Þingvallavatn is sitting in the tectonic rift valley between the continents. As we are standing here by the lake, we can see the Langjökull glacier and the Skjadbreiður lava shield at over 1000 meters above sea level. Skjaldbreiður is considered one of the most beautifully formed of all shields, and is believed to have erupted for decades, maybe even a century. The landscape of the no man’s land rift zone floor slopes off from there to the south western part of the lake, where it is below sea level.The bedrock of in the lake consists mostly of postglacial lavas, which are most common in the central part of the graben. Also upper pleistocene pillow lava, móberg of subglacial origin and supraglacial lava, which are the most common at the outcrops of the graben. The western margin belongs to the Plio-Pleistocene where most of the rocks are basaltic.
The morphology and structure of the lake surroundings are very strongly dominated by faults with throws of locally more than 100 m towards the actively sinking and expanding graben. The graben floor shows signs of a southeastward tilt. This is particularly clear along the western margin.The lake began to form at the end of the last glaciation as a lagoon in front of the glacier over about 12.000 years ago. We can only imagine the water at the time being turbid with glacial melt water flowing into it.This changed in the earliest Postglacial when Eldborgir and Skjaldbreidur lava shields formed across the central part of the lake to the north, providing a barrier to surface inflow and also an effective filtering of the glacial waters. The lake area became greatly reduced and the glacial run-off water from the north, where the glacier was receding, was now filtered through the lava fields and reappeared in the lake as clear spring water.Today, the water is considered as clear as can be; as clear as your drinking water.No rivers run into the lake, yet Iceland’s largest spring water river flows out of it, called Sogið.