Pillow basalt more than a kilometer above sea level

There is a nice exposure of pillow basalt just by the road that connects the Teide NP in Tenerife to the eastern part of the island. I didn’t know its existence before but when I saw these rocks while driving through the misty cloud forest that surrounds the Teide I knew I have to stop and take a closer look.

I had no doubt that these are pillows. However, there is one annoying aspect that troubled me. This outcrop is approximately 1200…1400 meters above sea level. Why is this a problem? Because pillow basalt forms underwater.

I think there is rather small possibility that these pillows formed in a lake. If we assume that they formed in seawater then there really can not be another explanation that the whole island has been pushed up by more than a kilometer. This is rather remarkable. I thought that oceanic islands grow larger mainly by the addition of new lava and pyroclastic layers. They of course swell also if the volcano is active which Tenerife is, although the most active regions of the Canary Islands at the moment seem to be the western part of the archipelago (La Palma, El Hierro).

But now I know a better explanation. These are not pillows at all. This is just a classic and beautiful example of spheroidal weathering. There are no radial fractures visible which should develop in pillow basalt because of contraction while cooling rapidly and there seem to be no flash-frozen crusts.

However, it is a really nice example of spheroidal weathering. What is the reason that we find it there? First of all, volcanic rocks often weather to smectite clay which expands when wet. Repeated diurnal cycles of taking on and losing water help to form such a weathering pattern. The particular location is especially suitable for that type of weathering because there are lots of moisture — its a cloud forest surrounding the volcano. You will even see fog in the first picture.