CHALK:
Chalk (pronounced /ˈtʃɔːk/) is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. It forms under relatively deep marine conditions from the gradual accumulation of minute calcite plates (coccoliths) shed from micro-organisms called coccolithophores. It is common to find flint and chert nodules embedded in chalk.
GRANITE
Granite (pronounced /ˈɡrænɪt/) is a common and widely occurring type of intrusive, felsic, igneous rock. Granite has a medium to coarse texture, occasionally with some individual crystals larger than the groundmass forming a rock known as porphyry. Granites can be pink to dark gray or even black, depending on their chemistry and mineralogy. Outcrops of granite tend to form tors, and rounded massifs. Granites sometimes occur in circular depressions surrounded by a range of hills, formed by the metamorphic aureole or hornfels.
Granite is an igneous rock and is formed from magma. Granitic magma has many potential origins but it must intrude other rocks. Most granite intrusions are emplaced at depth within the crust, usually greater than 1.5 kilometres and up to 50 km depth within thick continental crust. The origin of granite is contentious and has led to varied schemes of classification. Classification schemes are regional; there is a French scheme, a British scheme and an American scheme. This confusion arises because the classification schemes define granite by different means. Generally the 'alphabet-soup' classification is used because it classifies based on genesis or origin of the magma.
Granite is a normal, geological, source of radiation in the natural environment. Granite contains around 10 to 20 parts per million of uranium. By contrast, more mafic rocks such as tonalite, gabbro or diorite have 1 to 5 ppm uranium, and limestones and sedimentary rocks usually have equally low amounts.
DIORITE
Diorite is an extremely hard rock, making it difficult to carve and work with. It is so hard that ancient civilizations (such as Ancient Egypt) used diorite balls to work granite. Its hardness, however, also allows it to be worked finely and take a high polish, and to provide a durable finished work. Thus, major works in diorite tend to be important.
One comparatively frequent use of diorite was for inscription, as it is easier to carve in relief than in three-dimensional statuary. Perhaps the most famous diorite work extant is the Code of Hammurabi, inscribed upon a 2 metre (7 ft) pillar of black diorite. The original can be seen today in Paris' Musée de Louvre[2]. A few large statues remain, including several statues of King Khafre in the Egyptian Museum. The use of diorite in art was most important among very early Middle Eastern civilizations such as Ancient Egypt, Babylonia, Assyria and Sumer. It was so valued in early times that the first great Mesopotamian empire -- the Empire of Sargon of Akkad -- listed the taking of diorite as a purpose of military expeditions.
GABBRO
Gabbro (pronounced /ˈɡæbrəʊ/) is a dark, coarse-grained, intrusive igneous rock chemically equivalent to basalt. It is a plutonic rock, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass.
The vast majority of the Earth's surface is underlain by gabbro within the oceanic crust, produced by basalt magmatism at mid-ocean ridges. Gabbro is dense, greenish or dark-colored and contains varied percentages of pyroxene, plagioclase, amphibole, and olivine (olivine gabbro when olivine is present in large quantities).
RHYOLITE
Rhyolite is an igneous, volcanic (extrusive) rock, of felsic (silicon-rich) composition (typically >69% SiO2 — see the TAS classification). It may have any texture from aphanitic to porphyritic. The mineral assemblage is usually quartz, alkali feldspar and plagioclase (in a ratio > 1:2 — see the QAPF diagram). Biotite and hornblende are common accessory minerals.
Rhyolite can be considered as the extrusive equivalent to the plutonic granite rock, and consequently, outcroppings of it often bear a resemblance to granite. Due to their high content of silica and low iron and magnesium contents, rhyolites polymerize quickly and form highly viscous lavas. They can also occur as breccias or in volcanic necks and dykes. Rhyolites that cool too quickly to grow crystals form a natural glass or vitrophyre, also called obsidian. Slower cooling forms microscopic crystals in the lava and results in textures such as flow foliations, spherulitic, nodular, and lithophysal structures.
BASALT:
Basalt (pronounced /ˈbæsɒlt, bəˈsɔːlt/) is a common mafic extrusive volcanic rock. It is usually gray to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or gray.
On Earth, most basalt magmas have formed by decompression melting of the mantle. Basalt has also formed on Earth's Moon, Mars, Venus, and even on the asteroid Vesta. Source rocks for the partial melts probably include both peridotite and pyroxenite (e.g., Sobolev et al., 2007). The crustal portions of oceanic tectonic plates are composed predominantly of basalt, produced from upwelling mantle below ocean ridges.
The term basalt is at times applied to shallow intrusive rocks with a composition typical of basalt, but rocks of this composition with a phaneritic (coarse) groundmass are generally referred to as dolerite (also called diabase) or gabbro.
ANDESITE
Andesite (pronounced /ˈændəsaɪt/) is an igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. The mineral assembly is typically dominated by plagioclase plus pyroxene and/or hornblende. Biotite, quartz, magnetite, sphene are common accessory minerals. Alkali feldspar may be present in minor amounts. The quartz-feldspar abundances in andesite and other volcanic rocks are illustrated in QAPF diagrams. Relative alkali and silica contents are illustrated in TAS diagrams.
Classification of andesites may be refined according to the most abundant phenocryst. Example: hornblende-phyric andesite, if hornblende is the principal accessory mineral.
Andesite can be considered as the extrusive equivalent to plutonic diorite. Andesites are characteristic of subduction tectonic environments in active oceanic margins, such as the western coast of South America. The name andesite is derived from the Andes mountain range.
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Topic: Learning about all our Dwarf Fortress Rocks! (Read 53355 times)