Science of the structure, composition and structure of the earth's crust (earth = grch. Ge), its physical properties and its history of development as well as the processes forming it. The term was first used in 1778 by the Swiss meteorologist Jean-André Deluc (1727-1817); before that, geognosy was in use. The earth's crust consists of three different rock formations. These are crystalline or solidified rocks, sedimentary or stratified rocks and metamorphic or transformed rocks.
These are formed by cooling and solidification. The deep rocks (also plutonites) are formed when rising magma slowly cools down long before reaching the earth's surface and mineralizes in large crystals. The most common crystalline deep rocks are granite, mica slate and gneiss. If molten magma cools down closer to the earth's surface or still in the vent of an extinct volcano, it crystallizes in fine and finest crystals due to the faster cooling processes, forming hard, dense solidified rocks such as basalt or obsidian. In the case of effusive rocks (volcanites), liquid magma eruptively emerges as lava to the earth's surface to cool in water or in the air. From the eruption ash precipitation, gas-filled tufa rocks are formed, while the porous solidified lava flows weather into fertile lava soils.