File:Narrows Basalt (Lower Pleistocene, 1.5-1.6 Ma) (Narrows of the Yellowstone River, northeastern Yellowstone National Park, Wyoming, USA) 1 (19510598353).jpg

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Summary

Description

Columnar-jointed basalt lava flows in the Pleistocene of Wyoming, USA.

The two horizons with closely-spaced vertical fractures shown above are 1.5 to 1.6 million year old basalt lava flows exposed along the eastern walls of the Narrows of the Yellowstone River in Yellowstone National Park. Basalt is a mafic (dark-colored), aphanitic (finely-crystalline), extrusive igneous rock composed principally of plagioclase feldspar and pyroxene. Upon cooling and slight contraction, many basalt lava flows develop closely-spaced subvertical fractures called columnar jointing. The fractures define columns having (usually) pentagonal to hexagonal transverse cross-sections.

Below the lower lava flow is a conglomerate unit having clasts of volcanic lithologies. The conglomerate is part of the Sepulcher Formation (Washburn Group, Absaroka Volcanic Supergroup, Eocene). The volcanism represented by the Sepulcher Formation is unrelated to & much older than Yellowstone Hotspot volcanism.

Between the upper and lower lava flows is a Pleistocene glacial outwash unit, deposited during a glacier's melting phase. Above the upper lava flow is Pleistocene till, which is a poorly-sorted, non-bedded glacial deposit formed during a glacier's advancing phase.

Stratigraphy: Narrows Basalt, Lower Pleistocene, 1.5-1.6 Ma

Locality: Narrows of the Yellowstone River, just north-northeast of Tower Falls, northeastern Yellowstone National Park, Wyoming, USA (44° 53' 52.31" North, 110° 23' 07.47" West)
Date
Source Narrows Basalt (Lower Pleistocene, 1.5-1.6 Ma) (Narrows of the Yellowstone River, northeastern Yellowstone National Park, Wyoming, USA) 1
Author James St. John

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This image was originally posted to Flickr by James St. John at https://flickr.com/photos/47445767@N05/19510598353 (archive). It was reviewed on 10 October 2019 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0.

10 October 2019

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current00:14, 10 October 2019Thumbnail for version as of 00:14, 10 October 20194,000 × 3,000 (3.52 MB)Ser Amantio di NicolaoTransferred from Flickr via #flickr2commons
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