Glossary - Fundamentals of the Physical Environment - page 875

Batholith A large mass of intrusive (usually granitic) igneous rock emplaced at depth in
the core of an orogen ; eventual exposure at Earth's surface indicates the removal of
many kilometres of overlying rock.
Beach A gently sloping, concave sand and gravel accumulation occupying the foreshore
and backshore , the product of net onshore sediment movement.
Beach cusp The crescentic component of a transient, sinuous line cut in a beach by
breaking waves and caused by turbulence in the swash zone.
Bed form A feature developed in soft sediment by fluid motion across its surface; it
involves the entrainment or deposition of sediment and is therefore representative of
fluid velocity, flow conditions and sediment particle size.
Bedding plane A planar boundary separating two layers of sediment and marking a
break in the continuity of sedimentation or materials; this discontinuity is likely to be
exploited during subsequent weathering and erosion.
Benioff subduction or B-subduction The downward displacement of oceanic crust into
the mantle beneath continental (or other oceanic) crust by virtue of its greater density,
leading to metamorphism and melting; named after geologists Wadati and Benioff and
less commonly known as Wadati-Benioff subduction .
Benthic Of the sea- or lake-bed environment.
Berm A sand or shingle bank with a steep seaward face and flat top marking the upper
limit of the swash zone on a beach.
Beta diversity The diversity within a defined area which reflects changes between
habitats; measured by the degree of change in a community index along a transect in
the field.
Bifurcation ratio The ratio of the number of streams of one order to those of the next
highest order, providing a measure of the connectivity of the stream network; see also
stream order .
Biocomplexity Biodiversity studies which include social and economic considerations.
Biodiversity 'The variability among living organisms and the ecological complexes of
which they are a part; this includes diversity within species, between species and of
ecosystems' (UN Conference on the Environment and Development, 1992).
Biogenic sediment Sediment produced by the biological activity of living organisms and
consisting wholly or partly of their remains or derivatives.
Bioherm An alternative name for a reef which stresses its biogenic origin.
Biomass The total weight of living biological organisms within a specified unit (area,
community, population).
Biome A major ecological community extending over large areas; the dominant plants
have a similar physiognomy.
Biosphere The zone occupied by living organisms at the common boundary of Earth's
lithosphere, hydrosphere and atmosphere and depending for its raw materials on
geological fractionation and photosynthesis .
Biotic climax The interacting complex of plants, soils and animals which develops in a
specified region in response to climate, environmental factors and time.
Bioturbation structure A sedimentary structure, such as a burrow or cast, produced by
the motion or behaviour of living organisms.
Black body An ideal radiating substance which emits and absorbs all the radiation
appropriate to its absolute temperature.