Grotta Gigante is located on the Trieste Karst, a high rocky plateau that surrounds the city
consisting mainly of calcareous/carbonate rock (limestone).
Limestone is a sedimentary rock formed between 120 and 40 million
years ago, on ancient tropical seabeds by the mass deposition and accumulation of
calcareous hard parts from dead organisms, such as shells.
As millions of years passed by, these deposits reached a great thickness until they started to compact and harden, both under their own weight and due to chemical processes caused by the water circulating inside.
20 milion years ago
Starting from Miocene 20 million years ago, there were several phases of tectonic uplift of
the seabed (orogeny/mountain building), which led to the exposure of the rocks previously formed
in these basins, that turned into actual dry continental areas.
These rocks were covered with a terrigenous material (Flysch), which once eroded away
by wind and river water, exposed the calcareous limestone on the surface.
From that point onwards the meteoric agents such as rain and surface water started a slow process of superficial chemical dissolution – which is still ongoing! – while rivers, increasingly enlarging
and hollowing out the cracks created by the orogenic uplift, soon left the
surface, creating underground hydrographic networks.
Once the water seeped further down, it left behind empty underground spaces, nowadays referred to as “caves”!
10 million years ago
The formative history of the Grotta Gigante cave itself starts in the Miocene circa 10 million years ago (Tortonian plane). It is estimated that the underground river courses actively hollowed out the rock for a period lasting between 5 – 6 million years, until the river water abandoned the cave in order to create newer and deeper pathways approximately 4 million years ago.
4 million years ago
However, within the next 3 – 4 million years the cave’s morphology was substantially by series of collapses and natural structural modifications. The Great Cavern for instance is the result of the massive collapse of two separate river caves that were previously located one over the other, and the disintegration of the ceiling between these two caves lead to unification of the two caves.
In this way the Grotta Gigante finally acquired its present day structural stability, creating the right conditions for the colonization of the cave with calcite concretions known as speleothems (stalactites, stalagmites, flowstones etc.)
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