Mt. Tabor is not considered an active volcano, nor is it classified as dormant; it is scientifically categorized as extinct. This prominent natural landmark, located within the city limits of Portland, Oregon, possesses a distinct conical shape that often prompts questions about its volcanic history. The hill’s form clearly suggests a geologic past involving eruption, which is why it stands out as a unique feature in an otherwise urban environment. Understanding its current status requires looking into the specific type of volcano it is and the broader regional geology.
The Geological Structure of Mt. Tabor
Mt. Tabor is a classic example of a cinder cone volcano, which is the simplest type of volcanic structure. It was formed by a relatively short-lived eruption that built up a steep, conical hill from fragments of solidified lava. These fragments, called scoria or cinders, were ejected from a single vent and piled up around it.
The volcano is a product of the Plio-Pleistocene era, with its last eruption estimated to have occurred around 350,000 years ago. Evidence of its formation is visible where construction revealed layers of volcanic cinders, which were later used in the park’s development. Over the intervening millennia, the original, sharper cone shape has been significantly rounded and subdued by natural forces like wind and water erosion. This weathering process is one of the physical signs that the structure has been inactive for a vast period of geologic time.
Differentiating Volcanic Status
Volcanologists use three classifications to define a volcano’s potential for future eruption: active, dormant, and extinct. An active volcano is one that has erupted recently and is expected to erupt again within a timeframe relevant to human lifespans. A volcano classified as dormant is currently quiet but remains capable of erupting because its magma supply is still viable.
Mt. Tabor is categorized as extinct because its internal plumbing has been permanently shut down and its source of magma has been cut off. The energy and heat required to cause an eruption are no longer present beneath the cone. With no evidence of eruption for hundreds of thousands of years, and no current magma supply connected to the surface, the chance of Mt. Tabor erupting again is considered negligible. This classification provides a definitive answer regarding the risk posed by the individual hill itself.
The Regional Volcanic Field Context
Mt. Tabor is only one of over 80 volcanic vents that comprise the Boring Lava Field, a large collection of small volcanoes in the Portland-Vancouver metropolitan area. The entire field is named after the nearby community of Boring, Oregon, and extends across a large region. While the individual vents, including Mt. Tabor, are extinct, the Boring Lava Field as a whole is classified as potentially active.
The most recent eruption within the field occurred at Beacon Rock approximately 57,000 years ago, indicating that magmatic activity is not completely finished in the region. Unlike major Cascade Range stratovolcanoes, such as nearby Mount Hood, the Boring Lava Field is supplied by small, isolated pockets of magma that rise from the mantle. This monogenetic style of volcanism means that new eruptions, if they were to occur, would likely form an entirely new vent rather than reawaken an old one.
Geologists estimate that a new eruption within the Boring Lava Field occurs roughly once every 15,000 years, based on its long-term history. The probability of an eruption affecting the highly populated area remains very low. However, the presence of the field ensures that the region maintains a low-level volcanic hazard, which is monitored and factored into regional risk assessments.