Lab 5_ Earth's Magnetic Field (Emma Born).docx
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Name: Emma Born
Lab Section:
EPS 50: Fall 2022
LAB 5: EARTH’S MAGNETIC FIELD
Due one week from today
at the start of your lab section
Introduction
The Earth's magnetic field is one of the most important properties of this planet. Studies in
the 1960s of the Earth's magnetic field and magnetic anomalies led to the discovery of plate
tectonics, which is one of the most important theories about Earth's dynamics. Earth's
magnetic field also shields us from most cosmic radiation from space. In some ways, Earth’s
magnetic field behaves in the same way that magnetic fields on ordinary magnets behave. In
this lab we will be exploring some of the properties and effects of Earth's magnetic field.
1. Magnetic Field Lines
Magnetic fields are produced by moving charged particles. Magnetic field lines describe the
structure of a magnetic field in three dimensions. If we place an ideal compass needle free to
turn in any direction (unlike the usual compass needle, which stays horizontal) on any point on
a magnetic field line, then the needle will always point along that field line.
Field lines converge where the magnetic force is strong, and spread out where it is weak. For
instance, in a compact bar magnet (which approximates a dipole), field lines spread out from
one pole and converge towards the other. In other words, the field lines are closest together
at the poles, where the magnetic force is strongest. This is very similar to the behavior of field
lines in the Earth's magnetic field.
Magnetic field lines:
●
never cross.
●
point from the North to the South magnetic poles.
●
must always connect in continuous loops (no magnetic monopoles).
●
are closer together where the magnetic field is stronger.
1) Place your compass in the bar magnet’s magnetic field in each spot indicated by
the circles below. In each circle, draw an arrow showing where the north arrow on
the compass is pointing (showing the field direction) (12 pts)
With a group, get a bar magnet, a sheet of paper, iron filings, and a compass. Place the
paper over the bar magnet and carefully sprinkle the iron filings on the paper (near the
magnet) until a distinct pattern emerges. The iron filings should align with the magnetic field.
2) In the space below:
a)
Draw the orientation of the magnet (label N-S) and iron filings (4 pts)
b)
Draw magnetic field lines, showing the field direction with arrowheads (5 pts)
2
3)
Where along the bar magnet is the magnetic field strongest? (It may be helpful to
refer to your sketch in Question 2) (3 pts)
The magnetic field is strongest at the dipoles. We know this because there is a higher density of
filings (and thus a higher density of magnetic field lines) at the poles.
Align the north end of a bar magnet a few cm from the south end of another bar magnet.
Place a piece of paper over the two magnets and sprinkle iron filings in the region between
the magnets.
4)
In the space below:
a)
Draw the orientation of the magnets (label N-S) and iron filings (4 pts)
b) Draw and add arrowheads to field lines between the magnets, showing the
direction of the magnetic field (confirm with your compass) (5 pts)
5) What would ideal field lines look like INSIDE a bar magnet (i.e., in which direction
do they point)? Draw a bar magnet (N-S) and magnetic field lines outside AND inside
the magnet, labeling with arrows to show the field direction. (3 pts)
3
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2. Earth's Magnetic Field
Earth’s magnetic field, shown as if a giant bar magnet were placed at the Earth’s
center and slightly inclined (11°) from Earth’s rotation axis.
A
magnetic dipole field
is the field produced by a north
magnetic pole and a south magnetic pole in close proximity.
The above figure shows an idealized dipole field for Earth.
The magnetic field lines show the direction of the magnetic
field and presently point to the north magnetic pole.
There
is
something
surprising
about
this
fictitious
bar
magnet in Earth:
Its
south
(negative) pole lies at Earth’s
north magnetic pole
, or “magnetic north.”
You can see
why this must be so by considering that, in the absence of
any significant magnetic fields other than the Earth’s, the
north
end
of
a
compass
needle
points
toward
Earth’s
magnetic north, which is in the general direction of the
geographic North Pole.
However, the magnetic pole is not located at the same
exact position as the North Pole.
Declination
is the angle between magnetic north (the
4
direction in which a compass needle points) and true north (the north geographic pole). It is
often noted on maps, and also
changes in space and time
depending on where magnetic
north is. The figure above shows an example of magnetic declination, showing a compass
needle with an "easterly" variation from geographic north. N
g
is geographic or true north, N
m
is
magnetic north, and δ is magnetic declination.
6) Illustrate a declination of 17°W on the compass below. Label geographic north
(
N
g
)
, magnetic north (
N
m
)
, and the angle between them (δ). (5 pts)
Magnetic Inclination
In general, the magnetic field lines of Earth are not parallel with the surface of the planet
(except at the equator), and dip into or out of the Earth at a certain angle based on magnetic
latitude. The angle between the magnetic field and the horizontal is called
inclination (
i
)
,
which is positive (+) when the field lines point into the ground, and negative (-) when the field
lines
point
out of the ground. Magnetic inclination varies from
90° (perpendicular to
surface) at the magnetic poles to 0° (parallel to surface)
at the magnetic equator.
The relationship between inclination and latitude for a magnetic dipole is given by:
tan(
i
) = 2tan(
ϴ
)
This means that for a location on an Earth with an ideal dipole field, if you know the magnetic
latitude (
ϴ
) you can find the inclination (i), and vice versa.
7) Use the figure below to complete the table:
a) Find the latitude of points A, B, C, D, and E by measuring the angle between
5
the magnetic equator (thick black line) and a line that connects the middle of
the bar magnet to the point on the surface (A line is already sketched) (5 pts)
b) Measure the magnetic inclination angle (
i
) at each point using a protractor.
For a given latitude, the inclination is the angle between a line tangent (flat) to
the Earth’s surface and a line tangent to the field line that intersects the
surface. Make sure to record the appropriate sign (+,-) for inclination (5 pts)
A
B
C
D
E
Latitude (
ϴ
)
Measured Inclination (i
x
)
Theoretical Inclination (i)
6
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8) Now use a calculator to find the theoretical inclinations for a dipole field at the
magnetic latitudes you measured for A, B, C, D, and E. (5 pts)
9) Why do D and E have similar values of inclination? (2 pts)
As seen in the picture D and E have similar latitudes on the Earth and thus should have
similar
values
of
inclination
(though not the exact same inclination because of the
variability of the Earth’s magnetic field as well as the alignment of the magnetic north
pole, which doesn’t perfectly align with the geographic north pole)
10) Is magnetic inclination
always
the same magnitude on a given latitude line? Why
might there be some variability? (2 pts)
No, first off, there is variability on the Earth’s magnetic field which causes the angle of
inclination to not be totally constant at a given latitude. Also, the Earth’s magnetic dipole
is not at an orientation of 90 degrees (inside the Earth), and thus the angle of inclination
will vary at certain latitudes.
Below
is
a
world
map
of
the
magnetic
inclination
of
Earth
as
determined
by
measurement-based models in 2010. The inclination contour lines are each 20 degrees apart
(positive in the northern hemisphere and negative in the southern hemisphere).
7
11) What is the approximate current magnetic inclination of Berkeley, CA? (2 pts)
Approximately 60.05 degrees is the current magnetic inclination of Berkeley
Earth's magnetic field is slowly changing over time, on time scales that range from several
years to millennia. Such changes are referred to as
secular variation.
The figure below
shows how the declination in London has changed from approximately 10°E in the late 16th
century to 25 °W in the early 19th century, before returning to a current value of about 3°W.
8
All elements of the magnetic field change with time – not just the declination. For example,
the total field intensity in Toronto has decreased 14% during the last 160 years.
Magnetic
poles
are just the time-averaged direction of the magnetic field, where the effects of secular
variation are (ideally) averaged out. Many magnetic measurements spanning long periods of
time (at least several 1000s of years) are needed to derive a statistically robust magnetic pole.
12) What is the average declination for London over the last 400 years? (Find the
declination for ~5 points on the curve and take the average) (3 pts)
1600: 11 deg (E)
1700: -10 deg (W)
1800: -23 deg (W)
1900: -16 deg (W)
2000: -3 deg (W)
Avg = (11 + -10 + -23 + -16 + -3)/5 = -8.2 -> 8.2 degrees West
13) Is secular variation relatively fast or slow compared to the average time between
Earth’s magnetic reversals? (3 pts)
Secular variation is relatively fast (months to centuries) compared to the average time
between Earth’s magnetic reversals (thousands of years)
14) Does secular variation lead to magnetic reversals? Why or why not? (3 pts)
no, secular variation does not lead to magnetic reversals. We know this because the
two events operate on different time scales and thus one cannot be the cause of the
other
3. Paleomagnetism
A key factor in establishing the theory of plate tectonics was the recognition of past reversals
in the Earth’s magnetic field and the mapping of normal and reverse magnetic stripes on the
ocean floor. The convection of Earth’s liquid outer core produces a magnetic field that,
at
present,
causes
compasses to point toward magnetic north
. This polarity is called
normal polarity.
9
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Paleomagnetism
is the study of Earth's past magnetic field as it is recorded in the rock
record. The basic premise of paleomagnetism is that a rock sample can acquire and hold the
direction of the magnetic field that existed at the time and place the rock was formed. This is
termed the
magnetic remanence
of the rock.
The earth contains numerous elements that are generally classified as
ferromagnetic
(can
possess magnetization in the absence of an external magnetic field). These elements can
form minerals and therefore rocks with strong magnetic properties. A small proportion (<5%)
of
a
typical
crustal
rock
will
be
made
up
of iron-bearing magnetic minerals such as
magnetite
and
hematite
. There are two principal ways by which the ancient magnetic field
direction can become "frozen" into rocks during their formation:
1)
As magma cools to solidify and form igneous rocks, it cools through the Curie
points of its magnetic minerals (about 580°C for magnetite). The
Curie point
is the
temperature above which certain materials lose their permanent magnetic properties
(ferromagnetism). Above the Curie point, these crystals are magnetized in the direction
of the ambient magnetic field, but will stay aligned only in the presence of that
magnetic
field (paramagnetism). As the crystals cool below the Curie point, the
ferromagnetic properties allow their magnetizations to “lock in,” aligned to the ambient
magnetic field.
Therefore, the paleomagnetic field direction may be recorded when an
igneous rock cools, and can persist for millions and even billions of years. This process
is called
thermal remanent magnetization (TRM).
10
2)
Sedimentary rocks can also preserve a record of the Earth's past magnetic field,
but since they have not been cooled from a high temperature they contain no TRM.
They do, however, contain grains of magnetite and hematite that have been eroded
from igneous rocks. These fine grains behave like small magnets or compasses. As
sediments slowly settle through a water column in lakes or in the oceans, particles of
magnetite
are
free
to
align
with
the
ambient
magnetic
field.
As the sediments
accumulate on the lake or sea floor and gradually become compacted and cemented
into sedimentary rocks, this magnetic field gets locked into place. This is called
depositional remanent magnetization (DRM).
15) Which type of rock and characteristic remanent magnetization is better for
studying a detailed record of secular variation: TRM or DRM? Why? (5 pts)
TRM is better for studying a detailed record of secular variation because it is consistent. As lava
cools and forms igneous rocks, the orientation of the magnetic field is locked in. This process is
much quicker than depositional remanent magnetization as well as much more consistent and
reliable.
New oceanic crust inherits the
Earth’s magnetic field at the time
it
solidifies.
Reversals
of
the
Earth’s magnetic field therefore
cause reversals of the oceanic
crust's magnetization. Thus the
seafloor
has regions where its
magnetic
minerals
point
north
(normal) and areas where they
point
south
(reverse).
These
magnetic
rocks
produce highs
and lows in the local magnetic
field.
Because
they
were
originally
produced
at
the
mid-ocean ridge, the anomalies are oriented parallel to the mid-ocean ridge. We can map
these anomalies by towing an instrument called a “magnetometer” behind a ship and
measuring the strength of the local magnetic field. The resulting map of the sea floor will have
a distinctive magnetic signature.
11
The width of a normally or reversely polarized strip depends on two factors:
1. The length of time the field is in a given polarity
2. The rate of spreading of the ocean floor along a mid ocean ridge axis
Imagine that the field is reversed for 1 million years. During that time all the basalt that forms
at the mid-ocean ridge will be reversely polarized. A spreading rate of 50 mm/yr will produce
a band 50 km wide of reversely polarized rock.
Marine Magnetic Anomalies
(A)
The black line shows positive anomalies as
recorded by a magnetometer towed behind a
ship. In the cross section of the oceanic crust,
positive
anomalies
are
drawn
as
black
bars
(normal
polarity)
and
negative
anomalies
are
drawn as white bars (reverse polarity).
(B)
Perspective
view
of
magnetic
anomalies
shows that they are parallel to and symmetric
about the ridge.
Magnetic Time Scale -
Magnetic anomaly identification numbers are given on the top and absolute
time scale in millions of years (Ma) is given on the bottom. Normal magnetic anomalies are shown
in black, and reversed polarity is shown in white. The absolute age of the anomalies and their
polarities can be read directly from the chart. For example, anomaly 4 is positive and ~7 million
years old.
Restoration of the South Atlantic Coastline 50 Million Years before Present
From the magnetic striping on either side of the Mid-Atlantic Ridge, we can deduce that the
12
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African and South American plates are moving away from each other, carrying the continents
of Africa and South America with them, respectively. By using the pattern of magnetic
lineations shown on the map on the next page,
we can reverse the spreading process
and restore the positions of the African and South American coastlines to a time
when a particular set of magnetic lineations were being formed on the Mid-Atlantic
Ridge.
For the purposes of this exercise we will use anomaly number 21, which, according to
the magnetic lineation time scale, was formed roughly 50 million years ago.
16) On the map of the South Atlantic Ocean below:
a) Draw a red line over each of the magnetic lineations of anomaly number 21 on
the South American side of the Mid-Atlantic Ridge. Connect the segments of
the anomaly with a red line drawn along the connecting portions of the
fracture zones. (2 pts)
b) Attach
a
piece
of
tracing
paper
over
the
map
and
repeat
the
process
described above for anomaly 21 on the African side of the Mid-Atlantic Ridge.
(2 pts)
c) With the tracing paper still in place, trace the coastlines of Africa and South
America
on
the
tracing
paper
with
black
pencil.
Also,
trace
the
map
boundaries and the 20° South latitude line in black pencil. (2 pts)
d) Detach the tracing paper and slide it toward South America until the red line
on the tracing paper matches the red line on the map. When the two lines are
matched as closely as possible, hold the tracing paper in place and trace the
coastline of South America in red pencil on the tracing paper. Trace also the
20° South line on the tracing paper in red pencil. (4 pts)
The map you have constructed on the tracing paper shows the Mid-Atlantic Ridge as it
existed when magnetic anomaly 21 was being formed. Your tracing paper also shows the
relative positions of segments of the coastlines of Africa (black pencil) and South America (red
pencil) as they were approximately 50 million years ago. This reconstruction is based on the
assumption that the continents of Africa and South America were fixed to their respective
plates during the spreading process over the past 50 million years: that is, the continents
moved with respect to each other because the tectonic plates to which they were attached
moved as spreading continued along the Mid-Atlantic Ridge.
17) What is the evidence that the movement of the two plates was not strictly in an
east-west direction? (2 pts)
13
The orientation of the 20 degrees south line has shifted by about 20 degrees, meaning the
two plates also shifted in the North-South direction. The spacing of the isochrons is also
indicative that there is a shift in the North-South direction. Where there is larger spacing,
there must have been tectonic activity that induced a spreading of the plates in the
North-South direction.
18) Was the Earth's magnetic field normal or reversed at the time represented by
your map on the tracing paper? (2 pts)
At the time of anomaly 21, the magnetic field was normal.
(Above) Map of the South Atlantic Ocean showing part of the Mid-Atlantic Ridge in black bars, with
east-west fracture zones, and selected magnetic anomalies. The ages of the numbered anomalies or
magnetic
lineations
can
be
determined from the magnetic time scale on page 12. (From
Magnetic
14
Lineations of the World’s Ocean Basins
, © 1985 the American Association of Petroleum Geologists.
Since the polarity of the magnetic field at a given time is recorded in rocks that formed at that
same time, polarity can act as a chronometer (“time-measurer”), helping to tell us when the
rock was formed. In addition,
the inclination angle (
i
) of the "frozen" ancient magnetic
field will tell us the ancient magnetic latitude, the paleolatitude (
ϴ
)
, of the location
where the rock was formed. To get
ϴ
from
i
, use the relationship you learned in Part 2.
The measured declination of a sampled rock (corrected for present day declination)
tells us the angle between the present magnetic north and paleomagnetic north,
indicating the amount of rotation the rock has experienced
.
Note that we have no
method
to
determine
paleolongitude.
Paleomagnetic
information
is
important
in
plate
tectonics because if the magnetic latitude determined from the rock is not the same as it is
today for the site where the rock was collected, then either the magnetic pole has moved, the
site has moved, or both. These types of data form the main observations that are used in
plate reconstruction models.
Describe some of the main limitations of plate reconstruction models:
19) How might magnetic records in rocks get compromised or destroyed? (2 pts)
Metamorphism, erosion, geological uplift
20) How old is the oldest
widespread
oceanic crust? Does the oceanic crust made
at the mid-ocean ridge give us a complete paleomagnetic record of Earth? (2 pts)
The oldest widespread oceanic crust is 200-400 million years old. This oceanic crust will not give
us a complete paleomagnetic record of the earth because the earth is much older than 200-400
million years old (it is 4.6 billion years old).
21) Give two reasons why good paleomagnetic data might be difficult to obtain from
a rock that is a few billion years old. (3 pts)
Because the rock is so old, it is possible the rock has undergone metamorphism– the
magnetic properties of the rock have been destroyed (if the rock has been heated to
above the curie point), or plate tectonics have shifted the sample and thus cannot give us
good paleomagnetic data.
15
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Laboratory analysis of the paleomagnetic direction in rock samples of Pennsylvanian age from
South America indicates that their magnetic inclination is
+70°
and their declination is
83°E
(present value). The current location of the rock, where the sample was taken, is
20°S, 55°W
.
22) Indicate with a star on the map the present day sample location in South
America, and draw an arrow indicating magnetic declination. (2 pts)
23) At what paleolatitude were these samples formed? Mark the location with an “X”
on the map below. (2 pts)
24) Using your map, comment on the nature of tectonic drift of the South American
continent since the Pennsylvanian (magnitude and direction of movement, evidence
of rotation, etc). (4 pts)
16