Seafloor Spreading 5.4 p. 141-147

Seafloor Spreading Until the 1950’s little was known about the ocean floor, but after sonar was invented, a complex ocean floor with mountains, valleys, trenches, and ridges. The most complex mountain range was found in the center of the Atlantic. Scientists wondered what could have caused these mountain ranges.

Marianas Trench Deepest part of the earth Has barely been explored 36,000 feet deep at the “Challenger Deep” Can fit Mt. Everest in the hole

Mid Ocean Ridges A ridge of mountains that are very deep under the oceans and wind along the Earth’s like seams on a baseball. At these ridges new crust is being made from molten material that reaches the surface.

Sea-floor spreading The process of continually adding new material to the sea floor while other material was being recycled back into the mantle. In sea-floor spreading the sea floor spreads apart along both sides of a mid ocean ridge as new crust is added. The ocean floors move like conveyor belts carrying the continents along with them.

Seafloor Spreading at Mariana

Harry Hess In 1960 scientist Harry Hess suggested an explanation. His theory called seafloor spreading stated that hot molten material is forced upward to the surface at the mid-ocean ridge. It flows sideways carrying the seafloor away from the ridge. New magma then fills in the empty spots, then solidifies and forms the new seafloor. Three pieces of evidence showed this theory to be correct.

Evidence from Molten Material Scientists dived 4 km to the ocean floor aboard the submarine ALVIN where at the mid ocean ridge they found strange rocks shaped like pillows or toothpaste squeezed from a tube. These rocks form only from rapidly cooling magma under water.

Underwater lava

Evidence from Magnetic Stripes The magnetic field on the Earth has reversed itself many times over the course of history When that occurs the atoms within the rocks that have magnetic properties align themselves with whichever way is North at the time. Over time there are stripes that align with the magnetic field.

Evidence from Drilling samples In 1968 the Glomar Challenger drilled 6 km down in the ocean floor. Samples were brought up and they determined the ages of the rocks. The younger rocks were closest to the mid ocean ridge and the oldest were closest to the continental shores.

Subduction at Trenches If the seafloor is spreading- dose it keep getting wider and wider?????? NO- As rock reaches the continents the ocean floor plunges under the continents at sites called deep ocean trenches. This is where the ocean crust bends downward and the ocean crust sinks back into the mantle.

The Process of Subduction Subduction is the process of ocean crust returning to the mantle by sinking over millions of years. The reason the oceanic crust can sink is because it is more dense that the continental crust.

Subduction and the Earth’s Oceans The ocean floor is renewed about every 200 million years The Pacific ocean is shrinking in size because the mid-ocean ridge is producing less new rock than is returning to the mantle while the Atlantic Ocean is expanding because it only has a few trenches but many ridges.

The Theory of Plate Tectonics 5.5 p. 150-154

Lithosphere’s Eggshell texture In 1965, Wilson observed cracks in the continents similar to those on the ocean floor and proposed that the lithosphere is broken into separate plates. These plates carry pieces of continents and ocean floor, or both. This combined the theory of continental drift and seafloor spreading into one theory called plate tectonics.

How plates MOVE The plates are in constant motion moved by convection currents deep in the mantle. As the plates move they collide, pull apart, and grind into one another which causes volcanoes, mountain building, and trenches.

Plate Boundaries Three types of boundaries Convergent Divergent Transform

Divergent Boundaries The boundary between two plates that are pulling apart is called a divergent boundary. Two examples of divergent boundaries are the Mid-Atlantic Ridge and the Great Rift Valley in Africa.

Convergent Boundaries When the seafloor cools, becomes dense, and sinks, or where two plates collide it is called a convergent boundary. There are three types of convergent boundaries

Ocean-Continent Collision When an ocean floor plate collides with a continental plate, the ocean floor plate is denser and goes under the continental plate. This is called a subduction zone. Volcanoes and trenches occur at subduction zones; because the ocean floor plate is melting as it descends under the continental plate. The Andes Mountains in South America is an example.

Ocean-Ocean Collisions Ocean-Ocean Collisions occur when two ocean plates collide, or when one plate cools and begins to sink. A deep sea trench is formed as one plate goes under the other and islands and volcanoes are also formed as new magma is forced to the surface. The islands of Japan were formed this way

Di-Continental Plate Collide When 2 continental plates collide neither is less dense than the other so no subduction occurs, therefore the two plates crumple upward forming mountain ranges.

Appalachian Mountains A look at rocks exposed in today's Appalachian mountains reveals elongated belts of folded and thrust faulted marine sedimentary rocks, volcanic rocks and slivers of ancient ocean floor, which provides strong evidence that these rocks were deformed during plate collision. The birth of the Appalachian ranges, some 480 million years ago, marks the first of several mountain building plate collisions that culminated in the construction of the supercontinent Pangaea with the Appalachians near the center. Because North America and Africa were connected, the Appalachians formed part of the same mountain chain as the Anti-Atlas in Morocco. To the northeast, the same mountain chain continued into Scotland, from the North America/Europe collision.

Transform Fault Boundaries Transform fault boundaries occur when two plates slide past each other and are moving either in opposite directions or in the same direction at different rates. When 2 plate slides past each other, an earthquake will occur. The San Andreas fault in California is the most famous transform fault boundary.

Effects of Plate Tectonics Two of the main effects caused by plate tectonics are volcanoes and earthquakes. Plate tectonics also form all of the rifts and mountains to form. Tension forces cause the plates to stretch and tilt blocks forming a fault-block mountain. If two blocks separate a rift valley can occur.

Compression Forces Compression causes massive forces creating mountains as they smash together the plates.