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. This molten material had a different composition than the preceding material. In a process called plutonism , the new molten material solidified within the existing rocks that had been produced by volcanism. This plutonism occurred 85 to 90 million years ago.