The rainbow at the end of the ash: Barranco del Medio, Gran Canaria

– Sorry, honey, we will be here for a while. Enjoy your book while I shoot the rainbow!

I grabbed the camera and jumped out of the car. In front of me, the most colourful mountainside I had seen, a pallette of green, red and purple.

Gran Canaria is an island of steep mountains. Its long and wingding roads cling to the mountainside, and crawl up and down the steep valleys, the barrancos, like buckling snakes of asphalt. Magnificent views are around every turn, and there is always a surprise: What’s next?


Barranco del Medio is one of those surprises that make me pull the car over, pull the hand brake and run out with the camera. We geologists are famous for looking more at roadcuts and the views than the road, and this place makes me even more dangerous in the traffic than usual. The Chosen One does not always appreciate these ad-hoc field work stops, butt his time was different: She reckognised the beauty of the colorful volcanic ash.

For that is what it is: Fine grained ash. Spewn out by the volcanoes that made the Canary Islands.


Most volcanic ash is dark, black or grey. But when exposed to the elements after it settles, oxygen in air, rain and ground water can react with elements in it, to create colourful new minerals. It is the common chemical process called oxidation. We all know it, and hate it when it changes iron into red rust, rust that can fasten screws make tools useless and etch holes in boats.

The red colour in the ash is simply rust, created from oxidised iron from the minerals that make up the ash. Purple also comes from oxidation, when the process produces the iron mineral hematite, which is black in big grains, but reddish to purple in very fine grains.


The key to pretty colours is then; enough iron in the original ash, and allow for water with oxygen to circulate afterwards. Likely, there was a period of silence after these beds were deposited, which allowed for the water to flush through.

The green colour comes from clay minerals in the ash, mainly chlorite. Green may actually also come from traces of iron in the clay:

Different colours happen because iron, as part of rust or other minerals, gives away some of the negatively charged electrons it has in the outer shell of each atom. In rust and hematite, it gives away three electrons, and each atom becomes overall positively charged, Fe3+. In chlorite, it gives away only two electrons, and becomes Fe2+, and it is this differece that causes the various colours.

Finally, sulphur also plays a role in the colour play: Volcanic ash may contain lots of sulphur, with its well known yellow and orange colour.


Time flies in good company. Trying various camera angles, exposures, the whirls, layers, patterns of the rocks. There is always a new composition to try, another vantage point, another light as clouds drift by and turn the sunlight on and off. The colours show in new nuances as the dusk falls on.

On occasions like this, I loose the feeling of time. Just taking pictures, a kind of Zen where the outside world ceases to exist – and at the same time, the light changes constantly push to move, recompose, change filters… Until the bell rings.

– Dear, I need to go to you-know-where, and I need it now!

No such thing here. Reluctantly, I accept that there is a time for everything, and throw the stuff into the car, heading off to find you-know-what beyond the end of he rainbow.


02-06-2015 21-14-56

How to get there:

Gran Canaria is a major tourist destination, and served by a zillion airlines; charter, low-fare and good-old ones. From Norway, both SAS and Norwegian fly almost daily. Car rental is inexpensive, down to less than 100€ per day. Obviosly, both plane fares and car prices depend on season.

02-06-2015 21-16-09

Drive route, the fast way: Take the motorway GC-1 south from Las Palmas or the airport, until its end in the Tauro valley. Follow GC-200 up the valley, direction Mogan and further.

02-06-2015 21-16-53

Drive route, the scenic way: Follow the mountain roads across Gran Canaria, for lots of steep scenery: At Vecindario, follow GC-500, then GC-65, C-815, GC-60, GC-605 until joining the GC-200 at Pie de la Cuesta.

02-06-2015 21-17-53

5 responses to “The rainbow at the end of the ash: Barranco del Medio, Gran Canaria

  1. Since I am only a geographer, not much of a a geologist, I was wondering about the colours when passing by the sight. Now I found the answer. Thank you very much.

  2. Thanks so much for this! I’m going to Gran Canaria shortly and was looking for geological features to visit. This is going on my list.

  3. Thank you so much for explaining the beauty that we encountered yesterday. My children were asking how it happened and I hate it when I don’t know the answer. We went to the coastal village of St Nicholas at the bottom of the mountain and found a beautiful green pebble on the beach which my daughter is sure came from the rainbow on the mountain.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.