Abstract

This paper introduces digital processing of back-scattered electron (BSE) imagery as a microscopic approach to measure porosity from in situ dissolution of minerals. Four case studies exemplify this technique. Cases 1 and 2 explore the initiation and maintenance of weathering forms. In case 1, alveoli start in the Sedona area of Arizona when sandstone porosity exceeds ∼32%. This threshold is probably the point at which intergrain cohesion is reduced enough for shear stresses to erode grains. Case 2 examines the maintenance of gnamma pits and polygonal cracks on a basalt boulder on the island of Maui, Hawaii. Rock dissolution progresses while the surface of the rock is preserved under coatings of silica glaze. When a porosity of ∼37%–47% is reached, the weathering rind loses cohesion and spalls. Then, the protective silica glaze starts to accrete again, and another cycle begins.

Cases 3 and 4 involve measuring rates of dissolution over thousands of years. Case 3 concerns rock dissolution in weathering rinds formed on ventifacted aplite boulders. Weathering rinds lost mass for the last 14 k.y. at a rate of 40 ± 15 g/m2/k.y., and for the last 17 k.y. at a rate of 43 ± 16 g/m2/k.y. Dissolution rates increased over time to 67 ± 23 g/m2/k.y. for the last 60–65 k.y. Case 4 addresses the classic topic of which variable is most important in chemical weathering: temperature, precipitation, or microenvironment. In situ measurements of plagioclase dissolution in ∼3-k.y.-old basalt flows reveal that warmer temperatures enhance rates of plagioclase dissolution by about 0.07%/°C, when precipitation and microenvironment are controlled. Plagioclase dissolution increases as precipitation increases at higher elevations, even though temperature decreases. However, microenvironment is a more important control on plagioclase dissolution; organic-rich positions (under lichens) weather two to seven times faster than adjacent organic-poor positions away from epilithic organisms and rock coatings.

Rates of rock weathering are often established indirectly, by measuring the erosion of weathered material. In contrast, this microscopic method measures only in situ weathering. Conventional measures of rock weathering usually involve units, for example, depth of weathering pits or thickness of weathering rinds, that are not readily comparable to other data. In contrast, cases 3 and 4 illustrate that in situ measurements of rock and mineral porosity can yield data on mass weathered per unit area over time. This information is comparable to mass balance approaches in watershed- and soil-based weathering research.

First Page Preview

First page PDF preview
You do not currently have access to this article.