Glossary - Fundamentals of the Physical Environment - page 748

Shear stress The tangential stress which acts on a unit
of Earth material and may lead to shearing.
Sheet flow Surface water flowing as a thin, continuous
film rather than concentrated in a channel.
Sheeting structure A fracture or other planar discon-
tinuity formed or exhumed in rock mass by the removal
of overlying material accompanied by elastic strain release .
Shield The crystalline or high-grade metamorphic core
of stable continental crust or craton .
Shoaling The alteration of wave height, form and
velocity through sea-bed friction as it enters shallow water,
transforming it into a breaking wave .
Shoaling wave An incoming wave experiencing sea-bed
effects or shoaling .
Shore-normal Moving at right-angles to the shore, i.e.
directly landward or seaward.
Short-wave radiation The radiant energy emitted by the
sun at wavelengths below 3 μm.
Sichelwanne A crescent-shaped scar on a glaciated
bedrock surface marking the removal of a flake by an
abrading rock or by high-pressure meltwater.
Sierra A high mountain range or cordillera charac-
terized by jagged, saw-tooth peaks.
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

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 worldwide 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
metamorphism 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.
Solution The change of state of a solid or gas into a
liquid by mixture with a solvent which forms an important
chemical weathering process.
Solvent A fluid capable of forming a solution with a solid
or gaseous substance.
Species diversity Number of difference species of a
defined taxonomic group in a given area. It is synonymous
with species richness .
Species richness The number of species of a defined
taxonomic group in a specified area.
Specific retention The volume of water retained by an
aquifer after gravity flow, sometimes measured as a ratio
of that volume to the volume of rock.

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 alka-
linity, calculated by dividing the content of exchangeable
sodium by the square root of the sum of exchangeable
calcium and magnesium.