Gabbro is a medium-grained, ultramafic igneous rock. It is made up of plagioclase feldspar and pyroxene. Gabbro is an intrusive igneous rock, meaning that it forms from the solidification of magma within the earth. Intrusive igneous rocks are medium- to coarse-grained because they cool slowly over time. While gabbro is not economically important, it often occurs in conjunction with rocks containing iron, titanium, nickel and platinum ores.
Gabbro can be found at current and ancient plate boundaries. Montana and South Africa have large foliated gabbro complexes. Unfoliated gabbro intrusions can be found in Palisades, New Jersey and in Ontario, Canada. The location of these gabbro units are formed along ancient plate boundaries. Some of these intrusions have formed from magma pushing towards the surface during orogeny, or mountain-building events.
Gabbro primarily occurs at divergent plate boundaries. Minerals partially melt in the mantle, become less dense than the rock around it, and rise to the ocean floor. As it hits the cool water, the lava cools quickly and wedges the plates apart by forming new rock. The basalt forms fine grained pillows on the ocean floor. Lower in the plate boundary intrusion, rock cools slowly, forming dikes, sills and batholiths of coarse grained rock within the Earth’s crust. The coarse-grained igneous rocks are ultramafic gabbros formed from the pyroxene and plagioclase found in the magma.