Cascades Volcano Observatory

The Boring Volcanic Field — Hills of the Portland Basin

Boring Volcanic Field map and Portland Basin on a hillshade image. ...

Boring Volcanic Field map and Portland Basin on a hillshade image. Boring Volcanics shaded in orange and blue, vents are red circles, and Cascade-arc volcanic products in purple.

(Public domain.)

Non-enthusiasts may consider the Boring Volcanic Field aptly named, but the title comes from its location, not from a lack of interesting characteristics.

The Portland Basin is peppered with isolated hills and hill clusters rising up to 650 ft (200 m) above the surrounding landscape. Many of these hills are low volcanic shields or small "monogenetic" volcanoes. The monogenetic cones and lava flows formed during single eruptive events. This collection of volcanoes, the Boring Volcanic Field, is named for the community of Boring, Oregon, located about 12 miles (20 km) southeast of downtown Portland.

The Field consists of more than 80 small volcanic vents and associated lava flows dispersed throughout the greater Portland-Vancouver metropolitan area. Additional volcanic centers probably lie buried beneath younger sedimentary deposits from the colossal late Pleistocene (21,000 to 15,000 years ago) Missoula Floods, which would have obliterated or buried small cinder cones, tuff cones, and maars beneath as much as 98 ft (30 m) of slack-water silt.

Beacon Rock (Washington), Boring Volcanic Field, is the central cor...

Beacon Rock (Washington), Boring Volcanic Field, is the central core of a cinder cone that remained intact after its carapace was stripped away by the Missoula Floods about 15,000 years ago.

(Credit: Scott, William. Public domain.)

The earliest eruptions within the Boring Volcanic Field occurred in the southern part of the Portland Basin, south of present day Oregon City. Volcanic activity between 2.6 and 2.4 million years ago produced widely dispersed lava flows, the Highland Butte shield, and several monogenetic cinder cones and lava flows.

After a pause of several hundred thousand years, eruptions resumed about 1.6 million years ago just north of the previous locus of activity. These eruptions produced the Mount Scott shield. Activity then moved to the east into the Cascade foothills where eruptions built the Larch Mountain volcano.

After about 1.3 million years, the area of similarly timed volcanic activity became larger, resulting in a wider distribution of eruptive centers through the basin. By 1 million years ago, volcanic activity had spread to all of the areas were we see Boring volcanoes today.

Beacon Rock is the youngest volcano in the Boring Volcanic Field and was erupted as a large cinder cone about 57,000 years ago. All that is left of the cone is the central plug, as the tephra (cinder) that once surrounded it was stripped away by the Missoula Floods about 15,000 years ago. Battle Ground Lake in Washington, a maarvolcano, was formed when magma encountered water and blasted through a 100,000 year old lava flow. Therefore we know it is younger than the associated lavaflow, and is probably the only other known Boring volcano to be younger than about 100,000 years.

The vents of the Boring Volcanic Field are not randomly dispersed but instead concentrated in clusters of 3-6 vents, which are commonly aligned and erupted similar magma types over short time spans. Basalt and basaltic andesite dominate the field, and andesites are rare but make up the large shield volcano of Larch Mountain.

Boring Volcanic Field volcanism, like that of the Cascade arc, is related to subduction of the Juan de Fuca oceanic plate beneath the western North America plate. However, the tectonic position of the field is unique, located to the west (or trench-ward) of the Cascade volcanic arc, which is defined by large stratovolcanoes such as Mount Hood and Mount St. Helens. The timing and composition of the magma that fed Boring volcanism suggest a genetic relationship with regional rifting that occurred during a similar time as activity started in the region.

Since activity started 2.6 million years ago, it is rare that 50,000 years passed without an eruption. However, all existing Boring Volcanic centers are extinct and the probability of an eruption in the Portland/Vancouver metro area is very low.