Dettifoss and Selfoss are two waterfalls, which are located close to each other at the Jökulsá á Fjöllum river. Dettifoss is the second of them, it drops deep into the Jökulsárgljúfur canyon. The canyon is 25 km long, 500 m wide and in many places 100-120 m deep, making it one of the deepest and most breath-taking canyons in Iceland.
Like with other waterfalls in the canyon, also Dettifoss' underlying structure consists of huge columns of basalt rock, which were originally formed by cooling lava.
The location of this earthcache is a place where you can see both waterfalls at the same time. You are standing on top of some basalt columns. Make your observations and answer the questions. Of course everyone is invited to walk closer to the falls and enjoy their awesomeness.
Jökulsá á Fjöllum river has its source in the Vatnajökull glacier, which is the largest glacier in Iceland and one of the largest in Europe. It covers more then 8 percent of the country. Jökulsárgljúfur canyon was carved out during repeated catastrophic floodings in Jökulsá á Fjöllum river, which originated in Vatnajökull glacier some thousands of years ago.
The floods presumably originated subglacially at Bárðarbunga volcano, which, with a height of 2.009 meters, is the second highest mountain in Iceland. The largest flood had a peak of about 500.000 m3/s. When the resulting flow of water in Jökulsárgljúfur canyon was above a certain threshold, it would start to topple and transport huge columns of basalt rock - originally formed by cooling lava. These columns are the structure underlying several large waterfalls in the canyon, including Dettifoss. In several destructive floods those basalt walls were eaten away very rapidly, which means some days only. Quoting an author of a study at the University of Edinburgh: "We think of natural environments as being formed over thousands of years, but sometimes they are shaped very suddenly."
So the canyon below Dettifoss was formed by catastrophic glacial floodings, which were caused by Bárðarbunga's eruptions under the glacier. Latest research concludes that the formation happened during three flooding events, which occurred two, five and nine thousand years ago. More about these research results can be read here.
Jökulsárgljúfur canyon can be divided into three parts. The southernmost part is a 9 km long, pure canyon with vertical walls that follows fault lines trending north-south. The canyon walls comprise lava flows and sedimentary units, with one at the foot of the canyon where two thick lava flows overlie it. The middle part of Jökulsárgljúfur is a 9 km long U-shaped valley with boulder and gravel terraces on both sides. The northernmost part of Jökulsárgljúfur is a canyon with vertical walls and ends in Ásbyrgi. Ásbyrgi is an oval depression bounded up to 100 m high vertical walls on three sides, with an opening facing north. In the middle of the gorge rises a cliff-bounded island; seen from air Ásbyrgi resembles a horse shoe.
But let us return to the location you are standing right now: As Bárðarbunga still has to be regarded as an active volcano, future forces from glacial floodings could cause the structure of the canyon and both waterfalls to change again, quickly and significantly. This could happen next week, next year, or in thousands of years. So enjoy being here today!
In order to log this earthcache please answer these questions by using the Message Center:
1) At the earthcache location you are standing on top of some basalt columns, at the inner side of a bend of Jökulsá á Fjöllum river, high above the current water level. What do you think: Would that be a safe place for observing another catastrophic flooding of the canyon? Why?
2) What do you think: In the case of another catastrophic flooding, would the waterfalls move upstream or downstream? Why?
3) At the earthcache location, directly at the edge of the canyon, there is a rock formation. What does it remind you of?
a) obelisk
b) globe
c) bench
4) There exists an Icelandic term which has been adopted in glaciological terminology in many languages. It describes a type of glacial outburst flood. Please name that term (one word).
5) Take a picture and upload it with your log. The picture must show Dettifoss or Selfoss from the western river side, together with you or something which uniquely identifies you.
You can log immediately after having sent the answers. In case there is a problem I will contact you, logs without answers will be deleted. Happy caching!
