Columbia Gorge Geology�and the Ice Age Floods
50+ Million Years of Geologic Chaos
It Hasn’t Always Been So Quiet
Lloyd DeKay, Ice Age Floods Institute - Columbia River Gorge Chapter
IAFI.org
About Geologic Time and Geologists
Geologists are basically detectives and storytellers
We think and work in very long time periods
The Earth is over 4.5 billion years old
OR and WA are ~50 million years old
Columbia River is� >20 million years old
The GORGE is only ~18,000 years old
The PacNW is a Jigsaw Puzzle
Terranes include island arcs, oceanic plateaus, oceanic crust, marine and terrestrial sediments, mantle rock (ophiolites) and igneous intrusions
PacNW accretion includes eastern WA terranes from ~175 MYA to Coast Range terranes ~20 MYA
~42-20 MYA
Debris off those volcanos slowly filled the basin behind the volcanic arc
John Day Fossil Beds National Monument
Volcanic islands stretched across Oregon and Washington, similar to the present-day Cascades
Siletzia�terrane
Columbia River Basalt 16.7-6.2MYA
300+ massive eruptions from ~20,000 fissure vents covered central OR & WA with basalt layers up to 3 mi thick
20+ lava flows filled ancient Columbia River channels
Lava flowed at up to 9mph, taking only about one week to reach the coast
Outpourings were associated with early hotspot locations along the Yellowstone Hot Spot track
Rise of the Cascades <5 MYA
Major Volcanism Shifted West Again ~5+ MYA
Columbia River downcut as the Cascades slowly rose
Rise of the Cascades Volcanos
From ~2.5 MYA relatively fluid lavas formed shield volcanoes
Mt Defiance @ sunset
Frosty Underwood Mtn
From ~520,000 YA stiffer lavas built-up strato-volcanoes
Mt Hood
Mt Adams
Last Ice Age ~18,000 YA
An ice Dam blocked the Clark Fork River
Lake Missoula was ~2000’ deep and covered an area equal to Lakes Erie and Ontario combined
Ancient shorelines above Univ of Montana, Missoula
Meltwater and runoff filled
Glacial Lake Missoula
Huge Floods
When the lake reached 2000’ it began floating and undermining the ice dam, eventually causing it to collapse
When the ice dam collapsed it released the lake water in a rampaging flood
Time after time the ice dam rebuilt, the lake reformed, and the ice dam collapsed again
This happened 40 to 120 times from 18,000-12,000 YA
Missoula/Spokane/Bretz Floods
Those Huge Floods, up to 1000' deep, raced across central WA at up to 60+ mph, scouring the landscape, stripping away debris, carving giant coulees, depositing giant gravel bars, and forming vast temporary lakes behind narrow gaps as they roared to the Pacific
Recent Geology
Bonneville Landslide ~1450 A.D.
Bonneville Dam
Bridge of the Gods Legend
Wy’east (Mt Hood)
Pahtoe (Mt Adams)
Loowit (Mt St Helens)
Columbia Gorge - Eagle Creek Fire
But It Was Actually A ‘Good’ Mosaic Burn
Oneonta Tunnel Was Severely Damaged
Fire Images Appeared Catastrophic
Firefighters Saved Multnomah Falls Area Lodge
In the end the Gorge survived, and may even be healthier
40+ Million Years of Chaos
Proto-Cascades
Columbia River Basalts
Mt Washington
Landslides, Earthquakes, Fires… What’s next?
Columbia River Gorge
Ice Age Floods
Cascade Mountains
Questions?
Local Columbia River Basalt Features
Stacked Basalt Layers – Horsethief Butte
Pillow Basalt - Hwy 197, I-84 exit south
Pillow Basalt
Basalt Columns
Basalt Arch – Catherine Creek
Tree Casts at the Syncline
Intraflow Sediment Squeeze-up
This ‘Teepee Structure’ contains petrified wood and baked mudstone.
Similar structures on both sides of the highway roadcut indicate they’re continuous
SR14 East of Chamberlain Rest Stop
Cascades-Related Local Features
Collapsed Lava Tubes – White Salmon River
Folded CRB – Coyote Wall, SR14 east of Bingen
Lava Tubes – Guler Ice Caves, Trout Lake
Plumose Basalt – I-84 @ Cascade Locks
Other Related Features
Ortley Thrust from a Hwy 30 turnout above I-84 east of Rowena
Right-lateral strike-slip thrust fault
Middle fault = 60 m vertical offset
East fault = 150m vertical offset
Thrusting sometime 10-3.5 MYA
Part of the crustal rotation producing Horse Heaven Hills
East
Middle
West
Ortley Strike-slip Thrust Fault
Hood River Pelagonite MAAR
A volcano erupted into water, �explosive steam expansion�blew cinders and lava blobs (bombs) away from the vent forming an inverted cone.
I-84 roadcut just west of Hood River
The rapid cooling and moisture caused the bombs to fracture and cinders to oxidize (rust)
Ancestral Columbia & Mt Hood Lahar
Troutdale Frm <2 MYA
Old Columbia River deposits
Mt Hood Lahar
~100,000 yrs old
Underwood Mtn Volcanics ~.5 MYA
SR14 east of White Salmon River
Kolk Ponds
Discovery Center (The Dalles)
Kolk Ponds
Flood waters in high turbulence areas developed giant whirlpools that could pluck out bedrock, creating depressions that now dot the local landscape.
As the flood waters passed on and receded, they generally deposited gravels that filled the depressions.
Streamlined Bedrock Outcrops
(Horsethief Butte, WA)
Turbulent Water-Cut Benches (opposite the Discovery Center – The Dalles)
Hanging Valleys with�Spectacular Gorge Waterfalls
Multnomah Falls off I-84