Cap 8-15 cm broad, convex, plano-convex to plano-depressed in age, even to irregular in outline; margin at first inrolled, then incurved, at maturity decurved to upturned, frequently wavy; surface viscid, often with adhering debris, appearing glabrous, but with appressed fibrils (use hand lens); color of immature unexposed specimens, pallid, becoming cream-buff, lemon-yellow, to tawny, in age brown to reddish-brown at the disc, sometimes over the entire cap except for a yellowish to tawny margin; context up to 3 cm thick, cream to lemon-yellow, unchanging, soft; odor like that of Suillus pungens, i.e. fruity, harsh; taste mild.
Pores 1-2/mm, oval to irregular in shape, pale-yellow in youth, yellowish in age, unchanging when bruised; tubes 1-2 cm long, colored like the pores, depressed at the stipe.
Stipe 4.0-6.0 cm long, 2.0-4.5 cm thick, solid, equal to enlarged or with a sub-bulbous base; surface of apex pale-yellow, not glandular, or if so, sparsely in age; lower portion more or less glabrous, pallid, developing brown areas with handling and age; context fleshy, colored like the cap context; partial veil ephemeral, seen as cottony fibrils in the button stage; annulus absent.
Spores 7.0-10.0 x 3.0-3.5 (4.0) µm, cylindrical to narrowly elliptical in face-view, subfusoid in profile, smooth, thin-walled, hilar appendage inconspicuous, contents granular or with one to several vacuoles; spores brown in deposit.
Solitary to clustered in duff of Pinus jeffreyi (Jeffrey pine); fruiting during the summer months; known from the Mt. Lassen region of the Sierra Nevada, but to be expected through much of the range of Jeffrey pine; fairly common in its preferred habitat but easily overlooked (see comments).
Untried, but likely edible.
Named for the volcanic region where it was described, Lassen National Park, Suillus volcanalis, like many Sierran fungi, develops largely under a thick layer of duff, thus its presence is often betrayed only by humps in the substrate. Fieldmarks include a summer fruiting habit, association with Jeffrey pine, a viscid, yellowish to tawny cap overlain with brownish fibrils, and a lack of glandular dots on the stipe. Suillus brevipes, as noted by Thiers in California Mushrooms, is similar. It also lacks glandular dots on the stipe apex when young, but differs in having a uniformly brown, viscid cap which lacks the yellowish tones and brown fibrils characteristic of Suillus volcanalis.