Correction to: Nature Geoscience, published online 14 August 2017.

In this Article, owing to a technical error, the Greek letter μ is missing from the scale bar measurements given in the caption for Fig. 2b-e; they appear as ‘50 m (b); 20 m (c); 1 m (d); 1 m (e)’, but should be ‘50 μm (b); 20 μm (c); 1 μm (d); 1 μm (e)’. We apologize for this error and any confusion caused.