Basalt is an almost black, fine-grained rock that is extruded in a volcanic eruption. The molten rock (or magma) from which it is formed has a high proportion of minerals such as feldspars and these are responsible for the dark colour of the final rock.
The thickest layers of basalt often result from molten lava flowing up great fissures in the crust. These flows cool rapidly as the lava spreads out over the surface and this rapid cooling causes the fine grain as many crystals start to form but none gets the chance to grow large.
As the layer cools and hardens, tensile stresses build up and it commonly fractures forming “columnar basalt”. The joints are always at right angles to the surface and the columns are therefore almost always vertical and generally hexagonal. If the lava is still moving as the stressing occurs then the columns may twist or curve.
The Jökulsárgljúfur canyon is the largest ravine in Iceland at 25 km long, about half a kilometre wide and around 100 metres deep. The wild glacial meltwaters of the Jökulsá á Fjöllum river have carved this during its descent from the Porunnarfjall mountain and crashes down in 3 great waterfalls:-
Sellfoss (only 10 metres high but very wide),
Dettifoss (45 metres high and 100 metres across) &
Hafragilsfoss (27 metres high).
Dettifoss is a majestic sight with 212 tons of water crashing over every second. It is thought to be the most powerful waterfall in Europe but this earthcache is about the rock surroundings and not the waterfalls because nowhere have I seen as much columnar basalt exposed as in the Jökulsárgljúfur canyon!!
In many parts of the world a small example of columnar basalt is so noteworthy that musical compositions are based on it (Fingal’s Cave, Iona) or myths built up around how it was formed (The Giant’s Causeway, N.Ireland)
However, here near Dettifoss there seems to be more columnar basalt than in all these places put together! It is everywhere you look!
To get to the listing co-ordinates you will almost certainly be in a car or on a bike. Make sure that you approach the falls from the east on road No. 864. This was one of the worst rutted roads I travelled on when I visited Iceland and, as the ridges & hollows run across the road, it was impossible to travel at more than 7- 10 km/hr in my hired car. < br /> You can park there and take a walk of around 2.8 km in order to complete the 5 questions & photo - not required but would be nice - to log this Earthcache.
1) Walk down towards Dettifoss and at N65 49.100 W016 22.890 stop and look to the far side of the gorge where there is a high cave. Count how many different layers of columnar basalt you can see from the level of the river to the top of the plateau. How many different volcanic eruptions do there appear to have been?
2) Closer to the waterfall you will see lots of mini waterfalls coming down the wall opposite. What appears to be supplying the water to make these waterfalls?
Then walk to Sellfoss at N65 48.311 W016 23.171 where you might take a photo of yourself with your gps & the waterfall to upload with your log.
Now return to the start of your walk via 3 places where you can take closer look at some of the basalt columns. First go to N65 48.344 W016 23.175 and face the low cliffs on your side of the falls. In front of you should be a small rocky knoll with a lot of moss on its left side.
3)On the top of the rock, the edges of two basalt columns (side by side) are outlined by fine lines of silt and moss. What geometrical shapes do these look like at first glance?
4) At N65 48.389 W016 23.138 there is a large basalt block which has detached from the cliff. Stand on top of the block and count the number of vertical sides that it has
5) Now got to W65 48.460 W0 16 23.172 where there is another huge basalt block with a domed top. Stand on this and count the number of sides
Optional extra! Drive to Hafragilsfoss 2.6 km down the road at N65 50.170 W016 23.989 and note the main colour of the surface rock.
My grateful thanks to kasperine for adopting this earthcache from me when I was getting overburdened with caching and keeping it going for approx 5 years before offering it back. Much appreciated help.