5. DISCUSSION

Regolith is continually produced and eroded at Seven Parks

REGOLITH PRODUCTION AND RESIDENCE TIME

The lack of defined stratification and structure in the regolith suggests the disaggregation and weathering processes are occurring in a continual process of denudation, as discussed by Stallard (1995). In essence, there is no well- defined point in time when bare bedrock began to form regolith or when the regolith began to be exported by transport processes.

The well-preserved Creede caldera complex is in a region that has been undergoing rapid regional uplift for the last 5 to 6 My (Curtis, 1975). Given various geological constraints, a conservative estimate of physical erosion in the study area is around 10 m/My, or 10 mm/y, thus, a 1 m thick regolith profile has an average residence time or about 10⁵y.

Mechanical weathering mechanisms expose new rock surfaces, presumably on an annual basis.

ROCK DISAGGREGATION BY SEGREGATION FREEZING

The Seven Parks soils remain frozen 8-9 months per year. Theoretical (Walder and Hallet, 1985) and experimental studies (Hallet et al., 1991; Akagawa and Fukuda, 1994) conclude that fracture propagation in rock by a process known as "segregation freezing" is optimal in systems residing in sustained, slightly sub-freezing (-3^o to -4^o C) temperatures.

This mechanism of crack production does not require saturation of cracks with water, only sustained sub-freezing temperatures and localized hydraulic conductivity.

In the Seven Parks Regolith, the sediment-particle size distribution and XRD patterns of various sediment size fractions are consistent with the rock simply crumbling into phenocrysts and microphenocrysts during regolith formation.

MAGNITUDE OF GEOMETRIC SURFACE AREA PRODUCTION

Petrographic inspection of weathered tuff fragments indicates that permeability within the unfractured rock is relatively low. For chemical weathering to occur during the annual snow melt event, geometric surface area must be created by breaking larger rock fragments into smaller, individual rock and mineral constituents (0.01 - 2 mm), which may be easily accessed by infiltrating waters.

For example, if a 20 mm diameter rock sphere is disaggregated into 50% phenocrysts (0.5 mm radius spheres) and 50% microphenocrysts (0.01 mm radius spheres), the geometric surface area increases by three orders of magnitude.

In the context of the Seven Parks regolith the most important consequence of rock disaggregtion is the exposure of surfaces of highly-reactive, interstitial glass.

EFFECT OF GLASS DISSOLUTION ON INFILTRATING WATER

In contrast to mineral phases, the residual glass phase in a vitric tuff is the most susceptible to dissolution and most likely to affect infiltrating water chemistry (Claassen and White, 1979). The K⁺ -rich glass in the matrix of the Snowshoe Mountain tuff, though volumetrically sparse, is highly susceptible to chemical dissolution, the effect of glass dissolution on natural, infiltrating solutions would be to increase the K⁺/SiO₂ ratios, as observed in the vadose eaters in this study.

Glass dissolution has the effect of increasing K⁺/SiO₂ in infiltrating waters.

USGS home page || USGS WRD National Research Program Web Page
The URL of this page is: http://wwwbrr.cr.usgs.gov/projects/SW_corrosion/GSA_poster/index.html
Comments and feedback: cgunther@usgs.gov
Last modified: Thursday, 16-July-98 15:50:14 MST