Friday Rock Blogging: Caliche
I’ve previously rockblogged about the crusty, water-soluble minerals known as evaporites. But you don’t need open pans or puddles to produce this kind of mineral precipitate – drying out shallow soil will work just as well. When this happens, dissolved calcium carbonates (et al.) will coat all kinds of surfaces and infiltrate all kinds of voids within the soil, from pebbles (shown here) and fractures to partly buried tree trunks. Often, repeated cycles of wetting and drying will produce a layer of evaporated gunk that cements the soil grains together just a few inches below the surface; this layer is commonly known as hardpan, or caliche. The crusty gunk itself is also called caliche; though some people reserve the term for calcium carbonates, others don’t care. Calcite is the commonest precipitate anyway. Then there are the people in India who call it kankar, which just makes me giggle.
One of the virtues of caliche is that it keeps a record of the kinds of chemical isotopes present in the water it came from. In an article in this week’s Science, Prosenjit Ghosh et al. use the isotopic record from old caliche nodules in Bolivia to deduce the uplift history of the Altiplano (or “eastern Andes” if you want). They claim that the Altiplano rose fast, about 1 mm/yr over a period of 4 million years, which is of tremendous interest to those of us who get paid to think about mountains. But since it’s seminar time, I’ll just leave it as… hardpan: not quite so shitty as gardeners would have you believe.