We determined block size distributions on the surfaces of Holocene silicic lava flows at the Inyo domes and the Medicine Lake volcano, and studied the development of blocks on the active Mount St. Helens and Mount Unzen lava domes to better understand the emplacement history of young viscous flows. We measured block chord lengths along perpendicular 25 m long transects within vent, jumbled, and ridged morphologic units. Vent regions generally contain the largest average block sizes and largest range of average blocks, whereas ridged areas tend to have the smallest average blocks. Observations at the active Mount St. Helens and Mount Unzen lava domes show that block size distributions reflect stress conditions during flow. High extrusion rates produce small primary blocks and lead to rapid fracturing of the flow surface, whereas low extrusion rates allow large slabs to form in the vent area and lead to less severe fragmentation. A dramatic increase in the size of blocks evident in active vent regions may indicate a significant decrease in eruption rate, and thus could signal the cessation of extrusion. However, if the extrusion rate is too high or the cooling rate too low, a rigid crust and accompanying blocks will not form on an eruptive time scale. Blocks may fracture through mechanical and thermal processes as they move downslope. Most silicic lava flows reach a steady state downslope, where the average block size at the surface remains in the 20–30 cm size range with increasing distance from the vent. Fines (blocks <12 cm) do not accumulate on the flow surface because they slip toward the flow interior through void spaces between surface blocks. We therefore expect long silicic lava flows to have blocky surfaces throughout their lengths, an important consideration for evaluation of planetary lava-flow emplacement.