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Geology 100 (part 2) > Earth magnetic field and paleomagnitism > Flashcards

Earth magnetic field and paleomagnitism Flashcards

1
Q

paleomagnitism

A

description of the geomagnetic field and its spacial(space) and temporal (time) variations

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2
Q

magnetic field can be visualized by

A

lines of force, such as those revealed by alignment of iron filings on a piece of paper above a bar magnet

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3
Q

earth’s magnetic lines of force point which direction

A

into the ground at north magnetic pole and outward at south magnetic pole

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4
Q

what kind of magnetic field

A

dipole (two-pole) magnetic field

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5
Q

at the equator, the lines ____ to earths surface
at mid-latitudes, lines ____ to surface
at magnetic poles, lines_____ to the surface

A

parallel
tilt at angle
perpendicular

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6
Q

relative strength of dipole

A

strength of the field is maximum at either end

at midpoint, the strength is half the value of the poles

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7
Q

magnetic field direction of dipole

A

lines of force leave one end of magnet (north pole) arc through space, and reenter the magnet at the other end (southern pole)

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8
Q

difference between magnetic pole and geographic pole

A

11.5 degrees

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9
Q

polar cusps

A

bend in magnetic field around earth (look at pic on page three)

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10
Q

earth’s magnetic field distorted by

A

solar wind

look at pic on bottom of page 3

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11
Q

aurora

A

caused by polar cusp- charged particles are pulled towards magnetic poles. Interacting with atmospheric gases, these ions create colorful glowing aurora observed in polar regions
In north- called Aurora borealis aka northern lights
in south called aurora australis

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12
Q

earth’s magnetic field is not uniform

A

intensity and direction of the field changes from one location to another but over time as well

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13
Q

how much of earth is dipole and nondipole fields

A

only 90% of magnetic field can be described as dipole

remaining 10% (nondipole) has more complex structure

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14
Q

secular (time related) variation

A

both dipole and nondipole parts of field are changing overtime and this secular (time related) variation is fasted for nondipole part.
Change in field strength occur on time scales of decades and indicate that fluid motions within geodynamo system are in the order of millimeters per second
One of the few “solid earth” phenomena that change significantly over a human lifetime

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15
Q

magnetic reversals

A

spontaneous reversals of magnetic field
reverses its direction at irregular intervals (ranging from tens of thousands to millions of years)
exchanging the north and south magnetic poles, as if magnet flipped 180 degrees

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16
Q

another way to create magnetic field

A

electric currents

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17
Q

what part of earth generates magnetic field?

A

convection in earth’s outer core

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18
Q

why scientists believe outer core is responsible for magnetic field

A
  1. made primarily of iron, good electrical conductor
  2. convective motions are very rapid in the liquid, low-viscosity outer core. These rapid motions stir up electric currents in conducting iron to create a geodynamo with a strong magnetic field
    Field results from the circulation of liquid iron alloy, an electrical conductor, in earth’s outer core- in other words, behaves like electromagnet
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19
Q

dynamo

A

an engine tha produces electricity by rotating a coil of conducting wire through a magnetic field-
geodynamo in earth’s outer core operate on same principles, except the work comes from convection in outer core

20
Q

is it possible to stir up electric curents in the liquid outer core (and why)

A

Yes! because:

  1. Liquid iron alloy is good conductor
  2. convection (buoyancy gradient/temp gradient??)
  3. earth’s spin
21
Q

what mineral do you need to study the magnetic field

A

magnetite aka iron oxide
all pieces of it are magnetic
magnetite from 5mil yrs ago same as magnetic field of earth at that time
always has 2 poles no matter what

22
Q

thermoremanent magnetization

A

an important property of many very hot, magnetizable materials is that as they cool below about 500 deg C, they become magnetized in same direction as surrounding magnetic field
=lava solidification
look at diagram on top of p 7

23
Q

depositoinal remanent magnetization

A

chips of mineral magnetite would become aligned in the direction of earth as they fall through the water. this orientation would be incorporated into the rock when the particles become lithified

24
Q

magnetic field reverses itself periodically?

A

NO! super haphazard

look at chart on top of p 8

25
Q

the seafloor as magnetic tape recorder

A

in WWII extremely sensitive instruments developed to detect submarines by magnetic fields emanating from their steel hulls
geologists made some slight modifications to these instruments and towed them behind research ships so they could measure local magnetic field created by magnetized rocks beneath the sea
rocks magnetized by earth’s magnetic field in whatever direction field is oriented at the time at which rock is formed
Scientists argued that ocean progressively widens as new seafloor (mafic lava cooling) created along a crack on crest of mid-ocean ridge
As seafloor splits and moves away from ridge, approximately half of newly formed basalt moves to each side, forming two symmetrical bands
Newer material fills crack, continuing the process

26
Q

present direction of magnetic filed aka

opposite orientation of filed aka

A

normal
reverse
in geologic past, magnetic field switched back and forth erratically between normal and reverse

27
Q

mid ocean ridge

A

symmetrical of normal/reverse aka positive anomaly (higher magnetic intensity) and negative anomaly (lower mag intensity)
present-day magnetic field is the average
anomaly pattern represents alternating stripes of normal polarity and reversed polarity sea floor

28
Q

magnetism of sea floor

A

LOOK AT PIC ON PAGE 9

29
Q

isochrones

A

contours that connect rocks of equal age

30
Q

weird time in magnetic field history

A

for 40 million years during cretaceous period magnetic field did not reverse- shows how random it is

31
Q

magnetic bands provided evidence in support of theory of

A

seafloor spreading

32
Q

magnetic bands (stripes) used to determine

A

ocean floor’s spreading rates

see last slide on page 12

33
Q

magnetic inclination

A

the needle’s angle of tilt depends on latitude

34
Q

at magnetic north ole, need it is (pointing which direction)

A

vertical

35
Q

intensity and structure of earth’s magnetic field are changing what sped? How often?

A

slowly and erratically

36
Q

how often magnetic field strength and direction reevaluated

A

every 5 years for 5-year period

because a wide range of commercial and military navigation systems are dependent on models of the magnetic field

37
Q

declination

A

component of magnetic field

indicates the difference, in degrees, between the headings of true north and magnetic north

38
Q

horizontal intensity

A

component of magnetic field

defines the horizontal component of the total field intensity

39
Q

vertical intensity

A

defines the vertical component of the total field intensity

40
Q

total intensity

A

the strength of the magnetic field, not divided into component parts

41
Q

magnetic declination

A

the angle between magnetic north (direction the north end of a compass needle points) and true geographic north

42
Q

when is declination positive

A

when magnetic north is each of true north

and is negative when mag north is west of true north

43
Q

magnetic declination varies in what respects

A

varies from place to place

and with the passage of time

44
Q

isogenic lines

A

wher the declination has the same value

45
Q

agonic lines

A

where declination is 0

Geology 100 (part 2) (5 decks)

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