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Constructive Plate Margin 

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A constructive plate margin is where new crust is created along an extensional plate boundary.
Extensional plate boundaries, under the oceans, called mid ocean ridges ,form long ridges of submarine volcanoes whicj erupt to form new oceanic crust and cause the oceans to widen or spread. As the two oceanic plates pull apart magma from below wells up and erupts to form new ocean floor. The basalt lava which erupts forms pillow lavas within which magmetic minerals are aligned with the earths magnetic field.

On land a constructive margin starts as a rift with volcanoes which if successful can split apart a continent to form a new oceanic crust.

Constructive plate margin USGSCross section Conservative margin