Monthly Archives: October 2013

Geological colors

Making maps with GMT is fun (well, some might argue…), what is not fun is to find color palettes for GMT (*.cpt files) which have the “official” colors for geological time periods coded up against absolute ages as proposed by various geological time scales. The International Commission on Stratigraphy has all their time scales on their webpage, downloadable as PDF files. The different stages are assigned color values according to what apparently the Commission for the Geological Map of the World (CGMW) has proposed. At some stage they even produced a PDF which had both, RGB and CMYK color values for the different subdivisions of the time scale (could not find this anymore when I recently searched for it), but now they only have a giant image on their website which specifies the color assignment.

Such a color assignment is very helpful if you want to make maps in vectorgraphics illustrations by hand and you only have a few polygons to color in. It becomes a massive pain, however, if you want to automate map making, for example, when generating a sequence of paleo-tectonic maps using GMT. So what is needed here is to have that color information not locked up in an image file or as unstructured text, but rather as data file where an RGB value is assigned to a stage/period/eon (as categorical color palette) so that one can use this information in other applications – such as GMT or GPlates, but potentially also QGIS or ArcGIS or Inkscape/GIMP/Photoshop.

The same goes for information about magnetic polarity chrons which are used in plate tectonic reconstructions – a simple assignment of normal/reverse magnetisation to either white or black colors using a certain geomagnetic polarity timescale.

Here are some examples of how the color scales look like when plotted with GMT’s psscale:

GTS 2012 - Eons

GTS 2012 – Eons

GTS 2012 - Eras

GTS 2012 – Eras

GTS 2012 - stages

GTS 2012 – stages

And the geomag timescale plotted is here:

Snapshot of Gee and Kent (2007) magnetic polarities plotted using the GeeK07.cpt file

Snapshot of Gee and Kent (2007) magnetic polarities plotted using the GeeK07.cpt file

I have made a set of CPT files (available as Git repository on my BitBucket site) for the GTS 2012, GTS 2004, and Exxon 88 geological time scales as well as the Gee and Kent (2007) geomagnetic polarity time scale. The files are available under a Creative Commons Share-Alike Attribution License. A zip archive of the data can be downloaded directly through this link. The files should also be incorporated into the fantastic CPT-CITY website over the next weeks.

If you feel like contributing to this work, just fork the Git repository or send me text snippets for the color scales by email.