On the hike up to Pico Bejenado, and at other sights in the park, you will see lava tubes in the rock by the side of the path. Originally, these were conduits in the rock through which lava would pass, formed when slow-moving lava cooled enough for a tube to form. The lava tubes visible at this spot appear quite small, but one system of tubes on Teide, Tenerife, is over 18km in length.
Note there's a QR code on a post here that your smartphone can read to take you to an online audioguide (if you can get a signal) for more explanations about the tubes.