Volcanic Process and Landforms Intrusive (within the lithosphere) forces dikes, sills (minor intrusions) batholiths, stocks, laccoliths (major) Extrusive (at the top of the lithosphere) volcanic vents, volcanic activity

Volcanic Form = f(Lava Composition) Acid (silica rich) lava: andesite, rhyolite most viscous, don’t flow freely, explode Basic lava basalt flows freely Ocean basins: basalt

Intrusions molten and mobile igneous rock cool and solidify w/o breaching the surface batholith, stock Batholith (pluton) generally granitic produced by hot plumes of rock (diapirs) ascending through cooler country rock Stock solidified remains of magma chambers lopolith: saucer shaped, generally gabbro composition (igneous mafic rock similar to basalt)

Dikes and sills Dike: vertical intrusion (1-10m wide) Sill: horizontal intrusion (10-30 m thick) http://www.nps.gov/history/history/online_books/geology/publications/pp/294-K/contents.htm e.g. Ship Rock. NM is a volcanic neck with dikes radiating from it

Laccoliths

Laccolith: low in height relative to horiz dimension; fed from stock or sill Stock: large vert height, roughly constant dia.; fed from magma chamber Jackson and Pollard, 1988

Mantle Plumes (Hot Spots) Plate tectonic processes drive the majority of Earth volcanism ocean plate spreading at mid-ocean ridges and subduction Not all volcanoes are at mid-ocean ridges or subduction zones Hot spots are “regions of long-lived and extensive volcanism not directly assoc. with plate tectonics”

Hawaii (in the middle of the Pacific plate) Examples Hawaii (in the middle of the Pacific plate) Yellowstone: largest active continental hot spot Hawaiian hot spot (red star) has left a trail of volcanoes as the Pac plate moved WNW. YHS has left an archuate depression of modified cont. crust. Arrow indicates direction of movement of N.Am. plate. Does not explain track to the NW. CRB occurred largely in that NW region 16 Mya depositing 150,000 km3 of basalt. Humphreys and Schmandt, 2011

The plume hypothesis “Hot mantle plumes rise from great depths through the large-scale circulation systems caused by plate tectonics to produce relatively fixed hot spots” narrow column of buoyant rock super-heated mantle plume: a narrow column of buoyant rock originating at the super-heated thermal boundary layer just above the core, and feeds a hot spot. boundary layer: gradient region (vel or temp). Plume rises and decompresses partially melts in the shallow upper mantle creating magma. Magma breaches the surface, flows as lava, solidifies as basalt. b) hot buoyant material detaches from thermal boundary layer, rises faster through its “chimney” than it can push through the mantle, inflates the head and elevates earth surface, decompression partially melts plume head, magma fractures the plate, rises through it, flood basalt is carried away as the plume tail continues to feed a series of volcanoes that become the hot spot track.

Yellowstone hot spot Began 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? began with the most recent flood basalt eruptions in history (CRB) 16 Mya from fissures in NE OR. CRB joined to Ystone by the Snake R plain. The 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?

Inconsistencies Bends in hot spot tracks that don’t correspond to plate motion Absence of uplift (Siberia) distribution of continental flood basalts

How to see a plume Seismic waves Limited by number/location of samples hot partially melted mantle => slower transmission of seismic waves cold/dry mantle => faster transmission Limited by number/location of samples 400 seismometers, 70 km apart across the US changes in ve could be due to anywhere along the sampled track

Yellowstone seismic images Humphreys and Schmandt, 2011 seismic speed at depth of 125 km. wave speed= red= slower, blue = faster. b) vert xs’s below the AA’ and BB’ lines. red is the plume. mantle is thought to be solid until depths of 160-190 km. blobby appearance suggests either plume is disrupted by subducting ocean lithosphere or plume interacts with parts of upper mantle whose minerals are experiencing phase transformations. off track location of CRB and Snake R plain suggest an ascending plume must have intercepted subducting ocean lithosphere or flattened at the base of the N Am lithosphere. Plume is not strong enough to plow vertically through obstructions. Suggests narrow, deeply seated convective plume operating in a more focused fashion than larger scale plate tectonic circulation in which they are embedded.