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EOS 170 > Lecture 2 - Earth Evolution and Structure > Flashcards

Lecture 2 - Earth Evolution and Structure Flashcards

1
Q

How did the solar system form?

A

it coalesced from a rotating dust & gas cloud formed during a supernova

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2
Q

Accretion

A

the process by which particles in a dust cloud coalesce

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3
Q

How did Proto-Earth form?

A

Accretion occurred when particles accumulated into planetesimals by gravitational attraction, and these accumulated into proto-planets.
These collisions generated intense heat.

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4
Q

How did Earth’s core form?

A

As temperatures rose from repeated impacts, Proto-Earth started to melt and differentiate. Molten iron and other heavy elements were pulled to its centre, forming a core.

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5
Q

How did Earth’s mantle form?

A

Silica (SiO2)-rich rocky material floated upwards, forming an outer mantle and liquid magma ocean.
Friction during melt migration generated yet more heat.

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6
Q

What is the heat generated as Earth formed called?

A

primordial heat

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7
Q

How is Earth’s accretion and differentiation recorded?

A

meteorites - many of which are fragments of proto-planets and planetesimals

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8
Q

What are stony meteorites?

A

Stony meteorites are a class of meteorite made of 75–90% silica (SiO2)-based minerals, and 10–25% nickel-iron alloy

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9
Q

What are chondrites?

A

They are the most common type of stony meteorites.
They have a bubbly texture (chondrules) which shows that these rocks have never been molten – they represent primitive material from before planetary differentiation.
These are the oldest rocks in the Solar System!

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10
Q

What are achondrites?

A

They are a type of stony meteorite, but they lack chondrules.
These originate from the (differentiated) outer silicate mantle of large planetesimals.

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11
Q

What are Stony-iron meteorites?

A

They contain roughly even amounts of silicates and nickel-iron alloy.
They represent the boundary region between the outer silicate mantle and inner metallic core.

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12
Q

What are Iron meteorites?

A

They are made of nickel-iron alloy, and originated in the cores of planetesimals or proto-planets.

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13
Q

Who was William Thompson (Lord Kelvin) ?

A

He was a physicist who, in 1862, estimated the age of the Earth by assuming that it cooled through conduction from an initially molten state.
Using estimates of the geothermal gradient and the melting temperature of rocks, he calculated Earth’s age as ~100 My (million years), far younger than geologists of the time imagined.

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14
Q

What did William Thompson (Lord Kelvin) assume about the earth?

A

He assumed that the Earth cooled through conduction, was rigid, homogeneous, and contained no other sources of energy.

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15
Q

Who was John Perry?

A

He was an assistant of Lord Kelvin. In 1895, he realised that the assumption that the earth was rigid was invalid, and that heat transfer by convection (not conduction) could keep Earth warmer for longer. He got an age of 2–3 Gy (billion years).

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16
Q

Who discovered radioactivity? Why is this important when trying to date earth?

A

Henri Becquerel, in 1896 (a year after John Perry’s discovery).
It provides an additional internal source of heat for the Earth, which was not accounted for by either Kelvin or Perry.

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17
Q

By what technique do we now determine the age of the Earth?

A

Radiometric dating which exploits the known decay rates of radioactive elements (e.g. U,Th, K).

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18
Q

What are the two major sources of earth’s internal heat?

A

decay of radioactive elements

primordial heat

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19
Q

What is radioactive decay?

A

The spontaneous disintegration of the original, parent, nucleus into a different, daughter, nucleus, accompanied by the emission of electromagnetic radiation.
The rate of decay (the half-life, t1/2 ) is different for different radioactive elements.

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20
Q

How long ago did the solar system form? Why do we assume this?

A

~4.56 Gyr (4,560,000,000 years) ago

Radiometric dating of chondrite meteorites

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21
Q

How long may planetary accretion and differentiation have taken? Why do we assume this?

A

~30 million years

Dating of achondrites, stony-iron and iron meteorites

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22
Q

Early Earth was much [?] than today.

Why?

A

… hotter …

Radiogenic heat production has diminished through Earth’s history as radioactive elements have decayed.

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23
Q

What is the outermost rocky layer of earth known as? how dense is it?

A

the crust

Crustal rocks have density 2–3 g/cm³

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24
Q

What is Earth’s average density?

A

5.5 g/cm³

since the density of the crust is only 2–3 g/cm³, density must increase inwards

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25
Q

What density are sedimentary rocks?

A

~2 g/cm³

26
Q

What density is granite?

A

~2.6 g/cm³

27
Q

What density is basalt?

A

~3 g/cm³

28
Q

How was Earth’s mass estimated in 1774?

A

Newton’s Law of Universal Gravitation

Schiehallion experiment

29
Q

How deep is the Kola Superdeep Borehole, USSR?

A

12km

30
Q

What are xenoliths?

A

Foreign rocks brought up by lavas from the underlying mantle.

31
Q

What is peridotite?

A

A xenolith which is the main constituent of the upper mantle.
Its density is ~3.3 g/cm³.
Its green colour is from the mineral olivine.

32
Q

What are Diamond-anvil lab experiments?

A

They show how minerals such as olivine react under increasing temperature and pressure.
They undergo a series of reactions to even denser forms, reaching ~5 g/cm³ at pressures expected of the lower mantle

33
Q

What are two types of earthquake seismic waves?

A

P-waves: faster, can travel through liquid
S-waves: slower cannot travel through liquid.
They are reflected or refracted at major boundaries in the Earth, such as the core-mantle boundary at ~2900 km depth

34
Q

What is the chemical structure of the crust?

A

~0.5% of Earth mass
T: ~0–1000 ºC
Silicate rocks, rich in Al, Na

35
Q

What is the chemical structure of the mantle?

A

~67% of Earth mass
T: ~1000–3000 ºC
Silicate rocks, rich in Fe, Mg

36
Q

What is the chemical structure of the core?

A

~32% of Earth mass
T: ~4000–6000 ºC
Fe-Ni alloy (metal)

37
Q

What is rheology?

A

Rheology describes how materials deform under stress.

38
Q

What is the formula for stress in the context of Rheology?

A

Stress = Force/Area

39
Q

What is Strain?

A

Strain describes the deformation that occurs due to stress

40
Q

What are the different types of stress?

A 
Compression stress:
- perpendicular to surface
- leads to contraction or shortening
Tension stress:
- perpendicular to surface
- leads to extension
Shear stress:
- parallel to surface
- leads to simple shear or distortion
41
Q

What are the various rheologies of solids?

A

Elastic – deformation is recoverable
Brittle – at the elastic limit, the material may fracture
Plastic – deformation is permanent

42
Q

What is ductile deformation?

A

Stressed for long periods, plastic materials may flow like a viscous fluid.

43
Q

What affects rheology? How?

A

Material, Time, Temperature and Pressure.
Abrupt stress, low temperature and low pressure promotes brittle failure.
Slow stress, high temperature and high pressure promotes plastic flow.
Some materials can therefore act in one way at short timescales but in another over long timescales.

44
Q

What is oceanic crust mostly made up of? Density? Thickness?

A

Basalt
Mg and Fe-rich rock, with ~48% silica (SiO2)
Density of ~3 g/cm³
Oceanic crust is typically ~10 km thick (but thinner under mid-ocean ridges)

45
Q

What is continental crust mostly made up of? Density? Thickness?

A

Granite
Na and Al-rich rock, with ~60% silica (SiO2)
Density of ~2.7 g/cm³
On average ~35 km thick (but <80 km thick under some mountain ranges).

[ <80km ???]

46
Q

Rheologically how is earth’s outer shell made up?

A

The crust and uppermost mantle are rigid solids fused together forming the lithosphere.
Below the lithosphere is a “soft” plastic upper-mantle layer, the asthenosphere.
Below the asthenosphere in the lower mantle is a “stiffer” plastic layer, sometimes termed the mesosphere.

47
Q

What is isostasy?

A

It describes a gravitational equilibrium where lithosphere floats on denser, plastic asthenosphere at an elevation dependent on its thickness and density (ρ).
It is the same as floating by displacing water (ρ = 1.0 g/cm³).

48
Q

What are the densities of styrofoam, wood and icebergs and how do they float relative to the water?
(Density of water = 1.0g/cm³)

A

Styrofoam has a density of 0.05 g/cm³ and it floats mostly above water-level.
Wood has a density of 0.5 g/cm³ and it floats about half above water level.
Icebergs have a density of 0.9 g/cm³ and they float mostly below water level.

49
Q

What is isostatic adjustment?

A

Where addition or subtraction of mass from a floating object leads to change in elevation.

50
Q

What does Isostasy explain?

A

Why continental mountain ranges and plateaus are underlain by thicker crust and lithosphere.
Why oceanic crust sits lower than continental crust, resulting in deep ocean basins.

51
Q

Over how many years does postglacial rebound occur?

A

Over thousands of years because the mantle flows slowly. This is recorded by raised beaches.

52
Q

How does the sun produce energy?

A

It is a nuclear fusion reactor, combining hydrogen nuclei to produce helium and releasing nuclear energy as heat and light.

53
Q

What is the solar energy flux at the top of the atmosphere?

A

340 W/m² on average (~1/3 is reflected back into space).

54
Q

What is the geothermal heat flux from the deep Earth to space?

A

~0.1 W/m²

55
Q

How does the sun’s energy affect Earth’s climate system?

A

Uneven heating of the oceans and atmosphere (high in tropics, low at poles) drives the hydrological cycle and ocean currents, and thus the Earth’s climate system and associated hazards.

56
Q

In what three ways are humans exposed to naturally occurring radioactivity?

A

radioactive elements in soil and rock
cosmic rays from space
food, drink and air

57
Q

What are plate tectonics?

A

the relative motions of Earth’s lithospheric plates

58
Q

What drives plate tectonics?

A

mantle convection

59
Q

What drives mantle convection?

A

The mantle is heated from below by residual primordial heat and from within by radioactivity.

60
Q

What is internal heat energy responsible for?

A 
earthquakes
landslides
volcanoes
creation of mountains
etc
61
Q

What are Kimberlite pipes?

A

They are an unusual type of extinct volcano that bring up material from as deep as ~450 km, including diamonds.

EOS 170 (22 decks)

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