THE STORY OF SEAFLOOR SPREADING Seafloor Spreading Lesson

THE STORY OF SEAFLOOR SPREADING Seafloor Spreading Lesson Presentation Mr. Fluharty

TECHNOLOGY TWO IMPORTANT TECHNOLOGIES HELPED SCIENTISTS STUDY THE OCEAN FLOOR: Echo-sounding equipment called sonar uses sound waves to measure water depth. The device uses the speed of sound in water to calculate the depth of the ocean floor. In this way, giant ocean ridges were discovered. A magnetometer is used detect small changes in the direction of magnetic fields on the ocean floor. Magnetometers, a kind of magnetic compass, was towed by ships to detect the strength and direction of the magnetic field in the rocks on the ocean floor.

MAGNETIC REVERSALS A geomagnetic reversal is a change in the Earth's magnetic field such that the positions of magnetic north and magnetic south are interchanged. The Earth's field has alternated between periods of normal polarity, in which the direction of the field was the same as the present direction, and reverse polarity, in which the field was the opposite.

GEOMAGNETIC TIME SCALE It was discovered in the 1960 s that there was a pattern of magnetic reversals over geologic time in the rocks on the ocean floor. These reversals showed a symmetrical pattern parallel to the ocean ridges. The magnetic pattern on one side of the ridge was a mirror image of the pattern on the other side of the ridge.

SEAFLOOR SPREADING The world is surrounded mid-ocean ridges, like the seems of a baseball. In their center is a rift valley, which is a deep, narrow crack in the Earth's crust. Lava rises to the surface through this crack and spreads on either side of the mid-ocean ridge. Because it is liquid, iron atoms in the lava align with the existing magnetic field, before hardening to form new ocean floor. The spreading ocean floor eventually meets the continental crust. Continental crust made of granite and is less dense and lighter than the ocean crust, which is made of basalt. Because of this difference in density, the ocean crust is pushed under the continental crust.

EARTH: IN THE PAST AND INTO THE FUTURE Pangaea Song