About 200 million years ago (Ma), in the Jurassic, the arrangement of the continents was quite different from the current one. So different that all the continents formed a single supercontinent, Pangea.
At this time, and due to the distancing of the tectonic plates, the fracture and separation of this supercontinent began, with the separation of North America from Europe and Northwest Africa, which, in turn, gave birth to an ocean that we know, the Ocean Atlantic.
Like all geological events, the process of “oceanization” (opening of an ocean) takes place over a very long time scale, and it was only in the Lower Cretaceous, between 145-135 Ma, that the South Atlantic opened, with the separation of America from Africa.
Thus, it appears that the fractures that gave rise to the Atlantic Ocean did not spread quickly, crossing the continents from one end to the other, as happens when a plate is broken, for example. Rather, it is a slow and progressive removal of the tectonic plates, with a dynamic identical to when opening a zipper on a coat.
Millions of years later, in the Upper Cretaceous (70-74 Ma), and still related to the opening of the Atlantic Ocean, the Iberian Peninsula was in rotation towards the rest of Europe. This movement, combined with other geological events, promoted a phase of intense magmatic activity. A very important example of this activity is marked in Portugal in the form of the Monchique igneous complex.
The question that arises at this moment is: how do these magmatic rocks, formed inside the planet, appear on the surface? For this, it is necessary to understand that our planet Earth has an internal structure composed of different layers, like an onion. In a simple way, the three main divisions are: crust (continental and oceanic), mantle (lower and upper) and core (inner and outer).
The magma that originated the nepheline syenite of Monchique was born in a magmatic chamber located in the upper part of the mantle, between 30 and 45 kilometers deep, a distance similar to going from Lagos to Monchique. After its birth, the rocks of Monchique rose to higher levels of the crust, between 3 and 10 km deep, through the fractures that were generated during all the geological evolution described above. This rise of the magma is due to the difference in densities between the magma and the rocks it passes through.
Magma, because it is very hot, between 800 and 1200 ºC, is less dense than the host rocks and, therefore, tends to rise, as happens when we heat water to make our teas, when water boils, expands and rises .
Then, the magma settled in the upper crust for a few million years, which allowed its cooling to be slow and, consequently, to develop minerals with a size visible to the naked eye.
Next to the chapel of Caldas de Monchique, it is possible to see the two types of nepheline syenites from Monchique, which are distinguished by having minerals with different dimensions. The differences are due to the fact that the ascent and installation of the magma took place in two distinct magmatic moments.
Finally, the appearance of the Monchique igneous complex on the surface was due to differential erosion processes, that is, the sedimentary rocks that surround this igneous complex are less resistant to erosive processes than the nepheline syenites and, therefore, suffered more erosion, making the Monchique mountain range more prominent.
It thus appears that Monchique is not a volcano. The Monchique igneous complex does not present the characteristics that typically define a volcanic apparatus, namely volcanic cone, volcanic chimney and crater, nor does it present rocks with minerals that are not perceptible to the naked eye.
Now that we can correct our friends by telling them that Monchique was born a little over 30 km deep and that it is not a volcano, you can ask them the following question: where, in the Algarve, is it possible to see and touch an ancient volcano?
I bet they won't know!
Author Lourenço Crispim (Living Science Center of Lagos)
NOTE: This article was developed within the scope of the project PaNReD – Natural Heritage transformed into Digital Teaching Resources, funded by the CRESC Algarve 2020 Program, through Portugal 2020 and the European Social Fund (ESF).