Most of Lómagnúpur is made of palagonite, but there are also layers of lava (e.g. pillow lava and lava columns) and sediments. It was built up within a million years. The lowest layers are about 2.5 million years old, the highest ones about 1.5 million. In addition, the mountain was in interglacial times and directly on the coast immediately after the last ice age 10,000 years ago, so that the sea also left its mark on it.