Question of the Day Question: What is a transform boundary? Name the one closest to us. Answer: ……… Turn In: Rate of Plate Motion

Magnetic polarity reversals and Plate Tectonics

Earth’s Magnetic Field Earth’s liquid outer core is made of iron Earth’s liquid outer core is made of iron As the outer core spins, it creates a magnetic field. 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 Like most magnets, the Earth has a magnetic north pole and a magnetic south pole the poles the poles of the magnet do not always line up with the real North and South Poles

Normal Polarity Igneous rocks cool from magma- contain magnetic minerals Igneous rocks cool from magma- contain magnetic minerals Magnetic minerals align with the earth’s magnetic field Magnetic minerals align with the earth’s magnetic field N S Earth’s magnetic field Igneous Rock with magnetic minerals pointing towards earth’s north pole

Reverse Polarity Sometimes the North and South Poles of the earth’s magnetic field switch Sometimes the North and South Poles of the earth’s magnetic field switch Called reverse polarity Called reverse polarity N S S N Normal Polarity Reverse Polarity

See a Polarity Reversal Polarity reversal clip Polarity reversal clip Polarity reversal clip Polarity reversal clip

Reverse Polarity When the magnetic field is reversed, igneous rocks will record it When the magnetic field is reversed, igneous rocks will record it Magnetic minerals always point toward the magnetic north pole, where ever it is Magnetic minerals always point toward the magnetic north pole, where ever it is 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 poles reverse some time between when one rock formed and when the other formed. 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 On the seafloor we see alternating bands of normal and reverse polarity rocks Bands are Bands are symmetrical on both sides of a mid ocean ridge

Spreading Centers We see symmetrical We see symmetrical bands because the two bands formed at the same time After they formed After they formed the bands were moved apart as the plates separated

sea floor spreading