Pre-reading for the Field: Volcanism of the Eastern Snake River Plain, Idaho
Summary of Selected Chapters from “Volcanism of the Eastern Snake River Plain, Idaho: A Comparative Planetary Geology Guidebook” (NASA 1977)
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Chapter 1: Introduction (Greeley and King)
-Basaltic volcanism appear to cover substantial areas of the terrestrial planets
-The Snake River Plain (SRP) is similar in morphology to many volcanic regions on the Moon/Mars/Mercury
-SRP is an optimal analogue owing to its good preservation state, lack of forests / heavy vegetation (which would impede radar), and good network of jeep trails
-Study is restricted to central and Eastern sections of SRP
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Chapter 2: Volcanic Morphology (Greeley)
-A VOLCANIC CONSTRUCT* is an accumulation of volcanic products. Examples include stratovolcanoes, basaltic plains, cinder cones, etc.
-Factors influencing the morphology of Volcanic Constructs*:
- a) Eruption style (Strombolian, Plinian, etc.)
- b) Proportion of liquids (lava), gases (volatiles), solids (tephra)
- c) Lava Viscosity (Temperature, composition, volatiles, degree of crystallization, flow character (laminar vs.
turbulent)
- d) Vent characteristics (shape, number, arrangement)
- e) Effusion rate/duration
- f) Pre-flow topography
-Volcanic morphology is largely a function of eruption style; for example:
-BASALTIC FLOODS: produce large volumes of hot, fluid lava that often form LAVA PONDS
-STROMBOLIAN ACTIVITY: repeated eruptions of tephra and pasty lava ejected short distances from the
vent, deposited to form CINDER CONES
-PELEEAN ERUPTIONS: Thick, pasty block lava flows in low volumes, forming DOME volcanoes
-Block lavas often characterized by ‘corded’ large-scale textures
-Volcanic materials include liquids (lava), gases (eg. Steam), and solids (pyroclastics, tephra such as ash/lapilli/blocks/bombs)
– Higher proportion of lava erupted = Lower profile (ie. flatter) volcanic profile (eg. Hawaiian)
-Higher proportions of solid ejecta = Steepening
-High volatile content = fire fountaining / cinder cones / spatter cones
-Low volatile content = high lava fluidity (ie. volatiles remain in solution)
-LAVA VISCOSITY
–>One of the most complex parameters governing lava morphology
–>Controlled by primarily by Temperature / Composition / Volatiles / Degree of Crystallization / Flow Regime (laminar/turbulent)
*Inverse exponential relationship between temperature/viscosity (ie. visc = exp(1/T) )
*Lunar ‘lavas’ (impact melt flows) are more fluid than any terrestrial ones
–>Composition: Governed by felsic vs. mafic content (ie. silica distribution)
–>Silica can form 3D networks that retard flow and increase lava viscosity!
-VOLCANIC VENTS
–>Shape: Central/Point (eg. Craters) vs. linear (fissures)
–>Central vents: spatter cones/ cinder cones / shields / domes
–>Linear vents: Plateaus / plains
–>Fissures feeding flood basalts can be huge: eg. Yakima Basalt (of Columbia River Plateau) is >130km long
–>Each fissure produces one eruption; subsequent fissures with new eruptive material are commonly ~parallel to previous fissures
–>Valley floors typically contain the thickest / most active part of a lava flow
-EFFUSION RATE/DURATION
–>Effusion rate is the primary factor controlling lava flow LENGTH
–>Units = [Lava Volume / Time]
–>ie. High effusion = extensive, ‘simple’ flows (ie. a single cooling unit, such as a flood basalt);
Low effusion = layered, ‘compouned’ flows comprised of multiple cooling units
-BASALTIC LANDFORMS
–>Diverse: range from highly fluid to very viscous flows
-FLOOD BASALTS: Form extensive, thick flows erupted at very high rates from fissure vents, producing
vast basalt plateaus
-VOLCANIC SHIELDS: Produced by high eruption rate, though resulting in lower total lava volumes than for flood basalts
–>Central vents
–>Sporadic eruptions, as opposed to continuous eruptions for flood basalts
–>Thinner flow thicknesses
–>Can have rapid outbursts of high volume, short duration flow
-BASALTIC PLAINS: Combine characteristucs of both flood basalt and volcanic shields
–>SRP epitomizes this: Lava flows 10m thick, erupting both from central vents producing low lava shields (LOW SHIELDS) and short fissures
*Size/surface feature morphology resembles smaller lunar maria
*Lava tubes / Lava flow channels common (as for shield volcanoes)
–>COMPOSITE CONES: Steep volcanoes surrounding central vents
–>Composed of alternating lava flows / tephra deposits
–>Indicates episodic eruption styles (HawaiianàStrombolian)
–>Eruptive phase results in extensive size (10-20km across)
—>Tephra phase produces steep slopes
–>Cinders accumulate ~at angle of repose (~30 degrees)
*Tephra accumulations are WELDED TOGETHER by the episodic lava flows, resulting in the
massive composite cone
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Chapter 3: Basaltic “Plains” Volcanism (Greeley)
-The central Snake River Plain may represent a distinctive style of volcanism, combining aspects of both flood basalts and classic basaltic shield volcanoes; the term ‘basaltic plains’ has been tentatively proposed for this intermediate volcanic regime
-Compound vs. Simple Lava Flows: Compound consist of multiple flow units (5cm-10m thick), while simple consist of single (typically thicker) flow units that cooled as one single body
–> Correlates to effusion rate: low = compound, high = simple
-In the Snake River Plain, predominant flow type is compound, manifested by typically thin units
-Typical features include hummocky pahoehoe, collapse depressions, pressure plateaus, pressure ridges, flow ridges
-4 main types of volcanic constructs in eastern Snake River Plain:
- a) Low shields
- b) Fissure flows
- c) Major tube flows
- d) Intra-canyon flows
-b) Fissure Flows
-Most fissure vents assoc. with rift zones
-Typically compound flows
-COTM flows consist of both aa and pahoehoe flows erupted from fissures along Great rift, covering nearly 1500km^2
-Characteristically, continuous effusion occurs along several km of the fissure
-However, ‘point-source effusion’ may occur either independently or contemporaneously with fissure-wide effusion, producing spatter cones (or cinder cones, as in COTM); or phreatic eruptions along fissures at King’s Bowl, where rising magma encounters ground water
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Chapter 11: Guide to the Geology of King’s Bowl Lava Field (Greeley et. al.)
-Kings Bowl flow amongst youngest, at only ~2100 years old
-Compound lava flow, fissure-fed; erupted as lava sheets from the fissure, as well as from numerous point sources along the fissure (identified by small spatter cones)
-Different parts of fissure active at different times (perhaps separated by as few as a couple hours)
-Kings Bowl flows: cover ~3km^2, consisting of several flow units, some of which ponded as small lava lakes, with individual
flow thickness typically <1.5m; feeder dike exposed in some places
-Includes many fresh basaltic features, including: spatter cones, feeder dikes, drainback of lava into the fissure, squeeze-ups (mushrooms?), ‘grooved’ lava, cross-cutting relations of lava and fractures, overlap relations of flow units, and a phreatic explosion crater, lava mounds
-‘Squeeze-ups’ described as bulbous masses of lava ranging in size from 0.5m to >2m, many being hollows
-Hypothesized to have resulted from molten lava that ‘squeeze’/oozed through crack on the lava lake crust, possibly in response to the pressure generated by the crust’s subsiding
-Others observed to have been impacted by ejecta from King’s Bowl
-Describe some ‘linear squeeze-ups’
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Chapter 14: Geological Guide To Craters of the Moon National Monument (Papson)
-Situated along northern border of Snake River Plain
-Contains many typical features of basaltic volcanism, including Quaternary lava flows, cinder cones, spatter cones, lava tubes, volcanic bombs, tree molds, etc..
-‘Plains’ volcanism dominant
-55 cones w/ associated lava flows + 14 fissures (many with spatter cones)
-27 distinct cinder cones; many other cones, but partly buried by younger flows
-Great Rift (part of Idaho Rift System) passes through COTM as a set of en echelon fissures striking N35W, up to 3km wide
-These fissures are major vents for the youngest lavas
-The entire rift system may be a continuation of the normal faulting that produced the mountain ranges to the N and S of the SRP
-Basalt Geochemistry: Highly evolved olivine basalt of an Fe/alkali-enriched lava series
-Basalt Mineralogy/Petrology: Sunset Cone flow contains phenocrysts of andesite, fayalitic olivine, titanomagnetitte, and fluorapatite in an intersertal brown glass matrix; could imply >80% crystallization of the parent magma
-Pioneer Mountains close to COTM consist of 2 rock types:
- a) Oligocene quartz latite vitrophyres and basaltic vitrophyres (known as Challis Volcanics)
- b) Miocene quartz monzonite to granodiorite intrusives
-All flows (except Highway) contain monoliths (fragments of cinder cones) from 0.5-200m long and up to 25m high
-Highway Flow: Flow units include pahoehoe, aa, and blocky
Wow, I’m impressed you read so much of this book! Do keep in mind, though, that it is now 40 years old, and some of the thinking about this region and volcanism in general has changed. In particular, although you will see many references to “a’a” in the park, I’ve never seen a true a’a there. We are now coming to realize that basaltic lavas can also take a “transitional” form (slabby or rubbly), in addition to the traditional “pahoehoe” and “a’a”. These forms may occur through the mechanical break-up of a partially solidified pahoehoe flow, rather than through the viscous tearing that produces a’a.
And although many of the lavas at COTM look an awful lot like basalt, their composition is much closer to andesite (they have very high silica content). This is a region of bimodal volcanism – you can see rhyolite domes off in the distance – and is thus more complicated than a traditional basaltic lava field (like you see in Hawai’i).
Thanks! Gavin pointed out some of the more relevant chapters. Thanks as well for pointing out how our thinking about the park’s geology has changed over the decades, and the subtleties of its lavas.