File:Large quartz-laumontite amygdule in quartz tholeiite basalt lava flow (Two Harbors Basalts, North Shore Volcanic Series, Mesoproterozoic, 1097-1098 Ma; Burlington Bay, Two Harbors, Minnesota, USA) 9.jpg
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English: Large quartz-laumontite amygdule in quartz tholeiite basalt 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 (www.flickr.com/photos/jsjgeology/albums/72157632266738191). 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: minerals.usgs.gov/science/midcontinent-rift-minerals/imag...). 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 along the shoreline of Burlington Bay at the town of Two Harbors, Minnesota. The rocks are quartz tholeiite basalts - they are part of the Two Harbors Basalts succession. Three lava flows are exposed at this site. The basalt itself has numerous gas vesicles that have since been filled with minerals (amygdules), resulting in amygdaloidal basalt. The structure at the center of the above photo is a relatively large amygdule filled with quartz and laumontite. Laumontite (= salmon-colored) is one of the zeolite minerals. Zeolites are often found filling or partially filling cavities in volcanic rocks. Laumonite is a hydrous calcium aluminosilicate mineral, CaAl2Si4O12·4H2O. The non-salmon colored material in the amygdule is quartz. Stratigraphy: Two Harbors Basalts, Upper Southwest Sequence, North Shore Volcanic Series, Keweenawan Supergroup, upper Mesoproterozoic, ~1097-1098 Ma Locality: shoreline exposure on the western side of Burlington Bay, eastern side of the town of Two Harbors, northeastern Minnesota, USA |
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| Source | https://www.flickr.com/photos/47445767@N05/22292771539/ |
| 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@N05/22292771539. It was reviewed on 8 October 2020 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0. |
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| current | 15:08, 8 October 2020 | | 2,263 × 1,657 (3.63 MB) | Ser Amantio di Nicolao (talk | contribs) | Uploaded a work by James St. John from https://www.flickr.com/photos/47445767@N05/22292771539/ with UploadWizard |
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