Volcanic Process and Volcanic Process and LandformsLandforms

• Intrusive (within the lithosphere) Intrusive (within the lithosphere) forcesforces– dikes, sills (minor intrusions)dikes, sills (minor intrusions)– batholiths, stocks, laccoliths (major)batholiths, stocks, laccoliths (major)

• Extrusive (at the top of the Extrusive (at the top of the lithosphere)lithosphere)– volcanic vents, volcanic activityvolcanic vents, volcanic activity

Volcanic Form = f(Lava Volcanic Form = f(Lava Composition)Composition)

• Acid (silica rich) lava:Acid (silica rich) lava:– andesite, rhyoliteandesite, rhyolite– most viscous, don’t flow freely, explodemost viscous, don’t flow freely, explode

• Basic lavaBasic lava– basaltbasalt– flows freelyflows freely

• Ocean basins: basaltOcean basins: basalt

IntrusionsIntrusions

• molten and mobile molten and mobile igneous rock cool and igneous rock cool and solidify w/o breaching the solidify w/o breaching the surfacesurface– batholith, stockbatholith, stock

• Batholith (pluton)Batholith (pluton)– generally graniticgenerally granitic– produced by hot plumes of produced by hot plumes of

rock (diapirs) ascending rock (diapirs) ascending through cooler country rockthrough cooler country rock

• StockStock– solidified remains of magma solidified remains of magma

chamberschambers

Dikes and sillsDikes and sills• Dike: vertical intrusion (1-10m wide)Dike: vertical intrusion (1-10m wide)

• Sill: horizontal intrusion (10-30 m Sill: horizontal intrusion (10-30 m thick)thick)

http://www.nps.gov/history/history/online_books/geology/publications/pp/294-K/contents.htm

Laccoliths

• Laccolith: low in height relative to Laccolith: low in height relative to horiz dimension; fed from stock or sillhoriz dimension; fed from stock or sill

• Stock: large vert height, roughly Stock: large vert height, roughly constant dia.; fed from magma constant dia.; fed from magma chamberchamber

Jackson and Pollard, 1988

Mantle Plumes (Hot Spots)Mantle Plumes (Hot Spots)

• Plate tectonic processes drive the majority Plate tectonic processes drive the majority of Earth volcanismof Earth volcanism– ocean plate spreading at mid-ocean ridges and ocean plate spreading at mid-ocean ridges and

subductionsubduction

• Not all volcanoes are at mid-ocean ridges Not all volcanoes are at mid-ocean ridges or subduction zonesor subduction zones

• Hot spots are “regions of long-lived and Hot spots are “regions of long-lived and extensive volcanism not directly assoc. extensive volcanism not directly assoc. with plate tectonics”with plate tectonics”

ExamplesExamplesHawaii (in the middle of the Pacific plate)Hawaii (in the middle of the Pacific plate)Yellowstone: largest active continental hot Yellowstone: largest active continental hot spotspot

Humphreys and Schmandt, 2011

The plume hypothesisThe plume hypothesis

• ““Hot mantle plumes Hot mantle plumes rise from great rise from great depths through the depths through the large-scale large-scale circulation systems circulation systems caused by plate caused by plate tectonics to produce tectonics to produce relatively fixed hot relatively fixed hot spots”spots”

narrow co

lumn of buoyant r

ock

super-heated

Yellowstone hot spotYellowstone hot spotBegan with the most recent flood basalt eruptions in history (CRB) 16 Mya from fissures in NE OR

Plume arrived at NE OR, flattened underneath the plate, redistributing the volcanism over a large area

But why the NW location of the w. Snake R plain?

InconsistenciesInconsistencies

• Bends in hot spot Bends in hot spot tracks that don’t tracks that don’t correspond to plate correspond to plate motionmotion

• Absence of uplift Absence of uplift (Siberia)(Siberia)

distribution of continental flood basalts

How to see a plumeHow to see a plume

• Seismic wavesSeismic waves– hot partially melted mantle => slower hot partially melted mantle => slower

transmission of seismic wavestransmission of seismic waves– cold/dry mantle => faster transmissioncold/dry mantle => faster transmission

• Limited by number/location of Limited by number/location of samplessamples

• 400 seismometers, 70 km apart 400 seismometers, 70 km apart across the USacross the US

Yellowstone seismic imagesYellowstone seismic imagesHumphreys and Schmandt, 2011