Sea-Floor Spreading

How is Earth magnetic?The Earth is a layered planet

consisting of crust, mantle, and coreThe outer core is a metallic liquid

that moves around, creating a magnetic field

The lithosphere moves around on top of this

What is magnetic reversal?

Every magnet has a north and south pole because magnetic domains are aligned

Earth’s magnetic poles periodically REVERSE

Magnetic North becomes magnetic South and vice-versa EVEN THOUGH the geographic poles don’t move

What is magnetic reversal?

How was magnetic reversal discovered?

When iron-rich lava cools, its domains align in the direction of the magnetic field

Studies of paleomagnetism provide a rock record of magnetic field reversal

How was magnetic reversal discovered?

In WWII, we developed magnetometers• Sensitive tools to detect steel-hulled

submarinesLater, when scientists used the

magnetometers to explore the ocean floor, they discovered a banding pattern in the rocks

What is Sea-floor Spreading?At the mid-ocean ridge, molten material rises from the

mantle and erupts. The molten material then spreads out, pushing older

rock to both sides of the ridge. As the molten material cools, it forms a strip of solid

rock in the center of the ridge.

Sea-Floor Spreading

What is Sea-floor Spreading?The process that continually adds

new material to the ocean floor. Evidences include: molten

material, magnetic stripes, and drilling samples.

Harry Hess, a WWII naval officier, first explained the mechanism in the 1960s

Why the wait??? We did not have the ability to explore the oceans depths previously!

Hess used sonar echo readings to map the ocean floor

When was sea-floor spreading theory developed?

How are sea-floor spreading and continental drift related?

They support one another!One reason continental drift was rejected was

because Wegener could not explain “HOW”The evidence that the sea-floor is younger

than continental crust provides a mechanism for continental drift

How are sea-floor spreading and continental drift related?

The evidence that the sea-floor is younger than continental crust provides a mechanism for continental drift

Sea-floor spreading is the force pushing continents apart!

How are sea-floor spreading and continental drift related?

The transfer of thermal energy via convection cells drives seafloor spreading and all tectonic plate movement

Convection cells circulate molten material from areas of high temperature to cooler regions in a circular pattern

How are sea-floor spreading and continental drift related?

As material in the lower mantle warms, it becomes less dense and more buoyant, causing it to rise

As material in the upper mantle cools. It becomes more dense and sinks due to gravity

Partner DemoPartner up and put your desks together so you are facing each

other with almost no space between the two desksHold the paper so that only the tiniest bit is exposed on each

sideVERY VERY sloooooooooowly, begin pulling the paper up by

sliding it across your deskAs you slide, one person should be coloring ALL of the paper as

it comes upWhen I say “Switch” the second person should start coloring

Partner Demo

Partner DemoUse the diagram to label the following parts on your

sea-floor spreading model:• Mid-Ocean Ridge• Strips of alternating normal and reversed polarity

(label with “NP” and “RP”• Oldest• Youngest• Subduction Zone

Analysis QuestionsImagine that as you pulled the paper, your hands

represented two continents that were once together, but moved apart as the sea-floor grew. How does this model provide evidence for continental drift?

When was the Earth formed? Based on this model, explain why the ocean floor is only 200 million years old.

Analysis QuestionsWhat happens at a subduction zone? What happens to

this old rock?Are the areas of normal and reverse polarity equal in

size? What does this lead you to believe about periods of magnetic reversal in history?

Just like on continental crust, the top of the ocean floor is covered in sediments. Do you think the sediments are younger, older, or the same age as the rocks below. WHY?

Bellringer 10/5/2011

“The sea is everything. It covers seven-tenths of the terrestrial

globe. Its breath is pure and life-giving. It is an immense desert

place where man is never lonely, for he senses the weaving of Creation

on every hand. “

~Jules Verne

20,000 Leagues Under the Sea

Bellringer October 6th:Watch the demonstration in the front of

the room and write down your observations about the movement of the food coloring

Do you remember how heat flows from physical science???

http://education.sdsc.edu/optiputer/flash/convection.htm