The historical lavas on Mount Etna provide a large body of data for examining the growth of aa flow fields. A flow field, the final product of an eruption, is composed of one or more flows. Flow generation depends on the ratio SR/SD of crustal retaining strength (SR) to stress driving the lava outward (SD), rate of flow thickening h, and the time for which h is sustained. At low SR/SD, lava advances by simple spreading, and new flows are formed by subdividing about topographic highs (bifurcation). At low h, growing crusts can contain thickening at all values of SR/SD. Overflow of chilled margins can occur at higher sustained h. Increasing SR/SD favors flow propagation by breaching of crusts. Flow growth is examined by extending the Bingham model of Hulme (1974). Cooling and discharge rate along a flow appear to be the main factors controlling the generation of new flows, with the result that long durations and low average effusion rates favor flow fields with large width/length ratios. Bifurcation generates flows randomly. Breaching and overflow generate flows systematically because of their dependence on cooling. With time, therefore, flow field growth evolves from predominant lengthening to widening. Such behavior is evident for Etnean flow fields, which can also be grouped into two populations on the basis of their width/length ratios. These populations may be related to two distinct mechanisms of magma ascent. ¿ American Geophysical Union 1988