Geological History of Caves and Conservation Values of the World Nature Heritage Phong Nha-Ke Bang National Park

In July 2003 Phong Nha-Ke Bang National Park in Quang Binh province was selected by UNESCO as a World Natural Heritage site for its diversity of geology and geomorphology and unique beautiful landscapes. The Phong Nha-Ke Bang limestone mountain area in general and the cave system in particular is the product of a long history of geological development starting from the Ordovician period (435 million years) to the present day. French geologists call the Phong - Ke Bang limestone area the largest limestone “desert” in the world. The geological history of the cave is from the Devonian period (410 million years ago) to the Permian period (250 million years ago). During this period, the Phong Nha Ke Bang area was an oval-shaped marginal basin, creating a  platform limestone, in which the calcite content accounted for over 80%. That is the most favorable factor to form caves and beautiful and unique landscapes. In the first stage, the sedimentary basin deposited lime mud with oval shape (in Devon) and then gradually expanded to an isometric shape in carbon-permian. After turning lime mud into limestone, this sedimentary basin was  uplifted during the Cenozoic period (from 65 million years to the present) and the first generation of caves began to form by 2 main fault systems NE-SW. and NW-SE and then two sub-fault systems N-S and W-E. The age of the caves is also calculated from the formation of the East Sea. Each tectonic phase is forming a cave generation. The higher the cave is located, the older it is, the youngest is the current generation of caves. It is one of the outstanding values ​​of Phong Nha-Ke Bang cave which is highly appreciated by UNESCO. Each generation of cave has a series of geological processes occurring, which are called exogenous geology or Karst: (1) spectacularly beautiful stalactites precipitate from the ceiling of the cave with cracks down according to the reaction equation. reversible (CaCO3 ↓+ CO2 + H2O ↔ Ca(HCO3)2 ); (2) stalagmites  and stone columns are precipitated from below by drops of bicarbonate falling from the ceiling of the cave; (3) cave pearls of spherical and oval sizes have different compositions and structures created by eddy currents in the hollow; (4) layers of travectin from mechanical and carbonate mixed materials according to transport mode, deposited simultaneously with flood flow. CaCO3 plays the role of cement to be precipitated and recrystallizated in the syngenesis period; (5) the formation of a spiral mushroom in Son Doong cave caused by a flood of travertin mud deposited at the same time as the eddy current surrounding a large limestone block in the middle of the flood; (6) the operation of the flood season has changed the diversity and magnificence of the landscape in the cave, which is created in the dry season.