Soils developed from volcanic ejecta have been extensively studied throughout the world in most climates, with little attention given to formation in cool, aridic climates. This study was conducted to determine secondary mineral synthesis in soils formed from volcanic materials in the cool, dry climate at Craters of the Moon National Monument and Preserve, Idaho. This area is the largest dominantly Holocene basaltic lava field in the contiguous United States. Eruptive periods occurred from 15,000 to 2,100 yrs BP with as many as 3,000 yrs between eruptions. A chronosequence was selected that comprised 3 cinder cones aged 2,100 yrs, 6,000 yrs, and 15,000 yrs BP. Particle size, x-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, and selective dissolution analyses were performed on bulk soil samples. Throughout a 1-m profile, coarse fragments range from 18-94% cinders, with the remaining fraction consisting of finer particles of volcanic ejecta that have important edaphic implications for plant growth parameters. Both crystalline and amorphous iron and aluminum oxides were identified in the less than 2 mm fraction of soil samples from all cinder cones. Greater percentages of these secondary weathering products were associated with the older cones, likely due to the time necessary for weathering to occur. The weathering sequence along this chronosequence involves primary minerals including olivine, pyroxenes, and volcanic glass that weather into varying amounts of allophane, imogolite, iron oxides, and metal humus complexes. Craters of the Moon tephra have a different suite of secondary mineral products than other Andisols throughout the region due to differences in magma composition.