Bellringer What three specific types of evidence did Wegener study that led to him creating the idea that the continents had moved?

Sea-Floor Spreading Notes

Mid-Ocean Ridges Since the mid-1900s, scientists have been using sonar to study the ocean floor. Sonar is a device that bounces sound waves off underwater objects. The longer it takes the sound waves to bounce back, the farther away the objects are. Using sonar, scientists found long mountain ranges on the ocean floor. They called these mid-ocean ridges because they run through the middle of all oceans.

Mid-Ocean Ridges In a few places, mid-ocean ridges poke above the surface and form islands. Iceland is the top of a mid-ocean ridge in the North Atlantic Ocean.

What is Sea-Floor Spreading? Sea-Floor Spreading is a process that slowly adds new rock to the ocean floors. Scientist Harry Hess came up with the idea of sea-floor spreading in 1960.

Sea-Floor Spreading Here’s how sea-floor spreading works. In the center of a mid-ocean ridge, melted rock pushes up through cracks in the ocean floor. The melted rock pushes older, solid, more dense rock away from both sides of the ridge. The melted rock cools and forms new solid rock at the center of the ridge, called a rift valley.

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Sea-Floor Spreading This process keeps repeating. Slowly, the ocean floor is pushed farther away from both sides of the mid-ocean ridge. At the same time, new rock keeps adding to the ocean floor in the rift valley. As a result, the ocean floors move like conveyor belts, carrying the continents along with them. mation.gif

Evidence for Sea-Floor Spreading In the 1960s, scientists began to look for and uncover evidence supporting sea-floor spreading. Using a submarine named Alvin, scientists were able to look into a rift valley and examine something called pillow lava, which is a special type of solid rock that only forms on the ocean floor when magma cools very rapidly. This proved that new molten material was being added to the ocean floor at these ridges.

Evidence for Sea-Floor Spreading As you already know, convection in Earth’s outer core causes the Earth to have a magnetic field. Well, occasionally this field flips and the south pole and north pole switch polarities. The last time this happened was about 780,000 years ago. Measuring the magnetic iron “stripes” on both sides of a mid-ocean ridge, scientists discovered that on both sides of the ridge, there are matching “stripes” that flip direction each time the poles reverse.

paleomag.html Click the “Normal” button in the upper-right to flip the polarity of Earth’s magnetic field.

Evidence for Sea-Floor Spreading In 1968, the Glomar Challenger used special equipment to drill into the ocean floor six kilometers beneath the ocean’s surface. This feat has been compared to using a sharp- ended wire to dig a hole into a sidewalk from the top of the Empire State Building. They successfully brought up rock samples from the ocean floor. After analyzing the rocks’ ages, they were able to determine that the rocks got older the farther away they moved from the ridge.

Subduction at Trenches Sea-floor spreading makes the ocean floors get wider. New rock keeps forming at mid-ocean ridges. Old rock keeps getting pushed farther away from both sides of the ridges. After millions of years, the oldest rocks will begin to form ocean trenches. At a trench, the dense basalt of the oceanic crust is forced to bend downward and begins to sink into the mantle. This process is known as subduction.

Subduction at Trenches Sea-floor spreading and subduction work together to create a balance in the amount of ocean that covers the planet. In the Atlantic Ocean, very little subduction occurs, meaning that the Atlantic Ocean is continuously getting larger at the Mid-Atlantic Ridge. Conversely, there is a massive amount of subduction in the Pacific Ocean on the western coast of Asia and the eastern coast of the Americas. This means that the Pacific Ocean (currently the largest ocean on the planet) is shrinking.