Plate Boundaries Help (page 2)

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By — McGraw-Hill Professional
Updated on Aug 31, 2011


A rift or upgrowth of the ocean floor, where plates are slowly edged apart by the filling of hot magma, is known as an ocean ridge . Ridges are formed along divergent boundaries where plates move slowly away from each other. Magma then rises into the crack between them, filling it, and hardening into rock. Figure 4-6 shows how this seafloor growth takes place.

Plate Tectonics Ridges


Fig. 4-6. Upswelling magma adds to seafloor spreading at divergent boundaries.

Most of the magma exiting the mantle today is found at ridges in the ocean floor and along plate edges. When magma pours out of cracks in the ocean floor, they build up a lip along the crack and form mid-ocean ridges. Ridges thousands of miles long can be found in the Atlantic Ocean, and around the plate borders of the Pacific plate.

Cooled magma (lava) also flows horizontally forming more ocean floor and piling up around vertical vents to form volcanic cones and islands like the Hawaiian Islands and the Galapagos chain. These are hot spots. The unending creation of solidified magma (rock) creates new seafloor and widens the ocean basins, a process called seafloor spreading .

When British geologists, Drummond Matthews and Fred Vine sampled rocks along the edges of ocean ridges, they found that the farther away they were from the ridge crest, the older the rocks. When this information was added to the idea of continental drift and seafloor spreading, it helped explain the puzzling increase of crustal landmass and supported the plate tectonics theory.

Nearly all of the ocean ridges are at the bottom of the oceans, but the Mid-Atlantic Ridge that stretches up the center of the Atlantic Ocean, emerges in a few places including Iceland, where geologists can measure its growth and characteristics.

Below the waves, photographs from submarines at great ocean depths show that rocks near ridge edges are clean and sharp. As the distance from the ridge increased, rocks became covered with sediment. At about 10 km (6 miles) from a ridge, the rocks are completely obscured from sight by layer upon layer of sediment dusted over them for millions of years. We will take a closer look at the hardening of sediment into rock.

Practice problems of this concept can be found at: Plate Tectonics Practice Test

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