“Rainbow” of a scoria cone

Scoria cones (or cinder cones) are conical volcanic hills that are mostly composed of mafic vesicular lapilli (known as scoria) and other pyroclasts. They are very common volcanic landforms but I am not going to focus on them today. Instead I would like to show you a couple of photos that show how the color of scoria changes depending on the distance from the centre of the cone.

Fresh scoria is black, oxidized scoria is reddish brown. Below is a photo that shows reddish weathered scoria at the center of the La Montaña del Palmar scoria cone in northwestern Tenerife. There is a smooth gradation from black to reddish scoria which clearly demonstrates that this is the same material. The interior is just oxidized. But why?

The interior of the cone was very hot during its formation because there was the vent that fed the volcanic hill with liquid magma. It obviously had to be the contributing factor because only the interior which was once hot is oxidized. Another thing we need to oxidize scoria is water. So the answer is most likely that it was very hot water (steam) that was involved in the process.

Such a smooth gradation from black to oxidized red color is a common phenomenon but we only see it when someone has revealed the internal structure of the cone for us. In this particular case we can see it because there is a quarry which extends into the heart of the cone.

The cone is mostly composed of lapilli-sized pyroclasts but some of them are much larger (there are many embedded in smaller lapilli in the wall behind). These rocks are known as bombs. They were partially molten when thrown out of volcano. I am holding one bomb in my hands. Good thing with these bombs is that they don’t explode. It is pretty safe to handle one when it isn’t hot anymore and not flying through the air at high speed like a cannon ball.

The coordinates of the cone: 28° 20′ 22″ N 16° 51′ 01″ W. Altitude 900 meters.

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