During the glacier periods the 2 kilometer thick glacier on top of all the land is pushing the land down up to 500 meters in some places. When the ice retreats the land gradually rises again. This takes a lot longer time and when it happens the 'landscape' underneath also changes. Magma finds new way to flow and volcanic activity increases.
After the finish of the last glacier period many volcanoes were formed. One of them Mt.Skjaldbreiður started forming about 7000 years ago when there were no glaciers in Iceland. Research shows no sediments between lava layers in the mountain which indicates it is formed in a single eruption. This eruption is believed to have been going on non-stop for from 100-400 years.
The mountain can be seen from the National park at Þingvellir (Thingvellir) on a sunny day. The National park is visited on the so called Golden circle tour in Iceland.