Columbia River Basalt Group

The Columbia River Basalt Group is the youngest, smallest and one of the best-preserved continental flood basalt provinces on Earth, covering over mainly eastern Oregon and Washington, western Idaho, and part of northern Nevada. The basalt group includes the Steens and Picture Gorge basalt formations.

Introduction

During the middle to late Miocene epoch, the Columbia River flood basalts engulfed about of the Pacific Northwest, forming a large igneous province with an estimated volume of. Eruptions were most vigorous 17–14 million years ago, when over 99 percent of the basalt was released. Less extensive eruptions continued 14–6 million years ago.
Erosion resulting from the Missoula Floods has extensively exposed these lava flows, laying bare many layers of the basalt flows at Wallula Gap, the lower Palouse River, the Columbia River Gorge and throughout the Channeled Scablands.
The Columbia River Basalt Group is thought to be a potential link to the Chilcotin Group in south-central British Columbia, Canada. The Latah Formation sediments of Washington and Idaho are interbedded with a number of the Columbia River Basalt Group flows, and outcrop across the region.
Absolute dates, subject to a statistical uncertainty, are determined through radiometric dating using isotope ratios such as ⁴⁰Ar/³⁹Ar dating, which can be used to identify the date of solidifying basalt. In the CRBG deposits ⁴⁰Ar, which is produced by ⁴⁰K decay, only accumulates after the melt solidifies.
Other flood basalts include the Deccan Traps, that cover an area of in west-central India; the Emeishan Traps, which cover more than 250,000 square kilometers in southwestern China; and Siberian Traps that cover in Russia.

Formation

Some time during a 10–15 million-year period, lava flow after lava flow poured out of multiple dikes which trace along an old fault line running from south-eastern Oregon through to western British Columbia. The many layers of lava eventually reached a thickness of more than. As the molten rock came to the surface, the Earth's crust gradually sank into the space left by the rising lava. This subsidence of the crust produced a large, slightly depressed lava plain now known as the Columbia Basin or Columbia River Plateau. The northwesterly advancing lava forced the ancient Columbia River into its present course. The lava, as it flowed over the area, first filled the stream valleys, forming dams that in turn caused impoundments or lakes. In these ancient lake beds are found fossil leaf impressions, petrified wood, fossil insects, and bones of vertebrate animals.
In the middle Miocene, 17 to 15 Ma, the Columbia Plateau and the Oregon Basin and Range of the Pacific Northwest were flooded with lava flows. Both flows are similar in composition and age, and have been attributed to a common source, the Yellowstone hotspot. The ultimate cause of the volcanism is still up for debate, but the most widely accepted idea is that the mantle plume or upwelling initiated the widespread and voluminous basaltic volcanism about 17 million years ago. As hot mantle plume materials rise and reach lower pressures, the hot materials melt and interact with the materials in the upper mantle, creating magma. Once that magma breaches the surface, it flows as lava and then solidifies into basalt.

Transition to flood volcanism

Prior to 17.5 million years ago, the Western Cascade stratovolcanoes erupted with periodic regularity for over 20 million years, even as they do today. An abrupt transition to shield volcanic flooding took place in the mid-Miocene. The flows can be divided into four major categories: The Steens Basalt, Grande Ronde Basalt, the Wanapum Basalt, and the Saddle Mountains Basalt. The various lava flows have been dated by radiometric dating—particularly through measurement of the ratios of isotopes of potassium to argon. The Columbia River flood basalt province comprises more than 300 individual basalt lava flows that have an average volume of.
The transition to flood volcanism in the Columbia River Basalt Group, similar to other large igneous provinces, was also marked by atmospheric loading through the mass exsolution and emission of volatiles, via the process of volcanic degassing. Comparative analysis of volatile concentrations in source feeder dikes to associated extruded flow units have been quantitatively measured to determine the magnitude of degassing exhibited in CRBG eruptions. Of the more than 300 individual flows associated with the CRBG, the Roza flow contains some of the most chemically well preserved basalts for volatile analysis. Contained within the Wanapum formation, Roza is one of the most extensive members of the CRBG with an area of 40,300 square kilometres and a volume of 1,300 cubic kilometres. With magmatic volatile values assumed at 1 - 1.5 percent by weight concentration for source feeder dikes, the emission of sulphur for the Roza flow is calculated to be on the order of 12Gt at a rate of 1.2Gt annually, in the form of sulphur dioxide. However, other research through petrologic analysis has yielded SO2 mass degassing values at 0.12% - 0.28% of the total erupted mass of the magma, translating to lower emission estimates in the range of 9.2Gt of sulfur dioxide for the Roza flow. Sulfuric acid, a by-product of emitted sulfur dioxide and atmospheric interactions, has been calculated to be 1.7Gt annually for the Roza flow and 17Gt in total. Analysis of glass inclusions within phenocrysts of the basaltic deposits have yielded emission volumes on the magnitude of 310 Mt of hydrochloric acid, and 1.78 Gt of hydrofluoric acid, additionally.

Cause of volcanism

Major hotspots have often been tracked back to flood-basalt events. In this case the Yellowstone hotspot's initial flood-basalt event occurred near Steens Mountain when the Imnaha and Steens eruptions began. As the North American Plate moved several centimeters per year westward, the eruptions progressed through the Snake River Plain across Idaho and into Wyoming. Consistent with the hot spot hypothesis, the lava flows are progressively younger as one proceeds east along this path. Previous to this eruptive period, it is believed the Yellowstone Hotspot created features like Smith Rock in Central Oregon and perhaps another flood basalt event known as Siletzia which underlies much of the Pacific Northwest coast with exposures in the Oregon Coast Range.
There is additional confirmation that Yellowstone is associated with a deep hot spot. Using tomographic images based on seismic waves, relatively narrow, deeply seated, active convective plumes have been detected under Yellowstone and several other hot spots. These plumes are much more focused than the upwelling observed with large-scale plate-tectonics circulation.
The hot spot hypothesis is not universally accepted as it has not resolved several questions. The Yellowstone hot spot volcanism track shows a large apparent bow in the hot-spot track that does not correspond to changes in plate motion if the northern CRBG floods are considered. Further, the Yellowstone images show necking of the plume at, which may correspond to phase changes or may reflect still-to-be-understood viscosity effects. Additional data collection and further modeling will be required to achieve a consensus on the actual mechanism.

Speed of flood basalt emplacement

The Columbia River Basalt Group flows exhibit essentially uniform chemical properties through the bulk of individual flows, suggesting rapid placement. Ho and Cashman characterized the -long Ginkgo flow of the Frenchman Springs Member, determining that it had been formed in roughly a week, based on the measured melting temperature along the flow from the origin to the most distant point of the flow, combined with hydraulics considerations. The Ginkgo basalt was examined over its flow path from a Ginkgo flow feeder dike near Kahlotus, Washington to the flow terminus in the Pacific Ocean at Yaquina Head, Oregon. The basalt had an upper melting temperature of and a lower temperature to this indicates that the maximum temperature drop along the Ginkgo flow was 20 °C. The lava must have spread quickly to achieve this uniformity.
Image:YaquinaHead 6382.jpg|left|thumbnail|Yaquina Head Lighthouse sits atop erosion-resistant Ginkgo flow basalt of the Frenchman Springs Member over from its origin.
Analyses indicate that the flow must remain laminar, as turbulent flow would cool more quickly. This could be accomplished by sheet flow, which can travel at velocities of without turbulence and minimal cooling, suggesting that the Ginkgo flow occurred in less than a week. The cooling/hydraulics analyses are supported by an independent indicator; if longer periods were required, external water from temporarily dammed rivers would intrude, resulting in both more dramatic cooling rates and increased volumes of pillow lava. Ho's analysis is consistent with the analysis by Reidel, Tolan, & Beeson, who proposed a maximum Pomona flow emplacement duration of several months based on the time required for rivers to be reestablished in their canyons following a basalt flow interruption.

Dating of the flood basalt flows

Three major tools are used to date the CRBG flows: Stratigraphy, radiometric dating, and magnetostratigraphy. These techniques have been key to correlating data from disparate basalt exposures and boring samples over five states.
Major eruptive pulses of flood basalt lavas are laid down stratigraphically. The layers can be distinguished by physical characteristics and chemical composition. Each distinct layer is typically assigned a name usually based on area where that formation is exposed and available for study. Stratigraphy provides a relative ordering of the CRBG layers.
Absolute dates, subject to a statistical uncertainty, are determined through radiometric dating using isotope ratios such as ⁴⁰Ar/³⁹Ar dating, which can be used to identify the date of solidifying basalt. In the CRBG deposits ⁴⁰Ar, which is produced by ⁴⁰K decay, only accumulates after the melt solidifies.
Magnetostratigraphy is also used to determine age. This technique uses the pattern of magnetic polarity zones of CRBG layers by comparison to the magnetic polarity timescale. The samples are analyzed to determine their characteristic remanent magnetization from the Earth's magnetic field at the time a stratum was deposited. This is possible because, as magnetic minerals precipitate in the melt, they align themselves with Earth's current magnetic field.
The Steens Basalt captured a highly detailed record of the Earth's magnetic reversal that occurred roughly 15 million years ago. Over a 10,000-year period, more than 130 flows solidified – roughly one flow every 75 years. As each flow cooled below about, it captured the magnetic field's orientation-normal, reversed, or in one of several intermediate positions. Most of the flows froze with a single magnetic orientation. However, several of the flows, which freeze from both the upper and lower surfaces, progressively toward the center, captured substantial variations in magnetic field direction as they froze. The observed change in direction was reported as 50⁰ over 15 days.