Lakeshore beach gravel (upper Holocene; derived from the North Shore Volcanic Series, Mesoproterozoic, 1.1 Ga; Burlington Bay, Two Harbors, Minnesota, USA) 1 : 無料・フリー素材/写真
Lakeshore beach gravel (upper Holocene; derived from the North Shore Volcanic Series, Mesoproterozoic, 1.1 Ga; Burlington Bay, Two Harbors, 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 |
|---|---|
| 説明 | Lakeshore beach gravel in the Holocene of Minnesota, USA.This is modern beach gravel along the northern shoreline of western Lake Superior. Most of the clasts are pebble-sized (= 4 to 64 mm). The pebbles are derived from lakeshore outcrops of basalt that are part 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 pebbles were transported and deposited by waves and longshore currents.Provenance: North Shore Volcanic Series, Keweenawan Supergroup, upper Mesoproterozoic, ~1.1 GaLocality: Lake Superior shoreline on the western side of Burlington Bay, eastern side of the town of Two Harbors, northeastern Minnesota, USA |
| 撮影日 | 2015-06-08 12:24:26 |
| 撮影者 | James St. John |
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