Interspersed among the folded structures in the Cantabrian Mesozoic basin of northern Spain are at least 12 diapirs with cores of plastic Triassic shale, evaporite, and ophitic igneous rock exposed at the surface. They are in a generally east-west alignment over a distance of about 130 km (81 mi). The Iza diapir is in the east part of the basin at the eastern termination of the diapiric trend.

Surface evidence, seismic work, and the data from four wells drilled on the structure have enabled workers to outline this unusual diapir. Most diapirs in northern Spain are expressed on the surface as irregularly circular depressions representing the intrusive mass. The surrounding beds dip away from the central depression, commonly very steeply, to form sharp ridges. In contrast, the Iza diapir is a buried wall or ridge of plastic rock--at least 5 km (3 mi) long by less than 1.5 km (1 mi) wide--intruded into a sedimentary section more than 4,410 m (14,468 ft) thick. Only the uppermost tip is exposed, and it is in a belt of indistinct outcrops up to 30 m (102 ft) wide. One of the wells drilled on the structure penetrated an inverted block of Upper Cretaceous sandstone above Paleocene carbonate rock; the block apparently was incorporated into the diapir.