Sea-Floor Spreading

1. Sea-Floor Spreading

2. The Mid Ocean Ridge is the longest chain of mountains in the world at more that 50,000 km long! That’s longer the total circumference of the earth!

3. A steep-sided valley splits the top of the mid-ocean ridge for most of its length.

4. The discovery of this range (using sonar) was the last piece needed by Harry Hess who, in the 1940’s & 50’s, suggested that the ocean floor moves like a giant conveyor belt, carrying the continents with it…a theory that came to be known as Sea-Floor Spreading.

5. Sea-Floor Spreading: the process that continually adds new material to the ocean floor. The sea floor spreads apart along both sides of the mid-ocean ridge as new crust is added, pushing the older material further away from the ridge.

6. Sea- Floor Spreading in a nutshell: • The ocean floors move like conveyor belts, carrying “their” continents (and/or parts of continents) along with them. • At the mid-ocean ridge, molten material rises from the mantle and erupts (leaks). • The molten material then spreads out, pushing older rock to both sides of the ridge. • This movement begins at the mid-ocean ridge and moves outward or away from the ridge.

7. Evidence #1: Molten Material The submersible Alvin found that “new” molten material fresh from the mantle was oozing from the cracks in the mid-ocean ridges.

8. Evidence #2: Magnetic Stripes Earth’s magnetic fields have reversed many times throughout Earth’s history. The iron in the magma/rocks originating at the mid-ocean ridge has preserved all these reversals by way of the orientation of the crystals (the direction they point). The iron lined up with the magnetic north that existed at the time the rock cooled! This can be thought of as a record or magnetic memory. • I wrote a journal article on this!

9. Evidence #3: Drilling Samples   • Samples from the sea floor have been brought up through drilling pipes. • Scientists have determined the age of the rocks in the samples. • They found that the farther away from the ridge the samples were taken, the older the rocks were. • The youngest rocks were always in the center of the ridges. • This showed that sea-floor spreading really has taken place. Tah-Dah!!!!!

10. Evidence #4: Wegener Need we say any more about our forgotten (and long unbelieved) hero of this story???

11. So, while the sea floor is spreading, subduction is occurring at the deep-ocean trenches (DOT) • The ocean floor plunges into deep underwater canyons called deep-ocean trenches. • A deep-ocean trench (DOT) forms where the oceanic crust bends downward • Where there are deep-ocean trenches, subduction takes place. • Subduction is the process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle.

12. Subduction at Deep-Ocean Trenches • Convection currents under the lithosphere push new crust that forms at the mid-ocean ridge away from the ridge and toward a deep-ocean trench. • New oceanic crust is hot. But as it moves away from the mid-ocean ridge, it cools and becomes more dense. • Eventually, gravity pulls this older, denser oceanic crust down beneath the trench, far from the mid-ocean ridge where it formed. • At deep-ocean trenches, subduction allows part of the ocean floor to sink back into the mantle

13. Subduction

14. Subduction at Deep-Ocean Trenches

15. Subduction and Earth's Oceans • The processes of subduction and sea-floor spreading can change the size and shape of the oceans • Because of these processes, Earth's ocean floor is renewed about every 200 million years. • 200 million years is the time it takes for new rock to form at the mid-ocean ridge, move across the ocean, and sink into a trench

16. Subduction in the Pacific Ocean • The vast Pacific Ocean covers almost one third of the planet. And yet it is shrinking. • The deep ocean trench swallows more oceanic crust than the mid-ocean ridge can produce • The ridge does not add new crust fast enough, so the width of the ocean is shrinking. • This is happening to the Pacific Ocean, because it is ringed by many deep ocean trenches (Ring of Fire).

17. The Atlantic Ocean is expanding • The Atlantic Ocean has only a few short trenches • As a result, the spreading ocean floor has virtually nowhere to go • In most places, the oceanic crust of the Atlantic Ocean floor is attached to the continental crust of the continents around the ocean. • So as the Atlantic's ocean floor spreads; the continents along its edges move with the floor. • Over time, the whole ocean gets wider • Which means one day we could have another…? Supercontinent!