The Explosive History of Praia da Luz

BY LUÍS AZEVEDO RODRIGUES

All the multicoloured layers of Praia da Luz are pages of a book, in which each page tells us a moment of the geological history of the Algarve.

The multicoloured rock sequences reveal the composition of the sedimentary rocks. Still, they also show us 120 million years of history, stories of advances (transgressions) and retreats (regressions) from the sea, a coastline that is constantly changing. The strata of fine sediments, such as marl and clay (green, red and violet tones) were deposited in environments further from the coastline.

In contrast, the sandstones (yellow tones), where, for example, we can observe the Nerineas (fossil shells), and are sediments deposited near the shoreline height, in an environment similar to a beach.

The cliffs of beautiful coloured layers close to which we now place our towel were once a deep seabed or a coastal area close to the continent of that time, or even a beach environment. How do we know this? We can see this from the characteristics of the rocks, for the size and types of its minerals, and its colours.

If we walk east, with the sea on the right, we reach the natural limit of Praia da Luz, a dark rocky barrier, almost black in some places, that almost prevents the walk. This barrier is Ponta das Ferrarias, popularly called Rocha Negra (Black Rock).

These are the remains of an ancient volcano, which was created around 70 million years ago, making it the same age as the Monchique formation, which is not volcanic. Despite not being as big or impressive as Vesuvius or Etna, we are still able to recognise part of the walls of the Luz volcano dating from the upper Cretaceous. This period of the earth's history immediately follows the famous Jurassic.

WHAT DO THE VOLCANIC REMAINS TEACH US AT PRAIA DA LUZ?

Rocha Negra

Approximately 70 million years ago, the Iberian Peninsula continued to be a "stone raft", relatively isolated from the rest of the European continent and increasingly far from the American continent. These paleogeographic changes, which have been accentuated for some tens of millions of years, are the consequence of the removal of the tectonic plates that precipitated the opening of the Atlantic Ocean.

If we press a chocolate gateau, whose exterior is solid, but whose interior is in a liquid state, the filling will come out, and we will be left with a plate of varied textures. This also happened about 70 million years ago in various parts of the Iberian Peninsula. With the great tectonic movements, several zones of weakness were created, which, as in the cake analogy, caused melting material from the Earth's interior to ascend to the surface. It was these materials that gave rise to the Rocha Negra volcano and the other smaller volcanic structures, such as the one that exists right in the centre of Vila da Luz, in a garden of a private house.

AND THE FOSSILS, WHAT GEOLOGICAL STORIES TELL US?

Photo © Filipe Palma

At Ponta da Calheta, on the west end (the right side of those looking at the sea), there is a mega-concentration of fossils - the gastropod Nerinea algarbiensis. The fossils of these animals are found in yellowish sandstones, in large quantities, and it is possible to observe that many of them are aligned. This fact is an indicator of the tidal movements of the upper Cretaceous, with a different coastline than the current one, since the tidal movement caused the alignment of its shells.

If we close our eyes, we may still be able to imagine these seas and coasts of 120 million years ago, when the Nerineas' habitat was close to the coast and where their shells are now deposited.

These are some of the stories that the geology and palaeontology of Praia da Luz have to tell us.

Luís is a Palaeontologist (PhD) and Executive Director of the Ciência Viva Centre in Lagos, lrodrigues@cienciaviva.pt

Scientific Tourism of the Ciência Viva de Lagos Center

The Ciência Viva Centre in Lagos accepts bookings for guided visits either to Praia da Luz or to the dinosaur footprints of Praia Santa or Praia da Salema. Groups / families up to 8 people.

The Centre also organises the same type of visits to the dinosaur footprints of Praia Santa and the footprints of Praia da Salema. www.lagos.cienciaviva.pt

Video produced by the Ciência Viva Centre of Lagos on Praia da Luz: www.bit.ly/325iHZp

Luz's Explosive Secret

BY SOPHIE SADLER

It was during lock-down that I was invited by a lady resident of Parque de Praia in Luz, to see a small mound. She had been told by a visiting geologist that it had volcanic origins and asked Tomorrow to investigate.

Being quite frankly glad to get out of the house, I went to view the site. Father Rob Kean kindly let me into his garden to view the mound, which adjoins his garden. I had previously never taken notice of this small hill but sure enough, recalling my geography A-level days; it did look like an off-shoot of the volcano, we know existed here that formed Rocha Negra.

My curiosity ignited I made it my lock-down mission to find out the truth. At this time, I was interviewing the director of the Lagos Science Museum, Luis Azevedo Rodrigues, for the story on his production of COVID-19 masks. I found the hillock on Google Earth and sent the image to him. He put me in touch with a volcanologist who wrote to me:

"The so-called Rocha Negra is in fact of magmatic origin. It consists of several pulses of magmatic activity with a composition similar to basalts. It also shows typical geological features that are likely to find in volcanic vents. So, it is more likely to be the feeder of a possible volcano that was between two to three km above. If a volcano was constructed, all the typical features (cone, crater, etc.) were eroded.

Typically, this type of igneous activity is prone to offshoots, and the place that is shown in the Google Earth image is precisely one of these offshoots. It has been recognised in the geological history of the region (indicated by the arrow in the image attached)."

Case concluded! Luis kindly has written the fascinating article on the geology of Praia da Luz to accompany this discovery. I hope you find it as intriguing as I did.