Mt Stapafell is a hyaloclastite mountain on the Reykjanes Peninsula to the southeast of the village Hafnir.  It is mostly made of pillow lava.  Olivine, one of the primary structures of the basaltic rock, dominates the lower part of the pillows.  Just south of Mt Stapafell is the ancient main route between Grindavik and the fishing outfits of Rosmhvalanes (“The Walrus Spit), where it is still possible to see the grooves of the feet of men and horses in the lava areas.  Mt Stapafell has greatly diminished in size because of extensive quarrying and some of it is lying underneath the runways of the Keflavik Airport.