Models and structure of the oceanic crust

Geophysical measurements have mostly confirmed the validity of the standard structure of the oceanic crust as deduced from seismic refraction measurements in the early fifties. The relatively small degree of variability of the oceanic crust structure contrasts strongly with the comparatively large variability of the continental crust. Layer 1 consists of unconsolidated or semiconsolidated sediments no older than Lower Cretaceous. Layer 2 probably consists mainly of basalt, but may be locally covered by a layer of consolidated sediments of about Upper Jurassic age, or older. The results fit very well with the spreading floor hypothesis. There are no definitive arguments either for or against the serpentinite composition of layer 3 but an important fact is the progressive thickening of layer 3 away from the axial zone. The problem of the creation of the crust by the sea-floor spreading mechanism should be investigated within the framework of the mobile lithospheric plate concept.