Dirt cracking as rock fracture-wedging process in the Mediterranean climate of Victoria, British Columbia, Canada

Dirt cracking physically wedges open fractures in desert bedrock via the synergistic processes of carbonate precipitation and the wetting and drying of clays in fracture fill. We use back-scattered and high-resolution electron microscopy along with ⁸⁷Sr/⁸⁶Sr analyses to find that dirt cracking also occurs in the hypermaritime Mediterranean climate of Victoria, British Columbia, Canada. ⁸⁷Sr/⁸⁶Sr analyses of calcium carbonate from four different sampling locations do not reflect an ocean source, but ⁸⁷Sr/⁸⁶Sr ratios correspond with the known composition of the sampled rocks. Carbonates derived from decay of host-rock minerals precipitate in narrow fractures, widening them. In addition to calcium carbonate, iron carbonate, barium carbonate, and calcium phosphate also precipitate in fractures. Dissolution of silicates, in association with carbonate precipitation, aids in the further penetration of carbonates. Fines falling into fissures includes smectites that undergo wetting (expansion) and drying (contraction) that further promotes fracture widening. Lead contamination, probably from mid-20th century automobile emissions, occurs from heavy metal scavenging by iron oxides. As studies of dirt cracking are few in number, and since the carbonate precipitation process has been observed in Antarctica, monsoonal Asia, and now in a Mediterranean climate, it is possible that dirt cracking is a common rock-decay process in biogeochemical settings dry enough for carbonate precipitation in rock fissures.