In geology , sial is an antiquated blended term for the composition of the upper layer of Earth 's crust , namely rocks rich in aluminium silicate minerals . It is sometimes equated with the continental crust because it is absent in the wide oceanic basins , but 'sial' is a geochemical term rather than a plate tectonic term. As these elements are less dense than the majority of Earth's elements, they tend to be concentrated in the upper layer of the crust.
13-484: The uppermost layer of the crust is called the sial, consisting of silicate and aluminium (Si = silicate, Al = aluminium). On average, the thickness of the sial is till 25 km from the surface. The continents are composed mainly of lighter rock material formed from silicon and aluminium, so the sial is thick over the continents and very thin or absent on the ocean floor, especially the Pacific Ocean. Average density of
26-467: A felsic rock is given according to the TAS classification of Le Maitre (1975). However, this only applies to volcanic rocks. If the rock is analyzed and found to be felsic but is metamorphic and has no definite volcanic protolith , it may be sufficient to simply call it a 'felsic schist'. There are examples known of highly sheared granites which can be mistaken for rhyolites. For phaneritic felsic rocks,
39-413: A lower density (2700–2800 kg/m) than the sima, which is primarily due to increased amounts of aluminium, and decreased amounts of iron and magnesium. The base of the sial is not a strict boundary, the sial grades into the denser rocks of the sima. The Conrad discontinuity has been proposed as the boundary, but little is known about it, and it doesn't seem to match the point of geochemical change. Instead,
52-455: A tridimensional network of SiO 4 tetrahedra connected to each other. Once released in water and hydrolyzed, these silica entities can indeed form silicic acid in aqueous solution. The term "felsic" is a derivation of the words " fel dspar" and " si lica". The similarity of the resulting term felsic to the German felsig , "rocky" (from Fels , "rock"), is accidental. Feldspar is from
65-420: Is granite . Common felsic minerals include quartz, muscovite , orthoclase , and the sodium-rich plagioclase feldspars ( albite -rich). In modern usage, the term acid rock , although sometimes used as a synonym, normally now refers specifically to a high-silica-content (greater than 63% SiO 2 by weight) volcanic rock , such as rhyolite . Older, broader usage is now considered archaic. That usage, with
78-669: Is a modifier describing igneous rocks that are relatively rich in elements that form feldspar and quartz . It is contrasted with mafic rocks, which are richer in magnesium and iron . Felsic refers to silicate minerals , magma , and rocks which are enriched in the lighter elements such as silicon , oxygen , aluminium , sodium , and potassium . Molten felsic magma and lava is more viscous than molten mafic magma and lava. Felsic magmas and lavas have lower temperatures of melting and solidification than mafic magmas and lavas. Felsic rocks are usually light in color and have specific gravities less than 3. The most common felsic rock
91-460: Is a petrologic field term used to refer to very fine-grained or aphanitic , light-colored volcanic rocks which might be later reclassified after a more detailed microscopic or chemical analysis. In some cases, felsic volcanic rocks may contain phenocrysts of mafic minerals, usually hornblende , pyroxene or a feldspar mineral, and may need to be named after their phenocryst mineral, such as 'hornblende-bearing felsite'. The chemical name of
104-801: The QAPF diagram should be used, and a name given according to the granite nomenclature. Often the species of mafic minerals is included in the name, for instance, hornblende-bearing granite, pyroxene tonalite or augite megacrystic monzonite , because the term "granite" already assumes content with feldspar and quartz. The rock texture thus determines the basic name of a felsic rock. Volcanic rocks : Subvolcanic rocks : Plutonic rocks : Picrite basalt Peridotite Basalt Diabase (Dolerite) Gabbro Andesite Microdiorite Diorite Dacite Microgranodiorite Granodiorite Rhyolite Microgranite Granite Acid-base Too Many Requests If you report this error to
117-605: The German Feldspat , a compound of the German Feld , meaning field, plus spat[h] , meaning mineral. In order for a rock (rather than a mineral ) to be classified as felsic, it generally needs to contain more than 75% felsic minerals (namely quartz, orthoclase and plagioclase ). Rocks with greater than 90% felsic minerals can also be called leucocratic , from the Greek words for white and dominance. Felsite
130-445: The boundary has been arbitrarily set at a mean density of 2800 kg/m. Because of the large pressures, over geologic time, the sima flows like a very viscous liquid, so, in a real sense, the sial floats on the sima, in isostatic equilibrium . Mountains extend down as well as up, much like icebergs on the ocean; so that on the continental plates , the sial runs between 5 km and 70 km deep. Felsic In geology , felsic
143-458: The contrasting term "basic rock" (MgO, FeO, mafic ), was based on an ancient concept, dating from the 19th century, that " silicic acid " (H 4 SiO 4 or Si(OH) 4 ) was the chief form of silicon occurring in siliceous rocks. Although this intuition makes sense from an acid-base perspective in aquatic chemistry considering water-rock interactions and silica dissolution, siliceous rocks are not formed by this protonated monomeric species, but by
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#1732780107111156-489: The next lower layer in Earth, which is rich in silica and magnesium and is often exposed in the ocean basins; and the nickel-iron alloy core , sometimes referred to as the 'Nife'. These geochemical divisions of Earth's interior (with these names) were first proposed by Eduard Suess in the 19th century. This model of the outer layers of Earth has been confirmed by petrographic, gravimetric, and seismic evidence. The sial has
169-446: The sial is 2.7 g/cm. Geologists often refer to the rocks in this layer as felsic , because they contain high levels of feldspar , an aluminium silicate mineral series. However, the sial "actually has quite a diversity of rock types, including large amounts of basaltic rocks." The name 'sial' was taken from the first two letters of silica and of alumina . The sial is often contrasted to the ' sima ' (another antiquated blended term),
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