Columnar-jointed olivine tholeiite basalt lava flow below Gooseberry Falls - Main Falls (Middle Falls) (Gooseberry River Basalts, North Shore Volcanic Series, Mesoproterozoic, 1097-1098 Ma; Gooseberry Falls State Park, Minnesota, USA) 1 : 無料・フリー素材/写真
Columnar-jointed olivine tholeiite basalt lava flow below Gooseberry Falls - Main Falls (Middle Falls) (Gooseberry River Basalts, North Shore Volcanic Series, Mesoproterozoic, 1097-1098 Ma; Gooseberry Falls State Park, Minnesota, USA) 1 / James St. John
| ライセンス | クリエイティブ・コモンズ 表示 2.1 |
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| 説明 | Columnar-jointed olivine tholeiite 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 (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 in Gooseberry Falls State Park, Minnesota. The rocks are quartz tholeiite basalts - they are part of the Gooseberry River Basalts succession. Three lava flows are exposed at this site. The polygonal blocky 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: Gooseberry River Basalts, Upper Southwest Sequence, North Shore Volcanic Series, Keweenawan Supergroup, upper Mesoproterozoic, ~1097-1098 MaLocality: just below Gooseberry Falls - Main Falls (Middle Falls), Gooseberry Falls State Park, just inland from the Northern Shore of western Lake Superior, northeastern Minnesota, USA |
| 撮影日 | 2015-06-08 13:42:22 |
| 撮影者 | James St. John |
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