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Volcanic history of Minnesota
Transcript of Volcanic history of Minnesota
Gabbro is an intrusive rock formed when molten rock is trapped beneath the land surface and cools into a hard, coarsely crystalline mass. It is the intrusive equivalent to basalt. Minnesota's best examples of gabbro are in the part of the 1.1 billion year old Midcontinent rift exposed in the large hills at Duluth, known as the Duluth Complex. The rock is dense. Geologists have determined that about one billion years ago, the earth began to split apart along the area now known as the North Shore. Lava flowed out onto the earth and cooled to form volcanic bedrock. Several lava flows can be seen at the Upper, Middle, and Lower Falls and south of the Gooseberry River along Lake Superior. These basalt lava flows are also the birthplaces of Lake Superior agates. About two million years ago, glaciers (up to a mile high) advanced into the region. Greenstone in northern Minnesota is somewhat younger than gneiss. Greenstone is a weakly metamorphosed (altered) basalt that is, as its name suggests, greenish to gray. This type of rock formed about 2.7 billion years ago when the area that is now northern Minnesota was part of a volcanic island arc, much like the islands of Japan are today. Granite is found throughout northern and central Minnesota. It varies in age from 2.6 billion years in the Minnesota River Valley and northern Minnesota to about 1.7 billion years near St. Cloud. Minnesota granites are composed predominantly of the minerals feldspar, quartz, mica, and hornblende. These rocks formed deep below the surface in the roots of major mountain ranges. Some of the oldest rocks in the world include the gneiss found in the Minnesota River Valley. The Morton Gneiss, which is 3.6 billion years old, is a coarsely crystalline, foliated metamorphic rock. The texture and mineral assemblage of the Morton Gneiss give clues as to how the rock formed. The fact that it is a crystalline rock with large visible grains indicates that it originated as a granitic igneous rock that cooled slowly beneath the Earth's surface. Thank You!!!