Travertine terracing: patterns and mechanisms - Tufas and Speleothems: Unravelling the Microbial and Physical Controls - page 341

depth, and the travertine morphologies are often
botryoidal or 'stromatolitic' with porous textures,
reflecting a diffusion-dominated regime often with
substantial biological activity. These 'shrubs' are
often assumed to have a bacterial origin (e.g.
Chafetz & Folk 1984) but can probably form from
purely inorganic processes (Jettestuen et al. 2006).
Over the rim and on the steep outside wall of the
terrace, water flows in a thin sheet and flow velocity
increases.

and microterracettes is fairly constant, on the order
of 4 - 5 mm, regardless of slope, thus producing
pools of much larger area in regions of small
slope (Fig. 4). In fact, given a constant height h
and a minimum inter-dam distance k, we can geo-
metrically predict the IDD as a function of slope
angle a:

h
tan a þ k

IDD ¼

Here, the travertine is usually more

compact.
A basic observation is that travertine terraces
seem to display scaling properties, with microterrac-
ettes often having similar morphologies as the
largest terraces. As a working hypothesis we
might postulate that a similar mechanism is respon-
sible for pattern formation at all scales, and that a
continuous coarsening process could lead from
smaller to larger terraces. However, at the smallest
scales surface tension exerts an important influence
on the hydrodynamics, producing 'bags' of water
overhanging the rim and a meniscus at the base of
the exposed drop wall (Fig. 3). In most localities
it can be observed that the height of terracettes

Data given by Pentecost (2005) seem to fit
this simple model well (Fig. 5). Conversely, if the
underlying slope is constant, the IDD will also be
nearly constant. This effect may contribute to the
possible characteristic wavelength which will be
discussed below. The 'unit-step phenomenon' can
be explained by the fact that the surface-tension
meniscus lapping onto the rear wall of the pool
can only reach to a certain constant height, allowing
free flow from the upstream rim over the wetted
terrace wall. Given the temperature control on
surface tension, terracette height might possibly
vary with temperature. At larger scales, surface

Fig. 3. Due to surface tension, 'bags' of water hang over the rims of these microterracettes at the Jotun hydrothermal
spring, northern Spitsbergen. Message board peg for scale.