Strain intensity can be assessed by the shape of deformable sub-spherical objects. Quartz aggregates in granitic rocks, pebbles in conglomerates, and gas vacuoles in basaltic rocks all form with a sub-spherical shape. Following homogeneous strain these spherical objects are changed into ellipsoids. By measuring the axial ratios of those ellipsoids and plotting them on a Flinn diagram, it is possible to determine where the shape of the strain and its intensity.
This method can only underestimate the finite strain as stronger objects record only a fraction of the total strain recorded by the bulk of the rock.
From a 2D section through a population of ellipsoidal objects of axial ratio Rf (sketch on the right), the Rf /Φ method allows the evaluation of: 1) Ri : the axial ratio of the objects before deformation, and 2) Rd : the axial ratio of the finite strain ellipse. This method is extremely useful when the initial shape of the strained objects is an ellipsoid.
In this method the axial ratio of the particles is plotted against Φ, the angle between the long axis of the particle and an arbitrary direction (red line below), usually that of the foliation. When there is no strain (sketch A) Φ varies from +90º to -90º. When the rock is strained (sketch B) the range of Φ decreases and the range of axial ratio of the particles (difference between Rfmax and Rfmin) increases. Ri and Rd are simple function of Rfmax and Rfmin.
