Sial
Encyclopedia
In geology
, the sial is the upper layer of the Earth's crust
made of rocks rich in silicate
s and aluminium
minerals. It is sometimes equated with the continental crust
because it is absent in the wide oceanic basin
s, but "sial" is a geochemical
term rather than a plate tectonic
term.
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 aluminium. The sial is often contrasted to the 'sima
,' the next lower layer in the Earth, which is often exposed in the ocean basins; and the nife
, the nickel
-iron
core
. These divisions of the Earth's interior (with these names) were first proposed by Eduard Suess
in the 19th century. This model of the outer layers of the earth has been confirmed by petrographic, gravimetric, and seismic evidence.
(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
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.
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
, the sial is the upper layer of the Earth's crust
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...
made of rocks rich in silicate
Silicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...
s and aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
minerals. It is sometimes equated with the continental crust
Continental crust
The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to more felsic, or granitic, bulk composition, which lies in...
because it is absent in the wide oceanic basin
Oceanic basin
Hydrologically, an oceanic basin may be anywhere on Earth that is covered by seawater, but geologically ocean basins are large geologic basins that are below sea level...
s, but "sial" is a geochemical
Geochemistry
The field of geochemistry involves study of the chemical composition of the Earth and other planets, chemical processes and reactions that govern the composition of rocks, water, and soils, and the cycles of matter and energy that transport the Earth's chemical components in time and space, and...
term rather than a plate tectonic
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
term.
Geologists often refer to the rocks in this layer as felsic
Felsic
The word "felsic" is a term used in geology to refer to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium....
, because they contain high levels of feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
, 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
The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with the chemical formula '. It has been known for its hardness since antiquity...
and of aluminium. The sial is often contrasted to the 'sima
Sima (geology)
Sima is the name for the lower layer of the Earth's crust. This layer is made of rocks rich in magnesium silicate minerals. Typically when the sima comes to the surface it is basalt, so sometimes this layer is called the 'basalt layer' of the crust. The sima layer is also called the 'basal crust'...
,' the next lower layer in the Earth, which is often exposed in the ocean basins; and the nife
NiFe
NiFe or Nife is a general shorthand expression for a mixture of nickel and iron . NiFe is used to describe nickel-iron batteries, various chemical reactions that involve a nickel-iron catalyst or component, and in geology to describe the general composition of the Earth's core.The affinity of...
, the nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
-iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
core
Planetary core
The planetary core consists of the innermost layer of a planet.The core may be composed of solid and liquid layers, while the cores of Mars and Venus are thought to be completely solid as they lack an internally generated magnetic field. In our solar system, core size can range from about 20% to...
. These divisions of the Earth's interior (with these names) were first proposed by Eduard Suess
Eduard Suess
Eduard Suess was a geologist who was an expert on the geography of the Alps. He is responsible for hypothesising two major former geographical features, the supercontinent Gondwana and the Tethys Ocean.Born in London to a Jewish Saxon merchant, when he was three his family relocated toPrague,...
in the 19th century. This model of the outer layers of the earth has been confirmed by petrographic, gravimetric, and seismic evidence.
Properties
The sial has a lower densityDensity
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
(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
Conrad discontinuity
The Conrad discontinuity corresponds to the sub-horizontal plane that takes place in the continental crust in which the seismic wave velocity increases in discontinuous mode...
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
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
liquid, so, in a real sense, the sial floats on the sima, in isostatic equilibrium
Isostasy
Isostasy is a term used in geology to refer to the state of gravitational equilibrium between the earth's lithosphere and asthenosphere such that the tectonic plates "float" at an elevation which depends on their thickness and density. This concept is invoked to explain how different topographic...
. Mountains extend down as well as up, much like icebergs on the ocean; so that on the continental plates
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
the sial runs between 5 km and 70 km deep.