Formation of Limestone
After the Earth’s crust hardened 4.5 billion years ago and the first continents were formed it took some more hundreds of millions of years before the formation of life on the Earth. The life began with aquatic bacteria and algae about 3.5 – 3.8 billion years ago. The vital activity took its humble course until the next revolutionary milestone – formation of multicellular organisms. Further succession of life was irruptive. During the Ediacaran Period (630 – 542 millions of years ago) the oxygen content of the atmosphere ascended from zero to 17% (now 21%). This produces evidence of extremely abundant, although primitive aquatic life, about what we unfortunately have very little knowledge. All organisms that were living those days were squashy, having left no petrifactions, only couple of random traces (e.g. in Australia, Canada and Russia).
Evolution took new direction in Cambrium, when the environment became overpopulated and competition emerged. For the first time such organisms occurred, that were feeding on other organisms. The weaker ones had to go into defending themselves and a hard outer shell suited for this purpose. First it was chitinous, further there was some phosphate added (source material of phosphorite) and finally calcium carbonate. Such kind of protective remedy was used by all groups of organisms – Cephalopods, Brachiopods, Trilobites, Corals, Sponges et al. Dead organisms sinked to the bottom in the course of time. During the hundreds of millions of years the remains of calcareous shells densified, forming the limestone.
In Estonian territory formation of limestone began 472 million years ago in the Ordovician Period and continued up to the end of the Silurian Period. Limestone deposits thickened very slowly, approximately 1-2 metres per million years. While suitable conditions for limestone formation in Estonia extended for 56 million years, then it is possible to measure the thickness of limestone deposits here in hundreds of metres. Elsewhere in Europe formation of limestone continued also during later periods.