The quick cooling doesn't allow the formation of large crystals so most extrusive rocks have small crystals or none at all. In some extrusive rocks, like pumice and scoria, air and other gases are trapped in the lava as it cools. We can see holes left in the rock where the bubbles of gas were located. The most common extrusive rock is basalt. 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.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.