There are several types of rock fragments:
- fragments torn from the walls of the lava conduit
- fragments of ultramafic mantle rock (not found in Iceland)
- fragments of silicic crustal intrusions in various stages of remelting
- gabbroic crystal aggregates related to the host magma, in which case the correct term is cumulates.
Hrólfsvík (-vík means inlet) is a well known locality for the last type.The cumulate nodules are abundant in a thick basaltic lava flow outcropping in a low sea cliff.
The source of the lava is not known nor is its age. Most of it is covered by sand and gravel exposed at the shore. The nodules, which are mostly round but some have sharp edges, mainly consist of plagioclase feldspar, some pyroxene and some are rich in olivine.
The crust of the Reykjanes Peninsula consists of an uppermost layer of extrusives and minor intrusions down to 5-6 km depth, followed by another 10-15 km of intrusives below, of which the lower part is gabbros. The xenoliths were long considered to be fragments from the gabbroic layer. More probably they accumulated in a solidifying magma batch that came to a temporary rest on its way to the surface.
Kristján Sæmundsson, 2010