The largest lava flows on Reykjanes Peninsula are lava shields. The oldest and largest of these are Sandfellshæð and Þráinsskjöldur, over 100 km² each. They formed in late glacial time, about 14,000 years ago, when sea level was about 30 m lower than today.
The top crater, Sandfellsdalur, has a maximum diameter of about 450 m. It had a lava lake, which was partly drained undeground, emerging here and there from parasitic swells, which is also indicated by rare collapse pits. The lava also spilled over the rim to form spongy pahoehoe. When the eruption came to an end, the lava lake drained away and slivers slid down into the crater forming tilted ramps on the inside. Lava shields form at a low extrusion rate, and the largest may be active for years. Their rock is rich in the mineral olivine. About a 5 km broad rift zone passes through Sandfellshæð with the southeastern boundary fault cutting across the crater. It continues northeast with larger throws across much older hills of subglacially erupted pillow lava.
Kristján Sæmundsson, 2010