Aa - consists of blocks [q.v.] and fragments of lava with rough, jagged, angular surfaces that are found in fissured flows.
Ablation - is the removal of the surfaces of ice or snow from a glacier or snowfield by melting [q.v.] sublimation [q.v.], and/or calving [q.v.].
Abrasion - is the physical wearing-down and/or grinding-down of a surface through friction and/or through impact by material carried in air, water, or ice.
Abyssal plain - is a flat seafloor area, usually the upper surfaces of land-derived sediment [q.v.] that accumulates in abyssal depressions, at a depth of 3– 6km, generally near to a continent from which the sediment is sourced. The larger plains, those to be found in the Atlantic, are hundreds of miles wide and thousands of miles long.
Accretion - is the growth of the continental masses [q.v.] over geologic time via the addition of marine sediments [q.v.]. These sediments are added on to the edges of the continents through tectonic [q.v.] collision with other oceanic or continental plates [q.v.].
Acid rock - is any igneous [q.v.] rock containing more than 66% silica [q.v.], typical of granites [q.v.] and their finer-grained equivalents; q.v. basic.
Aeolian Landform - (or eolian landform) is a landform shaped by wind, involving the erosion or deposition of weathered surface material. This includes landforms with some of the following geomorphic [q.v.] features: sand dunes, deflation hollows, and desert pavements.
Aeration - is a state in which all the pores of a rock or other substance are filled with air or other similar gas.
Agate - is a siliceous [q.v.] rock with alternating bands of chalcedony [q.v.] and variously coloured chert [q.v.].
Agglomerate - describes a pyroclastic rock [q.v.] in which fragments of volcanic and other rocks lie in a matrix of finer grained ash [q.v.] or tuff [q.v.], produced by the force of explosive volcanic eruptions.
Aggregate - is a collection of, or a consolidation of disparate elements, a mass of minerals formed into a solid rock.
Alluvial fan - is the land equivalent of a delta [q.v.]. It is composed of a mass of silts [q.v.] sands [q.v.], and gravels [q.v.] deposited by a river at a point where it moves from a steep gradient to a flatter gradient following a braided [q.v.] path and suddenly losing transporting power.
Alluvial Terraces - are flat elevated ledges built-up from unconsolidated [q.v.] alluvium on the slopes each side of a stream channel. They are formed when a stream down cuts into its flood plain [q.v.].
Alluvium - consists of sediments [q.v.] that have been carried by a stream or other water flow. The water deposits unconsolidated [q.v.] [q.v.] terrestrial sediment composed of sorted or unsorted sand, gravel, and clay.
Aluminium - is the most abundant metal [q.v.] in the Earth’s crust [q.v.], and is a major constituent of clays and feldspars. Its chief commercial ore [q.v.] is bauxite.
Aluminosilicate - is a silicate [q.v.] containing aluminium [q.v.] especially one in which aluminium replaces some of the silicon [q.v.] in the ion [q.v.] SiO4; specifically any of the numerous minerals of this kind, e.g. the clay minerals and feldspars [q.v.].
Amorphous - describes anything that has neither definite form nor structure.
Angle of repose - is the natural angle at which a slope settles without continuing to fall. Whatever the size of the particles, from fine-grained soil or sand or snow to large blocks of scree, the particular sediments will lie on the slope without tumbling further down, until some external factor like rainfall or climbers disturbs them.
Angular unconformity - is an unconformity [q.v.] in which the bedding planes of the rocks above and below are not parallel; that is to say, the beds above and below the unconformity dip [q.v.] at different angles.
Anticline - is a fold in a rock sequence that is (or that was at some stage of its development) concave downwards (convex upwards) – which is to say, all the strata [q.v.] slope downwards from a crest that may be circular, thus forming a dome, or they may be linear, thus forming a ridge [q.v. syncline].
Apparent dip - refers to the inclination [q.v.] of rock beds [q.v.] that you see in a rock face not at right angles to the strike [q.v.].It is always less than the true dip [q.v.].
Aquifer - is a layer of rock or soil able to hold and/or transmit quantities of water. Aquifers often contain a water supply for their area.
Arch - occurs where cliffs are of resistant rock and wave action attacks any lines of weakness, such as joints [q.v.] and faults [q.v.]. Sometimes the sea cuts inland and forms a cave; and if the erosion [q.v.] deepens the cave further, and/or if the same weakness is attacked from the other side of the headland, an arch may be formed. When the roof of the arch falls, the result is a stack [q.v.].
Archaean - relates to, or denotes the earlier part of the Precambrian [q.v.] era, and is characterised by the absence of life.
Arête - is a sharply chiselled, knife-edge ridge formed by glaciers grinding away on both sides of a ridge e.g. Crib Goch in Snowdonia.
Arkose - is a sandstone [q.v.] containing much detrital feldspar [q.v.] as well as quartz [q.v.], chiefly in the form of microcline [q.v.], which gives a pinkish colour to the rock.
Ash - is the name applied to a pulverised, fine-grained, pyroclastic [q.v.] rock blasted out of a volcano, with a maximum particle size of less than 4mm.
Ash flow tuff - is lava-like rock that breaks itself into pieces as it flows whilst solidifying; q.v. tuff.
Asthenosphere - is the zone in the Earth’s mantle [q.v.] that possesses plastic, malleable properties. It is located below the lithosphere [q.v.] at between 100 and 200km.
Asymmetric rock knob or hill - is a bedrock [q.v.] form with a gentle slope on one side created by glacial abrasion [q.v.] and a steep slope on the opposite side created by glacial plucking [q.v.]; q.v. also roche moutonée.
Atmosphere - is the envelope of gasses surrounding the Earth. Ist character in any particular time and place is a major determining factor in the creation of landscapes, being an agent of either or both erosion and deposition. It both carries different materials and removes them. The term weathering [q.v.] is a clue to this… q.v. geoclimatology.
Attitude - is the orientation of a planar [q.v.] or linear feature in three-dimensional space. Planar features that are not horizontal, such as tilted strata, are described by their strike [q.v.], or the azimuth [q.v.] of the intersection of the plane with a horizontal surface, and the dip [q.v.], or the magnitude of its inclination from a horizontal reference-line. The trend and plunge of linear features, such as the axis of a fold, describe the azimuth of the line and its deviation from horizontal.
Attrition - is the act or process of gradually wearing out, especially by friction or abrasion.
Aureole - is the zone surrounding an igneous intrusion, in which contact metamorphism [q.v.] has taken place.
Avalanche 1 – snow - is a mass of snow and ice tumbling down a mountain with extreme speed and pressure.
Avalanche 2 - rock - (otherwise known as debris-flow) is a fast-moving, turbulent mass movement with a high content of both water and rock debris. The more rapid debris-flows rival the speed of rock-slides [q.v.].
Azimuth - is the horizontal angle or direction or dip [q.v.] along a particular compass bearing.
