Which igneous rock is dark colored, cooled rapidly on Earth's surface, and is composed mainly of plagioclase feldspar, olivine, and pyroxene?
Basalt is a very common extrusive igneous rock. That means it cools rapidly from lava on the Earth's surface. As a result of this rapid cooling, crystals int he basalt have little time to grow and only reach a small size. Usually you need a hand lens or a microscope to see the individual crystals in basalt.
Said crystals are composed of mafic minerals, which are relatively low in silica content and give the rock an overall dark colour. These minerals are commonly pyroxene, olivine and plagioclase feldspar.
Basalt is an important rock because it's produced at mid-ocean ridges, where new oceanic crust is made. Basalt is therefore the main constituent of Earth's oceanic crust, so it supports our oceans! In some parts of the world, it also forms islands such as those of Hawaii or Galapagos.