Within Oregon, floods are a relatively common hazard which the area experiences after a
heavy rainfall.
However, in the long geologic history of the state, these floods didn't always involve
water, and at one point involved lava.
For a period of several million years, Oregon and Washington were repeatedly buried by floods
of lava which in some locations measured more than 13,000 feet thick.
These floods were part of a rare type of volcanic eruption termed a flood basalt.
Flood basalts represent the largest known effusive eruptions in the history of the planet,
and are frequently correlated with mass extinctions.
This video will discuss these series of eruptions termed the Columbia River flood basalts, which
permanently changed much of the landscape in the United States northwest around 16 million
years ago.
The Columbia River flood basalt eruptions began 17 million years ago when the top of
a mushroom shaped hotspot in the mantle termed the Yellowstone Hotspot permeated a large
volume of magma into the crust.
This hotspot soon began producing dozens of unusually voluminous shield volcanoes in far
western Idaho and eastern Oregon and Washington.
However, activity soon changed as eruptions grew more and more voluminous.
Instead of originating from a single vent, eruptions now originated from massive fissures,
sometimes surpassing 100 miles in length.
The entire length of these long fissures erupted during individual flood basalts, covering
immense amounts of land in molten rock.
Beginning around 16.7 million years ago, the peak phase of the Columbia River flood basalts
began.
During the subsequent period of activity, a 150 kilometer or 93 mile long fissure opened
up in far southeastern Washington State.
Soon, the entirety of this fissure began erupting lava, outputting 2,944 cubic meters per second.
For comparison, this is comparable to approximately 1,000 simultaneous Kilauea volcanic eruptions
in Hawaii occurring at the same time.
Over a period of 14 years, this eruption continued, flooding a large swath of Oregon and Washington
state in molten rock.
By the time this individual eruption had ended, an area equivalent in size to the combined
states of New Jersey, Delaware, and Connecticut had been covered in up to 500 feet thick of
lava.
Individual lava flows from this single eruption traveled up to 200 miles distant from its
source vent, marking a truly impressive eruption.
In total, 1300 cubic kilometers of molten rock was emplaced on the surface in this short
timespan.
Eruptions like the one just described repeated in both Oregon and Washington an average of
once every 3,400 years for the next 1.1 million years.
93% of all of the lava which would be eventually emplaced by this flood basalt was emplaced
during this timespan, after which eruptions became far less frequent.
Individual flood basalts soon began occurring once every 10,000 years, then 25,000, then
50,000 years.
By 15 million years ago, 98% of the total lava which would be output had already been
emplaced.
For the next 10 million years, far less frequent residual eruptions created a sequence of smaller
volume flood basalts, which even sent flows of lava into northern Nevada.
The last such large volume eruption occurred 5 million years ago by which time the Yellowstone
Hotspot was centered in eastern Idaho, as eruptions from its mantle plume were now highly
explosive instead of effusive, creating large supervolcanoes such as the modern Yellowstone
caldera.
During this 12 million year long timespan, 210,000 cubic kilometers of low silica lava
erupted onto the surface, covering more than half of Oregon and Washington in black volcanic
rock.
In total, about 400 distinct eruptions occurred.
Due to the period of time this impressive flood basalt has been dormant, it could be
classified as extinct and thus will never erupt again.
Today, the closest the modern area has to the now extinct ancient flood basalts are
large volume shield volcano forming eruptions such as those which formed Devils Garden and
the Jordan Craters.
Yet, these eruptions are less than 1% of the size of the average ancient flood basalt eruption.
One notable area where the aftermath of these voluminous flood basalts can be observed in
the Palouse River Canyon.
The edges of this canyon display dozens of individual flood basalt lava flows, which
are marked as distinctive cliff forming layers in its walls.
These basalt lava flows only became visible after millions of years of erosion, as the
flow of water carved away at the hardened volcanic rock.
I hope that you enjoyed this video.
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