tectonic extension to the south and compression to the north. The uneven distribution of vents within this forearc suggests a local zone of crustal expansion. Over the last 2.7 million years, the volcanic field has irregularly rotated clockwise and migrated to the northwest at an average rate of ± per year relative to the surrounding crust. This northwest trending is consistent with other faults in the nearby area. The Boring Lava Field represents the youngest episode of volcanism within the Cascade forearc, and while there is no evidence that they were associated with a slab window (a gap that forms in a subducted oceanic plate when a mid-ocean ridge meets with a subduction zone and plate divergence at the ridge and convergence at the subduction zone continue, causing the ridge to be subducted), they likely interacted with the regional mantle wedge. Composition and local setting The Boring Lava Field shows a similar composition to the High Cascades that run through Oregon and southern Washington state, with Pliocene to Pleistocene basalt lava flows and breccias. It was active during the late Tertiary into the early Quaternary. Within the field, lava shows a diverse composition overall, varying from low-K, tholeiitic to high-K, calc-alkaline eruptive products. Some of the low-K tholeiite deposits likely originated from vents closer to the High Cascades, and they are overlain by Boring Lava materials. J. M. Shempert, a geologist at Portland State University, proposed that mantle sources for the two different lava types may be different and that the calc-alkaline sources are more refractory. Like the surrounding High Cascades, Boring Lava Field erupted lava made of olivine basalt and basaltic andesite; these sub-alkaline basalts and basaltic andesite predominate among Boring Lava deposits. The olivine basalt deposits have fine to medium textures, and the basaltic andesite lava flow deposits have relatively little pyroclastic rock in them, suggesting that explosive eruptions were uncommon within the field. Dark gray to light gray in color, Boring Lava produces columnar and platy joints, which can be seen in Oregon east of Portland and in Clark County in Washington state. It is usually phyric, though one sample from Rocky Butte consists of labradorite with olivine phenocrysts that have been transformed to iddingsite. The Boring Lava reaches thicknesses of more than . Boring Lava has a more mafic (rich in magnesium in iron) composition than the nearby volcano Mount Hood, but they are similar in