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Magnetism
•Bar magnets have distinct N & S poles
– Break a bar magnet in two, & get two smaller
magnets
•Like poles repel; unlike poles attract
– Attraction & repulsion cause magnets to align
themselves with external magnetic fields
– Use magnetic field lines to denote magnetic field
about a magnetic source
– Field lines emerge from N poles, & enter S poles of
magnets
•Magnetic field generated within Earth, called
the geomagnetic field, surrounds us
Geomagnetic field
• Geomagnetic field lines are rarely horizontal and
rarely aligned perfectly N-S
• Angle between horizontal plane & magnetic field line =
inclination
– Inclination is ~ 0° at equator & ~ 90° at poles
• Angle between direction toward true north and
magnetic field line = declination
– Declinations vary from point to point on earth at any
one time, & vary from time to time at any one point
on earth
• Intensity, magnitude, or strength of field varies with
position on earth and with time
– Strength is greatest at poles & weakest at equator
Properties of magnets
• Moving electric charges generate magnetic fields
– In permanent magnets (ferromagnetic materials), magnetic
fields due to electrons moving about nuclei & those due spin of
electrons align & amplify each other
• When heated, ferromagnetic materials loose their
magnetism at a temperature called the Curie
temperature
– Curie temperatures for ferromagnetic minerals are 200-700°C;
minerals & rocks melt at 1000°-1400°C
• When cooled back below their Curie temperatures,
ferromagnetic materials will reacquire their magnetism
parallel to the ambient magnetic field
– Thermal remanent magnetism = reacquired magnetism parallel
to the ambient magnetic field
Geomagnetism
• Earth’s magnetic field is dominated by axially
symmetric dipolar component
– Other components exist, but are less important
• Dipolar axis is not exactly parallel to rotation axis
• Curie temperatures and geothermal gradient indicate
that there is no bar magnet inside Earth
• TRM & DRM provide data on Earth’s magnetic field at
different times in the past
– 1500 years ago, field strength = 1.5 x present magnitude
– 5500 years ago, field strength = 0.5 x present magnitude
– Field has always been dominated by dipolar component, with
dipole axis parallel to rotation axis on average
– The field has had reversed polarity at different times in the
past
Geomagnetic polarity reversals
• Recognize reversals in places where we have extensive
history of volcanism - many rocks that have cooled
through their Curie temperatures
• Roughly half of rocks have normal polarity; half have
reversed polarity
• Reversals are worldwide & restricted to specific times
in the past
• Pattern of normal & reversed polarity is sufficiently
distinctive that we can assign ages to rocks in which
we can recognize their TRM
–Pattern is a song with two notes of different duration;
distinctive rhythms in a well-known song
Sea floor magnetic patterns
•In ocean basins, recognize distinctive pattern
of linear magnetic anomalies
•Positive & negative anomalies occur where field
strength is greater than or less than expected
value
•Regions of positive & negative anomaly occur in
linear regions aligned parallel to MOR axial
valleys
•Can identify identical anomalies on opposite
sides of MOR
•Anomaly patterns truncated & offset by oceanic
fracture zones
Geomagnetism & plate tectonics
•Recognize geomagnetic reversal pattern in linear
magnetic anomalies, with partiular anomalies
corresponding to rocks of specific ages
•When sea floor volcanic rocks cool through
their Curie temperatures, they acquire TRM that leads to anomalies
•Linear magnetic anomaly patterns
–Confirm that rock farther from MOR is older
–Corroborate earthquake first-motion studies
that show plates to be diverging across MOR
–Give plate velocities of 1-10 cm/yr
Hot spots
• PT predicts that volcanism on earth is either
associated with divergent (MOR or rift) or with
convergent (DOT or mountains) plate boundaries
• There are about 20 locations where volcanism does
not match this pattern; volcanism occurs within a
plate or with unusual character for plate boundary
setting
• These so-called hot spots correlate with regions of
mantle upwelling; plumes of warm mantle rise, melt,
and cause intra-plate volcanism
• Mantle plumes are relatively fixed, i.e. they move
slowly relative to each other & we can use them to
fix the ‘absolute’ motion of plates
States of matter
•Matter commonly occurs in three forms
•Gases -expand to fill the container in which
we place them
•Liquids -conform to container's shape, even if
they cannot fill the container
•Solids -have fixed shapes and do not flow
freely
•H2O is unique because it occurs naturally in all
three phases (i.e. physically separable
entities) on Earth's surface
Gases
• Lowest density, highest energy form
• Individual molecules or atoms in a gas are disordered
• Molecules or atoms collide with others or the walls of
container to produce a measurable pressure
• On condensing, gas shrinks by a factor of about 1000
• Volume of liquid condensate is ~1/1000th volume of
gas
• Condensing gases give off heat (∆H of condensation),
even though the temperature remains constant
• Liquids have lower internal energy than the
corresponding gas
Liquids
• Higher density, lower energy form
• Atoms or molecules in liquids to cohere, but they are
not bound to specific neighboring atoms or molecules,
so liquids pour
• Most liquids are incompressible
– Liquids have densities nearly equal to those of corresponding
solids, suggesting that atoms or molecules in liquids are closely
packed
– When liquids change to solids, their volumes usually decrease
by 3 to 30%
• Liquids must give off heat to become solid; ∆H of
crystallization is usually 3–30% ∆H of condensation
Solids
• Highest density, lowest energy form
• Solids only slightly more dense than liquids; atoms or
molecules not really in closer proximity than in liquid
• Solids do not deform readily; atoms or molecules have
‘fixed’ positions
• Crystallinity, the ordered internal arrangement of
atoms or molecules, is characteristic feature of solids
• A single compound in different crystal structures
solids will exhibit different physical properties
– Diamond & graphite
– Ice I, ice II, etc. to Ice X