『Abstract
Spheroidal weathering, a common mechanism that initiates the
transformation of bedrock to saprolite, creates concentric fractures
demarcating relatively unaltered corestones and progressively
more altered rindlets. In the spheroidally weathering Rio Blanco
quartz diorite (Puerto Rico), diffusion of oxygen into corestones
initiates oxidation of ferrous minerals and precipitation of ferric
oxides. A positive ΔV of reaction results in the build-up
of elastic strain energy in the rock. Formation of each fracture
is postulated to occur when the strain energy in a layer equals
the fracture surface energy. The rate of spheroidal weathering
is thus a function of the concentration of reactants, the reaction
rate, the rate of transport, and the mechanical properties of
the rock. Substitution of reasonable values for the parameters
involved in the model produces results consistent with the observed
thickness of rindlets in the Rio Icacos bedrock (≒2-3 cm) and
a time interval between fractures (≒200-300 a) based on an assumption
of steady-state denudation at the measured rate of 0.01 cm/a.
Averaged over times longer than this interval, the rate of advance
of the bedrock-saprolite interface during spheroidal weathering
(the weathering advance rate) is constant with time. Assuming
that the oxygen concentration at the bedrock-saprolite interface
varies with the thickness of soil/saprolite yields predictive
equations for how weathering advance rate and steady-state saprolite/soil
thickness depend upon atmospheric oxygen levels and upon denudation
rate. The denudation and weathering advance rates at steady state
are therefore related through a condition on the concentration
of porewater oxygen at the base of the saprolite. In our model
for spheroidal weathering of the Rio Blanco quartz diorite, fractures
occur every 〜250 yr, ferric oxide is fully depleted over a four
rindlet set in 〜1000 yr, and saprolitization is completed in 〜5000
yr in the zone containing 〜20 rindlets. Spheroidal weathering
thus allows weathering to keep up with the high rate of denudation
by enhancing access of bedrock to reactants by fracturing. Coupling
of denudation and weathering advance rates can also occur for
the case that weathering occurs without spheroidal fractures,
but for the same kinetics and transport parameters, the maximum
rate of saprolitization achieved would be far smaller than the
rate of denudation for the Rio Blanco system. The spheroidal weathering
model provides a quantitative picture of how physical and chemical
processes can be coupled explicitly during bedrock alteration
to soil to explain weathering advance rates that are constant
in time.
Keywords: spheroidal weathering; saprolite; steady state; soil;
erosion』
1. Introduction
2. Geologic setting and observations
3. Spheroidal weathering model
4. Model results
5. Application too observed weathering profiles
6. Model implications
7. Weathering and denudation
8. Conclusions
Acknowledgements
References