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96-734: Before the Rove sediments were laid down, during the Archean Eon , the Algoman orogeny added landmass along a border from South Dakota to the Lake Huron region; this boundary is the Great Lakes tectonic zone . Several million years later a thin layer of hypervelocity impact ejecta from the Sudbury impact event was deposited on the older, underlying, Gunflint Iron Formation , and the Rove

192-461: A Superior National Forest sensitive plant, Canada Yew ( Taxus canadensis ), was found; and a state-listed species of concern, Blunt-fruited sweet cicely ( Osmorhiza depauperata ), has only four populations within 30 km (19 mi) of each other. Minnesota's Department of Natural Resources lists these vascular plants as being threatened: Rocky Mountain woodsia ( Physematium scopulinum ssp. laurentiana ), there are few isolated populations in

288-401: A foreland basin at around 1850 million years ago in the south and 1835 million years ago in the north. The Rove Formation was deposited in the northern basin. In the southern basin, crustal thickening caused high-grade metamorphism of the sedimentary fill by 1830 million years ago. A series of post-orogenic plutons intruded into the overlying rocks 1830 million years ago,; these plutons mark

384-526: A 1 m (1 sq yd) – so it is vulnerable to singular events which would alter its habitat. Archean Eon The Archean Eon ( IPA : / ɑːr ˈ k iː ə n / ar- KEE -ən , also spelled Archaean or Archæan ), in older sources sometimes called the Archaeozoic , is the second of the four geologic eons of Earth 's history , preceded by the Hadean Eon and followed by

480-806: A diameter greater than 10 kilometers (6 mi) every 15 million years. This is about the size of the Chicxulub impactor. These impacts would have been an important oxygen sink and would have caused drastic fluctuations of atmospheric oxygen levels. The Archean atmosphere is thought to have almost completely lacked free oxygen ; oxygen levels were less than 0.001% of their present atmospheric level, with some analyses suggesting they were as low as 0.00001% of modern levels. However, transient episodes of heightened oxygen concentrations are known from this eon around 2,980–2,960 Ma, 2,700 Ma, and 2,501 Ma. The pulses of increased oxygenation at 2,700 and 2,501 Ma have both been considered by some as potential start points of

576-468: A feature in later, more oxic oceans. Despite the lack of free oxygen, the rate of organic carbon burial appears to have been roughly the same as in the present. Due to extremely low oxygen levels, sulphate was rare in the Archean ocean, and sulphides were produced primarily through reduction of organically sourced sulphite or through mineralisation of compounds containing reduced sulphur. The Archean ocean

672-656: A few are deeper than 61 m (200 ft). A topographic map shows that the majority of the interconnected complex of tightly packed lakes with their valley-and-ridge landscape is primarily in Minnesota; the border of the landscape is the Canada–US border. This border is composed of the Pigeon River, and these Rove Formation lakes (going east to west): South Fowl, North Fowl, Moose, Mountain, Watap, Rose, South, Little North and Gunflint. The border runs approximately through

768-534: A few billion years in age. It is believed that 43% of modern continental crust was formed in the Proterozoic, 39% formed in the Archean, and only 18% in the Phanerozoic . Studies by Condie (2000) and Rino et al. (2004) harvp error: no target: CITEREFRinoKomiyaWindleyet_al2004 ( help ) suggest that crust production happened episodically. By isotopically calculating the ages of Proterozoic granitoids it

864-481: A few plausible models that explain tectonics of the early Earth prior to the formation of Columbia, but the current most plausible hypothesis is that prior to Columbia, there were only a few independent cratons scattered around the Earth (not necessarily a supercontinent, like Rodinia or Columbia). The Proterozoic can be roughly divided into seven biostratigraphic zones which correspond to informal time periods. The first

960-591: A north or somewhat east-of-north glacial movement. This direction is transverse to the general trend on the valleys and ridges. The lakes in the Rove Formation area are unique from other well-known linear bedrock lakes in North America because their long axes lie transverse to the general movement of the glaciers. When continental glaciers moved over the Rove Formation area, the ice was a hundreds of meters thick and its surface sloped gently southward from

1056-526: A period of increasing crustal recycling, suggesting subduction . Evidence for this increased subduction activity comes from the abundance of old granites originating mostly after 2.6  Ga . The occurrence of eclogite (a type of metamorphic rock created by high pressure, > 1 GPa), is explained using a model that incorporates subduction. The lack of eclogites that date to the Archean Eon suggests that conditions at that time did not favor

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1152-408: A single day nearly 1,850 million years ago . Evidence indicates a 16 km (10 mi) diameter meteorite collided with Earth in the current-day vicinity of Sudbury, Ontario, Canada, about 1,850 million years ago . The meteorite vaporized and created a 240 km (150 mi) wide crater (this is the second-largest impact depression on Earth). This impact is 770 km (480 mi) east of

1248-718: A solid layer. Ejecta from the Sudbury Impact was found in May ;2007 on the Gunflint Trail in Cook County, Minnesota. Geologists Mark Jirsa and Paul Weiblen from the University of Minnesota took advantage of the burnt-over landscape resulting from the intense, hot Ham Lake fire to explore the newly exposed geology along the Gunflint Trail. Jirsa picked up some rocks which turned out to be ash and debris from

1344-529: Is a lack of extensive geological evidence for specific continents. One hypothesis is that rocks that are now in India, western Australia, and southern Africa formed a continent called Ur as of 3,100 Ma. Another hypothesis, which conflicts with the first, is that rocks from western Australia and southern Africa were assembled in a continent called Vaalbara as far back as 3,600 Ma. Archean rock makes up only about 8% of Earth's present-day continental crust;

1440-550: Is a very rare species with limited distribution and restrictive environmental needs; Sticky locoweed ( Oxytropis borealis var. viscida ), is restricted to a single cliff in Cook County; Encrusted saxifrage ( Saxifraga paniculata ssp. neogaea ), 11 populations are known to exist in the Rove formation; and Smooth Woodsia ( Woodsia glabella ), there are small, isolated populations in the Rove Formation. During two sensitive plant surveys conducted in June and July 2003, and July 2004,

1536-419: Is about 150 m (490 ft) thick; this layer contains fine-grained greywacke, and silty and graphitic argillites. Greywacke is a sedimentary rock composed of a mixture of poorly sorted grains of sand, silt and clay particles. Argillite is a fine-grained sedimentary rock composed primarily of clay particles; they are essentially lithified muds and oozes. Greywacke is abundant in the middle unit and dominates

1632-781: Is an elongated oval straddling the North Shore of Lake Superior, mainly in Minnesota. Approximately the northwestern two-thirds lies to the northwest of the shoreline; the southeastern third lies to the southeast of the shoreline (so is under Lake Superior's waters). During the Middle Precambrian a shallow inland sea covered much of the Lake Superior region and formed the Animikie Group, which are layers of sedimentary rocks which unconformably overlies 2700-million-year-old Archean rocks. This group contains both

1728-484: Is evidenced by certain highly deformed gneisses produced by metamorphism of sedimentary protoliths . The moderate temperatures may reflect the presence of greater amounts of greenhouse gases than later in the Earth's history. Extensive abiotic denitrification took place on the Archean Earth, pumping the greenhouse gas nitrous oxide into the atmosphere. Alternatively, Earth's albedo may have been lower at

1824-494: Is one of the many flat-topped, steep-sided mesas along Lake Superior's northwestern shore south of Thunder Bay that are collectively known as the Nor'Wester Mountains . In Minnesota, the Rove Formation area shows a relationship of bedrock to topography, with its valley-and-ridge landscape. There are several series of steep, east–west oriented valleys which were created by the erosion of the exposed shale. The diabase-capped ridges between

1920-703: Is the Rove Slate Bedrock Complex Landtype Association. The sedimentary and diabase rocks are calcareous ; they produce a more basic, nutrient-rich soil compared to the poorer soils typical of the Canadian shield. The Royal River drains Royal Lake [which is about 1.2 km (3,900 ft) east of South Fowl Lake] into John Lake, both in Minnesota. The relatively rich soils, particularly in the Royal River drainage area, along with steep, moist, north-facing cliffs provide

2016-452: Is the most eastern part of Minnesota; it is a diabase sill about 152 m (500 ft) thick. Both the north and south coasts of the point expose Rove slates under and over the sill. Within the sill, rates of cooling and gravity have created an interesting distribution of rock types. The visible formation is in Minnesota and contains many east–west oriented ridges and valleys. Many lakes in this 5 to 8 km (3 to 5 mi) wide band along

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2112-441: Is the period of advancing and retreating glaciers . The Rove region has multiple tilted layers of volcanic rocks and easily eroded shale. The more resistant diabase dikes and sills remained, while the softer shales were bulldozed away by the glaciers. These former shale valleys filled with water, forming the many lakes in the region. The character of the slates made them especially well suited for glacial quarrying , much more so than

2208-645: Is the third of the four geologic eons of Earth's history , spanning the time interval from 2500 to 538.8   Mya , the longest eon of the Earth's geologic time scale . It is preceded by the Archean and followed by the Phanerozoic , and is the most recent part of the Precambrian "supereon". The Proterozoic is subdivided into three geologic eras (from oldest to youngest): the Paleoproterozoic , Mesoproterozoic and Neoproterozoic . It covers

2304-471: Is within the formation. Between North Lake, Ontario, and South Lake, Minnesota, is a low saddle of land right on the border; this is the divide. North Lake drains into the Rainy River and then to Hudson Bay. South Lake drains into the Pigeon River and then to Lake Superior. Topsoils are thin and poor because the glaciers had abraded down to the bedrock. The soils are clayey silt. Topographically this

2400-518: The Azoic age . Before the Hadean Eon was recognized, the Archean spanned Earth's early history from its formation about 4,540 million years ago until 2,500 million years ago. Instead of being based on stratigraphy , the beginning and end of the Archean Eon are defined chronometrically . The eon's lower boundary or starting point of 4,031±3 million years ago is officially recognized by

2496-631: The Great Oxygenation Event , which most scholars consider to have begun in the Palaeoproterozoic ( c.  2.4 Ga ). Furthermore, oases of relatively high oxygen levels existed in some nearshore shallow marine settings by the Mesoarchean. The ocean was broadly reducing and lacked any persistent redoxcline , a water layer between oxygenated and anoxic layers with a strong redox gradient, which would become

2592-478: The International Commission on Stratigraphy , which is the age of the oldest known intact rock formations on Earth. Evidence of rocks from the preceding Hadean Eon are therefore restricted by definition to non-rock and non-terrestrial sources such as individual mineral grains and lunar samples. When the Archean began, the Earth's heat flow was nearly three times as high as it is today, and it

2688-648: The Pembine–Wausau terrane collided with the southern margin of the Superior craton . From 1880 to 1850 million years ago, the region was volcanically active. This volcanism ceased 1850 million years ago when a fragment of Archean crust arrived from the South at the subduction zone . Collision of this Archean crust in the south with the Superior craton in the North caused a period of intense crustal shortening. Rocks of

2784-609: The Penokean orogeny added more land mass by accretion that occurred from the south. A few hundred million years later the proto-North American continent nearly split in half along the Midcontinent Rift , which is a bow-shaped rift extending from northeast Kansas, arcing through the present-day Lake Superior Basin and then angling southeast through Michigan. Then came a period of advancing and retreating glaciers. The more resistant diabase sills and dikes remained, while

2880-565: The Proterozoic . The Archean represents the time period from 4,031 to 2,500 Mya (million years ago). The Late Heavy Bombardment is hypothesized to overlap with the beginning of the Archean. The Huronian glaciation occurred at the end of the eon. The Earth during the Archean was mostly a water world : there was continental crust , but much of it was under an ocean deeper than today's oceans. Except for some rare relict crystals , today's oldest continental crust dates back to

2976-435: The habitat for these rare plants. For over a century this landtype association has been recognized as being ecologically and botanically unique; it harbors a rare assemblage of plants, including the rarest plants in Minnesota. Virtually all of the known sensitive plant species in this landscape association occur on the north side of the cliffs or in the Royal River drainage. Six vascular plants are unique in that they are at

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3072-609: The 1,100 km (680 mi) distant impact site; this is the farthest distance that Sudbury detrita has been found. The Midcontinent Rift (also known as the Keweenawan Rift) began about 1,100 million years ago ; it lasted for about 20 million years. After the Penokean Mountains had eroded away, the proto-North American continent nearly split in half along this rift zone. The 2,000 km (1,200 mi) bow-shaped rift extended from northeastern Kansas, through

3168-519: The Archean and become common late in the Archean. Cyanobacteria were instrumental in creating free oxygen in the atmosphere. Further evidence for early life is found in 3.47 billion-year-old baryte , in the Warrawoona Group of Western Australia. This mineral shows sulfur fractionation of as much as 21.1%, which is evidence of sulfate-reducing bacteria that metabolize sulfur-32 more readily than sulfur-34. Evidence of life in

3264-593: The Archean and remained simple prokaryotes ( archaea and bacteria ) throughout the eon. The earliest photosynthetic processes, especially those by early cyanobacteria , appeared in the mid/late Archean and led to a permanent chemical change in the ocean and the atmosphere after the Archean. The word Archean is derived from the Greek word arkhē ( αρχή ), meaning 'beginning, origin'. The Pre-Cambrian had been believed to be without life (azoic); however, fossils were found in deposits that were judged to belong to

3360-679: The Archean without leaving any. Fossil steranes , indicative of eukaryotes, have been reported from Archean strata but were shown to derive from contamination with younger organic matter. No fossil evidence has been discovered for ultramicroscopic intracellular replicators such as viruses . Fossilized microbes from terrestrial microbial mats show that life was already established on land 3.22 billion years ago. Proterozoic Eon The Proterozoic ( IPA : / ˌ p r oʊ t ər ə ˈ z oʊ ɪ k , ˌ p r ɒ t -, - ər oʊ -, - t r ə -, - t r oʊ -/ PROH -tər-ə- ZOH -ik, PROT-, -⁠ər-oh-, -⁠trə-, -⁠troh- )

3456-525: The Archean, the conditions necessary to sustain life could not have occurred until the Archean Eon. Life in the Archean was limited to simple single-celled organisms (lacking nuclei), called prokaryotes . In addition to the domain Bacteria , microfossils of the domain Archaea have also been identified. There are no known eukaryotic fossils from the earliest Archean, though they might have evolved during

3552-399: The Archean. Much of the geological detail of the Archean has been destroyed by subsequent activity. The Earth's atmosphere was also vastly different in composition from today's: the prebiotic atmosphere was a reducing atmosphere rich in methane and lacking free oxygen . The earliest known life , mostly represented by shallow-water microbial mats called stromatolites , started in

3648-442: The Canada–US border are in the elongated east–west valleys; included are Caribou, Clearwater, Crocodile, Daniels, Duncan, Dunn, Hungry Jack, Iron, Loon, Moose, Pine, Portage and South. In Ontario the Rove Formation is overlaid by a thick diabase sill. The Archean Eon lasted from 3,800 million years ago until approximately 2,500 million years ago . The Algoman orogeny occurred 2800 to 2,500 million years ago , and it marks

3744-470: The Late Hadean is more controversial. In 2015, biogenic carbon was detected in zircons dated to 4.1 billion years ago, but this evidence is preliminary and needs validation. Earth was very hostile to life before 4,300 to 4,200 Ma, and the conclusion is that before the Archean Eon, life as we know it would have been challenged by these environmental conditions. While life could have arisen before

3840-550: The Minnesota-Ontario border of the Rove Formation. Earthquakes shattered the ground hundreds of miles away and within seconds ejecta (cloud of ash, rock fragments, gases and droplets of molten rock) began to spread around the globe. It is estimated that at ground zero the earthquake would have registered 10.2 on the Richter scale. Seas covered the Rove Formation area and the Sudbury impact generated huge tsunamis. To put

3936-737: The Neoproterozoic Era at the end of the Proterozoic Eon, possibly climaxing with the hypothesized Snowball Earth of the Sturtian and Marinoan glaciations. One of the most important events of the Proterozoic was the accumulation of oxygen in the Earth's atmosphere. Though oxygen is believed to have been released by photosynthesis as far back as the Archean Eon, it could not build up to any significant degree until mineral sinks of unoxidized sulfur and iron had been exhausted. Until roughly 2.3 billion years ago, oxygen

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4032-592: The Palaeoproterozoic or Mesoproterozoic, according to molecular data. Classically, the boundary between the Proterozoic and the Phanerozoic eons was set at the base of the Cambrian Period when the first fossils of animals, including trilobites and archeocyathids , as well as the animal-like Caveasphaera , appeared. In the second half of the 20th century, a number of fossil forms have been found in Proterozoic rocks, particularly in ones from

4128-464: The Patrician center to the north. The base of the ice sheet encountered the cuesta topography relief of a few hundred feet. Because the ice over the valleys would be thicker than the ice over the ridges, obstructed extrusion flow would operate and resulted in the removal of the easily quarried slates of the valleys. The ridges would not be appreciably eroded, partly because the ice would be thinner over

4224-620: The Pembine–Wassau terrane were thrust up over and onto the Superior craton, forming a mountain range that covered all but the northernmost portion of Lake Superior, crossed parts of three US states (south-central Minnesota , northern Wisconsin , and the Upper Peninsula of Michigan ), and continued to the southernmost tip of Ontario , Canada . Loading of the lithosphere by these thrust sheets caused it to flex downwards, forming

4320-464: The Rove and Gunflint Iron formations . The Rove Formation is the youngest of the many Animikie layers; it consists of gently tilted fine-grained sediments. It is composed of greywackes and black shale , and contains lower concentrations of iron and taconite than the underlying Gunflint Iron Formation does. The Rove Formation consists of a lower argillite unit, a middle transition unit and an upper thin-bedded greywacke unit. The lower argillite unit

4416-556: The Rove region the magma didn't reach the surface; it intruded into fractures in the formation and slowly cooled to become diabase (rather than basalt ). These solidified bodies are the Pigeon River and Logan Intrusion diabases. The continent didn't split into two because the Grenville province (a microcontinent) was converging with the proto-North American continent to the east. This convergence applied compressional forces to

4512-409: The Sudbury Impact event occurred 1,850  ± 1 million years ago . Because of the closeness in dating and the nearness of the crater, the Sudbury Impact event is the likely source for the ejecta; these are the oldest ejecta linked to a specific event on Earth. In the Rove area this layer is about 7.6 m (25 ft) thick; this thin layer very likely represents the catastrophic events of

4608-548: The Sudbury meteorite impact in perspective, the Chicxulub impact on the Yucatán Peninsula occurred 66 million years ago from an object perhaps 60% the size of the Sudbury impactor; the results of this impact caused the worldwide extinction of many species (including dinosaurs). The Sudbury Impact would have had global ramifications; it is conjectured that this caused the end of the iron deposits. The impact fundamentally affected concentrations of dissolved oxygen in

4704-442: The assembly and destruction of one and perhaps several supercontinents . Evidence from banded iron formations, chert beds, chemical sediments and pillow basalts demonstrates that liquid water was prevalent and deep oceanic basins already existed. Asteroid impacts were frequent in the early Archean. Evidence from spherule layers suggests that impacts continued into the later Archean, at an average rate of about one impactor with

4800-486: The breakup of the supercontinent Columbia and prior to the assemblage of the supercontinent Gondwana (~500 Ma). The defining orogenic event associated with the formation of Gondwana was the collision of Africa, South America, Antarctica and Australia forming the Pan-African orogeny . Columbia was dominant in the early-mid Proterozoic and not much is known about continental assemblages before then. There are

4896-546: The center of the lengths of these lakes. The only Rove-typical east–west oriented lakes lying in Ontario are Arrow and North lakes. The preglacial drainage pattern was controlled by the rock structure. Ver Steeg reconstructed a preglacial drainage pattern that shows major streams flowing east in the slate belts. Short tributaries and short north–south segments of the major streams cut across ridges which formed small gaps that are still present. The Laurentian Continental Divide

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4992-465: The contiguous sills. A cuesta – a ridge formed by gently tilted sedimentary rock layers – topography had developed and was dominated by major east–west valleys with a few pronounced gaps in the intervening ridges. The Rainy Lobe of the Wisconsin glaciation was the most recent of the glaciation events, and it retreated about 10,000 years ago. Glacial scratches and grooves in Rove rocks indicate

5088-553: The crystalline remnants of the surviving Archean crust. These include great melt sheets and voluminous plutonic masses of granite , diorite , layered intrusions , anorthosites and monzonites known as sanukitoids . Archean rocks are often heavily metamorphized deep-water sediments, such as graywackes , mudstones , volcanic sediments, and banded iron formations . Volcanic activity was considerably higher than today, with numerous lava eruptions, including unusual types such as komatiite . Carbonate rocks are rare, indicating that

5184-522: The deciphering of Precambrian Supereon tectonics. It is known that tectonic processes of the Proterozoic Eon resemble greatly the evidence of tectonic activity, such as orogenic belts or ophiolite complexes, we see today. Hence, most geologists would conclude that the Earth was active at that time. It is also commonly accepted that during the Precambrian, the Earth went through several supercontinent breakup and rebuilding cycles ( Wilson cycle ). In

5280-471: The deep oceans of the Archean probably covered the continents entirely. Only at the end of the Archean did the continents likely emerge from the ocean. The emergence of continents towards the end of the Archaean initiated continental weathering that left its mark on the oxygen isotope record by enriching seawater with isotopically light oxygen. Due to recycling and metamorphosis of the Archean crust, there

5376-575: The early stages of the Midcontinent rift; zircon dating shows a time period of 1200 to 1100 million years ago. Puckwunge sandstone is exposed along an extremely narrow (feet-wide) band on the southwestern border of the Rove formation. The Puckwunge exposure begins at Raspberry Point within the Grand Portage National Monument on Lake Superior, Cook County, Minnesota, and extends for about 40 km (25 mi) inland to

5472-431: The east–west valleys terminate abruptly at either one or both ends when the bordering sills merge. The elevation of the lakes ranges from Rose Lake 's 465 m (1,526 ft) to Loon Lake's 532 m (1,745 ft) above sea level. Lakes on the opposite sides of a single ridge may differ in surface elevation by as much as 60 m (200 ft). Many of the rock-bound lakes have a depth of about 30 m (100 ft);

5568-416: The ejecta layer. Minnesota's Iron Range is composed of this layer of banded-iron formation. Most of the impact layer in the Rove area consists of beccia, a mixture of rock fragments which ripped loose from the sea floor during the earthquakes. The tsunamis jumbled the loosened bedrock and ejecta together; over time this layer was buried by younger sediments, cemented together and fused by molten rock to form

5664-510: The end of the Penokean orogeny. At the base of the Rove Formation, between the Rove and the underlying Gunflint Iron Formation, there is a lateral layer of shocked quartz and feldspar grains found within accretionary lapilli , accreted grain clusters and spherule masses. These pieces of debris indicate that the layer contains hypervelocity impact ejecta. Zircon geochronologic data shows that this layer formed 1878 to 1836 million years ago;

5760-591: The end of the crust-building Archean Eon. There were several episodes of continental collision , compression and subduction which resulted in mountain building during this time. Orogenic events are characterized by extensive metaphorism, granitic extrusions and unconformities. The Algoman orogeny added landmass along a border from South Dakota to the Lake Huron region; this boundary is the Great Lakes tectonic zone (GLTZ). Northeast Minnesota has 2700-million-year-ago exposed rocks formed during volcanic activity that

5856-426: The eon continued the massive continental accretion that had begun late in the Archean Eon. The Proterozoic Eon also featured the first definitive supercontinent cycles and wholly modern mountain building activity ( orogeny ). There is evidence that the first known glaciations occurred during the Proterozoic. The first began shortly after the beginning of the Proterozoic Eon, and evidence of at least four during

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5952-757: The evolution of eukaryotes via symbiogenesis ; several global glaciations , which produced the 300 million years-long Huronian glaciation (during the Siderian and Rhyacian periods of the Paleoproterozoic) and the hypothesized Snowball Earth (during the Cryogenian period in the late Neoproterozoic); and the Ediacaran period (635–538.8  Ma ), which is characterized by the evolution of abundant soft-bodied multicellular organisms such as sponges , algae , cnidarians , bilaterians and

6048-510: The extreme edge of their range or are disjunct from the main range of their species. They are the Maidenhair spleenwort ( Asplenium trichomanes L.), in Minnesota six small populations of 20 to 40 plants have been found in the Rove area; Ross's (or Short) sedge ( Carex rossii ), only three populations are known to exist in the Rove area; Large-leaved sandwort ( Moehringia macrophylla or Arenaria macrophylla ), evidence indicates that this

6144-735: The first half of the Ediacaran from 0.63–0.55 Ga, and the Belomorian, spanning from 0.55–0.542 Ga. The emergence of advanced single-celled eukaryotes began after the Oxygen Catastrophe . This may have been due to an increase in the oxidized nitrates that eukaryotes use, as opposed to cyanobacteria . It was also during the Proterozoic that the first symbiotic relationships between mitochondria (found in nearly all eukaryotes) and chloroplasts (found in plants and some protists only) and their hosts evolved. By

6240-419: The formation of high grade metamorphism and therefore did not achieve the same levels of subduction as was occurring in the Proterozoic Eon. As a result of remelting of basaltic oceanic crust due to subduction, the cores of the first continents grew large enough to withstand the crustal recycling processes. The long-term tectonic stability of those cratons is why we find continental crust ranging up to

6336-468: The formation; and Holboell's rock-cress ( Boechera retrofracta or Arabis holboellii var. retrofracta ), rare in Ontario and Minnesota. Minnesota's Department of Natural Resources lists the Nodding saxifrage ( Saxifraga cernua ) as endangered, one source refers it to being "very rare" and that Cook County has Minnesota's single colony with about a dozen plants – the entire population occupies less than

6432-515: The habitat to several rare plants which prefer living in narrow cliff areas in a sub-Arctic climate. In Ontario the Rove Formation is overlain by a thick diabase cap. The Rove Formation is in the Arrowhead Region of northeastern Minnesota, U.S., and extends into Ontario, Canada. In Minnesota it occurs along the U.S.-Ontario border from Gunflint Lake to Pigeon Point (both in northeastern Cook County) and northward into Canada. Pigeon Point

6528-665: The late Palaeoproterozoic, eukaryotic organisms had become moderately biodiverse. The blossoming of eukaryotes such as acritarchs did not preclude the expansion of cyanobacteria – in fact, stromatolites reached their greatest abundance and diversity during the Proterozoic, peaking roughly 1.2 billion years ago. The earliest fossils possessing features typical of fungi date to the Paleoproterozoic Era, some 2.4 billion years ago; these multicellular benthic organisms had filamentous structures capable of anastomosis . The Viridiplantae evolved sometime in

6624-625: The late Proterozoic (most recent), the dominant supercontinent was Rodinia (~1000–750 Ma). It consisted of a series of continents attached to a central craton that forms the core of the North American Continent called Laurentia . An example of an orogeny (mountain building processes) associated with the construction of Rodinia is the Grenville orogeny located in Eastern North America. Rodinia formed after

6720-530: The neighboring island arcs and deposited in a forearc basin. Greenstone belts, which include both types of metamorphosed rock, represent sutures between the protocontinents. Plate tectonics likely started vigorously in the Hadean , but slowed down in the Archean. The slowing of plate tectonics was probably due to an increase in the viscosity of the mantle due to outgassing of its water. Plate tectonics likely produced large amounts of continental crust, but

6816-546: The northwest. The Rove Formation is located to the northeast of the Puckwunge sandstone; none is southwest of the border. The Rove Formation is bisected by several bands of Pigeon River Diabase; these bands are in a west-to-east orientation. The bands of Pigeon River Diabase do extend southwesterly past the Puckwunge Formation. The Quaternary Period began about 2 million years ago and continues today. This

6912-424: The oceans were more acidic, due to dissolved carbon dioxide , than during the Proterozoic. Greenstone belts are typical Archean formations, consisting of alternating units of metamorphosed mafic igneous and sedimentary rocks, including Archean felsic volcanic rocks . The metamorphosed igneous rocks were derived from volcanic island arcs , while the metamorphosed sediments represent deep-sea sediments eroded from

7008-716: The oldest rock formations exposed on the surface of the Earth are Archean. Archean rocks are found in Greenland , Siberia , the Canadian Shield , Montana , Wyoming (exposed parts of the Wyoming Craton ), Minnesota (Minnesota River Valley), the Baltic Shield , the Rhodope Massif , Scotland , India , Brazil , western Australia , and southern Africa . Granitic rocks predominate throughout

7104-690: The rest of the Archean continents have been recycled. By the Neoarchean , plate tectonic activity may have been similar to that of the modern Earth, although there was a significantly greater occurrence of slab detachment resulting from a hotter mantle, rheologically weaker plates, and increased tensile stresses on subducting plates due to their crustal material metamorphosing from basalt into eclogite as they sank. There are well-preserved sedimentary basins , and evidence of volcanic arcs , intracontinental rifts , continent-continent collisions and widespread globe-spanning orogenic events suggesting

7200-423: The ridges and therefore less plastic , and partly because of the resistance of the diabase to both quarrying and abrasion by the ice. The weight of the ice sheet compressed the land and created depressions. As the glacier retreated, the weight and pressure were relieved from the surface of the land and the land rebounded; the rebound process continues, and is estimated to be 100 m (330 ft). In Ontario,

7296-460: The rift, preventing the complete splitting apart of the proto-North American continent. This rift was the last of volcanic or mountain-building activity in present-day Minnesota. The solidified lava flows have sagged, tilted and faulted; this created a basin up to 5,000 m (16,000 ft) deep along the rift zone. Proto-Lake Superior filled the basin. The Puckwunge Formation has buff to grey sandstone which comes from sediments deposited during

7392-485: The rift. The deposited lava along the North Shore of Lake Superior is 7,620 m (5 mi) thick. This was a fast-spreading rift; the resulting basalts show little interaction with the then-existing rock. These immense volumes of mafic lava were generated in two major pulses, mostly via a hot mantle plume . Along the North Shore of Lake Superior, one can see the solidified lava (igneous rock) most everywhere. In

7488-472: The same structural elements which control the ridge shapes. Most of the lakes show an asymmetrical bottom configuration, showing a steep subaqueous slope on the south shore. The northern-facing cliffs provide the habitat for a few species of endangered flora. The Rove Formation has landscape features that are not found elsewhere in Minnesota. Most of the valleys are occupied by chains of elongate lakes, many of which are bordered by solid rock on all sides. Many of

7584-434: The sea; the accumulation of marine sediments (known as banded-iron formations ) were almost instantaneously shut down. Banded-iron formations are massive deposits rich in iron oxides ; they accumulated at several periods in the Earth's geologic past. One extended episode of banded-iron formation buildup suddenly ended about 1,850 million years ago . In northeastern Minnesota these banded-iron formations lie immediately under

7680-554: The sessile Ediacaran biota (some of which had evolved sexual reproduction ) and provides the first obvious fossil evidence of life on Earth . The geologic record of the Proterozoic Eon is more complete than that for the preceding Archean Eon. In contrast to the deep-water deposits of the Archean, the Proterozoic features many strata that were laid down in extensive shallow epicontinental seas ; furthermore, many of those rocks are less metamorphosed than Archean rocks, and many are unaltered. Studies of these rocks have shown that

7776-404: The shales and greywackes from the Rove Formation of the Animikie Group are overlain by a 60 m (200 ft) cap of diabase. This diabase cap is a sill remnant, and most of the diabase is covered by a considerable thickness of mineral soil. The mesa at Russell Point (about 16 km (10 mi) south of Thunder Bay ) is a Logan sill of diabase over the softer Rove Formation metasediments. It

7872-443: The softer shales were bulldozed away by the glaciers. The north path of glaciation is transverse to the general trend on the valleys and ridges. As a result of erosion of sandstone and the erosion-resistant sills and dikes, the topography in Minnesota has repeated parallel hills and valleys. The tightly packed lakes in the narrow valleys are long and narrow, and they orient from the east to west. The cliffs in these narrow valleys are

7968-518: The southeastern corner of Nebraska, diagonally northeast through Iowa, through Minnesota along the current Minnesota-Wisconsin border, arced through the present-day Lake Superior basin and angled southeasterly through Michigan. The Midcontinent Rift is the largest-known continental rift in the world. The rift began as a hot spot of basaltic magma underneath the Lake Superior region; it extruded layers of lava up to 20 km (12 mi) thick and extending up to 100 km (60 mi) on either side of

8064-606: The time from the appearance of free oxygen in Earth's atmosphere to just before the proliferation of complex life on the Earth during the Cambrian Explosion . The name Proterozoic combines two words of Greek origin: protero- meaning "former, earlier", and -zoic , meaning "of life". Well-identified events of this eon were the transition to an oxygenated atmosphere during the Paleoproterozoic;

8160-567: The time was virtually all obligate anaerobic . A second, later surge in oxygen concentrations is called the Neoproterozoic Oxygenation Event , occurred during the Middle and Late Neoproterozoic and drove the rapid evolution of multicellular life towards the end of the era. The Proterozoic Eon was a very tectonically active period in the Earth's history. The late Archean Eon to Early Proterozoic Eon corresponds to

8256-853: The time, due to less land area and cloud cover. The processes that gave rise to life on Earth are not completely understood, but there is substantial evidence that life came into existence either near the end of the Hadean Eon or early in the Archean Eon. The earliest evidence for life on Earth is graphite of biogenic origin found in 3.7 billion–year-old metasedimentary rocks discovered in Western Greenland . The earliest identifiable fossils consist of stromatolites , which are microbial mats formed in shallow water by cyanobacteria . The earliest stromatolites are found in 3.48 billion-year-old sandstone discovered in Western Australia . Stromatolites are found throughout

8352-583: The upper unit. The complete thickness of the upper two units is about 900 m (3,000 ft). Gunter Faure and Jack Kovach, using Rb-Sr dating, determined the age to be 1635 ±24 million years old. The Resident Geologist Program, Geology of the Thunder Bay South District, reports an age of 1800 million years old. The Penokean Mountain Range formed in the Penokean orogeny 1880 to 1830 million years ago, when an oceanic island arc called

8448-489: The valleys slope gently to the south (4° to 15° from the horizontal); the northern faces are precipitous; they rise 60 to 140 m (200 to 460 ft) above deep, cold lakes, creating an asymmetrical cross-section profile. The asymmetrical cross section of the ridges is the result of bands of massive, poorly jointed rock alternating with highly jointed rock units. The valleys contain glacially quarried bedrock lakes; they are also asymmetrical in cross section because of

8544-444: Was determined that there were several episodes of rapid increase in continental crust production. The reason for these pulses is unknown, but they seemed to have decreased in magnitude after every period. Evidence of collision and rifting between continents raises the question as to what exactly were the movements of the Archean cratons composing Proterozoic continents. Paleomagnetic and geochronological dating mechanisms have allowed

8640-560: Was enriched in heavier oxygen isotopes relative to the modern ocean, though δ18O values decreased to levels comparable to those of modern oceans over the course of the later part of the eon as a result of increased continental weathering. Astronomers think that the Sun had about 75–80 percent of its present luminosity, yet temperatures on Earth appear to have been near modern levels only 500 million years after Earth's formation (the faint young Sun paradox ). The presence of liquid water

8736-767: Was in the form of seepage of lava from rifts in the sea floor. These lava flows began to rise up out of the ancient ocean to form the Superior craton ; the Superior craton later assembled into the Canadian shield , which became part of the North American craton . The Superior province is the largest preserved fragment of Archean crust, and the Canadian shield is the nucleus of the North American craton. The Proterozoic Eon lasted from 2,500 million years ago until 570 million years ago The Animikie Basin, measuring 700 x 400 km (420 mi x 240 mi),

8832-436: Was probably due to two factors: Exhaustion of the chemical sinks, and an increase in carbon sequestration , which sequestered organic compounds that would have otherwise been oxidized by the atmosphere. The first surge in atmospheric oxygen at the beginning of the Proterozoic is called the Great Oxygenation Event , or alternately the Oxygen Catastrophe – to reflect the mass extinction of almost all life on Earth, which at

8928-487: Was probably only 1% to 2% of its current level. The banded iron formations , which provide most of the world's iron ore , are one mark of that mineral sink process. Their accumulation ceased after 1.9 billion years ago, after the iron in the oceans had all been oxidized . Red beds , which are colored by hematite , indicate an increase in atmospheric oxygen 2 billion years ago. Such massive iron oxide formations are not found in older rocks. The oxygen buildup

9024-411: Was still twice the current level at the transition from the Archean to the Proterozoic (2,500  Ma ). The extra heat was partly remnant heat from planetary accretion , from the formation of the metallic core , and partly arose from the decay of radioactive elements. As a result, the Earth's mantle was significantly hotter than today. Although a few mineral grains have survived from the Hadean ,

9120-761: Was the Labradorian, lasting from 2.0–1.65  Ga . It was followed by the Anabarian, which lasted from 1.65–1.2 Ga and was itself followed by the Turukhanian from 1.2–1.03 Ga. The Turukhanian was succeeded by the Uchuromayan, lasting from 1.03–0.85 Ga, which was in turn succeeded by the Yuzhnouralian, lasting from 0.85–0.63 Ga. The final two zones were the Amadeusian, spanning

9216-607: Was then deposited on top of the ejecta; it is estimated that at ground zero the earthquake generated by the meteor impact would have registered 10.2 on the Richter scale . During the Middle Precambrian a shallow inland sea covered much of the Lake Superior region and formed the Animikie Group of sedimentary rocks overlying 2700-million-year-old Archean rocks. After the Rove Formation sediments were deposited,

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