Maxine Smith, the Geological Society’s Online Development Editor, is also a keen diver. Here she describes one of her dream diving sites – between two tectonic plates….

Some of us enjoy hitting rocks with hammers. Some of us like field trips. Some of us like to experience geological wonders from underwater – and one of these is the mid-Atlantic Ridge, in Iceland.

Silfra is a world-famous diving site in Þingvellir (Thingvellir), an Icelandic National Park. The park lies in a rift valley marking the crest of the Mid-Atlantic Ridge and the boundary between the North American and Eurasian tectonic plates. Silfra is a fissure that formed along the ridge line and cuts through a freshwater spring, thus filling the fissure with the crystal clear water.

Image c. Alison Fish

Although for most visiting divers it’s not the deepest dive site in the world (about 20 metres), it’s an interesting site as the geological activity in the area causes rock falls and collapses, creating tunnels and caverns.

From one of the pools at Silfra there is a tunnel referred to as the ‘toilet’. To get into the tunnel the diver has to go head first vertically down to a depth of 16 meters. To add to the excitement, all of the water coming from the Silfra cave system has to go through this narrow hole before reaching the main lake. This creates strong current in the tunnel which ‘flushes’ the diver straight down.

If this sounds a bit too exciting try the ‘Silfra Cathedral’ fissure, which is about 20 meters deep and about 100 meters long, and has vertical lava rock walls. The visibility at this site is so good that divers and snorkelers can see from one end to the other.

Other Icelandic dive sites are available! In the video below, for example, a diver visits Lake Kleifarvatn, a geothermal lake located on the fissure zone of a the Mid-Atlantic ridge, and  proves that it’s hot enough to boil an egg…

• With thanks to http://www.silfra.org/ and https://www.dive.is/

Geoadvent challenge update

Yesterday’s window was of course the beautiful Lulworth Cove – well done to everyone who guessed it!