Picture of the month, June 2015

Spheroidally weathered basalt from Turkey. (credit: Francisco Sousa)

Spheroidal weathering of lavas, easily confused with pillows, is also found in other homogeneous igneous rocks. It develops from rectilinear joint sets along which the groundwater responsible for breakdown of silicates initially moves. Hydration reactions begin along the joints but proceed most quickly at corners so that curved surfaces begin to develop. The concentric  banding that sometimes culminates in almost spherical relics may involve more than just rotting of anhydrous silicates as the reactions involve volume increases that encourage further rock fracturing. Other factors, such as elastic strain release may also encourage the characteristic concentricity Prolonged, intense chemical weathering leaves isolated, rounded corestones surrounded by saprolite, that can form boulder fields when the softer weathered material has been eroded away.

One thought on “Picture of the month, June 2015

  1. Yes (to different rock types). Although the detailed processes may differ from rock-type to rock-type, and closely concentric exfoliation layers are less developed, rounded residuals can form in sandstones (e.g. Goblin Valley, Utah: http://stateparks.utah.gov/parks/goblin-valley/) and in granites, for which the best known examples perhaps are the Dartmoor Tors. The classic model of their formation, by Linton and Brunsden, is reproduced here: http://www.devon.gov.uk/geo-two-bridges-quarry.pdf. The famous mountain of Montserrat near Barcelona shows spectacular rounded tor formation with rounded residuals and is formed in conglomerates: https://en.wikipedia.org/wiki/Montserrat_(mountain).


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