Basalt rocks form the Earth's crust. They are heavy, dark and grainy. The dark color of this type of rock comes from its high magnesium and iron content. Andesites have a lighter color compared to basalt because they contain more silica and less iron.
Rhyolites are light-colored or white. These rocks don't have much iron or magnesium; instead, they contain a lot of silica. Some rhyolitic rocks are very light because of the bubbles of gas that were trapped as they solidified.Learn more about Volcanoes