The basaltic crust of Iceland and surrounding areas can be divided into two parts, the upper crust and the lower crust. The upper crust is characterized by increasing P-wave velocity with depth. At the surface of the basaltic lava pile, the velocity can be as low as 2.0-3.0 km/s in fresh highly porous lava in the volcanic rift zones of Iceland and at the crest of the adjacent Mid-Atlantic Ridge (MAR). Elsewhere, the P-wave velocity at the surface is in the range of 3.0-5.0 km/s, mainly depending on the degree of secondary mineralization. From the surface the velocity of the upper crust increases more or less continuously down to the top of the lower crust where the velocity is nearly constant at 6.3-6.7 km/s. The continuous increase in velocity with depth in the upper crust is considered to be due to closure of fractures caused by lithostatic pressure and to an increasing content of secondary minerals which reduces the porosity. In addition to the general increase in velocity with depth, thin high-velocity layers are observed within the upper crust. These are interpreted as relatively dense parts of the lava pile with a low content of scoriaceous material. The thickness of the upper crust is in the range 3-6 km in Iceland, except in southeastern Iceland where it exceeds 10 km. On the Greenland-Iceland-Faeroe Rise it is in the range of 3-8 km but on the Reykjanes Ridge it is considerably less, close to 2 km. This implies that the structure of the Icelandic upper crust is more similar to that observed on the Greenland-Iceland-Faeroe Rise than that on the MAR. The lower crust is very homogeneous in terms of P-wave velocity. It is most likely also composed of basaltic lava and intrusive rocks of almost zero porosity and with considerable amounts of heavy secondary minerals such as epidote. In some cases there seems to be a low velocity layer at the base of the lower crust indicating partially melted material. There are no signs of any sudden changes in the velocity structure of the crust between Iceland and the adjacent oceans, but the crustal layers in Iceland are generally thicker than on the Reykjanes Ridge. The crustal thickness in Iceland is rather poorly mapped but seems to be of the order of 10-15 km. This thickness reaches as much as 30-35 km on the Iceland-Faeroe Rise and probably 25 km on the Greenland-Iceland Rise. The subcrustal material has a P-wave velocity of 7.0-7.4 km/s below Iceland and at the crest of the MAR, 7.8-7.9 km/s below the Iceland-Faeroe Rise and 8.4 km/s below the flank of the Reykjanes Ridge. There seems to be a sharp change in the subcrustal velocity structure at the edge of the Icelandic shelf. Multichannel reflection surveys in the Tjornes Fracture Zone north of Iceland reveal a sedimentary basin with up to 4 km of sediments. The western part of the basin is a prominent graben with numerous listric normal faults. This graben is an apparent continuation of the spreading axis on the Kolbeinsey Ridge.
