The Precambrian Era. The name means: "before the Cambrian period." This old, but still common term was originally used to refer to the whole period of earth's history before the formation of the oldest rocks with recognizable fossils in them. In the last few decades, however, geologists have found that there are some hard-to-discern fossils in some Precambrian rocks, so this period is now also known as the Cryptozoic or "obscure life" Eon (from the words "crypt" = "hidden," and "zoon" = "life").
    The Precambrian covers almost 90% of 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 all of geologic time prior to 600 million years ago.  The Precambrian was originally defined as the era that predated the emergence of life in the Cambrian Period. It is now known, however, that life on Earth began by the early Archean and that fossilized organisms became more and more abundant throughout Precambrian time.
    The two major subdivisions of the last part of the Precambrian are the Archean (oldest) and the Proterozoic.  Rocks younger than 600 Ma are considered part of the Phanerozoic.

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Below is another type of breakdown of Precambrian time.
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Hadean ("Hades-like") Era. This era began with the formation of the earth from dust and gas orbiting the Sun about 4.6 billion years ago. During this era the surface of the Earth was like popular visions about Hades: oceans of liquid rock, boiling sulfur, and impact craters everywhere! Volcanoes blast off all over the place, and the rain of rocks and asteroids from space never ends. It's hard to take a step without falling in a pool of lava or getting hit by a meteor! The air is hot, thick, steamy, and full of dust and crud. But you can't breathe it anyway: it's made of nothing but carbon dioxide and water vapor, with traces of nitrogen and smelly sulfur compounds! Any rocks that do form from cooling lavas are quickly buried under new lava flows or blasted to bits by yet another impact. Some people think that an asteroid as large as the planet Mars hit the Earth near the beginning of the Hadean era, completely smashing and melting the Earth and forming the Moon as part of the "splash!"  Wow!  No one has found any rocks on earth from this era. Only meteorites from space and moon rocks are this old.  If any life formed on earth during this era, it was probably destroyed.
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Archean ("Ancient" or "Primitive") Era. This era begins about a billion years after the formation of the earth, and things have changed a lot! Mostly everything has cooled down. Most of the water vapor that was in the air has cooled and condensed to form a global ocean. Even most of the carbon dioxide is gone, having been chemically changed into limestone and deposited at the bottom of the ocean. The air is now mostly nitrogen, and the sky is filled with normal clouds and rain. The lava is also mostly cooled to form the ocean floor. The interior of the earth is still quite hot and active, as shown by the many erupting volcanoes. The volcanoes form lots of small islands in long chains. The islands are the only land surface. The continents have not formed yet. The islands are carried over the surface of the earth by the movement of rock deep in the earth's interior. (This movement results from the loss of heat from the deep interior and is called plate tectonics.) Occasionally the small islands collide with each other to form larger islands. Eventually these larger islands will collide to form the cores of the continents we know today.  Thank goodness those pesky asteroids and meteorites are now mostly gone, so impact craters form only occasionally. What about life? If you look closely, you will see evidence of blue-green algae (actually simple bacteria) floating in the ocean. That's all there is! Just single-celled bacteria in the ocean. There is as yet no life on land. Life began in the ocean near the beginning of this era. The oldest known fossils - the remains of different types of bacteria - are in archean rocks about 3.5 billion years old.
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The maps below show the probable locations of Archean (early Precambrian) rocks, which in many cases formed the early continental cratons. Cratons are large areas of continental lithosphere (or crust) that have remained coherent and relatively rigid since the Precambrian. Laurentia, the North American craton, is one of the oldest and largest. It includes the Precambrian shields of Canada and Greenland, the covered platform and basins of the North American interior. Laurentia owes its existence to a network of Early Proterozoic orogenic belts. Many of the belts are collision zones preserving only the deformed margins of formerly independent microcontinents composed of Archean crust. Other belts contain accreted Early Proterozoic island arcs and associated intraoceanic deposits.
    Thus, we now know that cratons were the early "cores" of the continents; sedimentary rocks accreted onto the edges of these cratons later in geologic time, and then were folded and bent up into mountains as one craton collided with another, or were down-dropped when the cratons split (rifted) apart. Note that the craton for North America, now called the Canadian Shield or the Laurentian Shield, is located just to the north of Michigan, and includes parts of the western UP.

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The early, Precambrian continents were unlike those we know today: they were smaller and had surfaces of igneous rocks. No life lived upon them. Continued squeeze of the cooling, shrinking crust caused the pent up forces beneath to revolt many times and push the land masses higher, or crack their edges and there pour forth molten rock in quiet or explosive volcanic activity. High mountain ranges and plateaus were built or pushed up above the level of the ocean until finally the continents settled to the forms of continental blocks our geographies show today.
    However, the Precambrian oceans did not settle in the basins we know, but covered much of the continents with shallow seas in which the story of a billion years of earth history was to be recorded. The continental shelves are, even today, covered by shallow seas and there the geologic history of today is being recorded. What are these records made in the shallow seas--- these tablets of time on which the story of ages is told? When the first igneous rock cracked, when the first drop of water moved, the record began. When the atmosphere cooled and frost arrived the recording became more rapid. When plant and animal life became abundant the record became complicated. Changes of temperature caused the granitic rocks to flake off at the surface, gravity and moving waters carried the loosened rocks down the slopes, rolled them together, broke them into smaller and smaller particles, carried them to the sea where they became sediments that muddied the seawater. Some of the sediments were dissolved in the water, and as the process kept on through the ages the seas became salty. The sediments, sorted by waves and currents of the first oceans, settled to the bottom of the seas and were spread out on the sea floor. In the course of a long time, as layer upon layer of sediments were piled one on another, they were compressed, cemented, and consolidated into rock which we call sedimentary. Each layer or bed is a stratum; layered or bedded rock is said to be stratified.
    Various minerals in the ancient igneous rocks were made over into different sediments. The hard minerals like quartz collected together as sand, and being heavy, were not carried so far out to sea. They built up coarse and fine rocks which we call sandstones. Other minerals were broken up into fine clay and silt muds that were carried far from shore but eventually settled on the sea floor and compacted to a rock we call shale. Some of the minerals disintegrated to form lime, or calcium carbonate. As the igneous rocks were eroded some minerals dissolved and were carried to the sea in solution; there certain chemical reactions caused the lime to be precipitated and in places to build up great thicknesses of lime mud that eventually solidified into a rock known as limestone. Other chemical reactions caused iron minerals to be formed which in turn settled to the bottom of shallow seas, and later in the geological history gypsum and salt crystallized out of the sea water to form great beds of gypsum and rock salt.

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Proterozoic ("Early Life") Era. Well, here we are about 700 million years ago, near the end of the longest time period in geologic history. It began about two billion years after the formation of the earth and lasted about another two billion years! So what has happened in all that time?  Hmmmmm. There is a lot more land to be seen. In fact, there are two supercontinents, one visible across the equator on this side of the earth and another one on the other side. These huge masses of land formed by collisions of the many, many islands made by volcanoes during the Archean and most of the Proterozoic eras. The earth's interior has cooled some more, and there are fewer volcanoes than in the Archean. Even though the movements of the earth's surface we call Plate Tectonics are still very fast and continental collisions are frequent (every few hundred million years or so!), the centers or cores of the continents are now quite large and stable. In fact, geologists date the beginning of the Proterozoic Era by the age of the oldest continental rocks that have not been reheated or chemically altered. Life has not changed much during the last two billion years, but the few changes are significant. Life is still found only in the ocean, but somewhere around 1.7 billion years ago, single-celled creatures appeared that had a real nucleus. Another important change is about to happen: true multi-celled life is about to appear, some 30 million years before the end of the Proterozoic. These multi-celled creatures will have no hard parts like shells or teeth in their bodies, so their fossils will be hard to find. The atmosphere is about the same, mostly nitrogen, with a little water vapor and carbon dioxide. But what's this? Free oxygen released by the algae floating in the oceans is beginning to collect in the air. These single-celled plants have been producing oxygen for about two billion years, but up until now the oxygen has been combining chemically with iron and other elements to form great mineral deposits around the world. Paradoxically, this oxygen, which we must have to live, is poisonous to most of the life forms living on the Earth during the Proterozoic, so another great change in the types of life is about to occur.
    The earth at this time is also very cold, with huge, bluish glacial ice sheets visible across the supercontinent, even in the normally warm equatorial regions!   In fact, glaciers invaded Michigan at this time; this glaciation is referred to as the Gowganda glaciation.
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Now let's focus in on the one craton that affects Michigan: the Canadian Shield.  Michigan began when a shield-shaped mass of igneous rock, nearly two million square miles in area and centering about Hudson’s Bay, formed the original continent of North America which we refer to as the "Canadian Shield." The point of the shield extended southward across the area now the western half of the Northern Peninsula into Wisconsin, and its southern edge was a northward-bearing arc extending from the Lake Superior region to the Adirondacks. Southward was a shallow sea covering the remainder of the continental block. The granite floor of the sea had many depressions and ridges, but only the basin-shaped depression bordering the shield on the southeast need interest us. The basin shallowed scoop-fashion eastward, but the deepest part was about where the Southern Peninsula is now. That was the early setting of Michigan, the basal foundation of the State.
    The maps below show the extent of the Canadian Shield. To its east lies its unstable edge, where folding of the rocks would later cause the Appalachians to form. To its west and south, more stable parts of the continents existed, where only subtle down-warping and up-warping occurred. One of these "downwarped" areas would later become the Michigan basin, in which many thousands of meters of sediments accumulated and later became rock.

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During the first part of the Proterozoic, a 200 million year-period of quiet, which we name the Huronian from its record north of Lake Huron, thick sediments were laid down in a shallow sea trough that covered the Lake Superior region. In places thick sand was deposited; in other fine muds, and in other places pure lime, accumulated in the shallow but slowly deepening sea. Over the sand great masses of iron minerals accumulated, either by chemical action or by the work of iron forming bacteria, or by both and perhaps other means, until vast thicknesses of sand and iron sediments were built up, and the world’s largest iron deposit was in the making in Minnesota, Wisconsin, and Michigan; and in that far ago time the foundations of Michigan’s wealth and the automobile industry were laid in the old Huronian sediments we now find in the iron ranges of Marquette, Baraga, Iron, Dickinson, Menominee, and Gogebic counties.
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Source:  Photograph by Randy Schaetzl, Professor of Geography - Michigan State University

The map below shows in detail where the important boundary is between the Precambrian rocks (PC: of the Canadian Shield) and the Paleozoic-age sedimentary rocks of the Michigan Basin and surrounding areas (C: for Cambrian).
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As soon as water fell on the Canadian Shield and the first winds blew, rock weathering began. The rocks started to flake off and break up, and erosion was initiated. Sediments were carried to and deposited in the seas, and the first sandy beaches were formed. Several times in these early turbulent eons the land at the edges of the seas were lifted into high mountain ranges, like the Penokeean Range, only to be worn down and their sediments carried to the seas to build up new shores and to be spread on the sea floor to form stratified masses of sandstones, shales, and limestones. Each newly formed beach increased the land area and advanced the land-water boundary southward. With each successive uplift the sedimentary rocks were bent, folded, broken, twisted, contorted, and changed. Masses of hot igneous rock were forced into the broken formations, completely changing or metamorphosing them in many places so that the rocks made in the several periods of mountain building and sedimentation became a very complex mass of disturbed, broken (faulted), distorted rock --- the oldest rocks that underlie the western half of the Northern Peninsula. All we can see of the oldest rocks are the Huron Mountains and the other granites and their kindred of Marquette, Baraga, and Gogebic counties. Once they were a mountainous seacoast, but their summits were worn in turn and all but the highest buried under later sediments.


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Rumblings of a new geologic disturbance, mountain building, interrupted the quiet Huronian sedimentation of the Early Proterozoic, 2.5 - 1.6 billion years ago. Again mountain building, termed the Penokean Orogeny (1.8 billion years ago) lifted the sediments, now hardened to rocks, to mountain heights, and in so doing crumbled, folded, and contorted them. The sandstones were metamorphosed to quartzite, shales to slate, limestones to green and white marbles, and the iron sediments were folded and changed with them. The rocks were cracked and broken; they slipped and moved along faults, vertically and horizontally. Some rock blocks were shoved up onto others, some were tipped on end until a madly jumbled mass was made of the flat Huronian sediments. The red and black jaspillite of Jasper Knob near Ishpeming shows what happened. Into cracks in the rock warm and hot waters, heavily mineral-laden, made their way. These waters left deposits of minerals that are the joy of the collector, although the complex Huronian rocks in which they are found have been the lure, despair, and subject of great dispute among geologists for the last hundred years. The pink pegmatite dike in the glistening white marble of Felch Quarry tells a tale of long sedimentation of pure lime metamorphosed to marble, perhaps by the very molten rock that cracked and forced its way through the marble, then slowly cooled, forming large crystals of pegmatite.

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

This material has been compiled for educational use only, and may not be reproduced without permission.  One copy may be printed for personal use.  Please contact Randall Schaetzl ( for more information or permissions.