Duluth Complex
The Duluth Complex, the related Beaver Bay Complex (often treated as part of the Duluth Complex), and the associated North Shore Volcanic Group are rock formations which comprise much of the basement
Basement (geology)
In geology, the terms basement and crystalline basement are used to define the rocks below a sedimentary platform or cover, or more generally any rock below sedimentary rocks or sedimentary basins that are metamorphic or igneous in origin...

In stratigraphy, bedrock is the native consolidated rock underlying the surface of a terrestrial planet, usually the Earth. Above the bedrock is usually an area of broken and weathered unconsolidated rock in the basal subsoil...

 of the northeastern part of the U.S. state
U.S. state
A U.S. state is any one of the 50 federated states of the United States of America that share sovereignty with the federal government. Because of this shared sovereignty, an American is a citizen both of the federal entity and of his or her state of domicile. Four states use the official title of...

 of Minnesota
Minnesota is a U.S. state located in the Midwestern United States. The twelfth largest state of the U.S., it is the twenty-first most populous, with 5.3 million residents. Minnesota was carved out of the eastern half of the Minnesota Territory and admitted to the Union as the thirty-second state...

. The Duluth and Beaver Bay complexes are intrusive
An intrusion is liquid rock that forms under Earth's surface. Magma from under the surface is slowly pushed up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. As the rock slowly...

 rocks formed during the formation of the Midcontinental Rift system; they adjoin and are interspersed with the extrusive rocks of the North Shore Volcanic Group
Volcanic group
A volcanic group is a collection of related volcanoes or volcanic landforms. Note that the term is also used in a different sense when it denotes a suite of associated rock strata largely of volcanic origin; see group for details.-Notable volcanic groups:-See also:*Complex...

 produced during the same geologic event. These formations are part of the Superior Upland physiographic region of the United States, which is associated with the Laurentian Upland
Laurentian Upland
The Laurentian Upland is a physiographic province which, when referred to as the "Laurentian Region," is recognized by Natural Resources Canada as one of five provinces of the larger Canadian Shield physiographic division...

 of the Canadian Shield
Canadian Shield
The Canadian Shield, also called the Laurentian Plateau, or Bouclier Canadien , is a vast geological shield covered by a thin layer of soil that forms the nucleus of the North American or Laurentia craton. It is an area mostly composed of igneous rock which relates to its long volcanic history...

, the core of the North American Craton.


The Duluth Complex includes much of Minnesota's Arrowhead Region
Arrowhead Region
The Arrowhead Region is located in the northeastern part of the U.S. state of Minnesota, so called because of its pointed shape. The predominantly rural region encompasses of land area and comprises Carlton, Cook, Lake and St. Louis Counties. Its population at the 2000 census was 248,425 residents...

 north of Lake Superior
Lake Superior
Lake Superior is the largest of the five traditionally-demarcated Great Lakes of North America. It is bounded to the north by the Canadian province of Ontario and the U.S. state of Minnesota, and to the south by the U.S. states of Wisconsin and Michigan. It is the largest freshwater lake in the...

. From the west near Duluth, Minnesota
Duluth, Minnesota
Duluth is a port city in the U.S. state of Minnesota and is the county seat of Saint Louis County. The fourth largest city in Minnesota, Duluth had a total population of 86,265 in the 2010 census. Duluth is also the second largest city that is located on Lake Superior after Thunder Bay, Ontario,...

, it arcs north and northeast to about 48° north latitude south of Knife Lake, proceeds east at that latitude some five to twenty kilometers distant from and south of the Canadian border to about 90° west longitude where it joins the border at the Pigeon River
Pigeon River (Minnesota-Ontario)
The Pigeon River forms part of the US-Canada border between the State of Minnesota and the Province of Ontario west of Lake Superior. In pre-industrial times the river was a waterway of great importance for transportation and trade.-Geography:...

, and thence runs east near and along the border to Lake Superior. The Duluth and Beaver Bay complexes lie south of this line.

Near Lake Superior these intrusive formations intermingle in a complex mosaic with the rocks of the associated North Shore Volcanics, which also are relics of the Midcontinent rifting event. The Duluth and Beaver Bay Complexes extend a short distance under Lake Superior south of the present lakeshore, but in most places along and near that shore their southern reaches are overlain by the North Shore Volcanic Group.


Some 1,100 million years ago (mya) the North American craton began to split apart in the Midcontinent Rift. Over a period of some 15–22 million years, magma rose through the earth’s crust, separating the older formations and creating new rock in the area of the rift. The rock sequences thereby created are known as the Keweenawan Supergroup. Rocks of this group north of Lake Superior are the layers of the North Shore Volcanic Group and the adjoining formations of the Duluth and Beaver Bay Complexes.

The North Shore Volcanics originated c. 1109-1096 mya from hundreds of individual lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...

 flows, forming six distinct tilted and partially-stacked plateaus which total more than 8,000 meters in thickness. These tilt toward the syncline
In structural geology, a syncline is a fold, with younger layers closer to the center of the structure. A synclinorium is a large syncline with superimposed smaller folds. Synclines are typically a downward fold, termed a synformal syncline In structural geology, a syncline is a fold, with younger...

 under Lake Superior, as shown in the adjacent picture of the Sawtooth Mountains
Sawtooth Mountains (Minnesota)
The Sawtooth Mountains are a range of low, serrated ridges situated on the North Shore of Lake Superior in the U.S. state of Minnesota, extending about 30 miles from Carlton Peak near Tofte on the west, to Grand Marais on the east.-Description:...

, the slopes of which mirror those of the shoreline rocks. While principally basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...

ic, these flows also include rhyolite
This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellite system, see Rhyolite/Aquacade.Rhyolite is an igneous, volcanic rock, of felsic composition . It may have any texture from glassy to aphanitic to porphyritic...

s and other types. As part of the Middle Proterozoic Keweenawan sequence, these volcanic layers are part of one of the oldest, largest, and best-preserved plateau lava provinces in the world.

These volcanics created the "roof rocks" into which were emplaced the mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...

 formations of the Duluth Complex. Primarily formed after 1102 mya, the oldest formations are near Duluth, and the youngest to the northeast near Tofte
Tofte Township, Minnesota
Tofte Township is one of the three townships of Cook County, Minnesota, United States. The population was 226 at the 2000 census.-Geography:According to the United States Census Bureau, the township has a total area of 162.6 square miles , of which, 154.6 square miles of it is land and...

. Insulated by the overlying roof rock, upwelling magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...

 cooled slowly, and the mafic rock into which it cooled therefore is coarse-grained. These intrusions formed a sill
Sill (geology)
In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet...

 some 16 km thick, primarily of gabbro
Gabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....

, but with significant amounts of anorthosite
Anorthosite is a phaneritic, intrusive igneous rock characterized by a predominance of plagioclase feldspar , and a minimal mafic component...

 and other related granitic rocks. The Duluth Complex is one of the largest intrusions of gabbro on earth, and one of the largest layered mafic intrusions known. It covers an area of 4715 km2. The lower portion of the intrusion along the northwestern margin consists of ultramafic cumulates
Cumulate rock
Cumulate rocks are igneous rocks formed by the accumulation of crystals from a magma either by settling or floating. Cumulate rocks are named according to their texture; cumulate texture is diagnostic of the conditions of formation of this group of igneous rocks.-Formation:Cumulate rocks are the...

 with associated segregations of nickel, copper and platinum group elements. The upper differentiated portions include ilmenite
Ilmenite is a weakly magnetic titanium-iron oxide mineral which is iron-black or steel-gray. It is a crystalline iron titanium oxide . It crystallizes in the trigonal system, and it has the same crystal structure as corundum and hematite....

 bearing labradorite
Labradorite , a feldspar mineral, is an intermediate to calcic member of the plagioclase series. It is usually defined as having "%An" between 50 and 70. The specific gravity ranges from 2.68 to 2.72. The streak is white, like most silicates. The refractive index ranges from 1.559 to 1.573....

Anorthosite is a phaneritic, intrusive igneous rock characterized by a predominance of plagioclase feldspar , and a minimal mafic component...


Along its northern margin, the Duluth Complex adjoins older structures, the Archaen Ely Greenstones
Greenstone belt
Greenstone belts are zones of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that occur within Archaean and Proterozoic cratons between granite and gneiss bodies....

 (once believed to be the oldest exposed rock on earth), and the ore-bearing Mesabi
Mesabi Range
The Mesabi Iron Range is a vast deposit of iron ore and the largest of four major iron ranges in the region collectively known as the Iron Range of Minnesota. Discovered in 1866, it is the chief deposit of iron ore in the United States. The deposit is located in northeast Minnesota, largely in...

 and Gunflint
Gunflint Range
The Gunflint Range is an iron ore deposit in northern Minnesota in the United States and Northwestern Ontario, Canada. The range extends from the extreme northern portion of Cook County, Minnesota into Canada....

 iron ranges deposited as part of the Animikie Group
Animikie Group
The Animikie Group is a geologic group composed of sedimentary and metasedimentary rock, having been originally deposited between 2,500 and 1,800 million years ago within the Animikie Basin. This group of formations are geographically divided into the Gunflint Range, the Mesabi and Vermilion...

 from the Penokean orogeny
Penokean orogeny
The Penokean orogeny was a mountain-building episode that occurred in the early Proterozoic about 1.85 to 1.84 billion years ago, in the area of North America that would eventually become Minnesota, Wisconsin, Michigan and Ontario...

, a mountain-building event from Paleoproterozoic
The Paleoproterozoic is the first of the three sub-divisions of the Proterozoic occurring between . This is when the continents first stabilized...

 times. Those two Middle Precambrian ranges are thought to have once been joined, but intruding magma of the Duluth Complex baked and engulfed the center of the mountain chain, separating it into the two ranges present today.

To the east, the complex abuts and intrudes into the Rove Formation, an older structure composed of sedimentary rocks. Gabbro and diabase structures of the Duluth Complex trend generally from southwest to northeast, and the differential erosion has left a series of ridges comprising these harder mafic rocks rising from the softer sedimentary rocks of the Rove Formation. Elongated lakes lie in many of these depressions.

To the south near Lake Superior, rock strata of the Duluth and Beaver Bay complexes are interspersed with and underlie the extrusive rock of the North Shore Volcanic Group. The Beaver Bay Complex occupies the center of the North Shore Volcanics, and is slightly younger in age than the other mafic rocks of the Duluth Complex, dating from c. 1096 mya. The volcanics and more recent sedimentary rocks were once thought to be underlain by the Duluth Complex all the way across Lake Superior to Wisconsin, where gabbro formations also exist. The Duluth Complex was considered to be a giant lopolith
A lopolith is a large igneous intrusion which is lenticular in shape with a depressed central region. Lopoliths are generally concordant with the intruded strata with dike or funnel-shaped feeder bodies below the body...

, a lens-shaped structure depressed in the center, connecting gabbro exposures on opposite sides of the lake, but now is recognized to extend only a few kilometers south of Superior's North Shore.

Contemporary landforms

The Precambrian bedrock of the Duluth Complex and the North Shore Volcanics are not buried beneath layers of later sedimentary rock, as is common further south; much of this bedrock is close to or at the surface. Glaciers scoured away earlier soils, and as is typical of the Canadian Shield, the new topsoils are thin and poor, being derived from the rock beneath or nearby rather than from deep layers of glacial till, which is intermittent and relatively shallow over most of the region. Consequently much of the bedrock is exposed, except for the sediments and glacial till in the watershed of the Saint Louis
Saint Louis River
The St. Louis River is a river in the U.S. states of Minnesota and Wisconsin that flows into Lake Superior. The largest U.S. river to flow into the lake, it is 179 miles in length and starts near Hoyt Lakes, Minnesota. The river's watershed is in area...

 and Cloquet River
Cloquet River
The Cloquet River is a river of Minnesota. It is the main tributary of the St. Louis River.-Geography:The Cloquet river generally flows in a southwesterly direction which goes through Alden Lake, Island Lake Reservoir and Wood Lake before emptying into the St...

s inland on the west.

Gabbro outcroppings anchor both ends of the complex. They dominate the city which gave the Duluth Complex its name, and also form part of Pigeon Point
Pigeon Point, Minnesota
Pigeon Point is an isolated peninsula located at the northeast extremity of Minnesota. To the north of the point lies Pigeon Bay, which shares a shoreline with Canada and to the south is Lake Superior. The extreme end of the point is owned by the United States Coast Guard while the land up to the...

, the easternmost point of Minnesota. In between, Superior's shoreline
North Shore (Lake Superior)
The North Shore of Lake Superior runs from Duluth, Minnesota, United States, at the southwestern end of the lake, to Thunder Bay and Nipigon, Ontario, Canada, in the north to Sault Ste. Marie, Ontario, in the east...

 from Duluth to the international border has been likened to one long volcanic outcrop, albeit interrupted by parts of the Beaver Bay Complex, such as the anorthosite
Anorthosite is a phaneritic, intrusive igneous rock characterized by a predominance of plagioclase feldspar , and a minimal mafic component...

 cliffs at Split Rock Lighthouse adjacent to basalt flows. Prominent relics of volcanism include rhyolitic
This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellite system, see Rhyolite/Aquacade.Rhyolite is an igneous, volcanic rock, of felsic composition . It may have any texture from glassy to aphanitic to porphyritic...

 cliffs at Palisade Head
Palisade Head
Palisade Head is a large rock formation on the North Shore of Lake Superior in the U.S. state of Minnesota. It is within Tettegouche State Park but not contiguous with the rest of that park...

, basaltic lava flows at Gooseberry Falls, and the Sawtooth Mountains
Sawtooth Mountains (Minnesota)
The Sawtooth Mountains are a range of low, serrated ridges situated on the North Shore of Lake Superior in the U.S. state of Minnesota, extending about 30 miles from Carlton Peak near Tofte on the west, to Grand Marais on the east.-Description:...

 further east. Along the lakeshore can be found quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

-banded thomsonite
Thomsonite is the name of a series of tecto-silicate minerals of the zeolite group. Prior to 1997, thomsonite was recognized as a mineral species, but a reclassification in 1997 by the International Mineralogical Association changed it to a series name, with the mineral species being named...

 and agate
Lake Superior agate
The Lake Superior agate is a type of agate stained by iron and found on the shores of Lake Superior. Its wide distribution and iron-rich bands of color reflect the gemstone's geologic history in Minnesota...

 gemstones created by mineral infilling of gas cavities formed when the lava flows cooled.

Interior highlands include Eagle Mountain
Eagle Mountain (Minnesota)
Eagle Mountain is the highest natural point in Minnesota, at . It is located in northern Cook County and is in the Boundary Waters Canoe Area Wilderness and Superior National Forest in the Misquah Hills, northwest of the community of Grand Marais. It is a Minnesota State Historic Site.Eagle...

 and the other Misquah Hills
Misquah Hills
The Misquah Hills are a range of large hills or small mountains in northeastern Minnesota, in the United States. They are located in or near the Boundary Waters Canoe Area Wilderness within Superior National Forest...

. Most of the eastern part of the Superior National Forest
Superior National Forest
Superior National Forest, part of the United States National Forest system, is located in the Arrowhead Region of the state of Minnesota between the Canada – United States border and the north shore of Lake Superior...

 and its Boundary Waters Canoe Area Wilderness
Boundary Waters Canoe Area Wilderness
The Boundary Waters Canoe Area Wilderness , is a wilderness area within the Superior National Forest in northeastern Minnesota under the administration of the U.S. Forest Service...

 (BWCA) is located on the Duluth Complex, and its exposed Late Precambrian bedrock created many of the characteristic features of the region. The inland lakes lie in hollows formed by differential erosion of the gabbro intrusions. These depressions were given their final form by glacial scouring during recent ice ages, creating the irregularly-shaped and rocky-shored lakes which are hallmarks of the wilderness.


  • Green, John (2004): Volcanic and Sedimentary Rocks of the Southwestern Sequence of the North Shore Volcanic Group, Field Trip Guidebook, Proceedings 50, Vol. 2. Institute on Lake Superior Geology, 2004. Retrieved 2008-04-27.

  • Guilbert, John M. and Charles F. Park, Jr., The Geology of Ore Deposits, Freeman, 1986, ISBN 0-7167-1456-6.

  • Miller, J.D.; Green, J.C.; Severson, M.J.; Chandler, V.W.; and Peterson, D.M.; Geologic Map of the Duluth Complex and Related Rocks, Northeastern Minnesota, University of Minnesota
    University of Minnesota
    The University of Minnesota, Twin Cities is a public research university located in Minneapolis and St. Paul, Minnesota, United States. It is the oldest and largest part of the University of Minnesota system and has the fourth-largest main campus student body in the United States, with 52,557...

    (2001). Retrieved on 2008-04-10.




External links

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