knowledge has increased greatly, but there remains much to discover. Advances in technology and the development of precise scientific instruments have contributed greatly to our scientific knowledge.
In this unit, you will find out how some of these advances have transformed our vision of how the interior of our planet works. In this task, you will create a table that summarises the main techniques and technology used to study the surface and interior of the Earth, and the knowledge about the Earth that they have given us.
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1. WEGENER’S THEORY OF CONTINENTAL DRIFT
The theory of continental drift was accepted in the 1960s. Until then, there was a lot of disagreement between scientists. Some believed that the continents were immobile1, while others believed that they had travelled great distances over time.
The idea that gigantic masses, such as the continents, could move thousands of kilometres was hard to believe, but there were many signs that they did. The most suggestive of these was that the east coast of Africa and west coast of South America seemed to fit together.
1.1. The theory of continental drift
The German geophysicist and meteorologist, Alfred Wegener, was the first to find proof to explain why the coasts of these continents look like they could fit together and to demonstrate that the continents were joined together in the past. He discovered that the continental shelf2 of each continent fitted together exactly. He also found that mountain ranges had rocks of the same age and identical fossil evidence in both continents.
In 1912, he proposed his hypothesis of continental drift. According to his hypothesis, 225 million years ago, all the continents were joined together as one large supercontinent called Pangaea, which means the whole Earth. Over a very long period of time, the continents drifted apart to the positions they are in today.
Understand
2. Find the position of North America in Pangaea and describe its movement from 225 million years ago to the present day.
Apply
3. How many oceans were there 225 million years ago? Why do we have more oceans today?
4. Fossils have been found in India that are more similar to those found in Australia than those found in China. Can you explain this?
Map of the east coast of South America and west coast of Africa
PA
NG
E A
225 million years ago
65 million years ago
135 million years ago
now
America Africa
Asia
Australia
Europe
Antarctica
A
Wegener wrongly believed that the continents were made of a light crust that could slide over the ocean floor, as it was a continuous and denser layer. Today we know that the surface layer of the Earth, the lithosphere, slides over the fluid material in the mantle.
Understand
1. Why do think the coasts of these two continents look like they could fit together?
South America
Africa
Mid-Atlantic Ridge
1immobile: cannot move2continental shelf: shallow submerged edge of a continent
Wegener's theory of continental drift
71. Tectonic plates
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Key concepts
❚❚ Wegener believed that all the continents were one supercontinent (Pangaea) millions of years ago and that they slowly moved apart to their current position.
❚❚ He collected a lot of different evidence, but he couldn’t explain what had caused the continents to move.
Remember
8. Why was Wegener’s theory not accepted at the time?
Analyse
9. Why do you think new technology helped Wegener's theory to be accepted?
1.2. Wegener’s evidence for continental drift
Wegener found evidence to prove that the continents were once joined together:
❚❚ Geological evidence: Wegener found that the continental shelves fitted together even better than the coastlines. He also found identical rocks and geological structures on both sides of where the continents were once joined.
❚❚ Paleontological evidence: He found identical fossils of land organisms, such as reptiles and plants, on the continents that are now separated. These organisms could not have crossed the oceans that separate them today.
❚❚ Paleoclimatic evidence: It is suspected that the northern part of Pangaea was covered in large tropical rainforests, while the southern hemisphere was covered in glaciers. Wegener examined the moraines3 deposited by glaciers in the continents that would have been part of the south of Pangaea. He found that they had identical glacial moraines of the same age. The equatorial area had large amounts of carbon.
Wegener did not find evidence to explain the movement of the continents and his theory was rejected. Half a century later, Wegener’s theory was finally accepted due to advances in technology that improved our knowledge of the Earth's interior, the ocean floor and the distribution of earthquakes and volcanoes.
Understand
5. Listen, look at the diagrams above and answer true or false. Correct the false sentences.
Apply
6. Find information about the continental shelves. Are they more similar to the continents or the ocean floor?
Evaluate
7. Explain the presence of plant and reptile fossils in the middle of Antarctica using the theory of continental drift.
continental shelf
granite rock of the same age
coastline 400 million years ago
mountain ranges of the same age
matching continental shelves
glacier deposits from 300-250 million years old
Geological evidence Paleontological evidence
3moraines: material deposited after the movement of a glacier
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2. THE STRUCTURE AND COMPOSITION OF THE EARTH
During the last century there were huge advances in our knowledge of the Earth, although there is still much more to be discovered.
2.1. Methods for studying the Earth’s interior
❚❚ Core sampling and mining. With current technology, the deepest they can drill is 13 km, an insignificant distance compared to the Earth´s 6 371 km radius. Therefore, this is not a very efficient method for learning about the Earth.
❚❚ Studying rocks. We may not be able to enter the interior of the Earth, but there are natural phenomena which move rocks from the interior to the surface. These include erosion, which breaks down the rocks on the surface and exposes other deeper rocks; and volcanic eruptions that often drag fragments of the Earth's interior to the surface.
❚❚ Studying meteorites and asteroids. These are fragments from when the Solar System was created, that did not join others to form a planet. They tell us about the original materials that formed the Earth.
❚❚ Seismic methods. These are the most important methods. They measure the seismic waves produced by earthquakes or controlled explosions. These waves travel though the Earth’s interior and are recorded on seismographs, which give us information about the layers they have passed through. There are two types of seismic waves, P and S.
Waves Origin of name SpeedSubstances they travel
throughMovement they provoke
P Primary: they are the first to arrive to the surface.
Faster All. They travel faster in solids than liquids.
They make the terrestrial particles move in the same direction as the wave.
S Secondary: they arrive after the P waves.
Slower They travel through solids not liquids.
They make the terrestrial particles move perpendicularly to the wave.
Seismic discontinuity
By studying internal P and S waves, scientists discovered the existence of seismic discontinuities. These are places in the Earth’s interior where there are abrupt changes in the speed of these waves, indicating a change in the composition or the physical state of the materials they are travelling through. They show us the boundaries between different layers of the Earth.
Analyse
11. Look at the graph and answer the questions:
a) What does this graph show?
b) Which waves are moving faster, the S or P waves? Explain your answer.
c) How many seismic discontinuities can you see on the graph? What is their depth?
d) According to the graph, how many layers are there in the Earth’s interior?
12. What is the physical state of the Earth’s interior from 2 900 km onwards? Explain your answer.
spee
d of
sei
smic
wav
e (k
m/s
)
5 000
12
depth (km)
0 4 0003 0002 0001 000
9
6
3
0
Moho
6 000
Gutenberg
mantle outer coreinnercore
S wave
P wave
Hoba meteorite
Understand
10. Many meteorites like this one are made up almost exclusively of iron. What information can they give us about the Earth?
Apply
13. If there was a big earthquake in Australia, what waves would be detected on a seismograph in Spain? Explain your answer.
91. Tectonic plates
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Remember
15. Listen to the physical layers of the Earth being described and say if it is the lithosphere, asthenosphere, mesosphere or endosphere.
Analyse
16. In both diagrams, the thickness of the crust and lithosphere are very exaggerated. Calculate the thickness of the continental crust (30 km) in a 20 cm diagram.
2.2. The composition and dynamics of the Earth
The study of seismic waves using seismographs revealed that the interior of the Earth is divided into layers, similar to those of an onion. Each layer can be classified according to two criteria: its chemical composition or its physical state.
❚❚ Compositional layers. These are the crust, mantle and core. They are classified in order of increasing density and are separated by seismic discontinuity boundaries.
The Mohorovičić discontinuity or Moho separates the crust and the mantle. The crust is made up of less dense rocks, rich in silicon and aluminium, than the mantle, which is made up of denser rocks like peridotite and magnesium.
The Gutenberg discontinuity separates the mantle and the outer core. The core is denser than the mantle and is mainly made up of iron.
❚❚ Physical layers. The lithosphere is the solid and very rigid surface that covers the Earth’s surface. The asthenosphere and mesosphere layers are found under the lithosphere. They are solid, but are more flexible than the lithosphere. The endosphere consists of the core. It is mainly liquid (outer core) and the inner core is solid.
Physical layers of Earth
z
Apply
14. The mantle is made up of a peridotite, a rock rich in iron and magnesium. Explain why the mantle is denser than the crust.
Peridotite
6 371 km 6 371 km
inner core
outer core
lower mantle
upper mantle
oceanic crust 5-10 kmcontinental crust
30-70 km
Moho discontinuity continental lithosphere
oceanic lithosphere
asthenosphere
2 900 km
5 150 km
mesosphere
endosphere
outer core (liquid)
inner core (solid)
Gutenberg discontinuity
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2.3. Vertical movements: isostasy
The advancement of technology and the development of new techniques have increased scientists’ knowledge and understanding of why there are highs and lows along the surface of the Earth. These changes in elevation4 are due to an interaction between the lithosphere and the asthenosphere.
The lithosphere, the rigid surface layer of the Earth, rests on the asthenosphere, which although it is solid, has a flexible behaviour. If a heavy weight forms on the surface of the lithosphere, it sinks into the asthenosphere, making a dent in the surface of the Earth.
These vertical movements are very slow (1 cm/year) and, due to the rigidity and thickness of the lithosphere, large variations in mass are needed for them to occur. This is what happens in the formation or melting of ice caps, the deposition of large quantities of sediments and in the elevation or erosion of mountains.
Isostasy is the equilibrium between the lithosphere and the underlying asthenosphere. If the weight on the lithosphere increases, it is pushed into the asthenosphere, if the weight is reduced, it rises.
lithosphere
asthenosphere(flexible)
1. starting position 2. formation of an ice cap
5. rebounding 3. sinking due to weight
4. melting and rising
Isostatic rebound
Apply
18. Explain, using isostasy, if the lithosphere would rise or fall in these situations:
a) Erosion of a mountain range
b) Deposit of sediment in a sedimentary basin
c) Rising of a mountain range
d) Formation of an ice cap
mountain range
lithospheric depressionasthenosphere
Lithospheric depression
Understand
17. Using isostasy, explain what is causing the lithospheric depression.
4elevation: height, altitude
111. Tectonic plates
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Understand
19. Explain the difference between the crust and the lithosphere.
20. Which is thicker, the continental or oceanic lithosphere? If the average thickness of the upper mantle is 150 km, what is the thickness of each type of lithosphere?
2.4. The crust and the lithosphere
The lithosphere is the rigid surface of the Earth. It includes the crust and upper mantle. It is not a continuous layer, but is broken into pieces called plates.
The graph below shows the variation in the density, temperature and melting point of the materials in the Earth’s interior. As you can see, the melting point increases with depth. This is due to the effect of pressure, so iron, which melts at 1 550 °C on the surface, is solid in the centre of the Earth, at 6 000 °C.
Moho
flexible lower mantle(asthenosphere)
continental crust(30-70 km)
rigid upper mantle
oceanic crust (5-10 km)
continental shelf
cont
inen
tal l
ithos
pher
e
crus
tm
antle
oceanic lithosphere
The relationship between the crust, lithosphere and mantle
tem
pera
ture
(ºC
)
6 000
depth (km)4 0002 000
2 000
06 000
mantle outer core inner core
4 000
dens
ity (g
/cm
3)
2
4
6
8
10
12
0
density melting point temperature
Density, temperature and melting point of rocks related to depth
Analyse
21. What does the sudden change in density at 2 900 km represent?
22. Which areas are liquid and which are near to melting?
Analyse
23. Explain how we discovered that the Earth has a liquid core below a depth of 2 900 km.
24. Explain, using isostasy, why it is so difficult to completely fill a river basin with sediment or erode a mountain range on Earth.
Key concepts
❚❚ The analysis of seismic waves using seismographs is the main method used for studying the Earth’s interior.
❚❚ The Earth's interior is divided into layers according to two criteria: their chemical composition or physical state.
❚❚ The lithosphere, that includes the crust and rigid upper mantle, rests on the asthenosphere, which is flexible.
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3. STUDYING THE OCEAN FLOOR
At the beginning of the 1960s, it was said that we knew more about the landscape of the Moon than the ocean floor. But during the Cold War, nuclear submarines needed accurate5 maps of the ocean floor. This led to the development of ocean floor maps.
Ocean trenches. Narrow, deep channels, usually found next to continental boundaries or island arcs.
Volcanic archipelagos. A chain of volcanic islands next to hot spots.
Island arcs. Curved chain of volcanic islands, next to ocean trenches.
Continental slope. The area between the continental shelf and the abyssal plain.
Seamounts and guyots. Volcanic landforms. Seamounts have pointed tops and guyots have flat tops.
Abyssal plain. A large flat area on the ocean floor.
Newfoundland
Peru-Chile Trench
Hawaiian Islands
New Zealand Patagonia
California
An
des
Rocky Mountains
1
A
C
DG
2
3
4 5
6
8
5accurate: detailed, precise
131. Tectonic plates
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Understand
26. Use an atlas or Internet application such as Google Maps or Google Earth to find the names of the island arcs (1– 8) and the ocean ridges (A – G).
❚❚ Sonar
Detailed maps of the ocean floor are created using sonar. Sonar sends out sound waves and measures the time it takes for the echo to return from an object. From this measurement, the distance of objects can be calculated in order to map the surface of the ocean floor. This map shows the main ocean floor landforms.
Ship using sonar
Apply
25. Use the diagram to explain how sonar works.
Ocean ridges. Very long underwater mountain ranges.
Rift. Big faults found in ocean ridges.
Continental shelf. The edge of the continent that lies underwater.
Iceland
North Sea
NewfoundlandAlps
Ural Mountains
Himalayas
Red Sea
Rift
Val
ley
Madagascar
Patagonia
Caucasus Mountains
India
Scan
dina
vian
Mou
ntai
ns
B
E
F
H
7
Transform faults. Fractures in the oceanic crust that move horizontally. They cross the rift of ocean ridges perpendicularly.
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3.1. Oceanic landforms
The use of sonar, deep sea drilling and the marine magnetometer6 has completely changed our vision of the ocean floor and our understanding of the dynamics of the Earth. Maps of the ocean floor show that there are many different landforms, from flat plains (abyssal plains) to large volcanic mountains. Notable landforms are the Mid-Ocean Ridge and ocean trenches.
❚❚ Mid-Ocean Ridge. All the ocean ridges connect to form an enormous mountain chain of over 60 000 km long and 2 000 km wide. Along its length there is a central fault known as a rift, crossed by numerous perpendicular fractures called transform faults.
❚❚ Trenches. These are narrow, deep channels usually found next to continental boundaries or volcanic island arcs, especially in the Pacific. The deepest, the Challenger Deep, part of the Mariana Trench, reaches 11 km.
❚❚ Other landforms
❚• The abyssal plain is 4 km deep.
❚• Island arcs are archipelagos associated with trenches. They are some of the most active volcanic areas on the planet. For example, Japan, Aleutian Islands, Mariana Islands and the Philipines.
❚• Seamounts and guyots are ancient underwater volcanic landforms. Seamounts have pointed tops and guyots have a flat top.
❚•Volcanic archipelagos, such as the Hawaiian Islands, are chains of volcanic islands. They are not associated with trenches, like island arcs, but with hot spots.
Maps also show big differences between different oceans, especially between the Atlantic and the Pacific.
The Atlantic Ocean does not have many trenches, while the Pacific Ocean is almost completely surrounded by a large network of trenches. Some of these trenches are located along the continent; and some are located next to island arcs and inland seas.
Analyse
27. What differences can you see between these two oceans?
Create
28. Guyots are extinct and submerged volcanoes, with flat tops. Make a hypothesis to show the formation of guyots. What effect do you think their formation had on isostasy?
6marine magnetometer: an instrument used to measure magnetic fields under water
151. Tectonic plates
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3.2. The composition of the ocean floor
Before our exploration of the oceans, some scientists believed that the oceans were huge bowls that filled up with sediment. However, areas were found with no sediment or with a very thin layer.
During the 1960s, several oceanographic expeditions, such as the DSDP (Deep Sea Drilling Project), took samples from the ocean floor that revealed new information:
❚❚ The ocean floor is made up of volcanic rock covered in marine sediment. The volcanic rock has pillow-shaped formations, called pillow lavas, and magmatic rocks. They were found just under a thin layer of marine sediment.
❚❚ These rocks are very young; there are no ocean floor rocks older than 180 million years, which is very little compared to the Earth’s age (4.5 billion years). When they looked at where the different aged rocks were, they found that the rocks closest to an ocean ridge were younger than those further away from the ridge.
The diagram below shows the differences between the oceanic and continental crust.
Key concepts
❚❚ Until the 1960s, the geography of the ocean floor was virtually unknown.
❚❚ Notable ocean landforms are ocean ridges and trenches.
❚❚ The oceanic crust is younger, thinner and denser than the continental crust.
The diagram also shows that the continental crust is not only the areas of the continents above water; it also occupies the underwater shallow areas that surround them, such as the continental shelf.
pillow lava marine sedimentcontinental platform volcanic rock
metamorphicrock
sedimentary rock
granite
plutonicrock
gneiss gneiss
dykesgabbro rocks
Analyse
29. Compare the thickness, age and density of the continental and oceanic crust.
30. What happens to the thickness of sediments further away from the ridge? What could be the cause of this?
The difference between the oceanic and continental crust
Create
31. Describe the landforms found on the ocean floor that are not found on the continents.
32. Find information about the first big oceanographic expedition carried out by the Challenger, in the 19th century. Compare its technology with modern oceanographic ships.
Pillow lava
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4. THE BIRTH OF PLATE TECTONICS
During the Cold War there was a need to know what nuclear tests the enemy was carrying out. To do this, a network of seismographs was set up7 around the globe. These instruments detected any vibration of the ground, including those produced by atomic explosions and natural vibrations, which could be distinguished8 by analysing their seismic waves.
4.1. The distribution of earthquakes and volcanoes
All the natural vibrations recorded by the seismographs were mapped and compared to the location of volcanoes. These maps showed something surprising: earthquakes are distributed in thin lines, called seismic belts, and they are abundant in areas where volcanoes are found.
Scientist began to relate the release of magma from the Earth’s interior and seismic activity. They could also see that there was a relationship between the location of seismic and volcanic activity and the position of ocean trenches and ridges. They therefore concluded that seismic and volcanic activity must be related to their formation.
epicentres of earthquakesmain earthquakes in the 20th century location of volcanoes
Global distribution of major earthquakes and volcanoes
Analyse
33. Compare the map of the ocean floor with the above map. Where are the seismic belts located?
34. Find the names of ten active volcanoes or recent earthquakes. Does their location correspond to the map above?
35. What is the Ring of Fire?
Evaluate
36. All earthquakes on Earth are detected relatively close to the surface. Why do you think they are not detected in the lower mantle or the core?
7set up: way in which something is organised, planned or arranged8distinguish: recognise something as different
171. Tectonic plates
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4.2. Tectonic plates
Seismic belts showed that the lithosphere was fragmented, similar to pieces of a large jigsaw puzzle.
Depending on the type of boundary between plates, there are varying levels of risk of earthquakes and volcanic eruptions.
4.2.1. Types of plate
Lithospheric plates can be classified according to two criteria:
❚❚ Size. There are eight large plates and many smaller fragments called microplates.
❚❚ Type of lithosphere. There are oceanic plates, which are composed of oceanic lithosphere, continental plates, which are composed of continental lithosphere, and mixed plates, which are composed of both oceanic and continental lithosphere.
As Wegener discovered 50 years earlier, these plates are mobile. They can increase and reduce in size, fragment or join each other. The study of the dynamic nature of these plates is called plate tectonics.
A tectonic plate, also known as a lithospheric plate, is a fragment of the lithosphere. Each plate is separated by a seismic belt and volcanoes.
Solid pieces of lava floating on a lava lake.
Understand
37. What are the similarities and differences between the above photo and lithospheric plates?
Lithospheric plates map and their movement
Apply
38. Find out the names of the eight large plates shown in the map. Then find an example of an oceanic, continental and mixed plate.
39. Choose five pairs of plates and describe their movement relative to each other.
40. The Mid-Atlantic Ridge runs the length of the Atlantic Ocean. What do you notice about the movement of the plates that lie along it?
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4.3. Sea-floor spreading
Drilling surveys9 not only showed that the ocean floor contains many volcanoes, they also showed that the age of rocks formed from their lava flows varies depending on their position relative to the ridge.
The diagram below shows the ocean floor of the Atlantic. You can see that the rock closest to the Mid-Atlantic Ridge is the youngest and that there is no marine sediment, or only a thin layer, in this area. This is because not enough time has passed for the sediment to accumulate.
The plates at the Mid-Atlantic Ridge move apart, and when they do magma rises up from the mantle and flows out as lava through the rift. As this new lava moves up, it pushes out the older lava deposits. As the lava cools it turns into rock, forming symmetrical bands on both sides of the ridge and creating new ocean floor. This process is known as sea-floor spreading.
As the ocean floor expands by sea-floor spreading, the continents move away from each other. Science was finally able to explain how the continents could move such great distances. So at last, Wegener’s theory of continental drift was accepted by the scientific community.
Mid-Atlantic Ridge
from 0 to 65 million years old
from 65 to 130 million years old
from 130 to 180 million years old
rift
North America Africa
180 million years ago
130 million years ago
65 million years ago
present day
Apply
41. What do you think the Atlantic Ocean was like 65 and 130 million years ago? What has happened since then?
42. Explain the geographical history of the Atlantic Ocean using the above diagrams.
Ages of rocks in the Atlantic OceanSea-floor spreading during the separation of Africa and North America
9survey: an investigation of something
191. Tectonic plates
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4.4. The subduction of the ocean floor
The ocean floor continually expands from the ridges but the surface of the Earth does not expand, and no very old rocks were found on the ocean floor, so it was thought that there must be something destroying the crust.
The areas where it was suspected that this could happen were at the trenches. In the trenches, the oldest part of the ocean floor seemed to bend and sink into the mantle. So while oceanic lithosphere is constantly being formed at the ridges, it is also continually being destroyed at the trenches, by a process called subduction.
Subduction is the process in which the oceanic lithosphere moves under the continental lithosphere (or oceanic lithosphere in the case of island arcs) and into the mantel.
Key concepts
❚❚ Tectonic plates are fragments of the lithosphere, separated by seismic belts and volcanoes.
❚❚ The ocean floor is continually generated at a very slow rate at ocean ridges, by a process known a sea-floor spreading.
❚❚ As the process of sea-floor spreading generates new oceanic lithosphere, it pushes the oldest lithosphere under the continental lithosphere, or oceanic lithosphere in the case of island arcs, by a process known as subduction.
❚❚ Sea-floor spreading occurs at ridges and subduction occurs at trenches.
Understand
43. At ridges the plates move away from each other, this is a divergent boundary. At trenches we find convergent boundaries. What is the movement of the plates in convergent boundaries?
Analyse
44. The pictures below show the process of subduction at an ocean trench. Look at the pictures and then answer the questions.
a) Where does the magma that feeds the volcano come from?
b) What happens to the marine sediment that covers the ocean floor?
c) What physical properties of the oceanic lithosphere help it sink below the continental lithosphere?
45. The Andes Mountains contain many volcanoes such as Aconcagua. Use the illustrations above to describe how these volcanoes were formed.
Evaluate
46. Look back at the map showing global earthquake and volcano distribution on page 16. What are the differences between the Mid-Atlantic Ridge and the Pacific’s Ring of Fire?
Geologist Harry Hess compared the ocean floor to a conveyor belt. It rises up from the ridges, moves outward from the ridge and sinks down into the trenches as it reaches the continental lithosphere.
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5. THE THEORY OF PLATE TECTONICS
Hess’s observations about the extension of the ocean floor, along with the analysis of the global distribution of earthquakes and volcanoes, led to the theory of plate tectonics:
❚❚ The lithosphere is divided into large pieces called plates. They cover the surface of the Earth and fit together like a jigsaw puzzle.
❚❚ Most volcanoes and earthquakes, which are caused by the internal geological activity of the Earth, are located at the borders between the plates.
❚❚ The ocean floor is continually being formed at ocean ridges, by sea-floor spreading, and being destroyed at the trenches, by subduction.
❚❚ Plate movement moves the continents. Where two plates move apart new oceans are generated; where they come together and collide, mountain ranges rise.
With the new information about the ocean floor, earthquake and volcano distribution, it was clear to Hess that the whole lithosphere (the continental and oceanic) was constantly moving, and not just the continents, as Wegner had thought. Recently, thanks to global positioning system (GPS), the movements of the plates have been verified.
Plate tectonics explained many geological phenomena that appeared not to be related to each other:
❚❚ The distribution of seismic and volcanic activity.
❚❚ The present and previous distribution of the continents and oceans.
❚❚ The formation of mountain ranges.
❚❚ The formation and destruction of ocean floor.
❚❚ The distribution of minerals and fossil fuels.
In addition to describing the movement of the continents to their present position, plate tectonics allows us to make predictions about their future movement, as shown in the diagram below:
Analyse
48. Look at the map and compare it to a present day map. Where do you think new oceans, mountain ranges and continental rifts will form?
49. How will the relationship between the African and Eurasian plates change?
50. What is predicted will happen to the Australian continent? What will form in its place?
Predicted location of the continents in 150 million years
It is possible to locate a point on the Earth's surface with an accuracy of a few centimetres using satellites.
Understand
47. How has GPS helped to confirm the movement of the continents?
211. Tectonic plates
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5.1. How do the plates move?
The theory of plate tectonics has been accepted by the scientific community for more than 50 years, even though it is still not completely clear how the movements of the plates are produced. The current theory of plate movement has evolved over time from the original hypothesis.
5.1.1. Original explanation
It was widely accepted that the lithosphere ´floats´ on the asthenosphere. It was thought that the convection currents in the asthenosphere caused tectonic plate movement. It was believed that in the areas where hot currents rise and separate, ridges are formed. In the areas where currents cool and descend, trenches are formed.
Understand
51. Look at the diagram and explain how convection currents are generated, taking into account the differences in temperature and density that take place.
Convection currents
Fg: movement down the ridge because of gravity
Fp: movement because of the weight of the subducting plate
Fg
Fp
Mid-Ocean Ridge
Understand
52. What two forces are acting on the plate to cause it to subduct?
Evaluate
53. What force do you think is more important? Explain your answer.
Forces that act upon the oceanic lithosphere
5.1.2. Current explanation
New seismic tomography10 techniques have increased scientists’ understanding of the dynamics of the Earth's mantle and how the plates move. The lithospheric plates do not just float on the asthenosphere, but also actively contribute to their own movement in two ways:
❚❚ On elevated ridges, the force of gravity pulls down on both sides of the plate.
❚❚ Once a plate is being subducted, the weight of the plate pulls it down lower. The plate sinks down to the edge of the core.
In conclusion, heat rises from the core and makes the mantle ductile. It also generates convection currents in the mantle. These convection currents, combined with the forces of gravity and weight that act on the plate, cause the plates to move.
subduction zone subduction zone Mid-Ocean Ridge
lithosphere
asthenosphere
convectioncell
convectioncell
Diagram of convection currents according to the original theory
Earth’s internal heat, that keeps the Earth geologically alive, was created from the impact of all the different fragments that formed our planet, and from the disintegration of different radioactive elements found in the core.
Understand
54. Why is the Moon, despite being so close to Earth, a geologically dead object, with almost no internal activity?
10tomography: technique used for showing a representation of a cross-section of an object using X-rays or ultrasounds
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1
Apply
56. Where on Earth do we find steps 2 and 3 of the Wilson cycle?
57. Explain how the African Rift Valley and Red Sea will look in the future.
5.2. The Wilson Cycle
Wegener had no idea that there was at least one other supercontinent before the formation of Pangaea.
Canadian geologist John Tuzo Wilson (1909-1993) was the first to propose its existence. He suggested that throughout the history of the Earth there have been two cyclic processes of rifting and reuniting of supercontinents, this process is called the Wilson cycle.
5.2.1. Continental rifting and opening of ocean basins
A clear example of continental rifting is found in the Rift Valley in East Africa. As you can see on the map on the left, the continental rift is at a divergent plate boundary. When the plates move apart, magma rises up through the fractures as lava, pushing the continental lithosphere on either side further apart. If the process of rifting continues, this continent will end up splitting in two.
This is what has happened in the Red Sea. The Red Sea Rift separates the Arabian Peninsula from Africa. It once was a continental rift, but now it is an oceanic rift, generating oceanic lithosphere.
The Atlantic is an example of an expanding ocean. Its size is increasing steadily due to the production of new oceanic lithosphere (at the Mid-Atlantic Ridge) and the fact that it is no longer surrounded by subduction zones.
continentalcrust
continentallithosphere
Moho
continental rift
3. Narrow sea stage. The continent has completely split and separated. New oceanic lithosphere and a small ridge begin to form between the two sides.
continental crustoceanic crustocean ridgeactive volcanonormal fault
Understand
55. What type of plate movement is at the Rift Valley and the Red Sea Rift above?
1. A dome forms. The heat under the continent causes the crust to expand and lift up.
2. Continental rift stage. Large fractures appear which make the lithosphere thinner and lead to the formation of a continental rift.
4. Maturing ocean stage. The divergence continues and the formation of oceanic lithosphere due to sea-floor spreading increases. The Atlantic Ocean is an example of this stage.
The African Rift Valley and the Red Sea
231. Tectonic plates
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Understand
58. In your own words, explain how the movement of the tectonic plates causes the opening of new oceans and the closing of others.
Evaluate
59. Marine fossils have been found in the Pyrenees, the Alps or the Himalayas. How did they get there?
5.2.2. Closing of ocean basins
The oceanic lithosphere becomes progressively older the further it is from the ridge. It also becomes colder and denser. Eventually, this causes the lithosphere to sink down into the mantle, which results in the formation of trenches and subduction zones. From this stage onwards, the ocean, which until this point had been increasing in size, starts to shrink. This is the current situation in the Pacific Ocean, which has many trenches at its boundaries and is getting progressively smaller every year.
5.2.3. Continental collision
40 million years ago the Indian continent collided with the southern edge of Asia. Marine sediment had accumulated on the edge of both continents when there was an ocean between them. This sediment folded and deformed to create what is now the Himalayas. The marine fossil remains in these mountains are evidence that these rocks once formed part of an ancient ocean floor.
oceanic l
ithosphere
continentalcolision
suture
Key concepts
❚❚ Plate tectonics is a global theory that describes the movement of the lithosphere.
❚❚ The internal heat of the Earth and forces acting on the plates (gravity and their own weight) cause their movement.
❚❚ The cyclical opening and closing of oceans can be described by the Wilson cycle.
5. Shrinking ocean stage. The ocean begins to close because of subduction on its boundaries. The Pacific Ocean is currently at this stage in the cycle.
6. Convergence stage. The ocean has almost closed. The continents converge with marine sediments on their edges.
7. Continental collision stage. The boundaries of both continents and the sediment trapped between them are deformed.
8. Final stage. The continental masses are joined together and a mountain range is formed from the collision of the continents. Eventually the plate boundary will become inactive.
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1CONSOLIDATION
Wegener’s theory of continental drift
60. Answer these questions about the theory of continental drift:a) Who created the theory and when?
b) What did the theory state?
c) Describe three pieces of evidence that supported the theory.
d) Why were many scientists not convinced of the theory at the time it was published?
61. Wegener believed that the continents could slide over the ocean floor. Do you think that this is possible? Explain your answer.
62. When and why was Wegener’s theory finally accepted by the scientific community?
The structure and composition of the Earth
63. List the different methods used to study the Earth. Which is the most important?
64. If we conducted two drilling surveys, one on land and one in the ocean, 15 km and 3 km deep respectively, would we reach the mantle in any of them? Explain your answer.
65. Answer the questions:a) What discontinuity boundary separates the core and the
mantle?
b) What is the physical state of the materials found above and below this discontinuity boundary?
c) Which of the Earth’s layers is the thickest? The mantle, outer core or inner core?
d) Is the crust the same as the lithosphere? Explain your answer.
66. Look at the diagram showing P and S waves travelling through the Earth.
S wave shadow zone
only P waves
mantlecrust
core
earthquake epicentreP wave
shadow zone
a) What is the difference between P and S waves?
b) How do we detect them?
c) What is the S wave shadow zone?
d) What would the S wave shadow be like for the following planets?
• A planet with a core bigger than the Earth’s core.
• A planet with a core smaller than the Earth’s core.
• A planet with a completely solid interior.
67. What is isostasy? Explain how it is possible that the ocean floor at a depth of 0.5 km accumulates more than 3 km of sediment.
Studying the ocean floor
68. List three differences between the oceanic and continental crust, and between trenches and ocean ridges.
69. Can we say, from a geological point of view, that the continents are masses of emerged land? Explain your answer.
70. Indicate the position of the rifts and transform faults in the picture below, then define each term.
71. Why did scientists find much less marine sediment on the ocean floor than they had expected?
72. Describe the movement of the plates at ridges and trenches.
73. Knowing that sound travels in water at a speed of 1 500 m/s, calculate the depth of the ocean floor when the echo delay was 2 s, 6.5 s, 0.25 s and 22.5 s.
74. How did sonar help with the development of the theory of plate tectonics?
The birth of plate tectonics
75. Are earthquakes randomly distributed over the Earth or are they concentrated in certain areas?
76. What are the areas described above called? What do they represent?
77. Which ocean would expand faster: an ocean with ocean ridges or an ocean with ocean ridges and trenches? Explain your answer.
78. What is subduction and where does it happen?
79. The Mid-Atlantic Ridge passes through Iceland. What will happen to the island with the passage of time?
80. Does the west or east coast of South America have a higher risk of earthquakes? Explain your answer.
251. Tectonic plates
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81. Look at the graph below and answer the questions:se
dim
ent
thic
knes
s (m
)
distance from the axis of the ridge (km)
010
4035302520151050
20304050607080
a) What variables does this graph represent?
b) What can you conclude from the graph?
82. What landforms are created in the following subduction zones: oceanic-oceanic and oceanic-continental? Give an example of each.
The theory of plate tectonics
83. Continent A moves at a speed of 2.5 cm/year, and continent B moves at 4.5 cm/year. They are 560 km apart. Answer the following questions:
AB
a) When will continent A and B collide with each other?
b) What will form as a result of the collision?
c) What is happening to the oceanic lithosphere between these continents?
d) How many plates are represented? Classify them according to their lithosphere.
e) What will happen to the ocean with the passage of time?
f) Copy the drawing in your notebook and label the: continental lithosphere, oceanic lithosphere, subduction zone, magma and volcanic arc.
84. Mars is much smaller than the Earth and its interior has been cold for many millions of years. Do you think that it has tectonic plates? Explain your answer.
85. What is the Wilson cycle? Explain each step and give examples.
The baked apple theory
Wegener’s theory didn’t attract much attention at first, but by 1920, when he produced a revised and expanded version, it quickly became a subject of discussion. Everyone agreed that continents moved – but up and down, not sideways. The process of vertical movement, known as isostasy, was a foundation of geological beliefs for generations, though no one had any good theories as to how or why it happened. One idea, which remained in textbooks well into my own school days, was the ‘baked apple theory’, proposed by Eduard Suess, just before the end of the century. Suess suggested that as the molten Earth cooled, it had become wrinkled like the skin of a baked apple, creating ocean basins and mountain ranges.
Bill Bryson A short history of almost everything, Lulu Press, 2014
(adapted)
a) The text mentions two scientists, one believed that the continents were mobile and the other believed they were immobile. Who are they?
b) What theory is being discussed?
c) How did the scientists that believed the continents were immobile explain the formation of mountain ranges?
d) Do you think that science is a static or dynamic area of study? Explain your answer using the text.
READ AND UNDERSTAND SCIENCE
STUDY SKILLS
❚ Create your own summary of the unit using the Key concepts. Add any other important information.
❚ Copy the following diagram and add the missing information to create a concept map of the unit.
You can record your summary and listen to it as many times as you like to revise.
❚ Create your own scientific glossary. Define the following terms: continental drift, Pangaea, P wave, S wave, survey, seismic discontinuity, crust, mantle, core, continental shelf, lithosphere, isostasy, rift, trench, ocean ridge, Mid-Ocean Ridge, abyssal plain, continental slope, guyot, seamount, island arc, sonar, plate, seismic belt, transform fault, sea-floor spreading, subduction, Wilson cycle, continental rift, convection current.
...
which are separated by their movements create
... ... ... ...
Internal heat
causes the movement of
...
which are classified according to their
which are studied by
...
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3WORK AND EXPERIMENTATION TECHNIQUES
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Checking a scientific hypothesis: Hot spots
1. What data do we need to check Wilson’s hypothesis?
2. Look at the graph you have drawn. Do you see a relationship between the age of the islands and their distances from the hot spot?
3. Using your knowledge about isostasy and external geological processes, explain why the islands are getting progressively lower towards the northwest.
4. Calculate the average speed at which the Pacific plate moves over the hot spot. To do this, take a section of your graph and use the formula: velocity = distance/time. What direction does it move in?
5. In your own words, explain how these islands were formed.
6. The Emperor seamount chain is bent and stretches out north-east of the Hawaiian Islands. How would you explain this change of direction?
7. Just off the south-east coast of Hawaii there is an active submarine volcano, Loihi. Where did it come from?
8. Why are there no islands south-east of Hawaii?
1. Draw the x and y-axis of your graph. Place distances on the x-axis and ages on the y-axis.
2. Mark the values of each axis: distances in increments of 500 km and ages in increments of 5 million years. Look at your largest values and make sure that there is enough room to place them on your graph.
3. Mark all the points of all islands whose age and distance to the hot spot is known.
4. Draw a straight line through your graph, including as many points as possible.
Procedure
Scientists create a hypothesis to explain an initial observation. They then have to test it and either confirm that it is correct, or modify it, depending on the data they collected.
The origin of some archipelagos, like Hawaii, has intrigued geologists for many years. They undoubtedly have a volcanic origin, appear perfectly aligned, but only one island is volcanically active. The rest of the volcanic islands are inactive and get progressively lower until they become submerged underwater; like the Emperor seamount chain, which stretches north-east of the Hawaiian Islands.
John T. Wilson proposed a hypothesis based on the theory of plate tectonics. He hypothesised that these islands were formed as the tectonic plate slowly passed over a fixed point of heat from deep within the Earth: he called these points hot spots.
Materials
❚❚ squared or graph paper
❚❚ data for the age and distance to the hot spot
❚❚ drawing tools: pencil, ruler
❚❚ calculator
Hawaii
Hawaiian Islands
Hawaiian seamount
chain
Emperor seamount
chain
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1. Tectonic platesz
FINAL TASK+www
Technological advances and the history of science
To present the results of your research, make a summary table:
a) You can present your table on paper, using a word processing program such as Word, or as a large wall mural.
b) For each scientific instrument/technique you should include the name of the inventor, the decade in which it was invented and the information that it revealed about the Earth.
c) It may be helpful to include images; you could order these from oldest to newest to show the advancement in technology.
2. Presentation
Deep drilling revealed the volcanic nature of the ocean floor. By studying the microfossils present in the marine sediment deposited on top of the volcanic rock, it was discovered that the sediment was much younger that expected.
a) What scientific methods have allowed us to understand more about the Earth’s interior and the ocean floor?
b) What new information have they given us on these fields of study?
c) How have they changed our view of the planet?
1. Research
Follow these steps for your research:
Search for information
❚❚ Write down the technical advances that have appeared throughout this unit, for example, seismographs, sonar, GPS…
❚❚ Explain how each of these pieces of technology contributed to the knowledge of our planet.
❚❚ Describe how each of these scientific instruments/techniques function. The textbook gives a lot of detail about the functioning of some of these methods, for others you will need to use the Internet.
Organise the information
❚❚ Create an individual data sheet for each method that you investigate.
❚❚ Order the scientific instruments/techniques from oldest to most recent.
❚❚ Choose the format that you are going to use to present your results.
Obtain conclusions and verification
❚❚ Check with your classmates that you have not forgotten any piece of relevant technology.
❚❚ Verify that you have collected all the information that you need about each scientific instrument/technique.
❚❚ Check that your final piece of work shows how scientific knowledge evolves over time along with advances in technology, especially in scientific areas that cannot be studied directly, such as the Earth’s interior.
Procedure
❚ Answer the following questions to evaluate your work:
1. Have you included enough examples of instruments/techniques?
2. Have you included all the information required about each one?
3. Have you been able to order your information in a way that clearly shows how the advancement in technology has improved our knowledge of the Earth?
4. Have you taken care of your organisation, writing and visual representation?
SELF-ASSESSMENT
The oceanographic surveys that were carried out in the 1960s, such as the Deep Sea Drilling project (DSDP), Ocean Drilling Program (ODP) and International Ocean Discovery Program (IODP), changed our entire view of
the ocean floor. Their data helped confirm Wegener’s hypothesis of continental drift and revealed the internal dynamic nature of our planet as a surface of fragmented lithospheric plates, continually moving, growing and being destroyed.