by Bruce Rogers » Jul 27, 2007 5:47 pm
Listeros,
As a suppliment to the above comments, I offer this "Short Life History of a Shield."
1. Start with a crack in a cave wall.
2. Cover said crack with a thin flim of calcite to block any seepage, thud building up a hydrostatic head of ground water pressure behind the film.
3. Crack this film via earth tides (the solid part of the Earth also moves in diurnal waves just as the oceans do, but on a vastly smaller scale).
4. Allow mineral-laden water to seep out, boil off carbon dioxide, and deposit another thin film of calcite over said crack, blocking it.
5. Crack open the newly deposited film via next round of Earth tides in concert with ground water hydrostatic pressure built up behind the calcite film.
6. Repeat as necessary to form a circular, disk-shaped, hollow speleothem.
7a. If ground water conditions change, then increased water flow may deposit draperies along the crack to make the shield look like a pararchute - as in Lehman Caves famous Parachute.
7b. As an alternative, if the upper plate of the shield cracks open, then either another shield may form along this crack or possibly a fringe of helictites may form.
If one is lucky enough to view the inside of a broken shield one can see the concentric growth lines as well. If the shield grew tilted up at some angle from the horizon, lines of calcite may form across the inside of the lower disk representing "shore lines," so to speak, of where the mineral-laden water deposited horizontal welts of calcite.
Cheers,
Bruce rogers, Earth scientist on a good day