StraboTools

Quantification of field observations is an essential step in making them reproducible and shareable, but field geologists have few tools for quantifying field observations of important features such as foliation intensity, crystal alignment, vesicle elongation, joint intensity, and mineral proportions. Here we describe a mobile app, StraboTools, which offers two ways to rapidly and objectively quantify these variables. The edge fabric tool examines grayscale gradients in a photograph and summarizes them with the edge fabric ellipse. For deformation of a homogeneous material with passive markers, this ellipse tracks the strain ellipse. Edge fabric ellipses can be determined on the outcrop and make quick work (5 seconds) of formerly time-consuming and subjective strain-analysis tasks (e.g., Fry and Rf /? analysis). They are remarkably sensitive to subtle deformations that are difficult to see by eye. The color index tool determines the proportion of any component in the photograph whose grayscale level can be isolated (e.g., dark minerals in a granitic rock, feldspar phenocrysts in a lava, or blue epoxy in a thin section). Estimating proportions by eye has poor precision and accuracy; the color index tool is both accurate and precise if a suitable rock face is available. These tools can be used with photomicrographs and aerial photographs as well as in the field.

Field geologists have long awaited the development of a geo-tricorder that would give a rock's crystallization age, chemical composition, mineral composition, etc. in the field. Although this device is just a gleam in some developer's eye at the moment, we have developed an app, StraboTools (ST), that is a short step in this direction. ST currently contains two main tools. (1) The color index tool allows the user to make rapid, precise, and accurate measurements of the percentage of dark minerals (color index) or other components of a rock that can be isolated based on their grayscale value. The color index correlates strongly with bulk composition, so easy field determination allows mapping of, e.g., pluton zoning without laboratory analysis. (2) The edge fabric tool quantifies fabric in an image by examining the orientations of grayscale gradient vectors and determining an ellipse representing their orientations and magnitudes. For homogeneous deformation, this ellipse tracks the strain ellipse. The edge fabric tool can pick up remarkably subtle fabrics and can be used to rapidly quantify fabric orientation and intensity on the outcrop. Two more tools are currently in development. (3) The mode tool uses unsupervised classification to determine the modal percentages of minerals in photos where different minerals are distinguishable based on color. In testing on granitoid rocks, it reproduces observed modes with surprising accuracy. This tool may also offer a method for mapping pluton zonation in the field. (4) The grain size tool uses autocorrelation to produce a representative grain size (or grain cluster size). Asking the grain size of a plutonic rock is an ill-posed question because the different crystal species generally have lognormal distributions with different means and dispersions; the grain size tool offers an objective measure of this important parameter.