Sial
Sial is the upper layer of the Earth's continental crust, characterized by its silicic composition rich in silicon and aluminum, primarily consisting of granitic and other felsic igneous rocks.[1] The term "sial" is a portmanteau derived from "silica" and "alumina," coined by Austrian geologist Eduard Suess in his multi-volume work Das Antlitz der Erde (The Face of the Earth), published between 1883 and 1909.[2] This layer forms the foundational structure of continents and continental shelves, contrasting with the underlying sima (silicon-magnesium-rich mafic material) that predominates in oceanic crust.[1] With an average density of approximately 2.7 g/cm³, sial is less dense than sima (around 3.0 g/cm³), enabling continents to "float" higher on the mantle in isostatic equilibrium.[3] Its thickness varies significantly, averaging 30–50 km beneath stable cratons but reaching up to 70 km under mountain ranges due to tectonic thickening.[3] Sial constitutes a substantial portion of the total crustal volume, estimated at approximately 70% of the Earth's crust,[4] and is predominantly igneous in origin, though it includes sedimentary and metamorphic rocks overlying the granitic basement.[5] In the context of plate tectonics, the buoyancy of sialic continental crust resists subduction, leading to continental collisions that build mountain belts and reshape landmasses over geological time.[5] The formation and evolution of sial are linked to processes such as partial melting of the mantle and fractional crystallization, with evidence suggesting its development began in the Archean eon and continues through modern orogenic events.[5]Definition and Overview
Definition
In geology, sial refers to the compositional layer forming the upper part of the Earth's continental crust, consisting primarily of felsic rocks enriched in silica and alumina.[1] The term "sial" is a portmanteau of "silica" and "alumina", highlighting the dominance of aluminum silicate minerals in these rocks.[2] This layer underlies the continents and contrasts with the denser oceanic crust.[6] The term "sial" was coined by Austrian geologist Eduard Suess in his multi-volume work Das Antlitz der Erde (1883–1909). Alfred Wegener later used the term in his early 20th-century work on continental drift, as part of a simplified model distinguishing continental (sial) from oceanic (sima) materials based on their elemental composition.[6] Wegener used the term to describe the lighter, granitic-like rocks that form the bulk of continental masses. Granites and related felsic igneous rocks are the dominant lithologies in this layer, reflecting its overall silicic nature.[7] Although sial is now regarded as an older term in contemporary geological nomenclature, it persists in educational contexts for its straightforward illustration of crustal differentiation. This usage aids in conveying the basic chemical distinctions without delving into more complex modern models of crustal structure.[8]Overview in Earth's Structure
Sial constitutes the uppermost layer of the continental crust, positioned directly beneath the sedimentary cover and extending down to the Mohorovičić discontinuity, or Moho, which marks the boundary between the crust and the underlying mantle.[9] This discontinuity, identified through seismic wave analysis, typically lies at depths of 30 to 50 kilometers beneath continental regions, reflecting the thicker nature of continental crust compared to oceanic equivalents.[10] In contrast to the oceanic crust, which is primarily composed of denser basaltic material known as sima and lacks a sialic layer, sial is exclusively associated with continental settings, forming the foundational rocky framework of landmasses.[9] This distinction underscores the heterogeneous composition of Earth's crust, with sial covering approximately 29% of the planet's surface through the distribution of continents.[11] Sial plays a key role in the isostatic balance of Earth's lithosphere, where its relatively lower density allows the continental crust to "float" atop the denser mantle, maintaining gravitational equilibrium as described by Airy isostasy.[12] This buoyant behavior explains variations in crustal elevation, such as mountain roots extending deeper into the mantle to compensate for surface topography.[12]Composition
Chemical Composition
Sial, the felsic component of the Earth's continental crust, is dominated by silica and alumina, with typical major oxide compositions featuring 60-70 wt% SiO₂ and 15-20 wt% Al₂O₃, alongside lower abundances of iron (around 5 wt% as FeO), magnesium (2-3 wt% MgO), and calcium (3-5 wt% CaO).[13] This high silica content distinguishes sial from mafic materials, placing it firmly in the felsic category, where rocks exceed 65 wt% SiO₂ and exhibit lighter, more evolved geochemical signatures compared to oceanic basalts.[14] Geochemical models provide refined estimates of sial's bulk composition, drawing from analyses of exposed crustal rocks and sediments. A seminal estimate by Taylor and McLennan for the upper continental crust, which approximates sial, is summarized below alongside a more recent compilation by Rudnick and Gao:| Oxide | Taylor & McLennan (1985) (wt%) | Rudnick & Gao (2003) (wt%) |
|---|---|---|
| SiO₂ | 66.6 | 66.6 |
| Al₂O₃ | 15.2 | 15.4 |
| FeO* | 5.0 | 5.0 |
| MgO | 2.5 | 2.5 |
| CaO | 4.2 | 3.6 |
| Na₂O | 3.9 | 3.3 |
| K₂O | 3.4 | 2.8 |
| TiO₂ | 0.5 | 0.6 |