Describe the major differences between constructive and destructive plate margins

Constructive margins occur when two plates with the same density (continental or oceanic) move away from eachother, which causes magma from the mantle to rise to the surface. This creates new crust and, when this occurs in the ocean, is called sea floor spreading. The magma is very basaltic and can overtop the water creating islands like Hawaii. The most common example of a consutrctive margin is the Mid Atlantic Ridge that is formed as the North American and Eurasian plates diverge. Destructive margins occur when one plate is denser than another and move towards eachother. The denser oceanic crust subducts under the less dense continental crust and creates volcanoes as the magma rises through the continental crust, a trench along the margin and earthuquakes too as pressure builds up as the plate subducts. Crust is destroyed at these margins and the best example of this margin occurs where the Nazca plate subducts under the South American plate, creating the Andes mountain range and the Peru-Chile trench.