Lokrum is a green island, covered in fertile land and lush flora. However, what lies underneath that thin layer is clearly visible only at the shore: the whole island is made of thick layers of sedimentary rock – limestone and dolomite.
The sedimentary rocks hide an interesting geological story millions of years old – they used to be a part of the Adriatic-Dinaridic carbonate platform. The platform’s remains still make up the karst region in Croatia, from Karlovac, Gorski Kotar and Lika to Istria, the Croatian Littoral and Dalmatia, but its largest part has sunk under the Adriatic Sea.
The platform was an expansive shallow-water area with an irregular cluster of smaller and larger islands, beaches and shallow waters with deeper lagoons separating them. It was bordered by a reef built of organisms which could survive in spite of strong waves and currents from the surrounding deep sea, such as golden anemones, moss animals and mollusks.
The inner area of the platform was dominated by quieter depositional environments, with conditions for the growth of various plants and animals in the very warm, shallow and full of light sea, i.e. conditions similar to the ones in the Bahamas today.
Just a small part of this lush life left a trace in the rocks, mostly organisms with mineral skeletons, similar to snails, mollusks and corals. The remains of such organisms, i.e. fossils, help determine the age of layers.
In the layers of sedimentary rock that Lokrum is made of, along with sporadically numerous mollusks, various tiny unicellular organisms, visible only with a magnifier or a microscope – foraminifera, of which the most important are Moncharmontia apenninica and Scandonea samnitica, and plants – algae Thaumatoporella parvovesiculifera i Aeolisaccus kotori, are present. They indicate that the sediments were deposited during the Late Cretaceous, about 85 million years ago.
The slow depositing of limestone material (carbonate mud, skeletons of various organisms, fragments of older rock, etc.) throughout the long geologic periods resulted in thick rock layers, slowly transforming through complex processes the carbonate mud and sand into hard rock – limestone.
About forty million years after the sedimentation of rock which makes up the island of Lokrum, the Adriatic-Dinaric carbonate platform ceased to exist due to the movement of large parts of Earth’s crust – the ancient supercontinents Gondwana, that incorporated present-day Africa and India, and Laurasia, that incorporated present-day Europe and Asia. That singular event had a huge impact on the whole region located between them, including the Adriatic-Dinaric carbonate platform.
As the result of the collision, the mountain chain that stretches from the Alps to the Himalayas arose. The Dinaric Alps are part of that belt.
What used to be an undisturbed sequences of sedimentations, like a large cake with horizontal layers, had undergone a huge deformation, and the consequences are also visible on Lokrum. The sedimentary surface is more or less inclined, rippled, full of numerous deep cracks or separated by faults.
The deposits between Portoč and Bora Cove and the surroundings of the favorite Lokrum swimming spot DEAD SEA (Mrtvo more), connected with the open sea by an underwater passage, are highly tectonically disturbed.
Later geological processes, such as dolomization (a process by which dolomite is formed from limestone when magnesium ions replace calcium ions in calcite) and karstification (gradual dissolution of soluble carbonate layers due to chemically aggressive water, which causes irregularly wide tectonic cracks and an uneven rock surface) also impacted strongly the current look of the rocks.
The island’s coast is constantly exposed to waves, which during occasional stronger storms slowly but surely erode the coastal area, especially in the south part of the island.
THE ROCKS which are actually placed on the very seaside, had been fairly far from the sea only 10 – 15 thousand years ago. Namely, during the glacial periods, the last of which had then been at its culmination, due to the coupling of a large quantity of water in the vast ice covers, the sea level had been sometimes more than one hundred meters below the actual.
At that time, Lokrum was in fact not even an island: it was only a small hill. The final flooding of the earlier land by the sea had taken place approximately 6 – 10 thousand years previously, during the time of the origin of the most ancient civilisations.