File:Baptism River Basalt (North Shore Volcanic Group, Mesoproterozoic, ~1097 Ma; Split Rock Lighthouse, Minnesota, USA) 3 (23063082361).jpg
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Basalt lava flow in the Precambrian of Minnesota, 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. The outcrop shown above is the top of a shoreline cliff in Split Rock Lighthouse State Park, Minnesota. The rocks are part of a basalt lava flow in the Baptism River Basalt. This unit is dominated by basalt (subophitic basalt, ophitic basalt), but basaltic andesite and rhyolite are also present (see Miller et al., 2001). The columnar weathering pattern is due to columnar jointing. Many basalt lava flows contract slightly after solidification as they continue cooling. Contraction results in the formation of polygonal columns - columnar jointing. Some of the best-developed examples on Earth are Devils Tower (Wyoming), Devil's Postpile (California), and Giant's Causeway (Ireland). Stratigraphy: Baptism River Basalt, Upper Southwest Sequence, North Shore Volcanic Group, Keweenawan Supergroup, upper Mesoproterozoic, ~1097 Ma Locality: shoreline cliff outcrops just west of Split Rock Lighthouse, Stony Point, northern shore of western Lake Superior, northeastern Minnesota, USA Reference cited: Miller et al. (2001) - Geologic map of the Duluth Complex and related rocks, northeastern Minnesota. Minnesota Geological Survey Miscellaneous Map Series Map M-119. |
| Date | |
| Source | Baptism River Basalt (North Shore Volcanic Group, Mesoproterozoic, ~1097 Ma; Split Rock Lighthouse, Minnesota, USA) 3 |
| 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@N05/23063082361 (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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| current | 00:21, 10 October 2019 | | 4,000 × 3,000 (5.23 MB) | Ser Amantio di Nicolao (talk | contribs) | Transferred from Flickr via #flickr2commons |
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