l>Precambrian Era

THE PRECAMBRIAN ERAThe Precambrian Era. The name means: "before the Cambrian period." This old, butstill common term to be originally used to describe the whole duration of earth"s historybefore the formation of the oldest rocks v recognizable fossils in them. In the lastfew decades, however, 6294.orglogists have discovered that there space some hard-to-discern fossils insome Precambrian rocks, for this reason this duration is now additionally known as the Cryptozoic or"obscure life" Eon (from the native "crypt" = "hidden," and"zoon" = "life"). The Precambrian covers nearly 90% that the entire history of the Earth.It has been divided into three eras: the Hadean, the Archean and the Proterozoic.

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The Precambrian Era comprises every one of 6294.orglogic time prior to 600 millionyears ago. The Precambrian was originally defined as the era the predated theemergence that life in the Cambrian Period. The is currently known, however, the life on Earthbegan by the early Archean and also that fossilized organisms became an ext and much more abundantthroughout Precambrian time. The two major subdivisions the the last part of the Precambrian space theArchean (oldest) and also the Proterozoic. Rocks younger 보다 600 Ma are thought about partof the Phanerozoic.

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Below is another form of malfunction of Precambrian time.
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Source: UnknownArcheanArchean ("Ancient" or "Primitive") Era. This era begins around abillion year after the development of the earth, and things have changed a lot! Mostlyeverything has cooled down. Many of the water vapor that remained in the air has cooled andcondensed to kind a worldwide ocean. Even most of the carbon dioxide is gone, having beenchemically changed into limestone and also deposited in ~ the bottom that the ocean. The wait is nowmostly nitrogen, and also the skies is filled v normal clouds and rain. The lava is alsomostly cooled to kind the s floor. The internal of the planet is tho quite warm andactive, as shown by the many erupting volcanoes. The volcanoes form lots of tiny islandsin long chains. The islands space the just land surface. The continents have not created yet.The archipelago are brought over the surface ar of the planet by the motion of rock deep in theearth"s interior. (This activity results from the lose of warmth from the deep internal andis called plate tectonics.) periodically the little islands collide through eachother to form larger islands. At some point these bigger islands will collide to form thecores of the continents we recognize today. Thank goodness those pesky asteroids andmeteorites are now greatly gone, so impact craters form only occasionally. What about life?If you look closely, you will certainly see evidence of blue-green algae (actually basic bacteria)floating in the ocean. That"s all there is! just single-celled bacteria in the ocean.There is as yet no life ~ above land. Life began in the ocean close to the beginning of this era.The oldest recognized fossils - the continues to be of different types of bacteria - room in archeanrocks around 3.5 billion year old.ARCHEAN EARTH
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The maps listed below show the probable areas of Archean (early Precambrian) rocks, which inmany situations formed the early on continental cratons. Cratons are big areas of continentallithosphere (or crust) that have actually remained meaningful and reasonably rigid because thePrecambrian. Laurentia, the phibìc American craton, is one of the oldest and largest. Itincludes the Precambrian shields the Canada and also Greenland, the covered platform and also basinsof the north American interior. Laurentia owes its presence to a network of EarlyProterozoic orogenic belts. Plenty of of the belts room collision zones preserving only thedeformed margins of previously independent microcontinents written of Archean crust. Otherbelts contain accreted beforehand Proterozoic island arcs and also associated intraoceanic deposits. Thus, we now know that cratons to be the early on "cores" that thecontinents; sedimentary rocks accreted ~ above the edges of these cratons later on in 6294.orglogictime, and also then to be folded and also bent up right into mountains together one craton collided v another,or were down-dropped as soon as the cratons split (rifted) apart. Note that the craton because that NorthAmerica, now dubbed the Canadian Shield or the Laurentian Shield, is located simply to thenorth that Michigan, and includes parts of the western UP.

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ProterozoicProterozoic ("Early Life") Era. Well, below we are about 700 million years ago,near the end of the longest time duration in 6294.orglogic history. It began around two billionyears after ~ the formation of the earth and lasted about another 2 billion years! therefore whathas happened in all that time? Hmmmmm. Over there is a lot more land to it is in seen. In fact,there are two supercontinents, one visible across the equator top top this next of the earthand an additional one on the other side. These vast masses of land formed by collisions the themany, plenty of islands make by volcanoes during the Archean and also most of the Proterozoic eras.The earth"s interior has actually cooled part more, and there room fewer volcanoes than in theArchean. Even though the motions of the earth"s surface we contact Plate Tectonics arestill very fast and also continental collisions are regular (every couple of hundred million yearsor so!), the centers or cores of the continent are currently quite large and stable. In fact,6294.orglogists date the beginning of the Proterozoic Era through the age of the earliest continentalrocks that have actually not been reheated or chemically altered. Life has not readjusted much duringthe last 2 billion years, however the couple of changes are significant. Life is still discovered onlyin the ocean, but somewhere about 1.7 billion years ago, single-celled creatures appearedthat had a actual nucleus. Another important change is about to happen: true multi-celledlife is about to appear, part 30 million years before the end of the Proterozoic. Thesemulti-celled creatures will have no difficult parts favor shells or this in their bodies, sotheir fossils will be tough to find. The atmosphere is around the same, greatly nitrogen,with a small water vapor and carbon dioxide. But what"s this? cost-free oxygen exit by thealgae floating in the oceans is start to collection in the air. This single-celledplants have been developing oxygen for about two exchange rate years, yet up until currently the oxygenhas been combining chemically through iron and also other aspects to kind great mineral depositsaround the world. Paradoxically, this oxygen, which us must need to live, is poison tomost of the life develops living top top the Earth during the Proterozoic, therefore another good changein the species of life is about to occur. The earth at this time is also really cold, v huge, bluish glacial icesheets visible across the supercontinent, also in the generally warm equatorial regions! In fact, glaciers got into Michigan in ~ this time; this glaciation is referred to asthe Gowganda glaciation.PROTEROZOIC EARTH

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Source: UnknownNow let"s focus in on the one craton that affects Michigan: the Canadian Shield.Michigan began when a shield-shaped mass of igneous rock, practically two million squaremiles in area and also centering about Hudson’s Bay, developed the original continent ofNorth America i m sorry we describe as the "Canadian Shield." The point of theshield prolonged southward across the area now the western half of the north Peninsulainto Wisconsin, and also its southerly edge was a northward-bearing arc expanding from the LakeSuperior an ar to the Adirondacks. Southward was a shallow sea spanning the remainder ofthe continent block. The granite floor of the sea had many depressions and ridges, butonly the basin-shaped depression bordering the shield top top the southeast need interest us.The basin shallowed scoop-fashion eastward, however the deepest component was about where theSouthern Peninsula is now. That was the early setup of Michigan, the basal structure ofthe State. The maps listed below show the level of the Canadian Shield. Come its eastern liesits stormy edge, wherein folding of the rocks would later cause the Appalachians to form.To that west and also south, an ext stable parts of the continents existed, where only subtledown-warping and up-warping occurred. One of these "downwarped" areas wouldlater come to be the Michigan basin, in which numerous thousands of meter of sedimentsaccumulated and also later ended up being rock.

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Source: picture by Randy Schaetzl, Professor of location - Michigan State University

The map below shows in information where the crucial boundary is in between the Precambrianrocks (PC: of the Canadian Shield) and the Paleozoic-age sedimentary rocks of the MichiganBasin and surrounding areas (C: because that Cambrian).

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Source: UnknownAs quickly as water dropped on the Canadian Shield and the first winds blew, absent weatheringbegan. The rocks started to flake off and break up, and erosion to be initiated. Sedimentswere brought to and deposited in the seas, and the an initial sandy beaches were formed.Several time in these beforehand turbulent eons the land in ~ the edges of the seas were liftedinto high mountain ranges, like the Penokeean Range, just to it is in worn down andtheir sediments carried to the seas to build up new shores and to be spread out on the seafloor to type stratified masses of sandstones, shales, and limestones. Each freshly formedbeach boosted the floor area and advanced the land-water border southward. V eachsuccessive uplift the sedimentary rocks were bent, folded, broken, twisted, contorted, andchanged. Masses of hot igneous rock were required into the damaged formations, completelychanging or metamorphosing lock in countless places so the the rocks do in the severalperiods of hill building and sedimentation ended up being a very facility mass of disturbed,broken (faulted), distorted absent --- the oldest rocks the underlie the western half ofthe north Peninsula. All we can see of the oldest rocks room the Huron Mountains and theother granites and also their kindred the Marquette, Baraga, and Gogebic counties. As soon as theywere a mountainous seacoast, but their summits to be worn in turn and also all but the highestburied under later sediments.

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Source: UnknownRumblings the a new 6294.orglogic disturbance, mountain building, interrupted the quiet Huroniansedimentation of the early on Proterozoic, 2.5 - 1.6 billion year ago. Again mountainbuilding, termed the Penokean Orogeny (1.8 billion years ago) lifted the sediments, nowhardened come rocks, to mountain heights, and also in therefore doing crumbled, folded, and contortedthem. The sandstones were metamorphosed to quartzite, shales come slate, limestones to greenand white marbles, and also the stole sediments were folded and readjusted with them. The rockswere cracked and broken; castle slipped and moved follow me faults, vertically and also horizontally.Some absent blocks were shoved up onto others, some were tipped on end until a madly jumbledmass was made the the flat Huronian sediments. The red and black jaspillite that Jasper Knobnear Ishpeming reflects what happened. Into cracks in the rock warm and hot waters, heavilymineral-laden, made your way. These waters left deposits of minerals that room the pleasure ofthe collector, return the complicated Huronian rocks in which castle are uncovered have to be thelure, despair, and subject of good dispute amongst 6294.orglogists because that the critical hundred years.The pink pegmatite dike in the look white marble the Felch Quarry speak a story oflong precipitate of pure lime metamorphosed come marble, perhaps by the really molten rockthat cracked and forced its method through the marble, then slowly cooled, forming largecrystals that pegmatite.

Parts of the text above have been paraphrased from C.M. Davis’ Readings in the location of Michigan (1964).

This material has actually been compiled because that educational usage only, and may no be reproduced without permission. One copy might be printed for personal use. Please contact Randall Schaetzl (soils