Mount Tabor is a prominent landmark in Portland, Oregon, known for its city park status and historic open-air reservoirs. This elevated feature offers panoramic views of the city, often prompting a geological question: Is this urban hill truly a volcano? The answer is yes. Mount Tabor is a genuine volcanic feature, making it one of the few volcanoes located within a major United States city.
Classification as a Cinder Cone
Mount Tabor is geologically classified as a cinder cone, a relatively small and simple type of volcano. This structure forms from a single vent where gas-charged lava is violently ejected into the air. The lava fragments quickly cool and solidify into porous, lightweight rock pieces called scoria or cinders before falling back down around the vent. This accumulation of pyroclastic material builds a steep-sided, cone-shaped hill that is generally circular or oval at the base.
The composition of a cinder cone differs significantly from towering stratovolcanoes, such as nearby Mount Hood, which are built from alternating layers of lava flows and ash. Mount Tabor is composed mostly of loose, fragmental material. This structure explains why the feature is small, reaching an elevation of 636 feet, with its shape clearly defined by the angle of repose of the cinders. Volcanic cinders confirming this composition were discovered in 1912 during park renovations.
Part of the Boring Volcanic Field
Mount Tabor is one of more than 80 small volcanic vents that collectively form the Boring Volcanic Field (BVF). This extensive field stretches across the Portland-Vancouver metropolitan area. The BVF is known as a monogenetic field, meaning each vent erupted only once. Other local features, including Powell Butte, Rocky Butte, and Kelly Butte, are also cinder cones belonging to this field.
The BVF is distinct from the larger Cascade Range volcanoes, which are linked to the Cascadia Subduction Zone. The volcanism is considered tectonically anomalous for its location west of the main Cascade arc. Geologists suggest the eruptions are related to regional crustal extension and fracturing, allowing magma from the mantle to rise locally. Activity began around 2.7 million years ago, producing mafic lavas like basalt.
Extinct Status and Future Activity
Mount Tabor is classified as an extinct volcano, a designation based on its age and the nature of its formation. A volcano is considered extinct when it has not erupted in the past 10,000 years and is not expected to erupt again because its magma source has solidified or been cut off. This is a firmer status than being merely dormant, which describes a volcano that is currently quiet but still capable of future activity.
The last eruption within the entire Boring Volcanic Field occurred approximately 57,000 years ago at Beacon Rock, well before the 10,000-year cutoff for active status. Cinder cones are typically monogenetic, meaning they erupt once and then the conduit connecting the magma chamber to the surface seals off permanently. Consequently, the probability of Mount Tabor erupting again is essentially zero. While the overall Boring Volcanic Field could experience a new, localized eruption from a different vent, the frequency of such events is extremely low.