Portland has the largest population of any city in all of Oregon. However, many people
who live in this city are not aware that it is built on a potentially active volcanic field,
much like Naples is in Italy. The volcanic field I am referring to is responsible for
a number of small mountains and hills which interrupt the dense cityscape of Portland.
It is called the Boring volcanic field, and is named after its proximity to the town of Boring.
This video will discuss this volcano, and mention what its future hazards are.
It may seem strange to call the Boring volcanic field potentially active, since it hasn't erupted
in a long time. Yet, geologists like myself can't rule out the possibility of a new volcanic
cone erupting once again, since the field has experienced previously longer periods of dormancy.
The Boring volcanic field describes a series of more than 80 known eruptive
vents over an area of 1500 square miles across both Oregon and Washington.
This vast volcanic field began forming 2.6 million years ago, when magma related to plate subducted
reached the surface about 10 miles southeast of Oregon City. This magma erupted in a tall fountain
of lava, after which lava which it quickly flowed downhill due to its low viscosity. Over the next
200,000 years, additional eruptions from a range of vents created a 9 mile wide shield volcano
known as Highland Butte. Today, the remnants of this volcano are difficult to notice, as
it contains an average slope of only 1.3 degrees. After another small vent erupted to the northeast,
this volcanic field then became dormant for 800,000 years. Then, 1.6 million years ago,
lava erupted onto the surface around what is now the town of Happy Valley. The volcano
which was constructed by the subsequent series of eruptions was called Mount
Scott. The slopes of this volcano were greater than the prior eruption, which indicated that
erupting lava now had a greater viscosity and did not travel as far from its erupting vent.
By 1.3 million years ago, the range of this volcanic field began to expand
outwards in every direction. Instead of being confined to a small northward trending patch
of land, new vents now formed over a much wider stretch of surrounding landscape. One set of these
subsequent eruptions created both Larch Mountain and Pepper Mountain to the east of Corbett.
Both of these shield volcanoes were constructed out of a black volcanic rock called basalt.
Over the next 500,000 years, dozens of new volcanic cones formed,
including some in the state of Washington. During this time, the magma at depth slowly
gained more and more silica as it leached surrounding rock. As a result,
the style of eruption on the surface began to change. Instead of forming shield volcanoes,
new eruptions began to form steeper cinder cones. 300,000 years ago, three of these cinder cones,
thus constructing Mount Tabor, Powell Butte, and Rocky Butte which can be seen from downtown
Portland. Today, at Mount Tabor, you can observe several rock outcroppings of layers of ash and
basaltic andesite which built this volcano. Then, 60,000 years ago, magma moved toward
the surface about 20 miles north of Portland within the state of Washington. This heat caused
an underlying patch of groundwater to suddenly flash to steam. This pressure continued to build,
when suddenly the ground exploded in what is called a phreatic eruption.
This eruption formed a 1500 foot wide crater which is now filled with Battle Ground Lake.
The last known eruption from this volcanic complex occurred 36,000 years ago, and formed a cinder
cone known as Beacon Rock. This cinder cone was then stripped of the majority
of its edifice during a catastrophic flood during the last ice age. What remained was
the original hardened lava core, which was harder than the surrounding volcanic rock.
Since then, no additional eruptions have occurred. Although all of the existing
vents are extinct, the overall Boring volcanic field is classified as dormant.
Any hypothetical future eruption would create a new cinder cone out of lava
and ash, and would occur somewhere within the outline I have marked on screen. Do not worry,
the nightmare scenario of a volcano suddenly growing in downtown Portland is rather unlikely.
Overall, the odds of this field erupting each year is about 1 in 50,000.
I hope that you enjoyed this video. If you would like to request a specific topic, please leave
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