Lake Baikal is one of the oldest lakes of the world. It is about 25 mln years old and there are no signs of its getting older and disappearing like many other lakes. On the other hand, the latest research gave geophysicists the opportunity to assume that Lake Baikal is an ocean being born. It is proved by the fact that its shores drift apart with the rate of 2 cm a year like the continents of Africa and South America.
The assumption by I.D. Chersky that Lake Baikal formed in Laurence (Pre-Cambrian) period is not accepted. It seems that in Paleozoic and especially in Mesozoic periods there was a compound relief with great contrasts of heights which is proved by the presence in the Baikal land of tightly cemented Jurassic conglomerates and sandstone. They formed under the influence of fragmental rocks drift from the mountains onto lower lands of the Irkutsk basin and the cavities of Transbaikal land.
Little is known about Cretaceous or early Paleogene periods of the Baikal relief development because sedimentary deposits of that period have not been found. By the end of Paleogene the Baikal land had been a raised sloping and hilly country vast parts of which were covered with the cloak of eroded rocks. The remains of peneplene and strong crusts of erosion can be found in many places of the Baikal land (the Olkhon plateau, Olkhon island and others) and on the underwater slopes of the lake cavity.
In the end of Paleogene (about 23 mln years ago) – the lower Neogene - in the Baikal land sags developed which were filled with fresh waters. In these shallow cavities sands, clays and shingle deposited. Ancestors of modern Baikal molluscs and sponges appeared. The fossil fauna of Neogene and the analysis of spores and pollen give evidence that the climate at that time was warmer than in the present.
By the early Quaternary period (1.5 mln years ago) the tectonic movements had spread all over the south of Eastern Siberia. At that time the cavity of Lake Baikal formed and its mountain frame, similar to the present one, appeared.
In the late Pliocene and early Quaternary periods the freezing of the Barguzin, Baikal, Khamar-Daban and other mountain ranges started in connection with the climatic fall of temperature. In the period of their maximum development the glaciers reached the lake. Powerful processes of the glacial accumulation and exaration, erosion with glacial waters, as well as the level of the Baikal fluctuations connected with ice formation and melting, changed the Pre-Quaternary contours of the shore. In the valleys, on the shore and shelf of the north-eastern, north-western and south-eastern shores of the lake an intense accumulation of glacial sediments was going on. They were 100-120 m thick.
Now the cavity of Lake Baikal is considered to be the central link of the Baikal rift zone which appeared and was developing at the same time with the world rift system. The cavity is a little broader of the present lake cavity, but much deeper than it. The roots of the cavity are in the upper mantle, e.g. 50-60 km deep. Research shows that the bowels of the earth under Lake Baikal are anomalously heated. The cause of that phenomenon is not clear yet, but it is indirectly supported by magnetic and telluric probing, numerous outcrops of thermal waters onto the surface and changes of the heat flow gradients on the bottom of the Baikal. The light undercrust substance could raise the earth’s crust at the surfacing having broken it completely in some places. It could create the necessary preconditions for forming the present mountain ranges around Lake Baikal. At that time extension of the earth’s crust could take place, it was recorded by geophysical research under the lake cavity. This in its turn lead to opening of old and forming of new breaks, lowering of some blocks along them and forming of intermountain hollows – “rift valleys” of the Baikal type, including the Baikal cavity.
In the early stages of rift origin the undercrust substance heated to plastic condition rose up to the surface along the breaks and flowed out as basaltic lavas. Volcanoes were active in the Baikal land (in the Tunka rift valley) in the early Neolithic age (8-9 thousand years ago).
Tectonic movements which caused the origin of Lake Baikal are going on now which is proved by high seismic activity of the whole Sayan-Baikal arch (vault) raise and by young seismotectonic breaches found in the mountain frame of Lake Baikal.
There are earthquakes up to force 10-11 in the Baikal land. Some seismotectonic breaches appeared within living memory. Thus the Proval (Gap) gulf with the area of over than 200 sq. km formed in January 1862 in the northern part of the Selenga delta after a force 10-10.5 earthquake. The land suddenly sank together with villages, vast pastures and hayfields. The Proval is one of the biggest Baikal gulfs up to 5.5-6 m deep. An earthquake of similar force (9) with the epicentre in the middle cavity of the lake took place in August 1959 18-20 km away from the shore. As a result the bottom sank 15-20 m in the epicentre. Weaker earthquakes take place more often, and seismographs record up to 2,000 of them a year.