Basalt (玄武岩) and granite (花岗岩) have the honor of being the most important rocks in the crust. Most of the ocean floor is basalt, and most of the continents are granite. Basalt and granite have quite a bit in common. Both are igneous (火成岩)rocks, which means that they cooled from magma. Both have large amount of silicon and oxygen. But there are several important differences, which help explain how the earth works.
Granite is an extrusive igneous rock. Many kilometers below the Earth’s surface, molten rock called magma flows into cracks or underground chambers. There, the magma sits, cooling very slowly over thousands to millions of years. As it cools, elements combine to form common silicate minerals, the building blocks of igneous rocks. These mineral crystals can grow quite large if space allows.
Basalt is extrusive igneous rock. The magma or lava is made of a slush of crystals, liquid, and bubbles. When it breaks through the crust of the earth and erupts on the surface, it cools quickly. If the lava cools in less than a day or two, there is no time for elements to form minerals. Instead, elements are frozen in place within volcanic glass. Often, lava cools over a few days to weeks and minerals have enough time to form but not time to grow into large crystals. 
Basalt is a dark colored, fine grained, igneous rock. It's the most abundant rocks.
Basalt has three ways to form depending on how the eruption and where the eruption takes place. One is that the lava erupts under the open air, second is that lava flows to the sea, and third is that lava erupts underwarter. The ocean floor is almost completely made of basalt.
