Soil-water-rock interactions II: The formation of tors in arid and semiarid environments

The prominence of bedrock edifices above essentially featureless, erosional pediment surfaces underscores the crux of the 'pediment problem' that has baffled geologists since 1877(described in the Part I vignette on pediments). Why do tors (bedrock knobs), inselbergs (larger, more complex bedrock edifices), and mountains emerge so abruptly from the pediment surface--forming a 'piedmont junction'--despite the absence of faults or changes in rock type? More generally, why do tors and inselbergs protrude through the sediment blanket of pediments at all (Figure 1)? It is clear that in some environments bedrock knobs and edifices arise from tectonic uplift, or because localized areas composed of more resistant and durable rock eventually become unearthed because they wear away more slowly. However, rock masses protruding from beneath a sediment mantle in lithologically homogeneous and tectonically quiescent landscapes remain enigmatic. Some geologists have proposed a two-stage, 'etching' hypothesis where spatially varying bedrock weathering processes controlled by bedrock structure is followed by stripping of the sediment blanket, exposing competent granitic masses of low joint density (panel A, Figure 2). These bedrock forms may then continue to grow as the bare rock sheds water to its surrounding, eroding sediment surface.

Field observations from disparate parts of the globe--pediment passes in the Mojave Desert, California, and aqueduct tunnels in the Valley of a Thousand Hills in Natal, South Africa--however, reveal no difference in joint density between higher standing tors or inselbergs and intervening bedrock floors. This suggests that structure-related differential weathering is not responsible for tor formation in these environments. Under the two-stage etching hypothesis, these features would be remnants of exhumed, ancient (Tertiary) deep-weathering profiles created during times of greater effective moisture. Exhumation under the two-stage hypothesis invokes the stripping away of Tertiary weathering profiles, presumably caused by vegetation loss and increased erosion rates spurred by the onset of climatic change and aridity in the more recent geologic past. This stripping process would suggest the presence of a wide variety of piedmont slopes and pediment mantle thicknesses in different areas, yet pediment formation persists into the Quaternary. And, pediments exhibit a uniformly thin sediment blanket and gentle slopes (5-10°) that both vary little in magnitude across different pediments. But then what explains the existence of tors and inselbergs if rock type, rock structure, climate change, or tectonics are not to blame?

A likely explanation for the existence of tors in environments where lithologic templates or climatic variability cannot be invoked is that a feedback between sediment transport behavior and bedrock weathering initiates tor emergence and promotes tor growth. How does this work? Let's revisit our description of bedrock weathering and alluvial surface erosion from the Part I vignette on pediments (Figure 3). If sediment surface lowering rates (from erosion) are higher than the bare bedrock weathering rate (hatched area of Figure 3), thinning of the sediment blanket can trigger an instability leading to exposure of bare bedrock. Climate fluctuations, which may manifest themselves as periods of higher effective moisture (decadal to millennial timescales) and changes in local base levels, may cause mantled surfaces to lower at rates exceeding the bare bedrock weathering rate (erosion rates increasing from e to e', which alters the equilibrium sediment thickness on pediments from h_eq to h_eq', Figure 3). With a mantled surface lowering at such a rate, the form of the weathering rate curve predicts that when sediment thickness falls below a threshold value (see ''Instability'' in Figure 3), the sediment blanket will tend towards zero thickness. If this instability is triggered in a spatially heterogeneous pattern related to ephemeral stream patterns, for example, a tor field will tend to develop. Incipient tors will then grow due to accelerated denudation on mantled surfaces compared to bare rock surfaces (shown schematically in panel B, Figure 2). As the tors grow in height, they will also tend to develop the steep sides and angular junctions with the surrounding pediment that characterize classic tor fields found in the Mojave Desert, California, such as those on the flanks of Cima Dome or in Joshua Tree National Park. Subsequent shifts in climate or local base level that restore sediment surface lowering rates less than the bare bedrock weathering rate (e.g., e in Figure 3) will lead to a progressive decrease in tor height, ultimately leading to their disappearance. Tors in these environments thus represent possibly transient features related to fluctuations in climate or local transport conditions. It is important to note that under this hypothesis of tor formation, the invocation of a climatic shift or change in local sediment transport efficiency is only necessary to initiate the dynamic instability that leads to tor formation. A climatic shift that strips the sediment blanket off of the pediment surface is not required to produce a landscape ornamented with tors. In other words, under this hypothesis, tors are not preexisting features of the subsurface that simply require exposure through removal of the sediment blanket on pediments.

Figure 4 shows the result of incorporating the above tor-producing mechanism into a numerical landscape evolution model, as was done to explain pediment formation in Part I of this vignette series on arid landforms. Here, tors form where areas of localized enhanced erosion from stream flow has thinned the sediment blanket on the pediment beyond the threshold sediment thickness in Figure 3, initiating the instability in which bedrock becomes uncovered through this feedback.

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