The Midcontinent Rift System (MRS) or Keweenawan Rift is a 2,000 km (1,200 mi) long geological rift in the center of the North American continent and south-central part of the North American plate . It formed when the continent's core, the North American craton , began to split apart during the Mesoproterozoic era of the Precambrian , about 1.1 billion years ago. The rift failed , leaving behind thick layers of igneous rock that are exposed in its northern reaches, but buried beneath later sedimentary formations along most of its western and eastern arms. Those arms meet at Lake Superior , which is contained within the rift valley . The lake's north shore in Ontario and Minnesota defines the northern arc of the rift. From the lake, the rift's eastern arm trends south to central lower Michigan , and possibly into Indiana , Ohio , Kentucky , Tennessee , and Alabama . The western arm runs from Lake Superior southwest through portions of Wisconsin , Minnesota , Iowa , and Nebraska to northeastern Kansas , and possibly into Oklahoma .
33-489: The rock formations created by the rift included gabbro and granite intrusive rocks and basalt lavas . In the Lake Superior region, the upwelling of this molten rock may have been the result of a hotspot which produced a triple junction . The hotspot domed the rocks of the Lake Superior area. Voluminous basaltic lava flows erupted from the central axis of the rift, similar to the present-day rifting underway in
66-592: A holocrystalline mass deep beneath the Earth 's surface. Slow-cooling, coarse-grained gabbro is chemically equivalent to rapid-cooling, fine-grained basalt . Much of the Earth's oceanic crust is made of gabbro, formed at mid-ocean ridges . Gabbro is also found as plutons associated with continental volcanism . Due to its variant nature, the term gabbro may be applied loosely to a wide range of intrusive rocks, many of which are merely "gabbroic". By rough analogy, gabbro
99-460: A dioritoid typically has less than 35% mafic minerals, which typically includes hornblende. Gabbroids form a family of rock types similar to gabbro, such as monzogabbro , quartz gabbro , or nepheline-bearing gabbro . Gabbro itself is more narrowly defined, as a gabbroid in which quartz makes up less than 5% of the QAPF content, feldspathoids are not present, and plagioclase makes up more than 90% of
132-527: A gabbronorite. Gabbroids (also known as gabbroic-rocks ) are a family of coarse-grained igneous rocks similar to gabbro: Gabbroids contain minor amounts, typically a few percent, of iron-titanium oxides such as magnetite , ilmenite , and ulvospinel . Apatite , zircon , and biotite may also be present as accessory minerals. Gabbro is generally coarse-grained, with crystals in the size range of 1 mm or larger. Finer-grained equivalents of gabbro are called diabase (also known as dolerite ), although
165-478: A replacement commodity substituted for those commodities which become too expensive. Additionally the fixed stock of most mineral commodities is huge (e.g., copper within the Earth's crust given current rates of consumption would last for more than 100 million years.) Nonetheless, economic geologists continue to successfully expand and define known mineral resources. Mineral resources are concentrations of minerals significant for current and future societal needs. Ore
198-563: A semi-precious stone. Indigo Gabbro can contain numerous minerals, including quartz and feldspar. Reports state that the dark matrix of the rock is composed of a mafic igneous rock, but whether this is basalt or gabbro is unclear. Volcanic rocks : Subvolcanic rocks : Plutonic rocks : Picrite basalt Peridotite Basalt Diabase (Dolerite) Gabbro Andesite Microdiorite Diorite Dacite Microgranodiorite Granodiorite Rhyolite Microgranite Granite Economic geology Economic geology
231-411: Is pyroxene-plagioclase adcumulate . Gabbro is much less common than more silica-rich intrusive rocks in the continental crust of the Earth. Gabbro and gabbroids occur in some batholiths but these rocks are relatively minor components of these very large intrusions because their iron and calcium content usually makes gabbro and gabbroid magmas too dense to have the necessary buoyancy. However, gabbro
264-514: Is an essential part of the oceanic crust, and can be found in many ophiolite complexes as layered gabbro underling sheeted dike complexes and overlying ultramafic rock derived from the Earth's mantle . These layered gabbros may have formed from relatively small but long-lived magma chambers underlying mid-ocean ridges . Layered gabbros are also characteristic of lopoliths , which are large, saucer-shaped intrusions that are primarily Precambrian in age. Prominent examples of lopoliths include
297-496: Is based on the relative percentages of plagioclase, pyroxene, hornblende, and olivine. The end members are: Gabbros intermediate between these compositions are given names such as gabbronorite (for a gabbro intermediate between normal gabbro and norite, with almost equal amounts of clinopyroxene and orthopyroxene) or olivine gabbro (for a gabbro containing significant olivine, but almost no clinopyroxene or hornblende). A rock similar to normal gabbro but containing more orthopyroxene
330-497: Is called an orthopyroxene gabbro, while a rock similar to norite but containing more clinopyroxene is called a clinopyroxene norite. Gabbros are also sometimes classified as alkali or tholleiitic gabbros, by analogy with alkali or tholeiitic basalts, of which they are considered the intrusive equivalents. Alkali gabbro usually contains olivine, nepheline, or analcime , up to 10% of the mineral content, while tholeiitic gabbro contains both clinopyroxene and orthopyroxene, making it
363-480: Is classified as mineralization economically and technically feasible for extraction. Not all mineralization meets these criteria for various reasons. The specific categories of mineralization in an economic sense are: Geologists are involved in the study of ore deposits, which includes the study of ore genesis and the processes within the Earth's crust that form and concentrate ore minerals into economically viable quantities. Study of metallic ore deposits involves
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#1732771886109396-452: Is classified as olivine gabbro or gabbronorite respectively. Where present, hornblende is typically found as a rim around augite crystals or as large grains enclosing smaller grains of other minerals ( poikilitic grains). Geologists use rigorous quantitative definitions to classify coarse-grained igneous rocks, based on the mineral content of the rock. For igneous rocks composed mostly of silicate minerals, and in which at least 10% of
429-404: Is concerned with earth materials that can be used for economic and industrial purposes. These materials include precious and base metals , nonmetallic minerals and construction-grade stone . Economic geology is a subdiscipline of the geosciences; according to Lindgren (1933) it is “the application of geology”. It may be called the scientific study of the Earth's sources of mineral raw materials and
462-661: Is to basalt as granite is to rhyolite . The term "gabbro" was used in the 1760s to name a set of rock types that were found in the ophiolites of the Apennine Mountains in Italy. It was named after Gabbro , a hamlet near Rosignano Marittimo in Tuscany . Then, in 1809, the German geologist Christian Leopold von Buch used the term more restrictively in his description of these Italian ophiolitic rocks. He assigned
495-664: The Afar Depression of the East African Rift system. The southwest and southeast extensions represent two arms of the triple junction while a third failed arm extends north into Ontario as the Nipigon Embayment . This failed arm includes Lake Nipigon , Ontario. The rift system may have been the result of extensional forces behind the continental collision of the Grenville Orogeny to
528-984: The Arctic in Nunavut to near the Great Lakes in Northwestern Ontario . The Proterozoic Nonesuch Shale formation in the Keweenawan Rift contains enough organic carbon (greater than 0.5%) to be considered a potential source rock for petroleum. Oil identified as Precambrian has been found seeping out of the Nonesuch Shale in the White Pine mine in Michigan. A few deep wells were drilled to explore for oil and gas in rift rocks as far southwest as Kansas. No oil and gas were found, but
561-777: The Bushveld Complex of South Africa, the Muskox intrusion of the Northwest Territories of Canada, the Rum layered intrusion of Scotland, the Stillwater complex of Montana, and the layered gabbros near Stavanger , Norway. Gabbros are also present in stocks associated with alkaline volcanism of continental rifting . Gabbro often contains valuable amounts of chromium , nickel , cobalt , gold , silver , platinum , and copper sulfides . For example,
594-479: The Merensky Reef is the world's most important source of platinum. Gabbro is known in the construction industry by the trade name of black granite . However, gabbro is hard and difficult to work, which limits its use. The term "indigo gabbro" is used as a common name for a mineralogically complex rock type often found in mottled tones of black and lilac-grey. It is mined in central Madagascar for use as
627-803: The Upper Peninsula and Isle Royale contains major native copper deposits in Keweenawan-age rocks associated with the rift. A copper mining industry was developed in Precolumbian times, reactivated in the 1840s and continued for more than a century. Some low-grade copper and nickel deposits, among the largest in the world, also exist in the Duluth Complex north of the lake. Once thought to be uneconomic to mine , these deposits have attracted renewed interest from resource companies. White hydrogen could be found or produced in
660-485: The Mid-continental Rift System at scale for a renewable hydrogen economy . Water could be pumped down to hot iron-rich rock to produce hydrogen and the hydrogen could be extracted. Gabbro Gabbro ( / ˈ ɡ æ b r oʊ / GAB -roh ) is a phaneritic (coarse-grained and magnesium- and iron-rich), mafic intrusive igneous rock formed from the slow cooling magma into
693-509: The QAPF content, and plagioclase makes up more than 65% of the total feldspar content. Gabbroids are distinguished from dioritoids by an anorthite (calcium plagioclase) fraction of their total plagioclase of greater than 50%. The composition of the plagioclase cannot easily be determined in the field , and then a preliminary distinction is made between dioritoid and gabbroid based on the content of mafic minerals. A gabbroid typically has over 35% mafic minerals, mostly pyroxenes or olivine, while
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#1732771886109726-579: The east which in part overlaps the timing of the rift development. Later compressive forces from the Grenville Orogeny likely played a major role in the rift's failure and closure. Had the rifting process continued, the eventual result would have been sundering of the North American craton and creation of a sea. The Midcontinent Rift appears to have progressed almost to the point where the ocean intruded. But after about 15–22 million years
759-603: The explorations did make some deep rock samples available. These include two "dry holes" drilled by Amoco : a 7,238 ft (2,206 m) well in Alger County, Michigan in 1987 and 1988, and one in Bayfield County, Wisconsin to a depth of 4,966 ft (1,514 m) in 1992. In 1987 Amoco also drilled a 17,851 ft (5,441 m) dry hole that penetrated rift sediments in Iowa. The Michigan Copper Country in
792-482: The far-reaching impact that extractive industries have on society, the economy and the environment. The purpose of the study of economic geology is to gain understanding of the genesis and localization of ore deposits plus the minerals associated with ore deposits. Though metals, minerals and other geologic commodities are non-renewable in human time frames, the impression of a fixed or limited stock paradigm of scarcity has always led to human innovation resulting in
825-792: The feldspar content. Gabbro is distinct from anorthosite , which contains less than 10% mafic minerals. Coarse-grained gabbroids are produced by slow crystallization of magma having the same composition as the lava that solidifies rapidly to form fine-grained ( aphanitic ) basalt . There are a number of subtypes of gabbro recognized by geologists. Gabbros can be broadly divided into leucogabbros, with less than 35% mafic mineral content; mesogabbros, with 35% to 65% mafic mineral content; and melagabbros with more than 65% mafic mineral content. A rock with over 90% mafic mineral content will be classified instead as an ultramafic rock . A gabbroic rock with less than 10% mafic mineral content will be classified as an anorthosite. A more detailed classification
858-501: The mineral content consists of quartz , feldspar , or feldspathoid minerals, classification begins with the QAPF diagram . The relative abundances of quartz (Q), alkali feldspar (A), plagioclase (P), and feldspathoid (F), are used to plot the position of the rock on the diagram. The rock will be classified as either a gabbroid or a dioritoid if quartz makes up less than 20% of the QAPF content, feldspathoid makes up less than 10% of
891-520: The name "gabbro" to rocks that geologists nowadays would more strictly call "metagabbro" ( metamorphosed gabbro). Gabbro is a coarse-grained ( phaneritic ) igneous rock that is relatively low in silica and rich in iron, magnesium, and calcium. Such rock is described as mafic . Gabbro is composed of pyroxene (mostly clinopyroxene) and calcium-rich plagioclase , with minor amounts of hornblende , olivine , orthopyroxene and accessory minerals . With significant (>10%) olivine or orthopyroxene it
924-567: The practical application of the acquired knowledge. The study is primarily focused on metallic mineral deposits and mineral resources. The techniques employed by other Earth science disciplines (such as geochemistry , mineralogy , geophysics , petrology , paleontology and structural geology ) might all be used to understand, describe and exploit an ore deposit. Economic geology is studied and practiced by geologists. Economic geology may be of interest to other professions such as engineers, environmental scientists and conservationists because of
957-794: The rift failed. The Midcontinent Rift is the deepest closed or healed rift yet discovered; no known deeper rift ever failed to become an ocean. Lake Superior occupies a basin created by the rift. Near the present lake, rocks produced by the rift can be seen on the surface of Isle Royale and the Keweenaw Peninsula of the Upper Peninsula of Michigan, northwest Wisconsin, and on the North Shore of Superior in Minnesota and Ontario. Similar rocks are exposed as far south as Interstate Park near Saint Paul, Minnesota , but elsewhere
990-480: The rift is buried beneath more recent sedimentary rocks up to 9 km (6 mi) thick. Where buried, the rift has been mapped by gravity anomalies (its dense basaltic rock increases gravity locally), aeromagnetic surveys , and seismic data. A slightly older but possibly related geologic feature is the 2,700,000 km (1,000,000 sq mi) Mackenzie Large Igneous Province in Canada, which extends from
1023-450: The term microgabbro is often used when extra descriptiveness is desired. Gabbro may be extremely coarse-grained to pegmatitic . Some pyroxene-plagioclase cumulates are essentially coarse-grained gabbro, and may exhibit acicular crystal habits. Gabbro is usually equigranular in texture, although it may also show ophitic texture (with laths of plagioclase enclosed in pyroxene ). Nearly all gabbros are found in plutonic bodies, and
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1056-558: The term (as the International Union of Geological Sciences recommends) is normally restricted just to plutonic rocks, although gabbro may be found as a coarse-grained interior facies of certain thick lavas. Gabbro can be formed as a massive, uniform intrusion via in-situ crystallisation of pyroxene and plagioclase , or as part of a layered intrusion as a cumulate formed by settling of pyroxene and plagioclase. An alternative name for gabbros formed by crystal settling
1089-439: The use of structural geology , geochemistry , the study of metamorphism and its processes, as well as understanding metasomatism and other processes related to ore genesis. Ore deposits are delineated by mineral exploration , which uses geochemical prospecting, drilling and resource estimation via geostatistics to quantify economic ore bodies. The ultimate aim of this process is mining . The study of sedimentology
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