Geology
HOW MOUNTAINS GROW
Throughout geological development, rocks are formed by melting in the depths of the Earth, by volcanic activity and sedimentation in the sea and freshwater basins. Due to tectonic movements during mountain-building processes, surface rocks mix with rocks of the Earth's mantle, where they are transformed at high temperatures and pressures - metamorphosis. The formation of deep rocks - granitoids continues. With more mountain-building processes, huge masses of these rocks come to the surface, forming high mountain ranges, which are denuded by futher activities - exposed, grinded off and moved, to finally form a surface relief, the landscape of planet Earth. Large tectonic events, Cadomian, Caledonian and Hercynian orogeny formed a massive mountain range with ninethousand‘s peaks. Their today‘s remains, hills several hundred meters high and low mountain ranges, still speak of their former size. The last Alpine orogeny, which includes Pyrenees, Tatras, Carpathians and Himalayas, began its development in the Late Cretaceous, about sixty million years ago, and continues till today as a result of continental plate‘s movement. African plate pushes the Eurasian plate, Alps are being folded and volcanism spreads - Vesuvius, Etna, Stromboli, Aeolian Islands. Indian subcontinent pushes the Asian plate, Himalayas rises and the volcanism of Krakatoa and the island of Java wakes up. Rugged peaks of these high mountain ranges remind us of earlier beauty of long time grinded peaks from former orogeny.
ABOUT OLD VOLCANOES
Volcanic manifestations are constant part of the geological evolution of our planet since its birth 4.6 billion years ago to the present day. For the formation of the Czech landscape are especially important two major volcanic events. Both are linked to mountain-building processes. The first to the Variscan branch of the Hercynian orogeny, so to the Paleozoic, the era 300 - 285 million years ago, to the end of the Carboniferous and the beginning of the Permian. The former volcanoes covered steep slopes of Podkrkonoší with lava flows. This bubble-rich lava after solidification and strengthening is known today as basalt-type rock melaphyr. The bubbles, which are rife at the edges of lava flows, were later filled with various minerals, but especially with brightly colored silicic acid solutions, and gave rise to the well-known Giant Mountains agates, amethyst druses, smoky quartz and crystals. The second major volcanic events took place in the Cenozoic era and are linked to the mountain-building process of Alpine orogeny. The volcanic activity at that time left behind nappes of volcanic rocks, rhyolites, trachyts, andesites, phonolites and basalts, started about 80 million years ago, and is divided into three period. The first volcanoes began their activity in the northwest of Bohemia in Lužice region, and after their eruptions no rocks remained - they were denuded over time. In the second period, that took place 40 - 18 million years ago, the landscape of Bohemian central mountains was formed. The beauty of old volcanoes, volcanic fills and old lava tuffs often with xenoliths are admired to this day (Milešovka, Oblík, Hrdišťany, Boreč ...). During tectonic movements sometimes older surface rocks, such as sediments, got into the upper mantle, where they were processed by pressure and high temperature, partially transformed and became heterogeneous inclusions in igneous rocks. By volcanic eruptions they came again to the suface together with volcanic rocks as older pieces of rock trapped in younger rocks – xenoliths. Similarly by tectonic activity emerged to the surface rocks transformed - metamorphic - eclogites, skarns, granulites and erlans, containing various ore minerals and garnets. After erosion and dispersion of these rocks, the garnets together with other minerals as heavier components appeared in alluvium. Today they are found as precious Czech garnets - pyropes. The third period of strong volcanism, which is related to Alpine orogeny, affected Jeseník area and dates back to about two million years ago. The older Paleozoic volcanism 300 Ma ago (Ma is a symbol for million years) shaped the landscape of Podkrkonoší with melaphyre rocks and agates. The younger Cenozoic about 70 - 2 Ma ago created Bohemian central mountains and Jeseník with a varied range of younger volcanic rocks containing Czech garnets and other interesting minerals.
Igneous Rocks:
| Extrusive young | Rhyolite (Liparite) / Trachyte / Andesite / Basalt |
| Extrusive old | Quartz porphyry / Feldspar porphyry / Porphyry / Diabas / Spilite / Melaphyry |
| Venous | Aplite / Pegmatite / Mineta |
| Abyssal | Granite / Syenite / Diorite / Gabro |
Igneous rocks are mainly composed of quartz, feldspar and mica, often amphibols and pyroxenes appear.
Metamorphic Rocks (Transformed):
Gneiss, Granulite (white stone), Erlan, Mica schist, Phyllite, Amphibolite, crystalline limestones, chlorite and talc slates.
Gneiss originating from igneous rocks is orthogneiss, paragneiss was formed by a transformation of sediments.
Sedimentary Rocks (Deposited):
They were formed by a deposition of eroded material of the original rocks - clastic rocks, or by chemical segregation from solutions and biogenic substances. Without distinction of origin, we will name:
limestones, dolomites, conglomerates, clays, loess, sandstones, quartzites, greywackes, arkoses, roof and stone slates, claystones, siltstones, gaizes, travertine, chalk, diatomite, lydite, cherts, phosphates, evaporates (salt deposits), coal, diesel and others.
Illustrations: Akad. mal. Jitka Bílá
