Sea-Floor-Spreading.pdf

Continental Drift Theory HOW THE DRIFTING TOOK PLACE? Sonar - a device that bounces sound waves off under-water objects and then records the echoes of these sound waves. The time it takes for the echo to arrive indicates the distance to the object. Mid-Ocean Ridge – the longest chain of mountain...

Continental Drift Theory HOW THE DRIFTING TOOK PLACE? Sonar - a device that bounces sound waves off under-water objects and then records the echoes of these sound waves. The time it takes for the echo to arrive indicates the distance to the object. Mid-Ocean Ridge – the longest chain of mountains in the world---these are divergent plate boundaries. MID-ATLANTIC RIDGE - An undersea mountain chain in the Atlantic Ocean. - Has a gigantic cleft about 32-48 km long and 1.6 km deep - Offset by fracture zones or rift valleys Sea-Floor Spreading THE PROOF-----SEAFLOOR SPREADING An American scientist named Harry Hess, together with Robert Dietz, proposed the seafloor spreading theory in 1962. The Sea floor spreading theory states that new ocean crust is being created at mid- ocean ridges and destroyed at deep-sea trenches. This is proof that the plates are moving along on a “conveyor belt” so Wegener’s idea on continent drift was correct. The force responsible for driving or moving the plates is convection currents _____________________. Convection Currents occur within the mantle of the earth when hot magma rises and cool magma sinks  Magma, because it is hotter and less dense, is forced up towards the ocean floor cooling off when it touches water. When the magma hardens, a small amount of new ocean floor is added to the Earth’s crust. As more magma rises and cools it pushes the new sea floor away from the ridge. This process is called sea floor spreading. Sea-Floor Spreading Subduction – Process by which the ocean floor sinks beneath a deep- ocean trench and back into the mantle; allows part of the ocean floor to sink back into the mantle Sea-Floor Spreading PROOF THAT THE SEA FLOOR IS SPREADING 1. The oldest ocean floor rocks ever found are 180 million years old. Remember the Earth is 4.6 billion years old. This proves that ocean floor is being destroyed therefore all ocean floor rocks are young compared to the age of Earth. ROCKS AT THE OCEAN FLOOR ARE YOUNGER THAN THOSE AT THE CONTINENTS PROOF THAT THE SEA FLOOR IS SPREADING 2. The rocks closest to the ocean ridge were younger than the rocks found further from the ridge. This means that new rocks are formed at the ridges and push the older rocks away from the ridge. ROCKS ARE YOUNGER AT THE MID- OCEAN RIDGE ROCKS FAR FROM THE MID-OCEAN RIDGE ARE OLDER PROOF THAT THE SEA FLOOR IS SPREADING 2. The rocks closest to the ocean ridge were younger than the rocks found further from the ridge. This means that new rocks are formed at the ridges and push the older rocks away from the ridge. SEDIMENTS ARE THINNER AT THE RIDGE 3. Geologists also discovered that the rocks found at oceanic ridges showed the magnetic reversals that the Earth has undergone. Evidence for Sea-Floor Spreading: Magnetic Reversals ▪ Some of the most important evidence of sea-floor spreading comes from magnetic reversals recorded in the ocean floor. ▪ Throughout Earth’s history, the north and south magnetic poles have changed places many times. ▪ When the poles change places, the polarity of Earth’s magnetic poles changes ▪ When Earth’s magnetic poles change places, this change is called a magnetic reversal. Sea-Floor Spreading Evidence from Magnetic Stripes – Rocks that make up the ocean floor lie in a pattern of magnetized stripes which hold a record of the reversals in Earth’s magnetic field  Magnetic Reversals occur When magma cools, the iron cools into the mineral magnetite. It lines up parallel to the Earth’s present magnetic field. This iron is like compass needles, pointing north. So when the rock hardens, a record of the Earth’s magnetic field at that time is locked in stone-- --so to speak!! 2. Studies of these magma flows have showed a magnetic reversal over geologic time. 3. A magnetic reversal (called a FLIP) is a change in Earth’s magnetic field. North pole becomes south pole and south becomes north. 4. A magnetic field that has the same orientation as Earth’s present field is said to have a normal polarity 5. A magnetic field that is opposite to the present field has reversed polarity Sea-Floor Spreading