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Differential weathering effects and mechanisms

The physical and chemical characteristics of the two differential weathering effects, case hardening and core softening, are examined to determine their formation mechanisms by investigating several field areas exhibiting differential weathering effects. The terms differential weathering effects, factors, mechanisms, processes, morphologies and their cause and effect relationships are defined in the context of the overall problem.

Because differential weathering effects are defined on the basis of spatial variations in relative and absolute hardness, a portable field instrument has been developed to measure rock hardness as manifested in the abrasion resistance of the material.

The design and operation of the instrument as well as results from standard materials are discussed in light of abrasive wear theory. The way in which the instrument removes material appears dominated by abrasive wear mechanisms. However, the concept of hardness implied by such mechanisms is profoundly different for rock than for homogeneous materials, and the effective hardness calculated for rock material using this instrument is more insensitive to mineralogy than expected, and is sensitive to the character of the intergranular bond.

At the first locality, Valley of Fire, Nevada, cavernous weathering of the Aztec Sandstone results from the differential weathering effect of case hardening. The case-hardened crust is an induration phenomenon consisting primarily of host rock, calcite cement, kaolinite and finegrained quartz. The calcite occurs in a wide range of concentrations (0.001 to 5.0 wt%). The hydrated calcium borate, colemanite, was also found as a non-cementing hardening agent on two outcrops and can be used as a tracer constituent. In all cases kaolinite and quartz were the major constituents of the indurating materials by weight and are necessary components of the crust. Eolian deposition and interaction with meteoric water were determined to be the primary differential weathering mechanism within the Valley of Fire.

At Catavina, Baja California, tonalite exhibiting cavernous weathering is found to be core-softened. Soft cores are more chemically weathered than the exterior rock as indicated by higher kaolinite contents. Hematite formed from the leaching of biotite occurs in coatings on rock surfaces, but the hardening effect of the coating is insignificant compared to the core-softening of the interior. The hardness, measured by the abrasion resistance hardness tester, is inversely correlated with kaolinite content in the tonalite. A one-dimensional water flow model was developed for core-softened, cavernously weathered boulders, and indicates that during infiltration and dessication the moisture flux through a boulder's surface is greatest at the interior cavern wall because of changes in the hydraulic conductivities induced by core softening.

The differential weathering effects developed in the Ferrar dolerite within the Labyrinth of the Dry Valleys, Antarctica are caused by two different mechanisms. The primary mechanism is precipitation of a brown, iron-stained silica coating in the exterior rock of outcrops and joint blocks. This is also true for the case-hardened Beacon Sandstone. Precipitation can occur in the rock's outer few millimeters to centimeters, thereby decreasing the exterior rock's permeability, and consequently its susceptibility to chemical weathering. The coating's effect on a dolerite block's internal moisture regime is modeled for the case of saturated flow, and shows that the contours of the pore water flow mimic subsequent morphology. Weathering of material underlying the coating results in core softening of the dolerite. In dolerite blocks of intermediate size, expansion of the interior owing to weathering can cause the less weathered outer zone to separate into an array of polyhedral cracks. Further weathering and removal of the underlying rock by the combined action of hydration, salt weathering and eolian processes leads to the development of cavernous weathering.

A less common differential mechanism occurs in the bottom of the Antarctic Labyrinth troughs in which an eolian polish develops on rock surfaces exposed to the austral winter winds. Development of the polish protects the underlying material with similar, but less dramatic effects, than accompanies the presence of the silica coating.

Exposures of the Bishop Tuff in the Mono Basin exhibit the differential weathering effect of case hardening. Early devitrification along joint planes to an average depth of 1 cm greatly increased the resistance of the joint faces to weathering over that of the joint block interiors. The absolute and relative hardnesses between interior and exterior change systematically with exposure age, and cavernous weathering results only on outcrops with long enough exposure ages, on the order of ten to twenty thousand years.

The Towel Creek Tuff in Cottonwood Basin, Arizona, weathers into peculiar forms: conical-shaped tepees which show cavernous weathering as a result of case-hardening by calcite precipitation in the exterior rock. Calcite contents of different materials are observed to vary directly with the abrasion hardness of the material. Basal surfaces are formed at the base of the tepees by heterogeneous fluvial erosion and the cavernous hollows are initiated in these zones. Although infiltration of meteoric water into the tepees occurs through all surfaces, moisture flow during dessication of the tepees occurs primarily through the basal surfaces and the lower cavernous hollows. Equilibrium aqueous chemistry limits the interior rock's carbonate content, but calcite can accumulate at the rock exterior.

Because of the overall differences in the intergranular bonding character between crystalline materials such as granite and clastic materials such as sandstone, the results of this study indicate that crystalline rocks tend to core-soften whereas clastic materials case-harden. Clastic materials will be affected by redistribution of secondary cements and greater accumulation at an interface can result in case hardening. In clastic rocks therefore, the hardness of different areas can either increase or decrease with time. On the other hand, a crystalline rock in a weathering environment will have its intergranular and intragranular bonds disrupted by chemical alteration. Spatial variations in disruption can result in core softening or case softening, but the hardness of all areas will decrease with time. Accumulation of secondary cements can often enhance differential effects in crystalline rocks but without case hardening the rock.

Differential weathering effects and mechanisms