How is granite made from basalt
Author: Torsten Purle (steine-und-minerale.de) | Last update: 07.05.2021
Basalt - properties, formation and use
English: basalt | French: basalt
Pliny and the Basalt of Ethiopia
The name basalt is mentioned for the first time in the records of the Roman natural scientist Pliny the Elder (23 to 79 AD). In "Naturalis historia" Pliny describes a stone from Ethiopia whose color and hardness remind him of iron.
Since the stone also seemed suitable for the manufacture of weapons due to its hardness, the Romans gave the stone the name, derived from the Egyptian, basalt - literally translated as hard stone.
Properties of basalt
Definition: Basalt is a basic (= low-silicon dioxide), igneous rock of extrusive origin - alternatively: extrusions, cast rock or volcanic rock, with a characteristic pentagonal or hexagonal column shape.
The The color of basalt is darkas is characteristic of rocks with a basic composition.
Often the rock appears blue, gray-blue, dark gray or black. As a result of weathering, writes the German mineralogist Carl Hartmann in 1854, the color changes and "becomes light gray, soon yellow and brown spots appear, a brownish crust forms".
The reason for the coloring of basalts are the rock-forming aggregates (= the minerals that make up a rock), which are at the same time to differentiate from other rocks or the basis for the definition of basalt.
The structure of basalt is fine-grained to dense; the xeno- and idiomorphic crystals are not easy to see with the eye due to their size - with the exception of feldspar basalt. As a result of eruptions or slagging, basalt often shows a direction of flow. Occasionally there are smaller cavities in the basalt or regulated inserts consisting of pyroxene or olivine nests.
The density of basalt is 3.0 to 3.5 g / cm3.
Main and secondary parts of basalt
The main mix parts dominating basalts with a share of more than five percent of the composition are through
With a share of up to 5%
the so-called secondary mixes of basalt.
The mineral composition of basalt can vary depending on the conditions in which it was formed and its regional origin.
Olivine basalt as a variety, for example, refers to relatively rich olives content in terms of quantity.
Feldspar basalt is a variety of basalt that shows clearly visible, larger crystals of feldspars.
Tholeiite basalt, named after the place Tholey in Saarland / Germany, is a basalt that is rich in plagioclase feldspars, augite and orthopyroxene.
The mineral stock of basalt can also be found in gabbro, which is why the rock is called plutonic equivalent of basalt applies.
Formation and distribution of basalt
Basalts are formed as representatives of the effusion rocks due to the rapid cooling of low-silica magmas that come to the earth's surface.
Therefore, basalt is assigned to the rock group of volcanic rocks - igneous rocks that arise on the earth's surface. (More details: The formation of igneous rocks)
Basaltic lava is characterized by a very high fluidity which is why the lava can form extensive rock coverings that cover both the mainland and almost the entire seabed of the world's oceans. In addition, basalt also forms in volcanic cones.
The cone-like shape of some basalt mountains gave historical mineralogists and geologists an opportunity to discuss the formation of basalt.
Karl Wilhelm Nose (1753 to 1835) observed that basalts "have certain characteristics of fire in them". Déodat de Dolomieu (1750 to 1801) goes on and writes that basalt "could be a product of underground fire" - insofar as he compares basalt mountains with the shape of volcanoes.
Honeycomb structure and Bérnard effect of basalt
Typical of basalt are geometric perfect, pentagonal or hexagonal (hexagonal) columns, or as Hartmann said in 1854: "Very often the stone, which is remarkably constant in its exterior, has a completely prismatic separation and forms beautiful columns".
The cause of the formation of hexagonally aligned basalt columns are thermodynamic processes during the cooling of the lava.
The effect that leads to the columnar alignment of the basalt becomes Bérnard effect called. Accordingly, on the one hand, the Heat of the rock melt on the underside and, on the other hand, through the atmospheric cooling due to the air on the top, vertically structured convection cellsthat appear hexagonal when viewed from above.
As the molten rock cools down further, contractions and tensile stresses occur, which is why the rock mass breaks apart following the hexagonal convection cell alignment.
All over the world there are numerous places where columnar basalt looks like something out of a picture book: the Giant's Causeway in Ireland is like a carpet of basalt, at the Svartifoss waterfall in Iceland one basalt column is lined up after the next, and at Scheibenberg in the Erzgebirge forms basalt palisades, which are popularly known as organ pipes.
On the other hand, basalts formed on the sea floor show a spherical, round shape on, spherical basalt. The so-called pillow lava hardens within a few minutes due to the enormous temperature difference between lava and sea water.
As "proof" that the so-called pillow lava actually cooled down very quickly, basalt, which was formed in the sea, shows coatings with rock glass.
Basalts occur worldwide - at hot spots under continental or oceanic plates, at spreading zones of mid-ocean ridges (between two oceanic plates that are drifting apart) and at subduction zones of plates, but also in areas with active or historical volcanism.
There are significant basalt deposits in Iceland; Greenland; the Canary Islands / Spain; Faroe Islands; in Ireland; Auvergne / France; Dekkan / India; Karroo / South Africa; Cameroon; Montana and the USA. Numerous basalt deposits are also known in Germany, which are evidence of volcanic activities in the geological past of the respective region; e.g. Rhön, Vulkaneifel / Eifel, Westerwald, Odenwald, Swabian Volcano, Vogelsberg, Siebengebirge, Lausitz, Pöhlberg, Scheibenberg and Bärenstein.
Sunburner basalt - weathering of basalt
The term sunburned basalt is not understood to mean a variety of basalt, but describes a form of weathering that is mainly observed in basalts and basanites.
So-called geological sunburn particularly affects basalts that are rich in foids, especially nepheline, where the transformation to analcime takes place in the course of weathering and the associated metamorphosis. At the same time there is an increase in volume due to the crystal growth of around five percent - comparable to the weathering form frost blasting.
The typical features of sunburner basalt include the white-speckled surface that looks cracked and crumbly. Within a few years, the hard rock disintegrates into smaller fragments or gravel. For this reason, sunburner basalt is not suitable for road construction. Due to the high mechanical stress on road traffic, the rock would disintegrate too quickly.
Importance and use of basalt
Basalt is a very compact natural stone that only slowly weathers and is therefore used as a millstone, road markers (basalt cross), gravel and gravel as well as masonry. Its use as a road surface / paving stone is no longer relevant, as basalt becomes slippery as a result of precipitation.
Basalt is often rich in solid copper, sapphires and iron ores, which is why the igneous rock is also important for the extraction of certain raw materials and minerals.
In the construction industry, basalt is melted and used for particularly resistant floor slabs, tiles or as a component of mineral wool. Basalt is also used to replace ice as a whiskey stone to cool whiskey without watering down the drink or affecting its taste.
⇒ rare earths
⇒ Neptunists against Plutonists: The basalt dispute from the Scheibenberg
⇒ Alexander von Humboldt and the minerals
⇒ Pliny, C .: Caius Plinius Secundus Natural History. Collection of the latest translations by Roman prose writers with explanatory notes under the supervision of Professors Bergsträser and Ostertag. Frankfurt am Mayn. 1787. Bey Johann Christian Hermann
⇒ Dolomieu, D. and Fortis, A. (1792): Letters from two foreign mineralogists on basalt
⇒ Nose, K. W. (1794): Tafeln about the formation and transformation of the basalt and the lavas
⇒ Bauer, J .; Tvrz, F. (1993): The Cosmos Mineral Guide. Minerals rocks precious stones. An identification book with 576 color photos. Gondrom Verlag GmbH Bindlach
⇒ Pellant, C. (1994): Stones and Minerals. Ravensburger nature guide. Ravensburger Buchverlag Otto Maier GmbH
⇒ Schumann, W. (1991): Minerals rocks - characteristics, occurrence and use. FSVO nature guide. BLV Verlagsgesellschaft mbH Munich
⇒ Maresch, W., Medenbach, O .; Trochim, H.-D. (1987): The colored natural guide rocks. Mosaik Verlag GmbH Munich
⇒ Murawski, H. (1992): Geological Dictionary. Ferdinand Enke Verlag Stuttgart
⇒ Murawski, H., Meyer, W. (2017): Geological dictionary. Ferdinand Enke Verlag Stuttgart, 12th edition
⇒ Schumann, W. (1994): Collecting stones and minerals; find, prepare, determine. BLV Verlag Munich
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