The Precambrian: Mystery and Myth (Part II)


A distant explosion.

Millions of miles away, two kernels of space dust begin to circle around one another, triggered by the shockwaves from the distant supernova. As the two specks circle, a third joins their tight gravitational dance. Four…five…six thousand, seven million, eight billion particles are drawn in by the movement. Nine trillion and counting. The momentum of 10 quadrillion particles continues to snowball around what had been empty space. The spinning becomes frantic at the center of this enormous disk. The wanton particles near the axis spin with such terrifying speed, a collision is inevitable. Then, it happens: two small atoms collide at nearly the speed of light, triggering an explosion of nuclear fusion. A helium atom is born in the fire.

And there was morning, the first day.

This ripple effect of spinning, colliding, and drawing in surrounding matter will slowly create a disk reaching more than 8 billion miles wide (the distance light travels in half of a current earth-day). As the swarm of atoms in the center of this solar system continue to collide, metal, dust, ice, and rubble begin to lump together at discrete distances from the disk’s center. About 100 million miles away, one such lump is beginning to form, like a raindrop sliding sideways on the window of a speeding car. The passing moments bring more droplets into this lump, as “raindrops” small and large smash together and gain a critical mass. There is no stopping it now; it grows too large for its own strength. Rocks melt. Metals melt. Fission. Like a centrifuge, the lighter elements are bullied towards the surface. Deep heat is distilled from deep cold as Earth enters its infancy.

One ancient writer painted the beginnings of time in a way that–though not scientific–would resound across the millennia:

“…the earth was without form, and void; and darkness was upon the face of the deep.”

Hadean Eon
An Artist’s Representation of the Hadean Eon

Hadean: Cosmologic Chaos

God as the “Geometer,” from The Frontispiece of the 13th century illuminated Bible Moralisee. Image Credit: Austrian National Library [Public domain], via Wikimedia Commons
The early earth was a chaos, a hell, a literal lake of fire. It is not surprising that the name of the first eon–Hadean, after the god of the underworld in Greek lore–has been applied to this earliest of eons. Older than old, this is the very image of primordial. The blistering heat of radiation and cosmic fission beset against the alien chill of a black universe; the black-body earth glowed red, and Darkness assailed its face. This is just the beginning, the opening of the first act of the Precambrian. 4,500,000,000 (4.5 billion) years ago.

Time before Time

Talking about events that happened this long ago is, in a literal sense, “pointless”: there are no “points” in time we can establish. Or, rather, none that we can delineate with great certainty. All we have are estimates, probably wrong, but also probably close. An “event” which happened “4.5 billion years ago” may have taken millions of years or more to complete, but we don’t have the data (or patience) to parse out the difference between 4,499,999,999 and 4,500,000,001 years ago. It’s not practical. It’s not possible. Time at this distance is “pointless,” or if there are points in time, they are smeared across unfathomable eons.

So, we say that the earth is just over 4.5 billion years old. At this “point” in time, the larval earth is roiling in its orbit, but it is not alone. Another large proto-planet about the size of present-day Mars is following a similar path. Time passes and they approach, and gravity begins to take hold. Positive feedback loop: the two protoplanets collide. The larger retains the “spoils” of this conquest–water, atmosphere, and much of the rock. The smaller bounces away and enters into a slow outward-spiraling orbit around the larger, and the Moon is born of heaven and earth.

The Precambrian Chrono-stratigraphic Units. Periods marked with a clock are delineated only by time, not by geology or paleoenvironment. Image Source: International Commission on Stratigraphy [link]
Less than 100,000,000 (100 million) years later (or so), a few rocks on the infant earth have begun to harden, no longer magma, even if surrounded by a magma ocean. As the rock matures, it is weathered. Small chunks break off and find themselves part of a crystallization process in which they do not entirely belong. Along with the normal crystal elements of silicon, oxygen, and zirconium, these “foreign” rocks that are present include iron, aluminum, uranium, yttrium, ytterbium, hafnium, and a mixture of rare earth elements. Through various means, these elements are “included” in a crystal that humans will later name Zircon. We are fortunate that Zircon was friendly to outsiders, because these crystals are the oldest timekeepers we have. The Zircon crystal acts as a shell, and the mineral inclusions are the clock hands, frozen in time, decaying in solitude.

As the earth continues to solidify on its edges, the moon’s surface solidifies completely. The embers of its youth have faded to the gray of age, brittle and hard. It has grown a shell, almost as if in preparation for the trials to come, because within 600,000,000 (600 million) years (or so), a stream of asteroids bombard the moon. Injured and bleeding out magma, a scarred moon survives these extraterrestrial missiles with craters and basins forming its iconic face. Somewhat defeated and spilling out the last of its magmatism, these basins fill in austere tranquility.

The larger rock around which this aging satellite revolves is not spared the attack, but because it is earlier on in its life, it is able to absorb, adapt, even utilize the asteroids to its advantage, scavenging their frozen waters. The earth continues to cool. At some “point” in Hadean time, parts of Earth reach a critical threshold: 122° Celsius (250° Fahrenheit and nearly 400 Kelvin). At well over water’s standard boiling temperature, this is the highest known temperature at which life can survive. Whether life begins at this point or later is uncertain. Regardless, with the fire and fitfulness of youth quickly fading away, Earth and its maturing satellite begin to emerge out of fire and into the planetary system that seems more familiar to us today.

Archean: Order from Chaos

As the magma on Earth’s surface continues to harden, seams are created in the new crust like a baseball. Unlike a baseball, these seams in the young crust are uneven, active, and are often spilling their contents onto the surface of the globe. Away from these seams, the lighter areas of crust that are forming are lifted above the geoid by the denser crust that line the large oceanic troughs between the cratonic “peaks.” In particular, rocks that would eventually be trod by Ojibwe, Cree and their descendants begin to rise above the other rocks, creating the oldest landmass we know today: the Superior Craton. They call this land Turtle Island a few short centuries before a lighter-skinned race of humans will re-name it “America” after one of their culture’s first explorers of the (erroneously-named, geologically speaking) “new” world. Amerigo Vespucci had mapped much of the American continents’ borders and was one of the first to propose it as distinct landmass from Afro-Eurasia.

Vespucci explored this new-yet-old continent from a boat that skimmed atop an ocean of liquid water, which is beginning to fill before and during the time the Superior Craton is forming. During this time, ice from asteroids combines with the water, ammonia, and carbon dioxide that is present in the proto-earth material and which is extruded from volcanoes. This liquefied water hurries to the lowest places where the oceanic crust rides lower on the mantle than the more buoyant continental crusts. The oceans are brimmed, and the early, methane-rich atmosphere clings tightly to its spinning parent of rock and water.

The evolution of the Earth–its temperature, the atmosphere, the oceans, and the differentiation of rocks–begin to march directly, if unknowingly, towards a singular destination: life.

Proterozoic: The Cradle of Life

By the time the Proterozoic Eon begins 2,500,000,000 (2.5 billion) years ago, prokaryotic life is already well-established on Earth. The cratonic seeds of each of the continents are beginning to form, and are in the process of agglomerating into a supercontinent known as Columbia.

Proposed make-up of Columbia Supercontinent. Image Source: Wikipedia

Cyanobacteria are already flourishing in the ocean at least 3,400,000,000 (3.4 billion) years ago. Though the sun radiates only about 80% of the energy it currently emits (and is about 10% smaller in size), these cyanobacteria begin to photosynthesize. Powered by this immature sun, cyanobacteria convert carbon dioxide and water (both of which are extraordinarily plentiful) into oxygen. Billions of bacteria are engaged in this practice for millions of years. Oxygen slowly begins to fill the atmosphere, and it dramatically changes the world as we know it.

The manner in which these Oxygen molecules begin to indiscriminately circulate throughout the gaseous ether befits their name: free oxygen. At first, the recently liberated oxygen produced by photosynthesis plays the role of thief, stealing electrons from elements such as iron. The larceny of free oxygen in Earth’s atmosphere has another, even more prominent role, however, in the development of Earth’s history than the deposition of the majority of the iron ore used today. It oxidizes methane–which is common in the atmosphere at this time–to carbon dioxide, and in doing so, oxygen radically decreases the atmosphere’s insulation effect. With the sun much less powerful than today, the Earth begins to cool.

This “Great Oxygenation Event” is one of those point-less points in time. It evolves over the course of 1,600,000,000 (1.6 billion) years or so, beginning about 2,500,000,000 (2.5 billion) years ago. For the first 600,000,000 (600 million) years, the concentration of oxygen in the miasmatic air shoots up from a negligible amount to somewhere in the neighborhood of 4% (just under 1/5 of its current concentration). With photosynthesis largely stopped due to the ice and snow, the oxygen concentration levels off for about 1,000,000,000 (1 billion) years, with land and water breathing in the extra oxygen that is produced during this time. Then, about 850,000,000 (850 million) years ago–in the final stages of the Precambrian era–the oxygen level begins to shoot up again, from 4% to over 20% in a short 400,000,000 years.

During the initial stages of this long event, Earth enters its longest glacial period, called the Huronian Glaciation. Many postulate that Earth was more or less frozen along the entire surface, even down through the tropics. “Snowball Earth” is the name given to this period.

Snowball Earth from Space
Artist Depiction of a Snowball Earth. Image Credit: NASA [link]
The Great Oxygenation Event and the Huronian Glaciation lead to one of the largest mass extinctions in Earth’s history. Though nearly all of the species alive at the beginning of this event are single-celled prokaryotes, many of them die off. These species have developed in the absence of oxygen (anaerobic), and are poisoned when the atmosphere shifts so dramatically.

However, this is also a moment of great triumph for life on Earth. Evidence of the first eukaryotic fungi and first eukaryotic bacterium date back to this time (though some eukaryotic species may be older), beginning the march to complexity that would explode at the close of the Precambrian.

For the first half of the great oxygenation event, the landmasses of the world are still huddled together in the Columbia supercontinent, as if for survival against the cold. They sometimes huddled too closely. Several collisional mountainbuilding belts are formed during this time, presaging mountain-building (orogeny) as we know it today, as the continents began to encounter one another on a large scale. Inevitably, as with any group of companions brought together under unique and trying circumstances, the various continents (or continent-seeds) that formed Columbia begin to drift apart, though with the scars and memories of their boisterous soiree.

This begins as the Paleoproterozoic transitions into the Mesoproterozoic. The term “mesoproterozoic” has three parts: Meso– meaning middle; protero– earlier or before; zoe- life. It is the middle era of a time when life existed, but not in the diversity that we know it today.  With relatively stable conditions book-ended by cataclysmic glaciations and roiling supercontinents, the Mesoproterozoic has been described as, well, boring. But like most middle siblings, the Mesoproterozoic may just be misunderstood.

The supercontinent Rodinia (“homeland” in Russian) begins to form in the Mesoproterozoic. At the center of the landmass, the Laurentian continent (later, “North America”) is hovering around the equator, rotated 90° clockwise from its current position. From the south, Amazonia (later, “South America”) crushes into Laurentia. Laurentia folds, bends, thrusts up into the newly-oxygenated atmosphere.

The evidence of this largely been lost, confused among the mountain-building episodes of more recent eons. But in several paces, you can still see the “skid marks” of this collision. One of these places is a small town of less than 2,000 people in French-speaking Quebec. Here, the most-recent mountain-shaping took the form of a canal devised by the British as a safeguard against a potential American attack following the hostilities of the War of 1812. But times have changed since then. The Grenville Canal has fallen into ruin with the rising of water behind a dam built in the 1960s, speaking of a bygone era. Similarly, the ruins of the Grenville Orogeny speak to those geologists who care to listen to the ancient rocks that are the youngest part of the Canadian Shield.

Grenville Canal Ruins
Ruins of the Grenville Canal. Image Credit: Radio Canada

The End of an Eon and the Beginning of Time

As the sun sets on the Mesoproterozoic, the last era of the Precambrian begins. Firmly in the Neoproterozoic, the Earth is undergoing dramatic changes that will not be undone. The first animals emerge. Ice–more ice than could be imagined–come and go and come again: the Cryogenian, an ice world. Rodinia has migrated south to form a daughter supercontinent, Pannotia. As the Precambrian comes to a close, this supercontinent is breaking open.

The end of the Precambrian is really more of a beginning. The final geological period of the Precambrian is the first with a stratigraphic Golden Spike. In a sense, this is where Geologic History truly begins, where geologists can first set the geologic clock by geology and not by arbitrary time.

Earth is leaving behind a history of myth and mystery. This is Earth, as we know it.

[This is the second post of a three-part series. The first is The Precambrian: An Origin Story and tells the story of the feud between the two geologists who discovered and named the Precambrian Eon. The third is titled: The Precambrian: 20,000 Feet Under the Trees and explores the story of the Precambrian in Illinois.

*The story presented in this post is based on hypotheses. All have scientific support, but some of them are better understood and delineated than others. All of them “make sense” in the broadest application of that term. Trying to understand the history of the Earth, the Sun, the Solar System, and the Universe is complex. This post is for illustrative purposes, but I hope it to be as accurate as the current science is.

This is not a scientific paper, but reference the following (usually easy-to-read) sources for the background to this story:

  • Supernova sparks the solar system spinning [link]
  • Sedna, an object orbiting the sun 8 billion miles from the Solar system’s center of gravity [link]
  • Birth of stratigraphy = planetary differentiation [link]
  • Hot early earth and cold surrounding universe [link]
  • Age of the moon’s solidified crust at 4.3-4.5 billion years [link]
  • Zircon “Clocks” [link 1, 2, 3]
  • Late Heavy Bombardment [link 1, 2, 3]
  • Life at 122 celsius [link]
  • Solar evolution (and less radiation in the past) [link]
  • Great Oxygenation Event [link]
  • Huronian Glaciation [link]
  • Vespucci finding new continent [link]
  • Mass extinction [link]
  • Emergence of Eukaryotes [link]
  • Grenville Orogeny [link]
  • Cryogenian ice age [link]

This site by BBC shows some helpful illustrations of Earth’s early timeline as well.