File:Vesicular tholeiitic olivine basalt (Boring Volcanic Field, Upper Pliocene, probably ~3 Ma; Crown Point roadcut, Multnomah County, Oregon, USA) 1 (26088896670).jpg

Vesicular tholeiitic olivine basalt from the Pliocene of Oregon, USA. (~3.7 cm across along the base)

Tholeiitic basalt (a.k.a. tholeiite) is a subalkaline variety of basalt. This sample is vesicular - it has a few moderately large-sized holes that were gas bubbles in the original lava. Olivine is present in the form of small phenocrysts. The rock has been described as a "coarse"-grained, olivine-phyric, low-potassium tholeiitic basalt. It comes from a crudely columnar-jointed, paleovalley-filling lava flow in the Boring Volcanic Field that represents a probable fissure eruption near Mt. Hood. Mt. Hood is a subduction zone stratovolcano, as are most of the volcanoes in the Cascade Range, which extends from northern California to Oregon to Washington State to British Columbia.

The Boring Volcanic Field is named after the Boring Hills and the town of Boring, southeast of the city of Portland, Oregon. The Boring Hills are volcanic centers or piles of fluvial gravels. The volcanic field is pretty well defined based on surface outcrops, but there are also some buried vents and subsurface intrusions. The Boring Volcanic Field has scattered, isolated vents and vent clusters - most are monogenetic, but there are some polygenetic vents such as Mt. Scott and Highland Butte. These vents had longer-lived eruptions that typical Boring Volcanic Field eruptions. All edifices tapped the same magma source. The field dates from the Late Pliocene onward. The youngest vent in the field is 57 ka (= a cinder cone that was washed away by Missoula floods, leaving a volcanic plug - Beacon Rock). Accurately dating Boring Volcanic Field samples has been problematic due to fine-grained groundmass and excess argon.

There is a diversity of lithologies in the field - a tremendous compositional variety. Some Boring Volcanic Field basalts are arc-like and some are not arc-like, based on different barium/niobium ratios (Ba/Nb). Mid-ocean ridge-like basalts are present in the field - low-potassium tholeiites. They look like mid-ocean ridge basalts at slow-spreading ridges. There are some calc-alkaline lavas in the Boring Volcanic Field as well - some are very enriched in potassium (arc-related lavas - high-K calc-alkaline basalts).

The Boring Volcanic Field occurs west of the Cascade Range axis. Something anomalous brought magma here, but the volcanic compositions are generally the same as in the Cascade Range proper, so the field is still subduction-related. There might be a discontinuity or a tear in the subducting slab that may account for the field's location. Small vent/edifice sizes in the field imply that magma rose upward quickly. Magma may have moved upward along extensional fractures, formed as a result of Juan de Fuca Plate subduction not being orthogonal with the North American Plate. This has produced transpressional and transtensional features. Subduction has been occurring in this area for ~40 million years.

Age: lower Upper Pliocene, probably ~3 Ma