Geoparque Mundial UNESCO

Nº33 – Metamorphic Contact

Geoparque Villuercas > Nº33 – Metamorphic Contact


At km 45.800 of the EX-118 road there is a junction with the CC-79 local road towards Fresnedoso de Ibor. On this road at km 2.200 we come to the area of contact between plutonic rocks (the granites of Bohonal de Ibor) and metamorphic schists. We can park in the entrance to an abandoned piece of road and observe the rocks along it, and with great care those found along a kilometre in the trenches of the new road, the cuttings of which show a great diversity of formations. The contact area extends from here towards Mesas de Ibor, and can therefore also be contemplated in other parts of the surrounding estates.


This geosite allows a didactic approach to geology and the explaining of contact metamorphism using granite batholiths. The landscape is extraordinary but the points of interest are often to be found on road cuttings, which does not allow their exploitation for geotourism.

Resultant metamorphic series as the level of metamorphism increases in schist-type rocks: slates, schists, mica-schists, and gneiss.



Towards the north of this area we can penetrate the area of granite boulders of Bohonal de Ibor. These are of coarse-grained granite with megacrystals of quartz, orthose feldspar, and two micas. We have here intensely fractured rocks with the typical geomorphology of granite boulder areas with their abundance of rounded blocks, boulders, and tors (see the geosite of the Berrocal de Peraleda). These rocks have been formed by rising molten magma (magmatic intrusion) and subsequent cooling inside the earth’s crust. This intrusion broke and assimilated within the magmatic mass the pre-existing rocks, in this case Palaeozoic shales which owing to the magmatic heat have had their texture and mineralogical composition transformed into slates or other schists. This phenomenon is known as contact metamorphism or thermal metamorphism as high temperatures predominate.

The transformations occur due to recrystallisation and involve the regrouping and orientation of the minerals of the slates. New minerals are generated to give rise to different metamorphic rocks depending on the degree of pressure and the temperature, which will be more intense as we approach the contact with the magmatic intrusion. This area is referred to as “contact aureole” or “metamorphic aureole”.

Along the slopes of the road we can encounter slates, micaceous schists, and mica-schists. On the surface these rocks show alterations and a micaceous lustre owing to the high content in the mica laminas (muscovites and biotites) of which they consist. They also show a typical slatiness or schistiness (a division into flat stones with breaks to create multiple parallel planes) which is defined by the laminate orientation of the micas that is acquired by the tectonic efforts generated by the locating of the granite batholite among the Palaeozoic slates.

Different filonian intrusions can also be observed on these slopes in the form of dikes (flat, narrow, and elongated in shape and resulting from magmatic intrusion) wedged into the fractures of slates, sandstones, and quartzites of the Palaeozoic era. Some are of the aplitic type with small crystals that are so light in colour they are almost white; in contrast others are pegmatite in type in which large grains of quartz, micas, and orthose feldspar have crystallised.