Geologists collected samples of lava rock from the sea bed surrounding these areas of volcanic activity, carefully recording the location and began piecing together the puzzle of these undersea lava flows. Scientists used the magnetic anomalies recorded in the lava rocks to date them - to tell how old they were and when they formed. Using this technique they discovered that the newest, youngest rocks were at the very center of the ridges and the oldest rocks were furthest away. That told them that the undersea volcanoes were the site of formation of new ocean floor and that the oceanic crust was being pulled apart along these rifts, or tears, in the ocean's crust. These were determined to be spreading boundaries of tectonic plates.
A spreading boundary is where the tectonic plates are separating, or coming apart. Some spreading boundaries are places where the crust is sinking downward as it is stretched thin - like in the East Rift Valley of Africa, where the Dead Sea is located. Many of the spreading boundaries are located deep in the ocean on the sea floor. These are places where volcanic activity is at a premium because the crust is being torn apart. New crust is forming when magma from the mantle deep down is forced upward out of the cracks where the plates are coming apart. Long chains of undersea mounts (like the mid-Atlantic Ridge) and volcanic islands (like Iceland) typically characterize these type of plate margins.
The direction of movement of the oceanic plates is laterally, away from the center of the ridge. (see the animation at left - courtesy of the USGS)
How do scientists account for volcanic activity that doesn't appear along tectonic plate boundaries? Hot spots ->
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