Geology of the Pacific Northwest
Suggested areas to focus studying:
Make sure you can label the physiographic/geologic provinces on a map of Oregon, and make sure you know the major geologic event
(or a small number of events) that defines each of them.
Coast Range:
55m volcanic islands off OR coast plastered to continent as slab subducted. Filled Colombia
Embankment.
50m Siletz volcanic accretion
Klamath Mnts: 150m accretion has stacked terranes of this region
Willamette Valley: Forearc basin due to dehydration of subducting plate.
High Cascades: 15m cascades look familiar to todays. N America shift, moving volcanic arc to where it is today.
Central Oregon: 65m-10m middle tertiary John Day Formation giving OR its current form.
Western Cascades: 40m debris and other sediments accumulate to form W cascades
Deschutes-Columbia Plateau: 16m Columbia river Basalts/50-250m Missoula Floods.
Blue Mnts: 100m material lopped off [accreted] onto continent through subduction.
High Lava Plains: 16m began at Idaho border. Related to hotspot and CRB.
Basin & Range: “ “
Owyhee Upland: “ “
Know what accretion is and approximately when the different portions of Oregon accreted.
Accretion is the “lopping off” of material onto the continent as the oceanic slab subducts beneath it. Blue mnts
accreted
100m yrs ago. Over past 150m yrs Klamath mnts built by accretion. 55m yrs ago volcanic islands accreted onto continent to form
Coast range.
Know where the major volcanic provinces of Oregon are and approximately when they erupted.
Western Cascades erupted 10m yrs ago, but few eruptions occurred. High cascades still erupting today, since 15m yrs ago.
Coastal range erupted as volcanic islands 150-100m yrs ago. Volcanism on the OR/ID border created the basin and range and high
lava plains.
Know which provinces have extensive sedimentary deposits, their approximate ages, and whether they contain marine or continental
sediments within them.
Klamath Mnts: Extensive sedimentary deposits. Progressively more oceanic in composition more westward. Olivine is
contained ever more in the terranes to the west.
Blue Mnts: 100m material lopped off [accreted] onto continent through subduction. Ophiolite sequences. 5 “exotic terranes”
What is the Yellowstone hotspot and how has it affected Oregon’s geology? How might it be related to the Newberry Volcano and the
basin and Range?
The Yellowstone hotspot is a fixed area of hot mantle plume against the underbelly of the continental crust. As
North
America moves SW, the hotspot remains, and we can see this in the heat flow it has left behind. The moving
North American
plate creates “corner flow.” This thermally weakens part of the lithosphere and causes extensional faulting in the Basin and Range.
Newberry volcano is a counterpart of the hotspot: the volcanism that created the basin and range at the OR/ID border has migrated to
where Newberry and the 3 Sisters are today, but it seems to be slowing as it nears the cascades.
What causes the Cascades volcanic arc, what defines how far from the coast it occurs, how has it moved around through time, and why
is it so narrow in Oregon?
The cascades volcanic arc is create by the subducting plate partially melting due to water within it, and that melt
rising
into magma chambers to create volcanoes. The steeper the subducting plate the closer to the coast the
volcanic arc. The volcanic
arc is narrow partly because of the steepness of the subducting slab and also because
the high cascades sit in a graben, in a zone
of normal faults.
What are the major volcano-related hazards associated with Cascades, and approximately how far from the volcano can they reach.
What volcanoes pose particular kinds of risk?
Lahars pose a great threat. Lahars are mudflows and they can reach up 10s of miles away from their source.
Tephra
or pumice and ash falls is another threat and these can reach, depending upon wind direction and the size of the eruption, hundreds
of miles. Big stratovolcanoes pose great risk of lahars and pyroclastic flow [rock and
gas]. Mt. Rainier in particular poses the
greatest risk of all the Cascadian volcanoes because it has more ice on it
than all the others combined. Were even a minor
earthquake to destablize the mountain, the resulting lahars could
flow and destroy many buildings and homes in the valleys
below. Mt. St. Helens gives a good example of just how
dangerous pyroclastic flows and lahars can be. 52 people were killed
and the landscape of stripped of its vegetation.
What are the different kinds of extrusive rocks (basalt, andesite, and rhyolite) and where would you find them in Oregon?
Richest in silica and lowest density to richest in iron & magnesium and highest in density:
rhyolite, dacite, andesite, basalt, komatitite
Rhyolitic rocks are found at Newberry and at other locations where enormous eruptions occurred. Andesite is found
stratovolcanoes. Basalt is found at the Columbia-Deschutes plateau and at various locations in the cascade range.
[intrusive: granite, tonalite, diorite, gabbro, peridotite]
What are the different kinds of faults (strike slip, reverse, normal) and where would you find them in Oregon?
Strike slip faults are transform faults and are two blocks passing each other. Reverse faulting is when the
at the
hangingwall is
climbing and the footman is falling. Normal faulting is when the footman is climbing and the
hangingwall is falling. Normal
faulting dominates the Basin & Range and causes extension, while most other types of
faulting are unfound in OR. Normal faulting is
also recently merging with the cascade range area.
What are the Columbia River basalts? What is their age, what are they made of (duh), where did they erupt, and how far did they flow?
These are basaltic eruptions that took 1.5m years to erupt from the NE region of OR. They flowed hundreds of miles—all the
way to the Pacific ocean and down the Willamette valley. The eruptions began 50m years ago. They’re basalt.
What is an ophiolite sequence and why do we find them in the Blue Mtns and Klamath Mtns?
An ophiolite sequence is the most common rock sequence; that of the ocean floor. At top are oozes->lava/basalt>pillow
lavas->sheeted dike sequence->gabbro->peridotite. We find these sequences in the blue and Klamath mnts because they were once
part of the ocean floor, but have since been accreted onto the continents.
What are the five major rock groups of the John Day National Monument? Know their ages and environmental significance of the fosils
within them.
Clarno: 54-37m; Indonesian tropical arc. [mudflows preserved fossils]
John Day group: 39-18m; high African savanna
Picture Gorge Basalt group: 16m; Columbia River Basalt sequence
Mascall Formation: 15-12m; low African savanna
Rattlesnake group: 8-6m; familiar grassland
What is Siletzia? When did it form, accrete, and what impact has it subsequently played in Oregon’s history?
A large volcanic sea chain that filled up the Columbia embankment. Began forming during early Tertiary. Sileztia
“docked”
by 50m. Siletzia is a large coherent block of oceanic crust. The accretion of siletzia caused the
subduction zone [and coast line] to
jump west, thus affecting the volcanism and subsequent geologic evolution of
OR from then on. The coast range and basin on the
continental shelf are protected by siletzia. Siletzia has thus
probably caused the block-like motion of all of the forearc. This all ties
into the NW motion of our entire forearc
and the possibility that it may one day end up in Alaska or cause California to break off.
Why does the coast go up between subduction zone earthquakes but down during them? How does this relate to tsunamis?
The subducting plate sticks to the continental crust, and causes the land upwelling. When the slippage occurs, the
subducting plate goes down and relieves the upwelling, causing the land to fall in elevation. This sudden change
also
causes tsunamis which then can have devastating effects on the newly lowered portions, and already low parts
of the coast.
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