3000 years ago the so-called Skúlatúnshraun flowed towards the shore west of Ásfjall. This was a low viscous pahoehoe lava thet to-day forms the shore between Cape Hvaleyrarhöfði and Straumsvík aluminium plant, on its way it dammed the breach at Ásfjall and then the the pond was created. In the age of the settlement, around AD 950, several eruptions occurred in the Grindarskörð and Brennisteinsfjöll volcanic field. Narrow fast flowing lava tongues rushed all the way to the shore affecting the Ástjörn pond, that then got its present day appearance.
Ástjörn has no surface outflow as most lakes and ponds in Reykjanes peninsula but its water level is regulated by the general groundwater height and fluctuates slowly along with the groundwater fluctuations.
The view from the top of Ásfjall is marvellous over Hafnarfjörður and all the Reykjavík area. The farm, Ás was in the neighbourhood and both the pond and the hill took their names from it. The area was indicated by diverse vegetation but in recent years the lupine has invaded the region eliminating the original flora. The Ástjörn area is a nature reserve.
Ástjörn can be visited from all directions. Paths have been made around it. On top of the Ásfjall hill is a observation platform with place names all over the Reykjavík, Hafnarfjörður and surroundings.
Árni Hjartarson, 2010