Geology of the Northern Velebit National Park is the source of its wealth and diversity. From the beginning, the National Park has organized and promoted geological research and has also issued a geological guide under the title: „From Sea Shoals to a Mountain“ (in Croatian).
The landscape of the Northern Velebit National Park is characterized by a distinctive karst topography with a diversity, multitude and level of development of karst forms that ranks it among some of the world’s most interesting karst areas.
Denuded karst peaks, ledges, towers, columns, sinkholes, grikes, fissures, snow-holes, caves, pits… are just some of the karst forms which make this area unique in its diversity, harmony and beauty and represent a trademark of the North Velebit National Park.
Crucial for the creation of this majestic karst topography were thick carbonate sediments, the tectonic activity, which formed the relief, and carbon dioxide-rich precipitation which dissolved the carbonate sediments.
As a result of dissolution of carbonate rocks by carbon dioxide-rich water, which has a very aggressive dissolving effect on calcite in carbonates, countless karst forms, small and large, were created both on the surface and underground. On the surface of rocks, various corrosion forms – karrens are found (furrows, solution pans, grikes, etc.), while underground we find numerous hollows such as pit caves, caves and caverns, usually predisposed by clefts created by tectonic movements.
Surface Karst Forms
Karrens (grikes, furrows, solution pans) are hollows in the rock of different size and shape. Their size and shape depend of the amount of precipitation, the composition of rocks (calcium carbonate - CaCO3 content of carbonate rocks), inclination of the terrain and rock fissures.
Grikes are created by corrosive action of water along cracks formed in the process of deposition of sediments or uplifting, folding and faulting of rocks.
Furrows (locally called žlibe), like solution pans, are formed in compact rocks under the impact of carbon-dioxide rich waters.
Sinkholes are cone, well, pan or irregular shaped depressions in the terrain. They can be formed by the collapse of an underground hollow roof in tectonically disrupted terrains or through corrosive action of water along cracks. They are shaped by the corrosive work of water.
Crags, towers, ridges, columns or sculptures are formed through selective decay in harder rocks that are more resistant to chemical and mechanical abrasion than the surrounding rocks.
Underground Karst Forms
Karst forms in the underground are formed through expansion of cracks or hollows due to corrosive and erosive (chemical and mechanical) action of water.
Pits are typical underground karst forms with vertical or steep channels with a total inclination of over 45°. On the other hand, caves are speleological features with horizontal or slightly inclined channels.
Caverns are underground hollows which are not physically connected to the surface and have no entrance. They are frequently discovered during construction works in karst areas.
Formation of Velebit Rocks
The Velebit karst rocks have formed in sea shallows on a journey over 6000 kilometres long, moving from an area south of the equator to the 45th parallel of north altitude. In this sea, through deposition of carbonate silt, crushed shells and skeletons of dead organisms (algae, foraminifers, bivalves, cnidarians, sponges etc.) hundreds and thousands of meters thick carbonate sediments were created. Deposited sediments hardened or cemented and were gradually bent, cleft, lowered or uplifted above the sea level. This movement was caused by the pressure of the African plate on the European plate. Stronger pressures lead to stronger movement and elevation of deposited “primordial Velebit” sediments, resulting in stronger folding, faulting, breakage and decomposition.
This is how the formation and growth of the mountain started about forty million years ago, but also lead to a process of decomposition, breakage and mechanical and chemical abrasion of uplifted cliffs. Yet, it was only twenty-tree million years ago that the period of strongest tectonic movements and main elevation started. The pressure of Africa on Europe is still present, and will be present in future, which means that Velebit will continue to grow.
Age of Rocks in the Northern Velebit National Park
Due to tectonic activity, rocks in the Northern Velebit National Park are of different age. Therefore, it is possible to skip tens of millions of years in just one step, with one leg standing on rocks older than 200 million years, and the other touching rocks formed “just” forty million years ago.
This area is composed of sedimentary, predominantly carbonate sediments, limestone, dolomite and clastic rocks composed of particles (fragments or clasts) created through decomposition of other rocks.
Found in the Štirovača area, the oldest rocks within the Park were formed through three different sedimentary processes during the Triassic period (extending from about 235 to 200 million years ago). The first sedimentary process took place in the shallow sea, the second after withdrawal of the sea, and the third during a period when the sea of small depth, not deeper than 15 cm, flooded the entire area again.
A large part of the Park lies on deposits created during the Jurassic period (extending from 200 to 145.5 million years ago). It was a long and quiet period, some fifty million years of calm, shallow seas in which plant and animal communities of algae, gastropods, bivalves etc. flourished. During this period the largest carbonate rock masses in the Park were created from these organisms.
In some other areas, however, this area is characterized by the presence of large dinosaurs, giant reptiles and other animals.
In the Park area, sediments formed during the Cretaceous period (extending from 145.5 to 65.5 million years ago) can only be found in the Rogić dolina locality. The shallow-sea environment of the Jurassic period continued, however, molluscs, especially bivalves of the rudist groups flourished during this era. Toward the end of the Cretaceous period, emersion occurred, and this is also when carbonate sedimentation in what is now the Velebit area ended.
The wider area of Hajdučki kukovi and Rožanski kukovi, as well as the coast-facing slopes of Velebit, are composed of rocks called Velebit breccias. These rocks were formed in ancient topographic depressions at a time of strong tectonic activity when the entire “primordial Velebit” area started to emerge. The Velebit breccias were formed through sedimentation of fragments created by breakage, karstification and crumbling of earlier uplifted sediments. As a result of hardening of these fragments and subsequent tectonic movements and uplifting, breccias can today be found even in the topmost peaks of north Velebit. It is in these deposits that entrances to some of the world’s deepest pits are found.
The youngest deposits to be found in the Park area were formed during the last 2.6 million years. They were created due to frequent changes of climate and exchange of glacial and interglacial periods. From melting glaciers moraine deposits were formed, consisting of till – unsorted and unstratified rock segments of varied size, ranging from millimetre to meter-sized ones. They are easily discernible in Bilensko Mirovo and Dundović Mirovo areas where they formed distinct topographical shapes like drumlins, eskers, kettles and others.