Weathering

3 parts to the “process of erosion”

G.K. Gilbert in the of “Land Sculpture” chapter of his Report on the Geology of the Henry Mountains first published in 1877: “The three general divisions of the process of erosion are 1) weathering, 2) transportation, and 3) corrasion. The rocks of the general surface of the land are disintegrated by weathering. The material thus loosened is transported by streams to the ocean. In transit it helps to corrade from the channels other material.”

Transport Limited Case

Weathering Limited Case

Weathered Profile

Regolith – from rhegos (blanket) and lith (rock).

Mobile regolith

Saprolite

Regolith

Saprolite – can be dug/augured, but retains rock structure; has not undergone strain. Presence indicates a lack of bioturbative activity and little or no hillslope transport.

Weathered Rock – fractured, chemically weathered, but not mobilized.

Mobile Regolith – detached, material in motion. Note strain profile. Can be called “soil” if segregated into horizons.

Strain Profile from the Oregon Coast Range

Strain calculated from concentration of Zirconium (Zr), an element which is unaffected by chemical weathering processes. The profile shows a 300% volumetric expansion within the mobile regolith layer. Near the surface, the jointing becomes closer to surface parallel, illustrating the "inflation" direction of the weathered rock layer.

Variability in Appearance of Weathered Profiles

Variations in thickness and degree of alteration of the weathered profile reflect differences in the rate at which material moves through the reactor and the rock weathering rate.

The Critical Zone The "heterogeneous near-surface environment in which complex interactions involving rock, soil, water, air, and living organisms regulate the natural habitat and determine the availability of life-sustaining resources". It is considered to occupy the region from the top of the canopy to the base of the groundwater. The CZ is a giant reactor, in which rocks are bathed in rainwater-derived solutions enriched in organic exudates and charged with dissolved gases (CO2). New water enters the reactor with each rainstorm. Chemical weathering reactions proceed because the minerals are out of equilibrium with the solutions. Products of the reactions (clays, oxides, solutes) are carried away in solution. Biologic agents (e.g. microbes, plant roots, burrowers) promote weathering. Within the upper portion of the reactor, temperature variations drive thermal expansion of minerals (water/ice), that crack open flow conduits, allowing the solution to access more unweathered surface area.

Denudation – Calculating via mass loss measurement

Denudation is the stripping of a covering of something, and in the case of geomorphology, we can consider this as the mass lost from a landscape both as solids and solutes (time rate of mass loss per unit area): Rivers rout these erosion products off of the landscape, so for any drainage basin, we can examine the concentrations of dissolved load and particulate load in the trunk river to obtain an estimate of steady-state denudation of a landscape. Blackboard Example

Is Dissolved Load a Significant Component of Landscape Denudation?