Transcript Magnetic polarity reversals and Plate Tectonics

Question of the Day
Question: What is a transform boundary? Name
the one closest to us.
Answer: … … …
Magnetic polarity
reversals and Plate
Tectonics
Earth’s Magnetic Field


Earth’s liquid outer core is made of
iron
As the outer core spins, it creates a
magnetic field.
Earth’s Magnetic Field
Like most magnets, the Earth has a
magnetic north pole and a magnetic
south pole
 the poles
of the magnet do
not always line up
with the real North
and South Poles

Normal Polarity


Igneous rocks cool from magmacontain magnetic minerals
Magnetic minerals align with the
earth’s magnetic field
N
Igneous Rock
with magnetic
minerals
pointing
towards earth’s
north pole
Earth’s
magnetic
field
S
Reverse Polarity


Sometimes the North and South Poles
of the earth’s magnetic field switch
Called reverse polarity
N
S
Normal
Polarity
S
N
Reverse
Polarity
See a Polarity Reversal

Polarity reversal clip
Reverse Polarity


When the magnetic field is reversed,
igneous rocks will record it
Magnetic minerals always point toward
the magnetic north pole, where ever it
is
S
Earth’s
magnetic
field
Igneous Rock with
magnetic minerals
pointing towards
earth’s north pole
N

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Side by side these rocks look like this:
This tells scientists that the magnetic
poles reverse some time between
when one rock formed and when the
other formed.
Magnetic Reversals
Spreading Centers
On the seafloor we see alternating
bands of normal and reverse polarity
rocks
 Bands are
symmetrical on
both sides of a
mid ocean ridge

Spreading Centers
We see symmetrical
bands because the
two bands formed at
the same time
 After they formed
the bands were
moved apart as the
plates separated

sea floor spreading