Basalt froms from basaltic lava is heavy, dark-colored, dense lava. Basaltic lava is dark because it contains high amounts of iron and magnesium. The black beaches of Hawaii are formed from eroded basalt. Another kind of intrusive rock is pumice which forms from granitic lava . This kind of lava is lighter colored, less dense and composed of silicon and oxygen.
Pumice cools off very quickly and also is associated with gaseous eruptions.
Therefore, pumice is very lightweight, and has very small crystals. Some pumice will float in water. Obsidian is also formed from granitic lava . Obsidian cools so rapidly that it has no crystals. Obsidian is also very dense.