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Geology F791 > Module 1: Earth structure and planetary geology > Flashcards

Module 1: Earth structure and planetary geology Flashcards

1
Q

What is the depth of the Lehmann Discontinuity?

A

5100km

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2
Q

What are the characteristics of the Lehmann discontinuity?

A
  • Boundary between material of the same composition but in different states
  • rocks change from all liquid in the outer core to a solid in the inner core
  • outer core - inner core
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3
Q

What is the depth and probable composition of the outer core?

A
  • 5100km - 2900km
  • liquid iron nickel
  • pressure is less than that of the inner core, allowing a liquid to exist
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4
Q

What’s the depth and probable composition of the inner core?

A
  • 6371km - 5100km
  • solid material due to extreme pressure
  • mixture of iron and some nickel.
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5
Q

What is the depth of the Gutenberg discontinuity?

A

2900km

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6
Q

What are the characteristics of the Gutenberg discontinuity?

A
  • change from silicate material to metallic (iron and nickel)
  • changes state from the solid lower mantle to the liquid outer core
  • p wave velocity decreases and S waves stop altogether
  • lower mantle - outer core
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7
Q

What is the depth and probable composition of the lower mantle?

A
  • 2900km - 700km
  • solid (S waves can travel through it)
  • made of the same type of silicate material as the stony meteorites
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8
Q

What is the depth and probable composition of the upper mantle?

A
  • 700km- average of 35km
  • solid silicate material which is less dense that that of the lower mantle
  • main rock is peridotite
  • part lies within the lithosphere while part is in the asthenosphere
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9
Q

What is the thickness of the crust in oceanic areas?

A

5-10km average 7km

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10
Q

What is the thickness of the crust in continental areas?

A

Up to 90km under highest mountains, average is 35km

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11
Q

What is the composition of the oceanic crust?

A
  • Rich in Fe and Mg
  • basalt (pillow lavas)
  • dolerite (dykes)
  • gabbro in layers
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12
Q

What is the composition of the continental crust?

A
  • Rich in Al and Si
  • granitic rocks
  • igneous, metamorphic and sedimentary rocks - deformed
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13
Q

What are the ages of oceanic and continental crust?

A

Oceanic - 200 Ma

Continental - 4000 Ma

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14
Q

What are the characteristics of the mohorovic discontinuity?

A
  • Boundary between the upper mantle and crust

- depth varies depending on whether it is below oceanic or continental crust

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15
Q

What is a rheid?

A

A solid material that flows

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16
Q

Why do P and S waves slow down in the aesthenosohere?

A

Less rigidity

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17
Q

Why are the characteristic of the lithosphere?

A
  • Rigid and brittle (temperatures are much lower)
  • Can’t flow but is carried by the underlying asthenosphere
  • broken into plates
  • contains part of the upper mantle and crust
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18
Q

What is used as indirect evidence of the structure of the earth?

A

Seismic waves, density and meteorites

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19
Q

P and S waves are example of what type of seismic waves?

A

Body waves (travel through the layers of the earth)

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20
Q

When do P and S waves slow down and speed up?

A

Faster - more rigid and more incompressible

Slower - rock becomes more dense

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21
Q

What is the shadow zone?

A

An area where no P and S waves are recorded

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22
Q

What are the angles of the shadow zone?

A

103 degrees - 142 degrees form the epicentre

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23
Q

Where is the shadow zone for S waves?

A

Beyond 142 degrees from the focus

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24
Q

What is the probable origin of the earths magnetic field?

A

The convecting mass of molten iron in the outer core generates electricity. This induces magnetism which generates more electricity and more magnetism and so on.

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25
Q

What is the self exciting Dynamo effect?

A

The continuos generation and destruction of magnetism and electricity.

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26
Q

Why must the earths magnetism be constantly gerneated?

A

The temperature of the outer core is well above the curie point at which materials lose their magnetism so it can’t be permanent.

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27
Q

What is paleomagnetism?

A

Ancient magnetism preserved in rocks

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28
Q

What is remnant magnetism?

A

Magnetism shown by rocks due to the alignment of their magnetic minerals according to the earths magnetic field at the time of their formation.

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29
Q

What is magnetic inclination?

A

The angle of dip of the lines of a magnetic field. It is the angle with the horizontal made by a compass needle.

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30
Q

How could you measure the latitude of a volcano at the time it erupted its lavas?

A

Work out the direction of the magnetic poles (a freely suspended compass needle lies exactly vertical at the magnetic poles). Then you work out where the magnetic equator is (compass needle is exactly horizontal). Next you measure the inclination of the rock.

31
Q

Why is paleolatitude important?

A

Helps to determine the position of continents over time and is used to date continental drift.

32
Q

How do mines and boreholes provide direct evidence for the structure of the earth?

A

Direct access via mines for coal, ores and diamonds. Samples of rocks can be brought back up and analysed.

33
Q

Whats the difference between a mine and a borehole?

A

Mine can be up to 5km deep whereas boreholes can be up to 13km deep.

34
Q

How does density provide indirect evidence for the structure of the earth?

A

Average density of the whole earth is 5.5g/cm3. The continental crust is 2.7g/cm3 and the oceanic crust is 2.9g/cm3. Therefore the core much be very dense to give such a high average.

35
Q

How do meteorites provide indirect evidence for the structure of the earth?

A

The meteorites could be similar in composition to the mantle and the core of the Earth. The metallic meteorites are much denser which suggest they are similar to the earths core. The less denser stony meteorites are probably similar to the mantle.

36
Q

What is geothermal gradient and how does it change with depth?

A

The rate of increase in temperature per unit depth in the earth. It increases with depth.

37
Q

How does volcanic magma provide direct evidence for the structure of the earth?

A

We can estimate the composition of the upper mantle by analysing the fragments of country rock from the upper mantle that have been brought to the surface via magma through kimberlite pipes.

38
Q

What is an ophiolite?

A

When two plates collide, sections of oceanic crust may be broken off a descending oceanic plate and thrust onto the edge of the continental plate instead of being carried down into the mantle.

39
Q

How do ophiolites provide direct evidence for the structure of the earth?

A

They may become exposed due to erosion allowing us to see the sequence of rocks.

40
Q

What happens to P and S waves in the outer core?

A
  • P wave velocity decreases due to the reduction in rigidity

- S waves stop all together because they can travel through liquids

41
Q

What happens to P and S waves in the inner core?

A
  • Both P and S waves move through the inner core

- S waves are generated by the P waves

42
Q

What happens to P and S waves in the lower mantle?

A
  • P wave velocity increases as it is more rigid and more incompressible
  • S waves can travel through it
43
Q

What mnemonic is used to order the planets?

A

My Very Easy Method Just Sums Up Naming Planets

Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto

44
Q

How old is the soar system?

A

4500 million years

45
Q

How was the solar system formed?

A

Giant molecular cloud of gas and dust collapsed. Material formed a rotating disc and as material was drawn to the centre, nuclear reactions were triggered forming the sun. Other materials in the disc of dust stick together, forming planets.

46
Q

Name four terrestrial planets

A

Mercury, Venus, Earth and Mars

47
Q

What are the characteristics of terrestrial planets?

A

‘Earth like’, dense inner cores, smaller, higher temperatures, closer to the sun

48
Q

Name four gas giants

A

Jupiter, Saturn, Uranus and Neptune

49
Q

What are the characteristics of gas giants?

A

Gaseous atmospheres, less dense, larger, lower temperatures, further from sun

50
Q

Which three planets does the moon Io belong to?

A

Jupiter,Mars and Venus

51
Q

How was volcanism on Io first discovered?

A

Fly-by’s of the outer gas giants

52
Q

What method is used to determine the age of rocks?

A

Radiometric dating

53
Q

What two factors are needed to work out the age of a rock?

A

Rate at which the radioactive isotope decays and the amount of radioactive isotope in an object when it formed.

54
Q

Which radioactive isotope is used?

A

Rubidium 87 - strontium 87

55
Q

Why is rubidium 87 used to date planets?

A

Very long half life and the daughter isotope is solid so unlikely to be lost from the rock.

56
Q

What is half life?

A

Time taken for half of the nuclei of a radioactive isotope to decay into another nuclear species.

57
Q

How do you work out half life?

A

Ratio of parent: daughter nuclei

58
Q

What three features does the Earths moon have?

A

Solid crust, mantle and core

59
Q

What is the surface of the Earths moon made up of?

A

Maria and highlands

60
Q

What is the maria?

A

Dark areas composed of basalt lava flows

61
Q

What are the highlands?

A

Light coloured areas composed of plagioclase rich rock

62
Q

What is the largest volcanic structure in the soar system?

A

Olympus mons - Mars

63
Q

What are the characteristics of Mars?

A
  • Large shield volcanoes
  • water trapped in polar ice caps
  • atmosphere = thin and mainly carbon dioxide
64
Q

What are the characteristics of Venus?

A
  • no oceans (desert surface)
  • thick spinning clouds that trap heat
  • very high temperatures
  • very bright (clouds reflect sunlight)
65
Q

Where is the asteroid belt located?

A

Between Mars and Jupiter

66
Q

What are meteorites?

A

Fragments of rock that fall to the earths surface from space

67
Q

Where do meteorites originate from?

A

Asteroids that lie in the asteroid belt

68
Q

What are the characteristics and interpretations of iron meteorites?

A
  • composed of an alloy of iron (fe and Ni)

- represents core of a planet

69
Q

What are the characteristics and interpretations of stony meteorites?

A
  • Composed of silicate minerals

- represent the mantle of a planet

70
Q

What are the Characteristics of carbonaceous chondrites?

A
  • type of stony meteorite
  • contain water and organic compounds
  • similar in composition to sun but with fewer volatiles
71
Q

What are impact craters?

A

Circular depressions with a rim of broken rock formed when meteorites collide with the earths surface.

72
Q

What is the evidence for impact craters?

A
  • material is ejected
  • Quartz grains are shocked and melted
  • rock strata tilted
  • material at depth is broken up
  • ejected material is inverted
73
Q

Why is the asthenosphere less rigid?

A

High temperatures cause partial melting (1-5%) of the peridotite which creates a film of melted minerals around each solid crystal allowing it to flow more easily (acts like a plastic).

Geology F791 (1 decks)

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