Hot Spot Vocab

flood basalt

regionally extensive, thick accumulations of basaltic rock extruded from fissures tapping a mantle source

dike

fracture-filling intrusion that cuts across existing structure

discordant

sill

fracture-filling intrusion that parallels existing structure

concordant

pahoehoe

basaltic lava that is extremely fluid

as it cools, the surface wrinkles as the interior continues to flow

usually very thin

ropy surface texture

aa

thick blocky lava flow

higher viscosity of the flow results in large chunks of solid lava falling down the front of the advancing flow

rough, jagged surface

silica

SiO4 molecules in lava

silica content ranges widely but is highest in felsic and intermediate volcanic rocks, lowest in mafic volcanic rocks

silica content controls gas escape and relative explosively of eruption

higher silica → more explosive

shield volcano

broad, low, dome-shaped volcano resulting from extrusion of fluid basaltic lava

pillow lavas

lava extruded underwater squeezes out of fractures on advancing flow front

always indicative of underwater extrusion

hot spot

surface expression of rising plume of heat sourced at core

as heat rises, eventually plastic asthenosphere physically rises, melting near surface upon decompression

snake river plain

low area across southern Idaho marking passage of North America over stationary Yellowstone hotspot

columbia river basalt

giant plateau of basaltic lava

one of the thickest sequences of basaltic rock in the world

extruded from fissures in southeastern Washington, spread as far west as central coast of Oregon

may be related to snake river plain-yellowstone hot spot

extruded from 15 to about 6 million years ago

steens mountain

block faulted mountain representing northern extrusion of the Basin and Range in southeastern Oregon

composed of basaltic andesite (steens basalt)

possibly related to Yellowstone hot spot

flow top breccia

fragmented top of a lava flow developed as brittle surface breaks as flow advances

columnar jointing

style of jointing in basaltic and sometimes andesitic lavas developed as surface cools, solidifies, and cracks

cracks propagate downward as cooling proceeds

pedalfer

soil type typical of Columbia River basalt

developed on flow tops and indicative of moist, warm conditions

iron-rich

intra canyon flows

lava flows that occupy pre-existing stream valleys or other topographic low spots

miocene

epoch of Cenozoic era during which snake river plain-steens mountain-columbia river basalt systems began to develop

spans period from 23 to about 5 million years ago

mantle plume

column of heat rising through mantle from source on the core

thought to represent hot spot at depth