Earth Structure and Global Tectonics Flashcards
(77 cards)
1
Q
How many layers of the Earth’s core can be observed?
A
3.7%
2
Q
What are the layers of the Earth? (top to centre)
A
Crust
Moho Discontinuity
Upper Mantle
Asthenosphere
Lower Mantle
Gutenberg Discontinuity
Outer Core
Lehmann Discontinuity
Inner Core
3
Q
What is the Moho layer?
A
Boundary between crust and upper mantle
Changes in characteristics of rock:
- more rigid
- more dense
- less compressible
Therefore the seismic waves speed up
4
Q
Why do waves bend when they move through the Earth?
What does this create?
A
pressure increases, so density of the rock changes which alters the speed of the wave and refracts the path more so it is not straight
- this can end up creating SHADOW ZONES
5
Q
What are shadow zones?
A
Areas in the Earth where seismic waves do not pass due to refraction or inability to travel through the state
Areas on the surface where seismographs cannot detect seismic waves after a seismic event such as an earthquake
6
Q
How are shadow zones created? Why are there no seismic waves?
A
- seismic waves are either refracted or stopped at the Gutenberg Discontinuity
- this is the core-mantle boundary where the material changes state from a solid to a liquid due to increased heat and pressure
- here, S-waves are stopped completely and P-waves are refracted
103 degrees - 142 degrees
7
Q
What evidence do we have to know all the layers of the Earth?
A
Geophysical measurements
Meteorite composition
Mantle Xenoliths
Mean Density Calculations
8
Q
What are the geophysical measurements?
A
Seismology
Conductivity
Geomagnetism
Gravitational Anomalies
9
Q
What is seismology?
A
The study of vibrations within the Earth using seismic waves
10
Q
What are a few example causes of vibration?
A
Earthquakes
Extraterrestrial impacts
Explosions
Storm waves hitting the shore
Tidal effects
11
Q
How can we measure seismology?
A
A seismometer measures seismic waves and produces a seismograph
- modern seismometers can measure movements smaller than one millionth of a millimetre
12
Q
What are the kinds of seismic waves?
A
Body waves = P-waves and S-waves
Surface waves = Love waves and Rayleigh waves
13
Q
What movements do the seismic waves make?
A
P-waves = longitudinal/compressional
S-waves = shearing motion (vertical)
Love waves = shearing motion (horizontal)
Rayleigh waves = elliptical/rolling
14
Q
Why do waves bend as they travel through the Earth?
A
Because the pressure increases, density of the rock changes which alters the speed of the wave and refracts the path more so that it is not straight.
15
Q
What happens to a seismic wave when it encounters a boundary between materials with different properties?
A
The energy splits into reflected and refracted (bent) waves
16
Q
What happens to the seismic waves when they pass through a boundary into a more dense rock?
A
The velocity increases
The wave refracts (bends) towards the boundary separating the layers
17
Q
What happens to the seismic waves when they pass through a boundary into a less dense rock?
A
The velocity decreases
The wave refracts (bends) away from the boundary separating the layers
18
Q
What is conductivity?
A
The changes in temperature as you move from one layer of the Earth to the next
19
Q
How is heat transferred within the Earth?
A
Conduction
Convection
Advection
20
Q
What is conduction?
Give an example of where this occurs
A
Where heat is transferred through a substance with no movement involved
- In the lithosphere, through rocks
21
Q
What is convection?
Give an example of where this occurs
A
Where heat is transferred by a substance due to buoyancy differences
- Currents in liquid mantle at hot spots or ocean ridges
22
Q
What is advection?
Give an example of where this occurs
A
Where heat is transferred through a substance by a fluid
- Water or magma through the crust
23
Q
Why is the interior of the Earth hot?
A
Formation of a proto Earth
Redistribution of siderophile elements
Radioactive decay in the core and mantle
24
Q
How do we know the interior of the Earth is hot?
A
- Magma, geysers and other geothermal activity at the surface
- Heat is constantly transferred from its source in the Earth to the surface
25
How does heat get transferred from its source in the Earth to the surface?
- Geothermal Flux
- Geothermal Gradient
26
What is geothermal flux?
The rate of heat energy transfer through a given surface over a period of time ((milli)watts per square metre)
27
How does geothermal flux work?
Heat from interior of Earth moves towards surface through conduction (in solid rock) and convection (in molten rock or magma), from the hotter layers (core and mantle) to cooler layers (crust and surface)
28
Give examples of areas of high geothermal flux and explain why it is high
Mid-ocean ridges/tectonic plate boundaries - highest geothermal flux as new crust is constantly being formed and magma is closest to the surface, giving out a lot of heat
Volcanic regions - high geothermal flux due to volcanic activity and magma chambers near the surface e.g. Iceland, Hawaii
29
Give examples of areas of low geothermal flux and explain why it is low
Areas of stable continental crust - old continental crust e.g. large landmasses (cratons) have lower geothermal flux because mantle beneath is more stable and cooler
30
What is the significance of geothermal flux?
Plate tectonics - rising heat from the core drives convection currents in the mantle. Heat causes mantle rock to become less dense and rise while cooler, denser rocks sink, driving movement of plates
Geothermal energy - understanding GF allows us to safely tap into geothermal energy sources - renewable - creates electricity
Geological processes - flow of heat affects processes e.g. volcanism
31
What is geothermal gradient?
The rate at which temperature changes with depth per kilometre (Celsius per km)
32
What factors affect the geothermal gradient?
Types of rock - different thermal conductivities e.g. igneous and metamorphic better than sedimentary
Tectonic activity - in tectonically active regions GG is higher because magma is closer to the surface e.g. plate boundaries/rifts
Crustal thickness - steep gradient in thinner crust and lower in thick continental crust where the heat is spread out over a larger area
33
What is an area of high gradient?
Over 30 degrees per kilometre
34
What is the average geothermal gradient of the crust?
25-30 degrees per kilometre
35
Where are areas of high gradient found?
Active tectonic areas or where magma is close to surface e.g. mid-ocean ridges or volcanic areas
36
What is an area of low gradient?
Below 20 degrees per kilometre
37
Where are areas of high gradient found?
In areas with tectonic activity or where magma is close to the surface
e.g. mid-ocean ridges or volcanic areas
e.g. Pacific Ring of Fire
38
Where are areas of low gradient found?
In areas of stable continental crust where heat flow is less intense
e.g. cratons or shields
e.g. Canadian Shield
39
How does geothermal gradient change with layers of the Earth?
Crust - gradient is pronounced, significant heat increases with depth
Mantle - going deeper, gradient becomes more gradual because although temp still increases, the material ability to conduct heat also increases so there are less rapid temp change per km
Core - temp is extremely high (up to 6,000 degrees) but gradient is less important for surface geology since heat flow is slow compared to crust
40
What is the importance of geothermal gradient?
Metamorphism - GG controls conditions which rocks undergo metamorphism
e.g. high temp + shallow depth = contact metamorphism near magma bodies due to higher GG
Volcanism - magma from mantle can more easily reach the surface due to volcanic activities - high GG
Geothermal energy - understanding GG allows us to tap into geothermal energy - renewable - generate electricity
41
How is geothermal gradient measured? (NOT UNIT)
Drilling deep boreholes into the crust and measuring how temp changes with depth. Data then used to calculate rate of temp increase per km
42
What is geomagnetism?
The study of the Earth's magnetic field
43
How do we measure geomagnetism?
What does the measurements show?
Electromagnetic surveys
tell us a bit about the composition of the crust and upper mantle
44
How do electromagnetic surveys work?
1. Transmit an electromagnetic field from the search coil into the ground
2. Any metal object (targets) within the electromagnetic field will become energised
3. They then retransmit an electromagnetic field of their own which can be detected by the receiver
45
What is resistivity?
the ability of a material to resist an electrical current
lower resistivity = a material that readily allows an electrical current
46
What is gravity?
What is the average gravitational pull of the Earth?
The force by which a planet or other body draws objects toward its centre
Av pull is 9.81
47
What can cause gravitational pull to change?
Different mass of objects/different density of rocks
48
How can we measure gravity?
Time of swing
quicker swing = greater gravitational pull
49
What does the measure of gravity tell us about the Earth?
allows us to start to map gravitational anomalies and this can tell us about the rocks
50
How do we measure time of swing?
T = 2π(root)L/g
time of swing = 2π(root)length of swing/pendulum/gravity
51
How do we measure gravity? equation
g = GME/r(squared)
gravity = mass/radius(distance from core)(squared)
52
What types of evidence do we have to know the composition of the core?
Introductory evidence
Density evidence
Meteorite evidence
Seismic evidence
53
What is the introductory evidence for the composition of the core?
We have no direct evidence of what is beneath Earth's crust because there are no boreholes that get from the crust to the mantle
- Deepest borehole is ~12km, distance to outer edge pf core is ~3,000km
54
What is the density evidence for the composition of the core?
1. Density of iron is nearly 8 and the density of nickel is nearly 9
2. Mean density of crustal rocks is about 2.7
3. Mean density of the Earth, calculated by dividing mass by volume, is about 5.5
4. Some volcanoes contain lumps of rock (xenoliths) that are believed to have home from the mantle, their density varies around 3.5
55
What is the meteorite evidence for the composition of the core?
1. Most meteorites appear to have come from the asteroid belt
2. The asteroid belt is a belt of objects between mars and Jupiter, some may have come from earlier larger bodies which had cores but were broken up by impact
3. Most meteorites are stony meteorite, but there are also iron meteorites, formed mainly of nickel-iron
4. The density of stony meteorites varies around 3 whilst nickel-iron meteorites have a density of around 7.5
56
What is the seismic evidence for the composition of the core?
1. Calculations from the velocities of seismic waves show that the density of the core is between 10 and 13
2. Seismic S-waves do not pass through the outer core, showing that it is fluid
57
What is the direct evidence for the composition of the Earth?
Crust
Mines and boreholes
Volcanoes
Ophiolite
58
What is the other evidence showing the composition of the core?
1. The density of the earth is higher than that of any other planet in our solar system
2. The Earth's magnetic field reverses in polarity at irregular intervals
3. Seismic P- and S-wave evidence shows that the distance from the surface to the outer edge of the core is ~3,000km
59
How does the asthenosphere facilitate plate movement?
- because the asthenosphere in the upper part of the mantle isn't solid rock, movement can happen
- this movement means plates on the surface can move, resulting in different plate boundaries and tectonic hazards
60
How does the theory of convection currents facilitate plate movement?
What are the arguments against this theory?
1. The immense heat and pressure from the core is transferred to the mantle, making it less dense and causing it to rise towards the surface
2. As the magma reaches the crust, it cools and becomes denser
3. The cooler, denser magma sinks back towards the core, dragging the tectonic plates on top, causing them to move and interact
ARGUMENTS AGAINST: insufficient traction to move such big things
61
How does the theory of ridge push/slab pull facilitate plate movement?
What are the arguments against this theory?
Which force is more significant and why?
RIDGE PUSH: (mainly divergent plate boundaries)
1. magma rises and cools (due to convection currents) at mid-ocean ridges, forming new oceanic crust
2. this new crust is initially buoyant and is elevated above the sea floor
3. where there is a ridge, there is a downslope, therefore plates move due to gravity as the crust cools and becomes denser and thicker, pushing everything along
SLAB PULL: (mainly convergent plate boundaries)
1. at subduction zones, denser plates sink below the other plates due to gravity
2. this pulls the whole plate with it
3. it can be kickstarted by ridge push which has the crust from the divergent plate boundary already moving
- slab pull is considered to be more significant as the weight of a large, dense, and sinking slab can exert a substantial pull on the rest of the plate
62
What are the theories as to how plates are forced to move?
1. convection currents moving plates by traction
2. ridge push/slab pull (slab pull theory)
But it is probably due to a combination of the two
63
How does the crust act as direct evidence for the composition of the Earth?
Old crustal rocks brought to the surface during Earth movements (tectonic?) are then exposed by erosion, showing that the upper continental crust is varied
CONTAINS:
- igneous
- sedimentary
- metamorphic
(a range, and mainly granite composition)
64
How do mines act as direct evidence for the composition of the Earth?
What are the limitations?
Allow us to access higher levels of the crust e.g. mines for coal, metal ores and diamonds
LIMITATIONS:
- drainage
- ventilation
- high working temps
- lifting the ore to the surface
- depth of mine: deepest = 4km gold mine in S. Africa
65
How do boreholes act as direct evidence for the composition of the Earth?
They can go deeper than mines, and we can get samples of rocks and microfossils as well as undertaking remote sensing
66
Give examples of boreholes
MOHOLE
- Mexico, 3660m below the ocean, reached 183m deep
- aimed to get through the crust to the boundary with the mantle
- abandoned due to cost issues associated with the depth
KOLA DEEP BOREHOLE, RUSSIA
- aimed to go 15km through the crust
- 300 degrees was way higher than expected, and the rocks were too plastic for the drill bits
SEISMIC SURVEYS
- 7km deep, expected to find basalt but instead found metamorphosed granite, heavily fractured and saturated with seawater from the minerals from the lower crust
67
How do volcanoes act as direct evidence for the composition of the Earth?
Volcanoes are fed by magma originating in the lower crust/upper mantle - this brings up samples of rock
BASALT LAVA - erupt at mid-ocean ridges
- formed by the partial melting of the upper mantle
- chemical analysis and volatiles of these basalts allow us to estimate composition of upper mantle
DIAMONDS
- diamonds and other minerals with compact structure occasionally appear in the volcanic pipe as igneous material
- to crystallise, need high pressure (depth 250km)
- magma carries up fragments of country rock torn from vents
The rock that enclosed the diamonds is KIMBERLITE ROCK
(also includes mantle xenoliths of Peridotite)
Kimberlite pipes are the result of explosive volcanism in the deep mantle
- looks like a conical vent hundreds of km across
- when comes within 2km magma is high pressure, explodes upwards and expands
68
What is ophiolite?
Give an example
sections of the oceanic crust broken off and thrust onto the edge of the continental plate during tectonic plate collision then exposed by erosion
e.g. Lizard Peninsula, England
- ophiolite millions of years old but very similar to modern day crust and mantle
69
How does ophiolite act as direct evidence for the composition of the Earth?
Oceanic crust sections can be ancient - and they are now on land so it's really easy to access them for studying, no need for disruptive boreholes
PERIDOTITE is at the base of the ophiolite sequence, from the upper mantle
- ultramafic igneous rock, olivine and pyroxene
- if ophiolite sequence is undisrupted and stays in original undeformed orientation, then its 7km - thickness of oceanic crust
70
What is the theory for plate tectonic theory?
Jigsaw theory - continental drift - Alfred Wegener
Fossil finding and rock types - studied Mid Atlantic Ridge in post war submarines an looked back to AB theory
71
What is the evidence for plate tectonic theory?
1. Geomagnetic properties of rocks
2. Geothermal data of hot spots and heat flows
3. Seismic Tomography
72
How do geomagnetic properties of rocks provide evidence for the plate tectonic theory?
PALAEOMAGNETISM
- when magnetic anomalies at Mid-Ocean Ridges form, we can see when they formed, showing the spreading of the crust and movement of the tectonic plates which results in the rock ending up where it was found
73
What is a magnetic field?
the region around a magnetic material or a moving electrical charge within the force of magnetism acts
74
Why does the Earth have a magnetic field?
THERE IS A MOVING ELECTRICAL CHARGE
1. Heat is transferred from the solid inner core to the liquid outer core
2. This leads to convection of the liquid iron of the outer core
3. Because iron is a metal and conducts electricity (even when molten), its motion generates a magnetic field
75
What is palaeomagnetism?
- magnetism preserved in the Earth's rocks
- rocks like basalt (which have high levels of magnetite)have remnant magnetism
- because of this, the crystals are orientated parallel to the Earth's magnetic field at the time as it cools from a lava flow
- this is known as palaeomagnetic inclination or dip
- this allows us to work out the magnetic north at the time and the latitude at which the rock was formed
76
How can the Earth's magnetic field change?
- weaken
- strengthen
- reverse polarity
77
How do magnetic anomalies form at mid-ocean ridges?
1. Magma rises up at a MOR and starts to cool
2. The magnetic properties in the rock align themselves with the direction of the magnetic field at the time
3. As the Earth's polarity changes so does the alignment of the rock
4. This then creates a symmetrical pattern either side of the MOR as the crusts move away from it
5. The bands of rock vary in width due to:
- timing of shifts in polarity
- amount of magma released in time period
- speed of plate movement
