Almannagjá Fissure

Here you will find the information board regarding “Sprunga opnest”. This is an Icelandic expression that can be literally translated as “a crack opens”. As @Sasha Taylor describes.
You will also find on the sign that cars drove through the Almannagjá until November 1, 1967. Only then was the ravine closed to wagons. Previously, the road was one of the main public roads through Þingvellir Park.

The hole in the trail
Until 2011 the gravel path in the Almannagjá fault extended all the way to the top. On March 31st 2011 a hole appeared in the middle of the gravel path in Almannagjá. After close investigation a larger and deeper fault beneath the trail was discovered. The fault was about 10 m deep and extended 15 m under the trail toward southwest, effectively a deeper extension of the ravine. The ravine beneath the road is narrower from top to bottom and was full of large rocks and boulders.Likely cause
The most likely cause for the hole in trail was that a large boulder in the upper layer of rubble had loosened and fallen down further into the crevice, followed by smaller rocks and gravel. A thin crust of gravel material remained and was slowly eroded by rain and snowmelt resulting in the hole that appeared in the trail. It is possible that the rocks were loosened by earthquakes in 2000 and 2008. The effect of the earthquakes was clearly noted in the Thingvellir even though the epicenter was far away. Later it was decided to build a wooden footbridge on top of the rift.Source: Local signage

Impressive trail with great views over the valley