LINKS BETWEEN ERUPTIVE STYLES, MAGMATIC EVOLUTION, AND MORPHOLOGY OF LOW-SHIELD VOLCANOES, SNAKE RIVER PLAIN, IDAHO: IMPLICATIONS FOR PLANETARY VOLCANISM

Connections between chemical composition, eruption style, and topographic features of two shield volcanoes on the Snake River Plain, Idaho were examined to see if differences in morphometry were related to magma composition or eruption styles. Such relationships could be used to infer important characteristics of shield volcanoes elsewhere in the solar system.

Despite their similar ages and geographic locations, two basaltic shield volcanoes—Kimama Butte (87 ka) and Rocky Butte (95 ka)—have strikingly different topographic profiles. Kimama Butte has a diameter of 9 km and a height of 210 m. In contrast, Rocky Butte is a broad 36 km wide shield that rises 100 m with less than 1° slopes.

Major- and trace-element compositions of the basaltic lavas are similar at the two shields, but distinct in Ni and Al₂O₃. The ranges in composition can be explained by fractional crystallization at 3 kbar (~10 km deep) involving Pl and Ol (incompatible TiO₂ ranges by a factor of ~2 at each volcano). Compositions of the phenocrysts are also similar at both shields (Fo_80-55 and An_78-52) but show variation with evolution. Calculated temperatures and magma viscosities overlap at Kimama Butte (1238 to 1154°C and 158 to 14 Pa·s) and Rocky Butte (1202 to 1152°C and 75 to 8 Pa·s). Because composition, volatile content, and temperature overlap at the two volcanoes, they are probably not important controls of volcano morphology.

Instead, eruption style is important. The vent crater at Rocky Butte developed as a large lava blister that inflated and then collapsed to form a crater in which a lava lake developed. Little spatter accumulated throughout the eruption. In contrast, high spatter mounds and spatter-fed flows flank the main summit crater at Kimama Butte. These late spatter and spatter-fed lavas have high phenocryst contents (>30%) and calculated magma viscosities extend ~80 Pa·s higher than those for Rocky Butte as result. Thus, we conclude that eruption styles and phenocryst contents play the most important roles in developing low-shield volcano summits, not composition or degree of magmatic evolution. Where eruptions shift from lava lake overflow and tube development to late fountaining with short, spatter-fed, phenocryst-rich flows, steeper, higher shields may develop.