It has been suggested that mafic volcanism in the vicinity of Yucca Mountain, Nev., is both recent (20ka) and a product of complex "polycyclic" eruptions. This pattern of volcanism, as interpreted by some workers at the Lathrop Wells volcanic complex comprises a sequence of numerous small-volume eruptions that become more tephra-producing over time. Such sequences are thought to occur over time spans as long as 100,000 years. However, paleomagnetic studies of the tephra and lava flows from

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... volcanoes near Yucca Mountain fail to find evidence of repeated eruptive activity over time spans of 10^ to 10^ years, even though samples have been taken that represent approximately 95% of the products of these volcanoes. Instead, the eruptions seem to have occurred as discrete episodes at each center and thus can be considered to be "monogenetic". Dates of these episodes have been obtained by the proven radiometricgeochronometer methods of K-Ar or ^Arft^Ar dating.