EPSC 201 – SHORT ANSWERS
Paleomagnetism and geomagnetic pole paths
If you payed attention to the biology
(plants and animals) across continents you see
similarities
Seems to have fit together for a time
South America and Africa split apart something
like 160 million years ago, they share animal
similarities up until this point
Mostly birds
There are common geological terranes
Same rocks in east Africa and brazil
Same age same kind
Similar geology in Mountain belts of north America as in Africa
Up until 180 million years ago, then they differ
Use
rocks from different continents to find the
magnetic pole at the time the rock was form,
Record the magnetic change over time
Turns out these curves line up if you reassemble
the continents
Paleomagnetism
Magnetic fields of ancient rocks indicate the orientation of the magnetic field relative to the rock,
at the time the rock formed. This record preserved in rock is paleomagnetism.
In a rock that formed millions of years ago, the orientation of the dipole representing the magnetic
field of the rock is not the same as that of present day Earth.
When lava starts to cool and solidify into rock, tiny crystals begin to grow. At first, thermal energy
cause the tiny magnetic dipole associated with each crystal to wobble chaotically. Eventually, the
rock cools, dipoles slow down and like tiny compass needles, align with the earth’s magnetic field.
Since magnetic dipoles of all grains point in the same direction, they add together and produce a
measurable field.
Paleopole
Supposed position of the earth’s magnetic north pole in the past.
Geologists wanted to track the change in position of the paleopole over time. They measure the
paleomagnetism in a succession of rocks of different ages from the same location on a continent
and plotted the successive positions of dated paleopoles on a map. This traced out a curving line
that came to be known as an
apparent polar-wander path.
They found that each continent has a different apparent polar-wander path. They realised it is
not the pole that moves relative to fixed continents, but rather the
continents that move relative
to a fixed pole
. Since each continent has its own unique polar-wander path,
the continents must
move with respect to each other.
Earth’s magnetic field
Flow in the liquid outer core creates the magnetic field.
o
It is similar to the field produced by a bar magnet
o
The magnetic pole is tilted ~11.5 from the axis of rotation
Geographic and magnetic poles are not parallel.
A compass points to magnetic N, not geographic N.
The difference between geographic N and magnetic N is called
declination.
It depends on:
o
Absolute position of the two poles
Geographic north
Magnetic north
Longitude
Curved field lines cause a magnetic needle to tilt.
