The Theory of

Seafloor Spreading

Seafloor BathymetryCreating Maps of the Ocean Floor

• Scientists were able to map the ocean floor using sonar, an Echo-sounding devices.

• The time it takes for sound waves to travel through the ocean and back is recorded to calculate ocean depth which is then converted to an image of the ocean floor.

Map of Ocean Floor

Mid-Ocean Ridges and TrenchesMid-Ocean Ridges and Trenches form the Boundaries of Earth’s Tectonic

Plates

HARRY H. HESS

• Scientist (geologist) who proposed the Theory of Seafloor Spreading in early the 1960 ‘s.

How Does Seafloor Spreading Work? 1. Magma rises through the

center of mid-ocean ridges.mid-ocean ridges.2. It cools and forms new

rock on either side of the ridge.

3. As more seafloor is created, the older floor is pushed away from the ridge and spreads the seafloor.

4. The oldest ocean floor becomes more dense because of the accumulation of sediment and forms trenchestrenches where the seafloor is recycled into the Asthenosphere in a process called subduction.

Subduction of older, more dense ocean crust into the mantle where it is recycled.

The Theory Says:

• The seafloor spreads apart along both sides of a mid-ocean ridge as new crust is being added.

• As a result the ocean floor moves like conveyor belts, carrying the continents with them.

The Ocean Floor is NOT NOT Flat; it contains a system of mountains, valleys and trenches that extend through the center of much of Earth’s oceans.

Mid-Ocean RidgesMid-Ocean Ridges – Underwater volcanic mountain range that is more than 56000 kilometers (35000 mi) long, where new seafloor is being created.

Rift Valley – The valley between the ridges where lava rises on the ocean floor and forms new rock.

Trench – Deep, steep, narrow crack in the ocean floor where old sinks back into the mantle and is recycled.

Evidence to support Sea floor Spreading

1. Evidence from Molten Material• Hardened rocks

shaped like pillows were discovered along the ridge.

• These rocks only form when lava cools quickly underwater.

• Pillow Lava (rocks) prove that new rock is being formed at ridges because it only forms under water.

Alvin Submarine

http://www.pmel.noaa.gov/vents/nemo/explorer/concepts/pillow_lava.html

The Discovery of Magnetic Stripes (of Rock)

• Using a Magnetometer hanging from a boat, two English scientists, Drummond Matthews and Fred Vine discovered magnetic bands on either side of the Mid-Atlantic Ridge.

2. Evidence from Magnetic StripingAs new Basaltic seafloor is created at the ridges, the iron particles in it magnetize differently proving that new seafloor was created at different times in our history.

Normal Polarity = N

Reverse Polarity = S

The iron lines up in the direction of the Earth’s strongest magnetic pole of the time and is locked into place when the rocks form proving that new rocks are always forming at the ridges.

Magnetic Zebra Stripes

3. Age Evidence from Drilling Samples

• The oldest ocean rock is only 180 million years old compared to the continents which are over 4 billion years old.

• Young rocksYoung rocks are found close to the ridge

• Old rocksOld rocks are found further away from the ridges.

Glomar Challenger research ship used to drill in the

seafloor.• Deep Sea Drilling

Ship

If New Seafloor is Always Created, Then Why Isn’t

the Earth Blowing up Like A Balloon?

Old seafloor is being

consumed or recycled

at the trenches!trenches!

• Places where seafloor is pushed back into the

Asthenosphere are called SUBDUCTION ZONES.

• Subduction occurs at the trenches.

So What is the Significance of

The Theory of Seafloor Spreading?

The Theory of Seafloor Spreading

Answers the Question:

HOW

the

Continents Moved!

As the seafloor spreads at mid-ocean ridges, the ocean floor

moves carrying the continents with it!