File:Narrows Basalt (Lower Pleistocene, 1.5-1.6 Ma) (Narrows of the Yellowstone River, northeastern Yellowstone National Park, Wyoming, USA) 5 (19510561513).jpg

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

The horizon with closely-spaced vertical fractures shown above is a 1.5 to 1.6 million year old basalt lava flow 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 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.

Above the lava flow is a Pleistocene glacial outwash unit, deposited during a glacier's melting phase.

Stratigraphy: lower 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) 5
Author James St. John

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This file is licensed under the Creative Commons Attribution 2.0 Generic license.
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This image was originally posted to Flickr by James St. John at https://flickr.com/photos/47445767@N05/19510561513 (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 (3.67 MB)Ser Amantio di Nicolao (talk | contribs)Transferred from Flickr via #flickr2commons

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