High Lava Plains
Area Between Blue Mountains, Basin and range, and Cascade Volcanoes
Bounded and cut by Faults
Moderate overall relief with little rain…great for geologist.
Primarily feature is volcanics
Created by extension processes
Age of Volcanics…17 mybp to present
Lots of really cool volcanic features (more in a bit)
Generally is thought of as Northern boundary of Basin and Range
no fault blocked mountains…only some large fault zones
Brothers Fault zone
Eugene-Denio zone
Walker rim zone
Green Ridge-Sisters zone
Volcanoes in this region
Pliocene through Holocene in age
Most built through Pliocene and Pleistocene with more recent activity capping a few of them
Many Volcanoes in this area are Bi-modal
Start as basaltic (mafic) but evolve to felsic
Age of volcanoes get younger as they go to the west.
Duck Butte: 12my Glass Butte : 4.9 my
Burns Butte: 7.5my Fredrick Butte: 3.9my
Squaw Butte: 5.7my China Hat: 0.9
Volcanic Features
Devils Garden:
Basalt complex
Lava tubes
Cinder cones
Pahoehoe flows
Some ‘a’a flows
Fissure Vents
Volcanic Maars:
Hole in the Ground
1 mile in diameter
Filled with debris
Big Hole
1 mile in diameter
Fort Rock
1/3 mile diameter
See lake levels (tuff ring eroded by Pleistocene lake)
Silicic Volcanism
Pumice Cones
China Hat
Cone in center of Paulina Lake
Obsidian Mountains
Glass Butte
Big Obsidian Flow
Newberry Volcano
Erupted originally as a shield volcano
One of the largest in Contiguous US
Occurs at the intersection of three fault zones
Many lava tubes, intercanyon flows, cindercones etc….
Over time lava changed to a more silicic composition….WHY??
Ideas on how volcanoes evolve
Longer residence time for magma
As volcanoes grow larger, magma stays in system longer
Magma can crystallize, fractionate and stratify
Silica rich component on top and erupts
Change in the source for the magma
Combination of both processes
Newberry’s caldera
4-5 miles in diameter
2 lakes: East lake and Paulina Lake
Lakes separated by central caldera eruptions
Ages of Newberry activity
Central Pumice Cone: 6400 yrs
Cinder Cones (NW rift): 5800-6200
Lava Butte: 6150
Lake obsidian flow: 6000 yrs
Big Obsidian flow: 1400 yrs
Snake River Plain
Down warped basin
Eastward progression of age of volcanics ending at Yellowstone
Bimodal volcanism
Possibly related to a hotspot pushing through continental crust
Many Cauldera type eruptions
McDermitt cauldera 1st evidence
15-16 mybp
Chain of Silicic cauldera eruptions Ending at Yellowstone
Basalt volcanism of Snake River Plain
Basalt tops silicic volcanic deposits
Created by rifts or fractures in crust
½ dozen major rifts form across axis of snake river plain
Great Rift
Very well exposed…extend ~60 miles
Craters of the Moon National monument
15,000 to 2100 years ago
cinder cones, shields, pahoehoe, aa, tree molds, tubes, spatter cones, bombs, intercanyon flows
Lava flows of this time changed course of Snake River
Lava dammed water at the narrows of Hells Canyon of border and created ancient lake Idaho
150 miles long and 50 miles wide
Find old shoreline by looking at deposits, fossils and pillow lavas
Columbia River Plateau
Two primary feature/themes dominate
Columbia River Basalts (CRB’s) 17-6 mybp
Missoula Floods 1500-1200 ybp
Region is East of Cascades and defined by area covered by CRB’s
If older rocks exist in area (and they probably do) they are totally covered by CRB’s
Columbia River Basalt Group
Flood Basalts
Covers about 179,000 km2
Erupted during mid to late Miocene
17 to 6 mybp
~40 lava flows total
Thousands of years between flows
Some soil horizons between flows
What caused the formation of the Fissures during this time?
Possible explanations:
Back arc spreading/upwelling
Rotation of Pacific NW
Mantle Plume
Can see a few of the dikes that represent the eruption vents
Created VERY thick lava flows
CRB’S
5 formations (youngest to oldest):
Yakima subgroup
1) Saddle Mountain Basalt
2) Wanapum Basalt
3) Grande Ronde Basalt
Picture Gorge Basalt
Imnaha Basalt
All formations have mutiple flows
Ex// Yakima subgroup has 21 flows
Imnaha Basalts
Oldest of the flows
16.8-17.3 mybp
~5% of total flows
67 flows total
Aprox 1 flow every 7,000 years
Originated from fissures near the Snake and Imnaha rivers in NE Oregon
Grand Ronde Basalt
15.5-16.8 mybp
85% of all CRB’s erupted
120 flows
~ 1 flow every 10,000 years
Flowed into W. Oregon
Originated from Chief Joseph Dikes
Two major members
Picture gorge basalts
Prineville basalts
Top of Grande Ronde basalts is known as the Vantage Horizon.
Soil horizon of ~15 mybp
Tree molds and Carbonaceous material
Often source of petrified wood
Wide spread throughout area
Great marker between Grande Ronde and Wanapum basalts
Wanapum Basalts
14.5-15 mybp
6% of all volume
36 flows (~1 flow every 20,000 years)
Reached Miocene coast by following present path of Columbia River and old path of Columbia river
Flows include
Priest Rapids
Roza
Frenchman Springs
CRB’s in Willamette valley mostly from this group
Portland hills area
West Salem hills
Silver Falls state park
Wanapum basalts responsible for changing course of Columbia river
Columbia River’s Course:
17mybp flowed from Washington, and emptied into Pacific just N. of Newport
15.4 million years ago:
The Ginko flow destroyed drainage and created series of shallow interconnected lakes
14.5 mybp
Rosalia flow forces channel northward 12 mybp
12 mybp
Bridal Veil channel was created as Pomona flow moved river again
6-2 mybp: Bridal Veil channel filled and current course created by folding and uplift
How can each member be identified?
Field methods
Jointing patterns
Magnetic Polarity
Crystal size and texture
Lab Identification
Major element analysis
Minor element analysis
After basalts are in place, deformation begins
Coincides with development of older Cascade mountain range (late Miocene early Pliocene)
Area folded in a N-S compression event which continues today
Creates anticlines and synclines
Mosier syncline
Some faulting also created due to this compression—Green River fault
Green River fault represents uplifted side of Cascade Graben.
Additionally during Pliocene:
Series of small localized volcanic eruptions occur
Black butte (basalt to andesite shield)
Tethrow butte near Terrabon
Squaw back butte etc…. 200 separate lava and ash flows. Fill many stream channels
Deschutes formation deposited at this time
Basalt and ash deposits worked by water
Glaciation begins in Pleistocene
Ice sheets and glacier filled valleys
Glaciers form Loess deposits
Loess
Wind blown glacial flour
Creates Palouse formation in Eastern Col. Plateau region
Very nutrient rich
Up to 100’ thick in places
Reason for rich farm country in this part of the NW.
Ice sheets and glacier filled valleys
Blocked Columbia river and created Grand Coulee dry falls
Caused large floods in Columbia plateau and into Willamette Valley
Missoula Floods
Glacier blocked valley near Missoula Montana
Undercut of glacier allowed flood waters to be released
Moved from MT through WA down through gorge and into Willamette valley
Occurred aprox 50-70 times in 3000 years
15,000 to 12,000 ybp