If this crater becomes very large, the walls of the crater might collapse to produce a feature called a caldera. If you watch much television, you have probably seen violent and explosive eruptions of volcanos. This is not always the case. Some eruptions are violent with material being hurled 100's of metres into the air. Gases mix with dust and ash to make great dark clouds.
Some eruptions are quiet and oozing.
We also need to look at the composition of the lava. One type is dark coloured with lots of water. The water often results in small bubbles or vesicles forming in the rock. Iron and magnesium are common elements in this lava and give it its dark colour. The rock type formed includes igneous rock like basalt.