Seafloor Spreading and Plate Tectonics

Harry Hess Admiral Harry Hammond Hess gathered sonar readings of the Pacific Ocean floor between battles during WWII. His data supported Wegener’s theory He hypothesized that magma rises up at mid-ocean ridges to create new ocean floor and that old ocean floor is being destroyed in trenches.

How do we know what the Ocean Floor looks like? Sonar (sound) bounces sound waves off the bottom of the ocean. This data is tells us relative depth and gives us the topography.

The parts of the Ocean Floor. . . TRENCHES Ridges

Ocean Floor Topography There are underwater mountain ranges (ocean ridges) longer and taller than on land. The Mid-Atlantic Ridge is about 25,000 mi long! Trenches are deep, steep canyons where a plate is being subducted. The deepest one, the Mariana Trench, is about six times deeper than the Grand Canyon and is the lowest spot on Earth.

Why is the ocean floor so young? (geologically speaking) The oldest part of the sea floor is 180-200 million years old – this is young compared to the oldest continental rocks which are 3.8 billion years old!

“SEA FLOOR SPREADING”:Ocean crust is being formed at ocean ridges and destroyed at deep-sea trenches. . . It sinks because ocean crust is denser than continental crust!

MAGNETIC HISTORY: The earth has a magnetic field around it generated by the inner core spinning in the outer core Right now, our compasses point “NORTH” because that is where the magnetic field lines enter the earth. Every several thousand years ( and for unknown reasons), the direction of the field lines will reverse.

BASALT contains iron crystals which act like compass needles. When magma is in a liquid state, they naturally point themselves in the direction of the magnetic field line entrance before it hardens. When the field reverses, they will also reverse directions. This creates alternating “age bands” where magma comes out at ridges, called ISOCHRONS.

This isochron map shows the various ages of the ocean floor This isochron map shows the various ages of the ocean floor. What pattern do you see? Was Harry Hess right? How come the Mid Atlantic Ridge is long, but only gets to about 2500 miles tall?

Wegener’s Missing Mechanism Harry Hess provided the missing link in Wegener’s Continental Drift Hypothesis. The continents do not move through the ocean floor – they get pushed along with the ocean crust as it is formed at ocean ridges! The movement itself of the plates come from convection currents in the mantle (hot goes up, cool goes down. . . )

Sometimes there are “leaks” through weak spots in the middle of a plate where a magma plume is poking through. . . These are called HOT SPOTS. . . Hawaii is a part of the Emperor Sea Mount chain, which is a “trail” of hot spots left behind in the Pacific Plate.

As the Pacific Plate passes over the magma plume, it pokes holes in the plate, creating low elevation volcanoes that turn into islands. Most of them are underwater! http://www.wwnorton.com/college/geo/egeo2/content/animations/2_6.htm

EVIDENCE! Evidence for Continental Drift ( Fossil and Climate evidence) = PALEONTOLOGY Evidence for Seafloor Spreading ( Isochrons, Ocean Ridges, etc.) = PALEOMAGNETISM