The Snake River Plain – Yellowstone Volcanic Province stretches from northern Nevada to present-day Yellowstone in a ~600 km volcanic belt across southern Idaho, USA. It has been the site of numerous very large rhyolitic explosive eruptions, and countless basaltic lava effusions.
Several aspects of the volcanism have made it the focus of intense research: (1) it is the youngest, and best-preserved example of an intra-plate large igneous province that has generated voluminous rhyolite, with widespread preservation of pristine glasses and ash shards; (2) the rhyolitic products are different (see below) to typical Plinian-ignimbrite eruption products elsewhere, leading to the term ‘Snake-River-type volcanism’. The styles and mechanisms of the unusual high-temperature eruptions are not fully understood. (3) The province is the best-known example worldwide of a trangressive intra-continental hotspot, where the focus of magmatism has been shifting systematically with time, from northern Nevada eastwards to its present location beneath Yellowstone. (4) The large igneous province holds the most voluminous low delta-Oxygen deposits in the world, and the origin of this isotopic signature remains controversial. Finally, (5) repeated explosive eruptions from the province have been of exceptional magnitude: Yellowstone is well-known for its three recent super-eruptions, but a much more protracted history of repeated super-eruptions from mid-Miocene times is now emerging.
Research at the University of Leicester and elsewhere, is underway to figure out how many super-eruptions there have been, how large they were, and how frequently they occur. The results of such enquiries should give us a much better idea about the frequency and environmental impact of super-eruptions globally, for example on world climate and biota.
A ‘hotspot’ in geology is a location on the Earth, typically away from the margin of a tectonic plate, that is the focus of intense magmatism. The location of a hotspot may remain fixed with respect to the Earth, so as a tectonic plate slowly moves (plate tectonics) the hotspot leaves a linear trail of volcanism across it, as at the Hawaiian islands.
Yellowstone is currently the site of an intra-continental hotspot. It is the site of a large thermal anomaly and volcanic activity. But about 16 million years ago the Yellowstone hotspot lay in northern Nevada, where ~3900 km3 of silicic magma erupted contemporaneously with effusion of Columbia Plateau flood basalts. It then slowly migrated eastwards, across southern Idaho, to its present-day volcanic field at Yellowstone. This eastward migration left a track of thick volcanic deposits in an unusual linear basin: this is the Snake River Plain. The eastward shift of volcanism can be explained by the westward migration of the North American plate above a geo-stationary hot, mantle plume (a hot convectional upwelling within the mantle) although other explanations have been proposed.
A pulse of rhyolitic volcanic activity in the central Snake River Plain between 13.0 – 10.4 Ma coincided with extensional opening of the West Snake River Rift, and generated widespread ash fall layers, vast rheomorphic ignimbrites and unusually large blocky lavas. Explosive eruptions of this type, or on this scale have not been witnessed by mankind, but we can learn about these catastrophic volcanic events from the geological record. This is important in order to assess their impact on the environment and on global climate.
The plume theory for Yellowstone – Snake River Plain volcanism is well established, although such continental magmatism differs from that of oceanic hotspots in several respects. Magma generated by lithospheric melting accumulated within the middle part of the crust in the form of a linear, composite basic sill beneath the Snake River Plain. This loaded the continental crust, producing a linear topographic basin (the Snake River Plain, filled with volcanic deposits) and the intense heat of the large volume of mafic magma caused melting and differentiation, with generation of rhyolitic melts in the upper crust.
At each successive location along the Snake River Plain volcanic activity seems to have initiated with major rhyolitic explosivity, closely followed at several centres by rhyolitic lavas. As the base of the rhyolitic successions within the Snake River Plain are not seen, it is not known whether or not undiscovered earlier basalt effusion preceded the rhyolitic activity.
Basaltic effusions continued for several million years after the rhyolitic eruptions had ceased at most locations along the plain, and the basalt lavas bury the rhyolitic volcanoes along the axis of the Snake River Plain. This concealment means that the rhyolite volcanoes are poorly understood: they are thought to be large-diameter, nested explosive caldera volcanoes, as seen elsewhere in the province (e.g. at Yellowstone), and each probably had a broad, low-relief or even a basinal topography, rather than a tall volcanic edifice.
Mid-Miocene rhyolitic eruption products in the central Snake River Plain are sufficiently atypical of silcic eruptions elsewhere, that the term ‘Snake River-type’ volcanism has been coined. Five characteristic features of Snake River – type volcanism are as follows: (1) Thick, parallel-laminated ash fall layers composed of coarse bubble-wall shards: pumice lapilli seen in Plinian eruptions worldwide, are scarce. (2) Unusually intensely welded and rheomorphic ignimbrites, in which both lithic and pumice lapilli are remarkably scarce. The ignimbrites are commonly flow-banded, rather than eutaxitic (with fiamme), and appear ‘lava-like’ in hand specimen, even far from source. They develop upper autobreccias, but can be distinguished from true lavas by their non-brecciated bases and gently tapering distal margins. (3) The presence of unusually long (≤40 km) and voluminous (10 – 75 km3) rhyolite blocky lavas. Rhyolite lavas elsewhere are commonly shorter, stubby lava domes and coulées. (4) High (>900 0C) magmatic temperatures, and (5) the most voluminous low-delta Oxygen volcanic rocks on Earth.
The magmatic processes and eruption mechanisms of Snake River–type eruptions are currently topics of considerable research interest. One reason for this is that Snake River eruptions had devastating consequences to the regional and, probably, global environment. Single eruption volumes exceeded 1000 km3. Fountaining explosive eruption columns generated radial ground-hugging pyroclastic density currents that carried searing-hot, partly molten rhyolitic droplets rapidly across the landscape, sterilising and enamelling it: some individual outflow ignimbrites exceed 60 m in thickness and cover thousands of km2. Fossil grass-like plant imprints can be found in glass on their undersurfaces, where the density currents seared the vegitation and baked the substrate soils to form bright red terracotta.
Large-volume explosive eruptions involving 10s to 1,000s km3 of magma are the most catastrophic form of volcanism and can cause abrupt regional obliteration and climatic perturbation. Given their size, high-temperature and unusual eruption products, the particular effects of those in the Snake River merit further research.
Mike Branney (principle investigator), Marc Reichow and Tom Knott at the University of Leicester (UK) with collaborators Rob Coe and David Finn (University of California, Santa Cruz), Michael Storey (Roskilde, Denmark) and elsewhere are investigating the sizes of individual Snake River Plain eruptions, by correlating individual deposits regionally. This is financially supported by the Natural Environment Research Council (NERC) and ICDP. The aims are to quantify the number of super-eruptions in the province, and the eruption frequencies (and hence magma productivity) through time. The ignimbrites closely resemble each other, and robust correlation is only possible employing a combination of techniques: field logging to define and characterise the individual eruption-units; geological mapping; and characterisation of each eruption-unit by whole-rock (XRF) and glass (LA-ICPMS) trace-element chemistry, mineralogy, and rock magnetism (TRM) together with high-resolution geochronology.
Snake River rhyolite lavas are exceptionally long and voluminous blocky lavas . They have a similar chemical compositions and mineralogy to the ignimbrites, and like some of the ignimbrites they have glassy (‘vitrophyre’) tops and bases, zones of lithophysal nodules, and thick microcrystalline (‘lithoidal’) centres, with complex flow-folds and sub-horizontal, ramifying platy or ‘sheeting’ joints. They may be distinguished from the abundant lava-like rheomorphic ignimbrites in the central Snake River Plain by their overall thick, lobate form with stubby, blunt flow margins, and the presence of well-developed basal autobreccias up to several metres thick. At individual eruptive centres the rhyolite lavas tend to post-date the ignimbrites and they did not flow onto topographic highs.
Rheomorphic ignimbrites are deposits of pyroclastic density currents that are so hot during emplacement that the hot glass particles (or droplets) agglutinate together and deform in a ductile manner during and just after deposition, while the ignimbrite is still very hot and degassing.
Key features formed by rheomorphism include elongation lineations (stretched and rodded, prolate fiamme and vesicles) associated with sheath folds. Also common are oblique fabrics, extreme attenuation and transposition of welding fabrics to form mylonite-like flow banding, with boudinaged fiamme, rotated lithics, and sheared vesicles, and the development of non-cylindrical intrafolial folds. Tops of rheomorphic ignimbrites may develop surface folds, upper autobreccias, and vitrophyre and pumiceous zones (formed by late-stage post-deposition exsolution), whereas basal contacts are typically non-brecciated vitrophyres. Underlying soils are commonly baked, and so emplacement of a rheomorphic ignimbrites effectively sterilises and enamels landscapes.
Rheomorphic ignimbrites are surprisingly common, and form a large component of several igneous provinces in diverse settings (e.g. Pantelleria, Canary Islands, Ethiopia and the East African Rift, North Atlantic Igneous Province, Sardinia, Colorado, Etendeka-Parana, and in ancient volcanic successions in the English Lake District, North Wales, and Glencoe). They are particularly abundant in the Snake River Plain, and there is increasing interest to understand how they are erupted and emplaced.
Latest research findings on Snake River Volcanism from the Leicester Group will be posted here — watch this space!”