How The Plates MoveSea Floor spreading and Subduction

Sea floor spreading

The process by which new ocean lithosphere is created as the older rock material is pulled away.

Sea floor Spreading …

Takes place where plate edges meet. Most often underneath the ocean

Molten rock (magma) spurts up between the plates.

As the magma is cooled by the ocean water it forms new crust.

This new crust creates ocean ridges

Sea Floor spreading

• This process keeps repeating so new crust is constantly being formed

• The crust increases in age the further it is from the ocean ridge

What is our evidence that the crust at the ridges is the youngest?

• Magma contains magnetic minerals

• These magnetic minerals align with the magnetic field of the Earth – they move toward the north pole

Magnetic Reversals

The (magnetic)North and South pole have switched places many times in Earth’s history

Switching places is called “magnetic reversal”

So the magnetic minerals have created bands or stripes in the crust

How the sea floor spreads -Ridge Push

Two plates come apart in the ocean

Magma rises to fill in the gap and when it cools it forms a ridge (extra rock crust)

The extra weight of the ridge (extra rock crust) puts pressure on the asthenosphere causing magma to rise again

Sea floor spreading features Rift – is the area of

separating plates Rift valley – is the

valley that forms between the ridges

Ridge - is the mountain that is formed by the build up of cooled magma

http://app.discoveryeducation.com/search?Ntt=sea+floor+spreading

Sea Floor Spreading Activity

So does the Earth keep getting bigger?

No! The Earth recycles itself in a process called Slab pull – or SUBDUCTION

As the ridges push out new crust the old crust is pulled back down into the mantle.

A slab pull is when the plates are pulled down into the mantle.

How Subduction works

At the Mid Atlantic Ridge the plates are pulling apart, at the Nazca and S. American plate the plates are colliding and subducting

What forms when subduction occurs?

Trenches and sometimes volcanoes

Trench – is a ditch like feature formed by subduction

Composite Volcanoes

Picture Citations

http://www.pmel.noaa.gov/vents/nemo/education/images/seafloor_spreading.jpg

http://www.womenoceanographers.org/Content/Profiles/MelanieSummit/images/LearnMore1.jpg

http://www.pmel.noaa.gov/vents/nemo/education/images/seafloor_spreading.jpg

http://www.answersingenesis.org/assets/images/articles/am/v2/n2/figure-2-c.jpg