Table T2. Distinguishing characteristics of basalt lava types.
Lava type
Must have
Commonly has
Commonly lacks
Must not have
Pahoehoe subtypes:p-type pahoehoe,spongy pahoehoe Smooth (continuous) flow top and base; glassy marginal selvage (0.2-1.5 cm thick); vesicular upper crust (15%-60% vesicles); lower vesicular crust (10%-50% vesicles) 0.3- to 80-m flow thicknesses; inflation features (e.g., tumuli); thick massive interior (0%-5% vesicles); compound flow lobes; segregation structures (e.g., vesicle cylinders) Angular and stretched vesicles Autobrecciation
Pillow lava Smooth (continuous) flow top and base; glassy marginal selvage (0.2-1.5 cm thick) Concentric microvesicular zones; pipe vesicles; compound flow lobes; intercalated with hyaloclastite Macroscopic vesicular zones
Slab pahoehoe Autobrecciated flow top; slabs of broken pahoehoe surfaces Aa and pahoehoe clasts in breccia; thin basal breccia

Spiny pahoehoe Continuous top and bottom spinose surface High degree of crystallinity
Autobrecciation
Rubbly pahoehoe Autobrecciated flow top; broken and intact pahoehoe lobes; coherent vesicular crust below breccia; lower vesicular crust Massive interior; distorted by rounded vesicles; smooth pahoehoe base Well-defined vesicular zones Basal breccia
Aa Autobrecciated flow top; slabs of broken pahoehoe surfaces 2- to 5-m flow thickness; clasts entrained within the core; core pushing into the flow-top breccia; 5%-20% vesicularity of clasts and core; minor eolian sediment infill Round vesicles; inflation features; segregation structures Smooth pahoehoe surfaces

Note: Modified from Keszthelyi (2002).