Question of the DayQuestion of the DayQuestion of the DayQuestion of the Day

Question: What is a transform boundary? Name the one closest to us.

Answer: … … …

Turn In:Rate of Plate Motion

Magnetic polarity Magnetic polarity reversals and Plate reversals and Plate TectonicsTectonics

EarthEarth ’s Magnetic Field’s Magnetic Field

EarthEarth’s liquid outer core is made of ’s liquid outer core is made of ironiron

As the outer core spins, it creates a As the outer core spins, it creates a magnetic field.magnetic field.

EarthEarth ’s Magnetic Field’s Magnetic Field Like most magnets, the Earth has a Like most magnets, the Earth has a

magnetic north pole and a magnetic magnetic north pole and a magnetic south polesouth pole

the poles the poles of the magnet do of the magnet do not always line up not always line up with the real North with the real North and South Polesand South Poles

Normal PolarityNormal Polarity Igneous rocks cool from magma- Igneous rocks cool from magma-

contain magnetic mineralscontain magnetic minerals Magnetic minerals align with the Magnetic minerals align with the

earthearth’s magnetic field ’s magnetic field N

S

Earth’s magnetic field

Igneous Rock with magnetic minerals pointing towards earth’s north pole

Reverse PolarityReverse Polarity Sometimes the North and South Poles Sometimes the North and South Poles

of the earthof the earth’s magnetic field switch’s magnetic field switch Called reverse polarityCalled reverse polarity

N

S

S

N

Normal Polarity

Reverse Polarity

See a Polarity ReversalSee a Polarity Reversal

Polarity reversal clipPolarity reversal clip

Reverse PolarityReverse Polarity When the magnetic field is reversed, When the magnetic field is reversed,

igneous rocks will record itigneous rocks will record it Magnetic minerals always point toward Magnetic minerals always point toward

the magnetic north pole, where ever it the magnetic north pole, where ever it isis

S

N

Earth’s magnetic field

Igneous Rock with magnetic minerals pointing towards earth’s north pole

Side by side these rocks look like this:Side by side these rocks look like this:

This tells scientists that the magnetic This tells scientists that the magnetic poles reverse some time between poles reverse some time between when one rock formed and when the when one rock formed and when the other formed.other formed.

Magnetic ReversalsMagnetic Reversals

Spreading CentersSpreading Centers

On the seafloor we see alternating On the seafloor we see alternating bands of normal and reverse polarity bands of normal and reverse polarity rocksrocks

Bands are Bands are symmetrical on symmetrical on both sides of a both sides of a mid ocean ridgemid ocean ridge

Spreading CentersSpreading Centers

We see symmetrical We see symmetrical bands because the bands because the two bands formed at two bands formed at the same timethe same time

After they formed After they formed the bands were the bands were moved apart as the moved apart as the plates separatedplates separated

sea floor spreading