A few small lava flows originating from the northern part of Mt. Lambafjöll erupted soon after the last glaciation. The rock type, called picrite, is the most primitive type of oceanic basalt. It is formed deeper in Earth’s mantle than more evolved basalt, such as tholeiite. Picrite is characterized by abundant olivine phenocrysts. One of these lavas, Laufrandarhraun, erupted from a remarkable crater. Hvammahraun lava flow almost filled it so that now only the northern rim is visible. At the close of the eruption, powerful phreatomagmatic explosions hurled rocks about 2.5 km away from the crater north to Höfuðreiðarmúli. On the crater rim there is a 2-3 m thick layer of rock debris and boulders of the same composition as the lava. The Laufrandarhraun lava is somewhat older than the Vedde-tephra which erupted from the Mýrdalsjökull glacier about 12,000 years ago.