Volcano plots show a characteristic upwards two arm shape because the x axis, i.e. the underlying log2-fold changes, are generally normal distribution whereas the y axis, the log10-p values, tend toward greater significance for fold-changes that deviate more strongly from zero. The density of the normal distribution takes the form . So the of that is and the negative is which is a parabola whose arms reach upwards on the left and right sides. The upper bound of the data is one parabola and the lower bound is another parabola. References External links NCI Documentation describing statistical methods to analyze microarrays, including volcano plots Description of volcano plots at MathWorks Bioinformatics Statistical charts and diagrams