Only the Clear Lake volcanics in California lie as far west in the coterminous United States. Unlike Clear Lake, Boring lava vents have been inactive for at least 300,000 years. The three dated samples show reversed remanent magnetism, but since tens of other determinations of Boring Lava have about equal normal and reversed magnetic polarities, the volcanoes were probably active from at least 2.7 million to less than 500,000 years ago. Lava from Highland Butte and Larch Mountain shield volcanoes from gently sloping plains covering many tens of square kilometers. Below this they were scoured by the cataclysmic Bretz floods from Glacial Lake Missoula around 13,000 to 15,000 years ago.
Boring lava is characteristically a light-gray phyric olivine basalt.
A specimen from Rocky Butte is predominantly labradorite, with phenocrysts of olivine, mostly altered to iddingsite. The volcanoes locally contain scoria, cinders, tuff, tuff breccia, and ash. The setting of the basalt field is puzzling and not understood. The vents lie well west of the crest of the Cascades, and those such as Kelly Butte , Mount Tabor , and the vents in the Portland Hills are in and even west of the Portland basin. The densest concentration of volcanic vents lies west of the town of Boring, where 20 centers occur within an area of about 36 square miles.