Download PDF - Speleogenesis

HYPOGENIC CAVE FEATURES
Figure 21. Theoretical development of simple and complex
spheres by condensation corrosion. Simple sphere develops
upwards and laterally (left); an irregularity in the ceiling produces a
new sphere development that fits within the previous one (right);
two neighbouring spheres diverge toward the greatest zone of heat
transfer (right). After Szunyogh (1989) as adopted by Audra et al.
(2007b).
this case; hence buoyant convection is not a contributing
factor. Another interpretation is that cupolas form in
subaerial conditions by moist air convection driven by the
heat from a pool of thermal water in a chamber that is
closed to outside airflow (Cigna and Forti, 1986). Acidic
vapor (especially when H2S is involved) is condensed onto
cooler cave walls. Szunyogh (1989) showed that spherical
pockets could be shaped in this way (Figure 21). Dreybrodt
(2003), Dreybrodt et al. (2005) and Lismonde (2003)
discussed complications with condensation processes
arising from heat release at the wall surface, which slows
down or terminates condensation. A sufficient gradient has
to be maintained to enable continuing condensation. When
this condition is met (e.g. above warm lakes in caves
located close to the surface), the development of cupolas
by condensation-corrosion (especially spherical and semispherical
ones) is a sound possibility. It is likely that this
process re-shapes original cupola-like forms created in
confined/phreatic conditions. The condensation-corrosion
mechanism does not serve to explain cupolas when they
occur through all parts of extensive 3-D systems with
vertical ranges of several hundred meters, including areas
quite far from where warm lakes at the water table could
be presumed (e.g. Monte Cucco in central Italy, Carlsbad
Cavern and Lechuguilla Cave in the Guadalupe Mountains,
USA, etc.).
There are additional arguments as to why the origin of
cupolas by condensation-corrosion should not be applied
too broadly. Cupolas are common in hypogenic caves for
which neither thermal nor sulfuric acid processes are
applicable, such as hypogenic caves in gypsum. Among
caves whose origin involved hydrothermal or/and sulfuric
processes, cupolas are common also in those where no
signs of water table effects are recognizable, such as maze
caves composed of small passages arranged in inclined
stories. In 3-D cave systems, cupola/domepit complexes
often extend upward from a base passage or chamber for
tens of meters and are terminated at, or interrupted by,
differently oriented stories of maze passages with which
the cupola/domepit complexes show clear functional
relationships. Their development due to condensationcorrosion
seems to be highly unlikely in such situations. In
a broader context, the water table model of hypogenic
speleogenesis is discussed in Section 4.5 in relation with
the Guadalupe Mountains speleogenesis.
Traditional interpretation of ceiling half-tubes (ceiling
channels) is that they are paragenetic features formed when
sediment choke of passages directs phreatic dissolution
upward (Ford and Williams, 1989; Lauritzen and
Lundberg, 2000). This is an obvious case in many epigenic
caves. However, half-tubes are commonly observed in
caves that have never been filled by sediments, such as in
most hypogenic caves. Their incompatibility with the
paragenetic model is especially evident in multi-story and
complex 3-D caves where half-tubes occur at different
levels, are connected by rising forms from below, and
connect to cupolas/domepits at higher ceiling elevations.
Pendants are residual pillars of rock between channels
cut into the ceiling. They are traditionally interpreted as
remnants of bedding plane anastomoses (when the main
body of passages had entrenched down) or as pillars
between closely-spaced paragenetic ceiling channels. This
fits well to observations in many epigenetic caves.
However, both explanations are not applicable to many
hypogenic caves where broadly braiding ceiling channels
(creating pendants in between them) best fit to the model
of buoyancy currents rising from multiple feeders at the
bottom.
Rising wall and ceiling channels are sometimes
explained as trails curved by degassing bubbles in phreatic
thermal CO2-H2S systems (Audra et al. 2002). However,
Palmer and Palmer (2000a) noted that the maximum depth
at which degassing to form bubbles can take place is
limited to a few meters below the water table at commonly
observed concentrations of these gases. Rising channels
are widespread in hypogenic caves at all levels within
vertically extended caves. Furthermore, similar rising
channels are common in gypsum caves and caves where
degassing of rising water and enhanced condensationcorrosion
cannot take place.
Widespread occurrence of the above features in the
characteristic suite of forms (which also includes feeders),
in a variety of caves that share a hypogenic origin in the
hydrogeological sense, strongly suggests their interrelated
origin as described in the previous section and depicted in
Figure 19. In hypogenic transverse systems, local
convection cells can develop from even small density
gradients (either thermal or solute) in mature caves under
39