Constructive plate boundaries are divergent zones where the Earth forms new crust through the cooling of lava. The Internet Geography website states that most of these boundaries occur under the ocean.
As the plates move apart, they form openings in the crust that allow hot magma to rise to the surface. As this magma cools, it increases the size of the plates. The rising magma forms shield volcanoes, according to HowStuffWorks.
The skin of the lava cools almost immediately as it contacts water, but the interior of this pillow formation cools at a much slower rate. The speed of cooling affects the grain size of the rocks that form. The skin is fine-grained and glass-like in appearance, while the interior has a much courser grain. The interior cooling takes thousands of years. As the new crust cools, it also shrinks, causing any newer formations to stand higher than the older ones. These constructive plate boundaries form long, wide ridges along the floor of the ocean known as mid-ocean ridges. Such ridges are only a few miles high, but hundreds of miles in width. According to About.com, the crest of these ridges is the line of volcanic activities, where black smokers form.
This pulling apart of plates is significantly different than what occurs at other boundaries. Their primary driving force is the slab pull, caused as the opposite end of plates sink into the Earth's mantle in subduction zones, due to their own weight.