spider diagram A useful plot, used in igneous petrology to show variations between two rocks or rock types for a wide range of elements. Usually, one rock is a standard type, e.g. a mid-oceanic-ridge basalt or a carbonaceous chondrite meteorite. Data for the test rock are ‘normalized’ to the standard by dividing the abundance for each element by the abundance in the standard and plotting the quotient, usually in order of atomic number. Elements which have the same abundance in the test rock as in the standard yield a quotient of 1; if the two compositions were identical the plot would be a straight line through 1 on the y axis. Elements more abundant in the test rock yield numbers greater than 1; those less abundant yield numbers less than 1. By connecting the points plotted by a single line, a spidery diagram of peaks and troughs is obtained which illustrates immediately systematic differences in composition, e.g. relative enrichment in incompatible elements.
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