Not so long ago scientists geologists knew about crustal structure only based on their own experience of its surface study and upper layers available during borehole drilling. Implicit ideas of what was located at a great depth were obtained using special equipment to learn properties of lower level - density, magnetic properties, electrical conductivity, elastic wave propagation velocity induced by natural or man-made earthquakes.
Nobody saw the internal structure of our planet. To have a general idea let’s look at usual chicken egg - we are not the first to take such a successful comparison. All of us knew what it consists about. So, the Earth consists of core, interlayer - mantle and outer shell and earth crust. These three components of the planet are in same ratio as yolk, white of an egg and shell, it is very fragile - throughout covered by deep fractures. Coming from the surface to its deep it is divided into three inhomogeneous layers: sedimentary cover, granitic and basaltic layers. The latter two are figuratively called so as they are similar by physical properties to granite and basalt. It should be noted that thickness of each layers both of continents and ocean bottom are different. Moreover, not all three layers present at once: there is no granite layer under marine sediments, no sedimentary mantle in the continents within the crystalline shields. Kola north and Khibiny in particular has all layers therefore this region was chosen appropriate for drilling Super deep borehole. Other point is that our region is rich in minerals.
Before this borehole was drilled at a depth of almost 15 km in Murmansk region, Pechenga district, the world’s record belonged to Americans. Why such a great job has been done? First of all, to check the depth of copper and nickel and other metals deposits; secondarily, to learn more about the Earth crust, its composition and physical properties of its layers; at last, to evaluate if the drilling equipment and instruments worked reliable as they were specially manufactured in Russia for this experiment.
Geological structure of the Khibiny can be visually seen for example using geological map. Such maps are usually made based on data of different studies: geological survey observations, study of geological drill holes and mine workings - pits, shafts, aerial and space photo interpretation and other observations. For instance, physical properties of subsurface as magnetic, electric ones and mineral and chemical composition of rocks, etc. are investigated.
In the wake of events of the long ago - billions, hundreds of million years ago - being fixed on geological map, the geologist restores the complicated history of our planet’s development. This geological investigation has been made for the Kola peninsula as well. In order to learn the succession of sediment deposition and what rock is older or younger, there is a simple rule: the oldest layers are in the deeper section and covered by younger ones. This can provide for assessment of mineralogical composition of the Khibiny mountains.
For a long geological history of the earth crust development many-kilometers long formations are accumulated consisting of sedimentary deposits. By gravity they go deep and exposed to high temperature and pressure action, friable deposits are turned into solid crystalline rocks such as gneiss and slates. The older the age of the rocks, the harder they suffered, the stronger they changed mineral and chemical composition and outer appearance. Therefore, sedimentary rocks are changing and became metamorphic. As a result of lateral pressure the layers are changed to small and large folds with more or less steep walls resembling cleaving stone. Besides, fold structures have faults and fractures and individual layers can move along it for a significant distance. Large masses of melted material of the Earth called magma is streaming to hollows formed. When magma is cooled down, it is crystallized into rocks - granite, diorite, diabase and gabbro. They compose large and small rock bodies or intrusions of different shape. If melted magma reaches the upper horizons of the Earth crust or comes out on daily surface it covers large areas. Lava flows occur of effusive rocks - basalts, pillow lava and other.
There are three long-term time periods or three eons - long-term geological time interval that unites several geological ages in the history of our planet’s development: the Archean - over 2 600 million years, the Proterozoic - 2 600 to 570 million years and the Phanerozoic - less than 570 million years. In order to determine time of sedimentary rock formation the geologists use fossil shells, skeletons, marine and land animals as well as plant roots, trunks and leaves that were on our planet. Some animals and plants lived and grew during strictly defined time period. After decaying they became sedimentation of its time. Such method of layer dating is called paleontology.
You know since biology lessons that organic life on Earth has appeared 500-600 million years ago. Geological time we are living now started only 1 million 500 thousand years ago and has a long name: modern epoch sedimentary (or anthropogenic) period of Cenozoic era.
Prevailing rocks of the Khibiny can be seen on the geological map of the Kola region where can be found representatives of all three types of rocks of all geological periods from the Archean to modern epoch. The region mainly consists of ancient complexes of gneiss, slates, amphibolite and intrusions of granites, diorites, gabbro and diabase. The age of all these rocks is 3.5-4 billion years. They form very complicated folded structure, elongated from north-west to south-east, i.e. along the long side of the peninsula. These folded structures are divided into different directions by many faults going to the depth. Therefore, on the map the rock of different age and composition are located, as it may seem at first sight, irregular and interchanged kaleidoscope.
Long ago the territory of the present-day Kola peninsula was a bottom of the World ocean. Marine sediments accumulated on its surface - limestone, sandstone and clay. Later the sea took its waters, bottom became a mainland, accumulated marine sediments had dispersed over a long distance. Traces of such sediments as sandstones and argillites can be still found in Sredny and Rybachii peninsula, Kil'din island and Tersky seacoast - villages Olenina, Kashkarantsy, Kuzomen', where are deposited in old gneiss and sink into waters of the Barents and White sea.
In the central part of the peninsula, in the Khibiny and Lovozero mountains as well as some places to the south the geologists have discovered and thoroughly studied the magmatic rock massif that consists of sodium, potassium, aluminum, phosphate, magnesium and calcium. Here is concentrated the large deposits of apatite-nepheline, iron ores and unusual mica - vermiculite and phlogopite. Rock massif of such kind are called central but not because it is located in the center of the territory but due to its structure reminds tree cut: rocks of different composition are deposited as a circles around some center. They are delivery channels of the old underground volcano of 350-400 million years old. Study of the massif brought a lot of interesting for the science: it turned out they are located in the earth crust fault joints and some of them like Khibinsky and Lovozersky still grow as though they squeeze out subsurface and keep the warms of the former volcano. Besides, during borehole drilling it was registered the gradual increase of the temperature by 1° C for each 40 meters although in the Kola North temperature increase by 1° C is typical for each 100 meters.