The lava emitted in eruptions at the Tseax Cone is fluid. Its lavas are made of basalt, a common grey to black or dark brown extrusive volcanic rock low in silica content (the lava is mafic) that is usually fine-grained due to rapid cooling of lava on the Earth's surface. Pāhoehoe is found at the volcano, which has a smooth, billowy, undulating, or ropy surface. A pāhoehoe flow typically advances as a series of small lodes and toes that continually break out from a cooled crust. It also forms lava tubes where the minimal heat loss maintains low viscosity. However, there is also basaltic lava at the volcano characterized by a rough or rubbly surface composed of clinker called ʻaʻā. The clinkery surface covers a massive dense core, which is the most active part of the flow. As pasty lava in the core travels downslope, the clinkers are carried along at the surface.