Rauðhólar, which translates to “red mounds” is a field of so-called pseudo craters or also called rootless cones, that is located just a few kilometers outside of Reykjavík along the Highway 1. It is about 1.2km2 in size and consists of scoria and spatter cones that have a very distinctive look and reddish color.
They formed about 5200 years ago during a volcanic eruption in the Brennisteinsfjöll volcanic system, with a Pahoehoe lava stream flowing all the way into Elliðaárdalur valley in Reykjavík, forming the Leitahraun. Originally there were about 150 cones and craters with the biggest ones reaching heights of over 20m. A big part of it was however used as building material, for example for roads in the past and therefore destroyed. Nowadays the area is protected.
Formation
The craters here have a special formation. They were formed during hydromagmatic explosions that occur when hot, molten lava flows over a wet land surface, in this case a small lake. They got their name “pseudo craters” because they don’t have actual vents that go down and feed lava like in real volcanic craters. The lava forming them is so called Pahoehoe lava, which is one of two common lava types in Iceland.
When hot lava comes into slow contact with water, a small crust builds around the lava, protecting it from extensive cooling. The lava then proceeds to inflate during the internal, slow cooling process and advances further into several directions in lobe like patterns. Because more lava is coming from behind and pushes the front, the crust around the lava may break, and the glowing hot lava comes into sudden contact with the wet surface and starts boiling and expanding. This leads then to the steam explosions that ultimately form the craters we can see here today.
Picture taken from Thordarson & Höskuldsson (2014)
Characteristics of the deposits
The cones have an interesting internal structure and also the rock material changes from the lower to the upper parts of the formations.
The steam explosions tore the magma apart and created characteristic deposits. In the lower part we can find lapilli scoria, which are small, roundish tephra fragments that build during their fall through the air. Those are alternating with thinner silt layers from the original lake sediments and black ash. Lava fragments that are covered with a skin of quenched lava, due to the rapid cooling when flowing over the lake, can also be frequently found.
In the upper part, bigger grain sizes dominate with spatter bombs that have fluidal surfaces and welded splatter which shows a higher explosive activity in those parts.
Picture taken from Thordarson & Höskuldsson (2014)
To log this geocache please answer the following questions and send the answers via my profile. You don’t need to wait for a log permission and can log immediately. It would be nice if you would include a picture of you at Rauðhólar.
1) Please take a closer look at the rocks at one of the craters for the following questions.
a) What are the different colors of the rocks? What do you think could be responsible for those colors?
b) Briefly describe the surface texture of the rocks. Can you see differences in the material between the different layers?
c) Do the rocks have a low or high density? What is the reason for that?
2) What is the name of the lake in which the lava probably flowed into?
I hope you enjoyed this earthcache:)
References
Thordarson, T., & Höskuldsson, Á. (2014). Classic Geology in Europe 3: Iceland. Dunedin Academic Press Ltd.