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Geophysics > Geophysics MIDTERM > Flashcards

Geophysics MIDTERM Flashcards

1
Q

Why are subduction zones curved?

A

Because Earth is a sphere, not flat. There are no straight lines. Subduction zones curve over our geometry.

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2
Q

What is a triple junction? Sketch showing all motion vectors

A

When three plates share a single point.

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3
Q

What’s bathymetry? What’s the problem with it?

A

The measurement of water depth in oceans, seas, and lakes.

The problem is that bathymetry is relative to sea level; which isn’t the same everywhere! Our MEAN sea level VARIES.

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4
Q

Continents are ______ in age, oceans are ______.

A

Continents are old; oceans are young.

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5
Q

Anomaly

A

Something different from what was expected.

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6
Q

Does a continent end at its shoreline or continental slope?

A

Continental slope (final major deposition area underwater)

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7
Q

What’s a paradigm shift?

A

When a new explanation reinterprets (or makes irrelevant) data to such a degree that people can’t really help mixing belief and science.

Exciting times for science, but scary for non-scientists to learn science is NOT a pile of known stuff… it’s the product of human discussion and interpretation.

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8
Q

Alfred’s Evidence of Continental Drift

A
  1. Continents fit like a puzzle
  2. Fossil assemblages & migrations across continents
  3. Direction of past glacier movements
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9
Q

What is isostasy?

A

The vertical balance of Earth’s crust.

Continents are granitic and float; underlying substrate is basaltic and is “floated-ON”. Granitic continents plow through the substrate to drift; a dynamic process.

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10
Q

Ridges vs Trenches

A

Ridges: Plates are being created (newer)

Trenches: Plates are being destroyed (subducted –> older)

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11
Q

How was the Ring of Fire Formed?

A

Subduction zones; where the oceanic plate slides under the continental plate. When the two plates converge, the oceanic plate is denser and heavier (basaltic), so it slides underneath, causing uplift/magma/stress release –> leads to mountain chains, volcanoes, and earthquakes!

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12
Q

Fault

A

A fracture zone between two rocks.

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13
Q

What are the largest mountain chains in the world?

A

RIDGES underwater!!

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14
Q

What drives plate movement?

A

The continents are long-lived, but the oceans come and go. Specifically, though; CONVECTION

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15
Q

Explain How Convection Drives Continental Drift

A

Convection: Warm molecules rise, cool molecules sink.

Material forms are oceanic ridges (spreading centers), moves through convection, and gets destroyed at oceanic troughs (subduction zones).

Convection currents move in the fluid molten mantle. In places where convection currents rise, plates move away from each other (seafloor spreading). In places where convection currents sink, plates converge.

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16
Q

Why is the mid-ocean ridge like a factory?

A

It inputs heat and mantle peridotites to output basalt/gabbro and “depleted” mantle peridotites.

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17
Q

Depth and ridges are…

A

HIGHLY CORRELATED

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18
Q

Why are seismic zones inclined?

A

Because they correspond to trenches; the inclination of subduction where stress is released along its subduction.

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19
Q

What were the two technical innovations that drove the conversion of continental drift into a theory of plate tectonics?

A

World War Submarines; sonar & magnetic sensors.

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20
Q

Plates behave by _______________

A

ROTATING

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21
Q

What are Euler poles?

A

A fixed mathematical point that describes the motion of a tectonic plate on a sphere is a rotation around that point.

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22
Q

Three Types of Margins

A
  1. Constructive: Plates move apart (creates oceanic crust)
  2. Destructive: Continuous upwelling of molten material (creates volcanic arcs)
  3. Strike-Slip Faults: Two plates sliding past each other (nothing created or destroyed)
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23
Q

What are Earth’s heat sources? What do they control?

A
  1. Sun’s radiation –> controls surface water cycle, rainfall, erosion
  2. Interior heat –> controls tectonics, metamorphism, evolution of core
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24
Q

4 Modes of Heat Transfer

A
  1. Conduction: Vibrating of neighboring molecules (strongest in solids- closest together)
  2. Convection: Movement of fluid particles
  3. Radiation: Propagation of waves or photons (no medium required)
  4. Advection: Movement of heated “bulk mass” (mostly in liquids or gas)
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25
Q

What modes of heat transfer are found in different portions of the Earth?

A

Solid Lithosphere: CONDUCTION
Solid Mantle: CONVECTION
Fluid Outer Core: CONVECTION
Solid Inner Core: CONDUCTION

RADIATION: Induced by hot lava and radioactive minerals.
ADVECTION: Through erosion, faulting, isostatic rebound, and tectonic movement.

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26
Q

How is convection possible in the solid mantle?

A

In the geologic time scale, the solid mantle does behave as a fluid (over a very long period of time)

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27
Q

Geotherm & Geothermal Gradient

A

Geotherm: A line or surface within or on the Earth connecting points of equal temperature.

Geothermal Gradient: The rate of change of temperature with respect to increasing depth.

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28
Q

Radioactive Heat Generation

A

One of Earth’s heat sources; produced by the decay of radioactive isotopes in the crust and mantle.

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29
Q

Steps of Measuring Heat Flow

A
  1. Measure temperature at different depths
  2. Plot temp vs depth
  3. Fit the straight line
  4. Estime the thermal gradient (vertical slope of line)
  5. Calculate the surface heat flow (with heat conduction law/equation)
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30
Q

Heat flow variation shows…

A

The plate tectonic boundaries! Hot at diverging, cold at converging– remember driven by convection!

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31
Q

Why was Kelvin’s calculated age of the Earth so wrong?

A

He didn’t consider temperatures at different depths– Earth is not homogeneous. In addition, convection and radioactive heat generation was unknown at the time.

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32
Q

Rayleigh-Bénard Convection

A

A buoyancy-driven flow in a container with a temperature gradient. As the fluid at the bottom heats up, its density decreases, so buoyant forces push the less-dense fluid up towards the cooler end of the container.

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33
Q

Ra~2000
Ra~10^5
Ra~10^6

A

Ra~2000 –> initiates convection (parallel lines of sinking/rising)
Ra~10^5 –> strong convection (bimodal pattern/perpendicular lines)
Ra~10^6 –> chaotic convection (hexagonal cells)

Note: If T=100C, MANTLE convection is chaotic!

34
Q

Why is the Earth’s magnetic field not perfectly dipole?

A

There’s deviations due to magnetic materials and complex geodynamo patterns.

35
Q

What are the possible reasons for no magnetic field on Mars?

A

It’s core is frozen! No convection or heat flow to lead to currents and magnetic fields.

36
Q

Does mantle convection also produce the geomagnetic field? Explain.

A

No. It’s not electrically conductive like the core.

37
Q

An aurora was visible in the sky from Kingston last year. Briefly describe how this phenomenon occurs on Earth. Are there auroras on other planets? Explain.

A

Charged particles from the sun interact with Earth’s atmosphere,
forming aurora or the northern lights (aurora borealis) or southern
lights (aurora australis).
* The solar wind disrupts the magnetosphere and alters the paths of
charged particles (electrons and protons), entering the upper
atmosphere (thermosphere/exosphere). This process causes ionization
and excitation, resulting in the emission of colourful and complex light.

38
Q

Induced Magnetization

A

When a magnetic material, is placed within a magnetic field, the magnetic material will produce its own magnetization.

39
Q

Types of Induced Magnetization

A
  1. Diamagnetic: Weak magnetitsm in opposite direction (ie halite, quartz, calcite).
  2. Paramagnetic: Weak magnetism in the same direction as the field (ie pyrite, biotite, hematite).
  3. Ferromagnetic: Strong magnetism in the same direction (magnetite, ilmenite, pyrrhotite).
40
Q

Curie Temperature & Curie Point Depth

A

Curie Temperature: The temperatures above which magnetic materials lose their magnetic properties.

Curie Point Depth: The depth at which rocks reach the Curie Temperature.

41
Q

Geodynamo Theory

A

Says the Earth’s magnetic field results from electric currents induced in the liquid outer core of the Earth.

42
Q

What causes a larger tilt between rotation axis and magnetic pole?

A

When the geodynamo forms closer to the surface!

43
Q

Explain polar wander.

A

Our magnetic pole is moving. It wanders a little offset from Earth’s rotation axis, and can even reverse.

44
Q

Do geomagnetic reversals cause extinctions?

A

NO!

However, radioactive particles can affect electromagnetic equipment and navigation. Even animals will have to adapt.

45
Q

What causes geomagnetic storms (weakening field)?

A

Solar activity like sunspots and flares interacting with our magnetic field when its differential spin “unravels”.

46
Q

What are sunspots?

A

Areas if intense magnetic field localization– formed by the tangling of magnetic field lines (differential rotation).

47
Q

Types of Remanent Magnetization

A
  1. Local: Rock has local stored field
  2. Persistent: Field would be present even if Earth’s turned off
  3. Oriented: Originally aligned with Earth’s field
  4. Measurable: Field doesn’t have to stay aligned with Earth’s
48
Q

Apparent Polar Wander vs True Polar Wander

A

Apparent: The perceived movement of Earth’s paleomagnetic poles relative to a continent assuming it’s FIXED IN PLACE.

True: ACTUAL movement of the poles due to whole-Earth rotation to its spin axis. The pole shift is due to variations in movement spin inertia (mass redistribution).

49
Q

What causes gravity/gravitational force?

A

A bowling ball on a trampoline.

(The space-time curvature induced by the mass energy)

50
Q

Acceleration due to gravity is __________ proportional to the mass and __________ to the radius.

A

Acceleration is directly proportional to mass.

Acceleration is inversely proportional to radius.

51
Q

Acceleration due to gravity is maximum where in Earth?

A

The Core-Mantle Boundary!! This is where gravitational acceleration reaches a maximum.

52
Q

Gravity Equipotential

A

A surface where the gravity potential is constant.

53
Q

What is the Geoid & Geoid Height?

A

Geoid: The “equipotential” surface that defines the SEA LEVEL. Everywhere on the sea surface, gravitational potential is equal.

Geoid Height: The distance between the geoid and reference ellipsoid of the Earth. If there is no “gravity anomaly”, the geoid IS the reference ellipsoid!

54
Q

What can cause a geoid anomaly?

A
  • Continental collisions
  • Postglacial rebounds
  • Subduction
    and more plate tectonic consequences.
55
Q

How do mountains affect the geoid?

A

Excess mass, above or below the reference ellipsoid, ELEVATES the geoid, while deficient mass depresses it.

56
Q

What is a surface gravity anomaly?

A

The change in acceleration due to gravity on Earth’s reference surface due to a mass anomaly.

57
Q

Types of Gravity Corrections

A
  1. Latitude Correction: At any given latitude. (Subtracted)
  2. Elevation Correction: If the measurement is made above the reference geoid. (Added)
  3. Bouguer (Topography) Correction: For extra mass above the reference geoid. (Subtracted)
  4. Terrain & Tidal Corrections: For steep topography or gravitational effects from the Sun/Moon.
58
Q

Describe the Wilson Cycle

A
  1. Rift → breaking up rocks like a zipper. Once there’s a gap, oceanic crust can form.
  2. Drift (outwards) → mid ocean ridge spreads stuff outwards
  3. Initiate Subduction → the spread stuff reaches continental crust and subducts
  4. Drift (inwards) → this pushes things together
  5. Collisions & Mountain Building → makes mountains
  6. Supercontinent.
59
Q

Explain Auroras

A

Solar winds → disrupts Earth’s magnetosphere & alters path of the solar charged particles → enters the upper atmosphere → interacts with atmosphere to ionize and release photos; beautiful coloured lights.

60
Q

Explain the Geodynamo Theory (Steps)

A
  1. Internal heat source from the inner core drives mantle convection
  2. Convection causes flow of molten iron; electrically conductive
  3. Earth’s rotation induces “helical” patterns of convective flow
  4. Magnetic field!
61
Q

Why is the Geodynamo Self-Sustaining?

A

A current moving along a helical path produces a magnetic field, and helical flows twist magnetic field lines to produce a current.

62
Q

Constants g & G

A

g = 9.8 m/s^2

G = 6.67x10^-11

63
Q

1 km = ?m
1 mu m = ?m

A

1km = 1000 m
1 mu m = 1.0x10^-6 m

64
Q

What’s a Concordia diagram?

A

Describes the U-Pb parent and daughter ages in a closed system if there were no initial Pb isotopes. DISCORDIA is the real line of parent/daughter ratios, caused by a disturbance and the system to lose Pb. The intersection of conc and disc is the AGE OF FORMATION (time the disturbance occurred).

65
Q

______________________ dating is widespread. It can be used on most rocks.

A

Potassium-Argon (K-Ar)

66
Q

What is postglacial rebound/glacial isostatic adjustment

A

The rise of land masses that were depressed by the huge weight of ice sheets. This makes the “water level decrease” (in some areas like the Hudson Bay).

67
Q

Relative vs Absolute Age

A

Relative: Relative age of a geological event/unit with respect to another (stratigraphy & paleontology).

Absolute: The actual age of the geologic event/unit (radioactive isotopes).

68
Q

Isotopes

A

Different number of neutrons. Unstable isotopes are radioactive.

69
Q

What is a half-life and what does it tell us?

A

The time required to disintegrate half of the original parent atoms.

It tells us which isotope dating is suitable for a certain age range!

70
Q

Which radioactive dating should we use when?

A

The expected age range should be about the same order as the dating half-life. For old rocks, short half-life isotopes may have already vanished!

U-Pb = 10^6 (OLD)
C-N = 1000 (YOUNG)

71
Q

Why is radioactive dating best in a closed system?

A

No contamination- no initial daughter atoms or foreign parent/daughters, OR escaping of target parent/daughters.

72
Q

Curie vs Closure Temperature

A

Cure: The temp at which rocks begin holding a magnetic signature.

Closure: The temp at which a rock isotopically stars recording an age (it has cooled enough so there is no diffusion of the parent/daughter out of the system).

73
Q

What radioactive isotope vs fission-track dating

A

U decays to Pb for example, but it also undergoes spontaneous FISSION, leaving damaged trails (etches) in the rock sample. We can count these to determine age.

74
Q

Why are meteorites good for dating the Earth?

A
  • Closed system
  • Pb to Pb isochrones
75
Q

What are some natural causes of Earthquakes?

A

Any natural, sudden release of energy.
- Plate margins
- Impacts
- Isostatic rebound
- Volcanic eruptions, etc.

76
Q

Types of Seismic Waves

A

Body Waves:
1. P Waves
2. S Waves

Surface Waves:
3. Love Waves
4. Rayleigh Waves

77
Q

P Wave

A
  • Move along the direction of propagation
  • Fastest; travels through all media
  • Early warning
78
Q

S Wave

A
  • Move perpendicular to the direction of propagation
  • Slower than P waves; travels only through solids
  • Stronger than P waves
  • Vertical and horizontal components (SV & SH)
79
Q

Love Wave

A
  • Interaction of SH waves on the surface
  • Move along the horizontal
  • Slower than body waves but faster than rayleigh
  • Powerful and damaging
80
Q

Rayleigh

A
  • Interaction of P & SV waves on the surface
  • Move along horizontal and perpendicular (rolling motion)
  • Slower than love waves
  • Powerful and damaging
81
Q

Why are surface waves so destructive?

A

The amplitude of surface waves lasts longer over a longer distance than body waves.

82
Q

What are some applications of seismology?

A
  • Exploration (ore deposits, oil, etc)
  • Engineering (seismic hazards)
  • Medical imaging (ultrasound)

Geophysics (2 decks)

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