While the crust underneath Iceland is without any doubts of oceanic type created at a mid-oceanic ridge, it clearly differs from normal oceanic crust by enhanced crustal thickness due to increased production of basaltic magma in the hot upper mantle. The results of different observation methods agree that the crust is at least 18 to 20 km thick, new interpretations of seismic data even indicate a thickness in excess of 40 km at some locations. While normal oceanic crust is formed by surface extrusions, shallow dikes and magma chamber accumulates at a depth of about 5 km – sources of crustal accretion which are also obvious in Iceland – the thick Icelandic crust demands a fourth source of accretion which is a deep region of dike and sill embedding. While the overall rate of accretion is obvious from the divergence rate of the plates, the amount of material injected in the different source regions is not. Under the assumption that surface lavas are quickly cooled after the eruption process to Icelandic surface temperatures and magma chamber filling as well as deep dikes and sills are embedded at basalt melt temperature, we studied three different scenarios of crustal accretion. In each case 60% of the entire injected material is attributed to (a) surface lavas, (b) magma chambers, and (c) deep dikes and sills. The effects of the accretion type on the crustal structure, temperature and mechanical behavior is quiet strong. The numerical formulation is based on solving the Navier-Stokes and energy conservation equations using a mass conservations equation defining Gauss-shaped influx fields accounting for the fourth accretion source regions defined above. In each case we followed the accretion process for about 10 Ma. For all creating types a thinning of the upper and middle crust is observed in the central region, crust created following type (a) is hot and relatively uniform with a thin seismogenic top layer formed by weakly inclined sheeted lava layers; type (b) crust shows a distinct boundary between upper and middle crust, a tendency to vertical lamination of the middle crust, and intra-crustal differential movements within a 200 km wide central zone; finally, crust of type (c) accretion is a down-building crust, with a relatively cold and thick seismogenic upper crust with deep reaching steep layering. Comparing our findings to various observations in Iceland, as temperature gradients, earthquake focal depths, and seismic profiles lead to the conclusion that the crust in Iceland is predominately formed by magma camber accumulates and deep dikes, lava flows, shallow dikes, and fissure swarm fillings may account for the upper 6 to 8 km of Icelandic crust.