Geochemistry

Caves in 2D

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Science  21 Jan 2011:
Vol. 331, Issue 6015, pp. 265
DOI: 10.1126/science.331.6015.265-b
CREDIT: THINKSTOCK

Vast expansive underground caves start out meagerly as small fractures in carbonate-rich bedrock such as limestones or dolostones. As slightly acidic water, which is undersaturated in carbonate, slowly percolates through the rock, carbonate dissolves according to the direction of flow. Geochemical models, however, predict this scenario to break down at depth because undersaturated solutions have very low penetration depths. Models designed to overcome this discrepancy are successful at explaining cave formation but treat fractures as simplified one-dimensional (1D) objects from which dissolution uniformly propagates, and rely on complicated chemical kinetics. By performing simulations of dissolution using a 2D model, Szymczak and Ladd observed that uniform dissolution fronts generated in just one dimension are mathematically unstable as soon as the fracture has any width. Instead, the model predicts that dissolution along fractures occurs at smaller, highly localized hot spots spaced out along a semi-regular wavelength. These hot spots can eventually develop into channels according to the size and orientation of the fracture relative to the flow direction. Over time, a single channel can eventually grow into a large cave or system of caves.

Earth Planet. Sci. Lett. 301, 424 (2011).

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