Once this molten material reaches the surface, it's called lava. The natural activity by which lava is produced is called volcanism . The volcanic unit on Aruba is called the Aruba Lava Formation . Some of the volcanic activity that created Aruba occurred under water. The rocks that were created under water have a distinct form.
Pillow lavas are round, black rocks that form under water.
The most common rocks in the Aruba Lava Formation are dolerite, tuffs, conglomerates and schists. Dolerite is a hard, black rock that is abundant in the Aruba Lava Formation. Tuff consists of consolidated ash and characterizes violent volcanism. These include conglomerates , which consist of fragments from preceding rocks. Once the volcanism was over, more molten rock flowed out from within the earth's crust, but did not reach the surface.