Tectonic.

We went to the National Park to have a look at the tectonic plates. Although not really my thing, the rest of the family were interested. We had a very informative (free) tour by a park ranger who explained about the geology and the history of Iceland.

We then went to see a volcano that had a lake at the bottom of it. ( see extra)

https://www.thingvellir.is/en/history/