The volcano's relatively steep slopes and dome-like shape led some early researchers to conclude that the volcano is built from viscous, silicic lavas or pyroclastics, rather than fluid basalts. However, more recent studies based on data from the Mars Odyssey, Mars Express, and Mars Reconnaissance Orbiter (MRO) spacecraft indicate that Tharsis Tholus is probably a basaltic shield or a shield with a stratocone component. CRISM data from MRO show that lava exposed on the caldera scarp is composed of high- and low-calcium pyroxenes. Olivine is present in abundance on the west flank of the volcano, but it appears to be present only in the dust that mantles the volcano. No phyllosilicates (clay minerals), sulfates, or iron oxides have been detected.