geology Geology (). is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth ...

, sial is an antiquated blended term for the composition of the upper layer of

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

's crust, namely rocks rich in

aluminium Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...

silicate mineral Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust. In mineralogy, the crystalline forms of silica (silicon dio ...

s. It is sometimes equated with the

continental crust Continental crust is the layer of igneous, metamorphic, and sedimentary rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as '' continental shelves''. This layer is sometimes called '' si ...

because it is absent in the wide

oceanic basin In hydrology, an oceanic basin (or ocean basin) is anywhere on Earth that is covered by seawater. Geologically, most of the ocean basins are large Structural basin, geologic basins that are below sea level. Most commonly the ocea ...

s, but 'sial' is a

geochemical Geochemistry is the science that uses the tools and principles of chemistry to explain the mechanisms behind major geological systems such as the Earth's crust and its oceans. The realm of geochemistry extends beyond the Earth, encompassing the ...

term rather than a

plate tectonic Plate may refer to: Cooking * Plate (dishware), broad, mainly flat vessel commonly used to serve food * Plates, tableware, dishes or dishware used for setting a table, serving food and dining * Plate, the content of such a plate (for example: r ...

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. 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 the sial is 2.7 g/cm³. Geologists often refer to the rocks in this layer as

felsic In geology, felsic is a grammatical modifier, modifier describing igneous rocks that are relatively rich in elements that form feldspar and quartz.Marshak, Stephen, 2009, ''Essentials of Geology,'' W. W. Norton & Company, 3rd ed. It is contrasted ...

, because they contain high levels of

feldspar Feldspar ( ; sometimes spelled felspar) is a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagiocl ...

, 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 Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...

and of

alumina Aluminium oxide (or aluminium(III) oxide) is a chemical compound of aluminium and oxygen with the chemical formula . It is the most commonly occurring of several aluminium oxides, and specifically identified as aluminium oxide. It is commonly ...

. The sial is often contrasted to the '

sima Sima or SIMA may refer to: People * Sima (Chinese surname) * Sima (Persian given name), a Persian feminine name in use in Iran and Turkey * Sima (Indian given name), an Indian feminine name used in South Asia * Sima (surname) * Sima (born 1 ...

' (another antiquated blended term), the next lower layer in Earth, which is rich in silica and

magnesium Magnesium is a chemical element; it has Symbol (chemistry), symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 ...

and is often exposed in the ocean basins; and the nickel-iron alloy

core Core or cores may refer to: Science and technology * Core (anatomy), everything except the appendages * Core (laboratory), a highly specialized shared research resource * Core (manufacturing), used in casting and molding * Core (optical fiber ...

, sometimes referred to as the 'Nife'. These geochemical divisions of Earth's interior (with these names) were first proposed by

Eduard Suess Eduard Suess (; 20 August 1831 – 26 April 1914) was an Austrian geologist and an expert on the geography of the Alps. He is responsible for hypothesising two major former geographical features, the supercontinent Gondwana (proposed in 1861) and ...

in the 19th century. This model of the outer layers of Earth has been confirmed by petrographic, gravimetric, and seismic evidence.

Properties

The sial has a lower

density Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...

(2700–2800 kg/m³) 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, the boundary has been arbitrarily set at a mean density of 2800 kg/m³. Because of the large pressures, over geologic time, the sima flows like a very

viscous Viscosity is a measure of a fluid's rate-dependent resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for example, syrup h ...

liquid, so, in a real sense, the sial floats on the sima, in isostatic equilibrium. Mountains extend down as well as up, much like

iceberg An iceberg is a piece of fresh water ice more than long that has broken off a glacier or an ice shelf and is floating freely in open water. Smaller chunks of floating glacially derived ice are called "growlers" or "bergy bits". Much of an i ...

s on the ocean; so that on the continental plates, the sial runs between 5 km and 70 km deep.

References

Sources

* Bates, R.L., and Jackson, J.A., (1987) ''Glossary of geology'' American Geological Institute, Alexandria, Virginia. * Dilek, Y. and Newcomb, S. (eds.) (2003) ''Ophiolite Concept and the Evolution of Geological Thought'' Geological Society of America Special Paper 373, Boulder, Colorado. {{Refend Petrology Structure of the Earth

Properties

See also

References

Sources