Thingvallavatn occupies a NE-SW elongated graben within the western branch of the axial rift zone in SW-Iceland. The topography of the graben floor slopes off from the Langjokull ice cap and Skjaldbreidur lava shield of over 1000 m altitude in the NE to below sea level in the southwestern part of the lake. The bedrock of the catchment consists of 1) Postglacial lavas that are most extensive in the central part of the graben. 2) Upper Pleistocene pillow lava and hyaloclastite of subglacial origin and subaerial inter- or supraglacial lava. These are most extensive at the periphery of the graben. 3) A small sliver on the western margin belongs to the Plio-Pleistocene series. Virtually all the rocks are basaltic. The lake basin is closed towards SW by Upper Pleistocene volcanic complexes of progressively younger age towards the west. Eruptions in the lake basin are confined to the northern extremity of these. The volcanic production diminishes rather abruptly towards the lake where it far from matches the rate of subsidence. 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 present lake began to form at the end of the last glaciation as an ice-dammed frontal lake. During and immediately after ice recession, the lake may have been turbid with glacial meltwater flowing into it. This situation changed in the earliest Postglacial as the Eldborgir and Skjaldbreidur lava shields formed across the central part of the catchment providing a barrier to surface inflow and an effective filtering of the glacial waters. As a result, the lake area was greatly reduced and the drainage from the main part of the catchment seeped into the lavas to emerge as springs at the shores of the lake. Smaller lavas have been erupted by crater rows SW of the lake, the youngest, Nesjahraun, about 2000 years ago. At the same time the phreatic tuff cone of Sandey was formed in the middle of the lake. Ash from this eruption was blown towards east and has a thickness of at least 40 cm on the nearest eastern shore, and thus must form a layer of considerable thickness on the lake bottom. After the postglacial lava shields were formed more than 9000 years ago the lake has been constantly gaining in area, mainly towards the NE by stepwise and latent subsidence of the graben floor. The total amount of subsidence during this period is more than 40 m between Almannagja and Hrafnagja. The horizontal extension of the graben over the same 9000 year interval has been estimated at about 70 m. The last tectonic event to hit the area occurred in 1789 when the Hengill volcanic system split up along its > 60 km long fissure swarm. Geological evidence indicates that subsidence along the northern shore of Thingvallavatn during this event was of the order of 1-2.6 m.