The typical scheme of the internal structure of a volcano, with a magma chamber located several hundred meters below the main crater, is the starting point of the formation of a caldera. However, only the most intense eruptions will lead to this geological phenomenon, since it is necessary an eruption with such a power that suddenly empty the magma chamber. As shown in the image below, a volcanic eruption with such power would cause the instability of the volcano, leading to its gravitational collapse. Once triggered, the process is unstoppable, since the beginning of the collapse increases the pressure in the magma chamber, further strengthening the eruption and feeding back the collapse of the volcano over its base.
A number of geological features characterize the volcanic calderas and help us to identify them in the field. Of course, the first is morphology: a depression with a diameter ranging from hundreds of meters to tens of kilometers, which can sometimes resemble an impact crater. If volcanism is still active after the cataclysm, there may be a resurgence, i.e. it can emerge a new caldera at the site of the first, and can also develop one or more new volcanic structures inside. Another typical feature is the presence, in the margins of the caldera, of a set of highly explosive or pyroclastic rocks such as pumice and ignimbrites. They are the result of the deposit of ash and lapilli projected into the atmosphere by large volcanic explosions which precede and accompany the collapse of the caldera.
In third place we have the tectonism. Especially in the circular escarpment bordering the caldera, and nearby, we can find a fault system that accommodated the collapse of the inner blocks of the volcano. A fourth feature is the development, in the final stages of volcanism, of a complex and extensive hydrothermal systems due to high temperature fluids emerging from the still hot magma and flowing upward through cracks and fractures, significantly modifying pre-existing rocks.
All these features, except those related to the presence of active volcanoes, are present in the calderas that can be found in the Cabo de Gata Natural Park. There are four main calderas. The first, the best known and most recognizable is the caldera of the Majada Redonda, which is accessed from the village of Presillas Bajas, close to Los Escullos. Other two are in Rodalquilar. The oldest covered almost from the Cortijo del Fraile to the sea in El Playazo, forming the present valley of Rodalquilar. The more modern, La Lomilla, is a resurgent caldera inside the Rodalquilar one. The Cerro del Cinto and Cerro de las Lázaras are the outcrops of the pyroclastic rocks (ignimbrites) of these two major volcanic eruptions, and in them, the processes of hydrothermal alteration are responsible for mineral deposits that have made this small Andalusian village famous.
The last of the large calderas of Cabo de Gata is the least known and perhaps the most surprising. It remains only one of the flanks in the cliffs bordering the Cerro del Fraile, specifically between Los Escullos and Punta de Loma Pelada. The rest of this great structure is under the Mediterranean Sea. This caldera shows particularly well the fault system characterizing their flanks and the hydrothermal circulation system that led to the formation of several deposits of economic interest that have been exploited in the past, and that seems to be associated genetically with the Rodalquilar one.
There are some other smaller and uncertain calderas, such as the one near Cala Higuera. We recommend visiting this and other calderas in Cabo de Gata with the guiding and accompaniment of an expert geologist to help you understand and identify all its amazing features and characteristics.