Basalts usually begin as a dark gray or black, but can develop a greenish or reddish crust when weathered. The grains of basalt are so fine that they cannot be distinguished without the help of a microscope. Under the sea, basalt flows are vast and are mostly found along oceanic ridges. Many islands in the oceans, such as Hawaii, are formed of layer upon layer of basalts erupted from underwater volcanoes. Extensive basalt flows can also be found in continental areas, often in the form of polygonal pillars that compose incredibly beautiful landscapes frequented by tourists.