Viscoplastic Models of Lava Domes

Abstract

It is proposed that lava domes possess a yield strength whose magnitude governs their shape and explosivity. Theoretical and laboratory modelling of the spread of viscoplastic material over a horizontal surface provide means of examining geological data on dome growth in terms of dome rheology. Experiments using kaolin slurries show that a growing dome maintains a state of static equilibrium such that its height, H, radius, R, density, ϱ, and yield strength, τ 0, are always related by H = 1.76 (τ 0R/ϱg)1/2, a relationship which is independent of the effusion rate. No concentric or radial structures developed on the models’ surfaces. Instead, slip plane traces spiralled out from the dome summit in clockwise and anticlockwise directions. These traces divide the surface into rhombohedral roughness elements which, although their size and roughness is greatest near the dome margins, have an order of magnitude spacing of τ0/ϱg. Labelled positions on the surface of a growing dome move radially outward but never reach the dome margin. The experiments predict that the distal surface of a growing low lava dome should experience radial shortening and circumferential stretching. The relation H = 5.75 R1/2 describes the growth of the 1979 Soufriere, St Vincent, dome well and implies a yield strength of 2.6×105 Pa. Similar yield strengths are calculated for other low lava domes irrespective of composition, suggesting that the strength of chilled magma forming the dome carapace is 105–106Pa. In contrast, more common Peleean domes are covered in rock debris and are pyramidal in shape such that tan-1 (H/R) = Ф the angle of repose of loose talus surrounding these domes (30°–45°). The talus apron rather than magma rheology governs the dome shape directly as these domes are too strong to spread very far under their own weight, indicating yield strengths of at least 106–107Pa. Pressures considerably greater than atmospheric (105Pa) must accumulate if Peleean domes are to deform. Explosive disruption is typical of Peleean domes but rare at low lava domes. This difference in hazard potential is accounted for by the larger yield strengths of Peleean domes.