Glossary - Fundamentals of the Physical Environment - page 910

Silicates A group of minerals built around the silicate tetrahedron , including the olivine,
pyroxene, amphibole, mica, feldspar and quartz group, and which form 95 per cent of
Earth's crustal rocks.
Silicate tetrahedron The highly stable silicate anion SiO 4 4− which forms the basic
building block of a wide range of silicate minerals.
Sill A tabular igneous intrusion which is accordant with existing rock structures.
Slab pull A gravitational component of sea-floor spreading, drawing cold, dense crust
into a subduction zone .
Slaking The disintegration of Earth materials on exposure to the air or hydration.
Slickensides A rock surface polished by shearing and abrasive removal of surface
roughness during movement along a fault .
Sliding resistance The sum of forces capable of resisting sliding on a slope, which
normally consists of the proportion of the normal stress of a rock or soil mass acting at
right-angles to the slope plus friction.
Slumping A translation failure involving shearing at the upper boundary of a moving soil
or rock mass and its downward rotation along a curved failure surface.
Sodium adsorption ratio ( SAR ) A measure of soil alkalinity, calculated by dividing the
content of exchangeable sodium by the square root of the sum of exchangeable
calcium and magnesium.
Soil An assemblage of loose and normally stratified, granular minerogenic and biogenic
debris at the land surface; it is the supporting medium for the growth of plants.
Soil catena The sequence of soils which occupy a slope from the topographic divide to
the bottom of the adjacent valley.
Soil separate A particle-size fraction of the mineral material in soil, i.e. sand, silt or clay.
Soil Taxonomy The Comprehensive Soil Classification System devised and used by the
US Department of Agriculture.
Soil texture The relative proportions of sand (2·0-0·05 mm diameter), silt (0·05-0·002
mm diameter) and clay (less than 0·002 mm diameter) mineral material in soils.
Soil zonality The concept which views the distribution of soils in world-wide zones
corresponding to climatic regions.
Soil - forming process Any process working to produce a soil from parent material.
Solar wind The outflow of charged particles from the sun that escapes the sun's outer
atmosphere at high speed. It may interact with the Earth to produce the aurora.
Solid solution A single crystalline mineral phase in which one element may substitute for
another without change of phase.
Solid - state recrystallization The reformation of less dense mineral species into denser
forms in regional meta-morphism without melting or other change of phase.
Solifluction A form of mass wasting involving the slow to intermediate flow of loose,
granular materials above their liquid or plastic limit ; often applied more narrowly to
such behaviour in the active layer of a permafrost environment and - incorrectly - to
soil creep .
Solod The original Russian term for solodic planosol .
Solodic planosol An acid soil with an illuvial Bt horizon which results from the
degradation of solonetz .
Solonetz An alkaline soil with an illuvial Bt horizon with a distinct columnar structure.