CRB: Columbia River basalts, aka Columbia Plateau/River lava flows, aka CR flood basalts

            a fine example of a large igneous province, a region of intense magmatic activity

            lava flows made of basalts and basaltic andesites (mafic to mafic-intermediate volcanics)

                other large basalt provinces:  Deccan lavas in India (65 Ma), Parana basalts (200 Ma)

    covers parts of WA, OR, ID presently.  today’s area about 165,000  km²

            at time of formation, probably covered 200,000 km².  small bits along Hood Canal!

    average thickness today slightly greater than 1 km = 2/3 mile

    total volume today around 175,000 km³, original volume larger.

    oldest lava flows approximately 17.5 Ma, most lava flows 15.5-16.5 Ma, youngest 6 Ma

            SWGI: from youngest Saddle Mtn to Wanapum to Grande Ronde to oldest Imnaha

            85% or so erupted in 1 million years or so during Grand Ronde time, very voluminous

    source of lava flows = many many cracks in crust in SE WA and NE OR and W ID

            fissures now filled with cooled magma = dikes.  early feeders buried by later lava flows

Composition and Emplacement and Flow Features

    continental basalts (aka tholeiites) produced by high degree of partial melting of dry mantle

                decompression melting of mantle related to mantle convection

            some of this primary magma gets modified by crystal separation = basaltic andesites

    basaltic magma very fluid because of its low silicon content, spreads out in giant sheets

            individual lava flows average about 20 m = 60 feet thick, but can be thinner and thicker

            individual lava flows can be traced continuously from ID to Pacific Ocean, over 500 km

    Big Burp Model:  basaltic magma pumped out of ground in huge volumes in short time

            individual lava flows created in just a few days

    Inflation/Hawaiian Model:  magma pumped out slower, lava flows fed from behind

            lava flows move forward slowly, lava flows get “inflated” with magma

    regardless, flows show many great features related to cooling, such as gas bubbles = vesicles

            hexagonal cracks propagate up and down and create columnar basalts

            lava enters rivers and lakes, creates pillow basalts and basaltic glass = palagonite

    lavas filled depression centered on Tri-Cities area, weight of lavas helped make depression deeper

Tectonics:  which plate tectonic settings could create this.  think: continental something

    continental rift: like Eithiopia today, northern extension of Basin and Range rifting to south

            N-S oriented feeder cracks:  check.  large volumes basalt:  check.  rhyolites:  NO

            timing is problem, B and R rifting mostly earlier to south

    continental hot spot:  some geologists have pointed the finger at the Yellowstone Hot Spot

                nearby:  check (but see next line).  big volumes:  check.  rhyolites:  NO. 

            would have been way south of feeder cracks at 15 Ma, so must get hot spot north somehow

    continental/oceanic subduction zone:  NO.  no primary intermediate magmas (andesites, etc.)

            basaltic andesites derived from basalts.  way too big for stratovolcano.  sz magmas to west.

    continent-continent collision:  NO.  basalts not produced in c-c collision

    impact model of Alt and Hyndman, based on lunar mare = basalts that fill impact craters

            where’s the evidence?  no impactites from 17.5 Ma, peak magmatism = 16 Ma

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