File:Amygdaloidal basalt (North Shore Volcanic Series, upper Mesoproterozoic, ~1.1 Ga; Grand Marais, Cook County, Minnesota, USA) (39619143950).jpg
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Amygdaloidal basalt from the Precambrian of Minnesota, USA. (public display, Geology Department, Wittenberg University, Springfield, Ohio, USA) Along the northern shore of western Lake Superior are numerous exposures of a lava flow-dominated succession called the North Shore Volcanic Series. This is equivalent to & the same age as the Portage Lake Volcanic Series of northern Michigan's Keweenaw Peninsula (<a href="https://www.flickr.com/photos/jsjgeology/albums/72157632266738191">www.flickr.com/photos/jsjgeology/albums/72157632266738191</a>). The North Shore and Portage Lake successions are ~1.1 billion years old and represent basalt lava flows, plus minor sedimentary rocks, that filled up an ancient rift valley. This old rift is the Lake Superior segment of the Mid-Continent Rift System, a tear in the ancient North American paleocontinent of Laurentia (see: <a href="https://minerals.usgs.gov/science/midcontinent-rift-minerals/images/MRS-map.png" rel="nofollow">minerals.usgs.gov/science/midcontinent-rift-minerals/imag...</a>). Tectonic rifting started along this tear, exactly like the modern-day East African Rift Valley. Laurentia's Mid-Continent Rift System started and then stopped and was subsequently filled and buried. This ancient failed rift is now exposed on either side of Lake Superior in North America's Great Lakes. This sample is vesicular basalt that has had its gas vesicles subsequently filled with minerals - such a rock is called an amygdaloidal basalt. Stratigraphy: North Shore Volcanic Series, Keweenawan Supergroup, upper Mesoproterozoic, ~1.1 Ga Locality: unrecorded/undisclosed site at or near the town of Grand Marais, Cook County, northeastern Minnesota, USA |
| Date | |
| Source | Amygdaloidal basalt (North Shore Volcanic Series, upper Mesoproterozoic, ~1.1 Ga; Grand Marais, Cook County, Minnesota, USA) |
| 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@N05/39619143950 (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. |
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| Date/Time | Thumbnail | Dimensions | User | Comment | |
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| current | 00:22, 10 October 2019 | | 2,450 × 1,620 (2.97 MB) | Ser Amantio di Nicolao (talk | contribs) | Transferred from Flickr via #flickr2commons |
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