Penny Boston <[email protected]> wrote:
>> This "ripple" pattern is observed only on icicles (and stalactites?)
>> that are growing via deposition on the exterior surfaces. Soda-straws and
>> helictites are fed from internal canals (water-drop diameter in straws;
>> capillary size in helictites), and can't be expected to show effects that
>> develop in an external film. Helictites are typically irregularly crooked
>> (thought to be the result of crystal-growth forces), and are not made of
>> regularly-spaced annular rings. In helictites, the nearest thing I have
>> seen to that is the rare "beaded" pattern in which some aragonite
>> helictites are composed of a string of repeating growths of conical clumps
>> of diverging crystals (e.g., in Silent Splendor and Breezeway Cave at
>> Manitou, Colorado). As far as I know, the mechanism that causes beaded
>> helictites has never been explained, so I can't rule out that development
>> of their structure may involve subtle chemical effects. In general,
>> however, helictites and stalactites are often found together, sharing the
>> same water source, and whether a helictite or stalactite grows from a
>> given point seems to depend on whether the source flow is enough to form a
>> hanging drop at the tip (which will make the resulting form a stalactite).
>>
>> ?Donald
>
>Dear All,
>
>I strongly suspect that there is a combination of physical factors
>involved in the production of the ridges, namely fluid flow regime (i.e.
>laminar vs turbulent flow at a very tiny scale) and the effect of subtle
>wind velocities coupled with issues of surface tension and capillarity.
>Once the standing ripple pattern is begun it is probably self-reinforcing
>because of enhanced precipitation on the ridges and less in the valleys.
>I have seen this same ridged pattern not only in icicles and stals of the
>calcite variety but also in soft sediment features of fine particles held
>together with microbial biofilm, microbe bodies, and mineralized micro
>crystals. So I think it is much less dependent on the materials and hence
>chemistry than it is on the physics. However, any additive that would
>have an effect on the physical properties, e.g. surface tension and
>wettability, capillarity, or the like is likely to have an effect like
>salt.
>
>Cheers,
>Penny
Very persuasive remarks.
--Donald
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