Halarauður is a welded fallout tephra which was created when the Krafla caldera formed 110,000 years ago, as a consequence of a large eruption from a shallow magma chamber. The layer is named after the colour (rauður meaning red), as it is oxidized on fracture surfaces. Otherwise, it is grayish and looks like a flow-banded lava. The composition of Halarauður is dacitic, formed by magma mixing of basaltic and rhyolitic magma. The layer contains xenoliths of basalt and gabbro. An accessible outcrop is at the top of a narrow gully on the western slope of Mt. Halaskógarfjall, where one can also see rock formations which are part of the southern slope of the pre-caldera Krafla central volcano. The gully is cut along a dike striking east-west. Halarauður increases in thickness to the north being about 10 m at the caldera rim by Sandabotnaskarð and about 50 m thick in the southwest of Hágöngur.