Static breakage of granitic detritus by ice and water in comparison with breakage by flowing water

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ABSTRACT

Temperature cycling between −4·5±1·0°C and 13·0±2·0°C in the presence of added water caused significant breakage of granitic detritus. Because cycling with only adsorbed water present also caused breakage, fragmentation is attributed to both the combined action of ice and adsorbed water and to the latter acting alone. Breakage evidently resulted from the gradual tensile opening of pre-existing cracks, weakening then splitting grains. Surviving unbroken grains show evidence of a fatigue effect. Quartz split dominantly along pre-existing subplanar microfractures whereas feldspar and biotite split along crystal cleavages.

Degree of breakage and product size distribution depend on the crystalline nature of the parent material, its previous history, and the nature and duration of the breakage process. With the first two factors the same, size distributions from adsorbed water breakage alone and from that due to adsorbed water plus ice differ slightly. Both contrast strongly with those of simulated fluviatile breakage. Whereas the latter preferentially produced 2–20 μm particles (probably debris of inter-particle collisions), static breakage split coarser grains wherever major weaknesses occurred, producing less selective product size distributions with greater proportions of loess-sized material (about 20--60 μ). Characteristic inflections in the size distribution curves of our experimentally produced debris are also shown by samples from the sola of some frost-affected soils.

Partially healed microfractures in plutonic quartz are normally spaced at about 1–10 μ—approaching the downward asymptotic comminution limit for brittle solids (about 1 μ). Surficial physical processes are capable of reducing only a small proportion of plutonic quartz to this size before its storage in sediments.

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