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The Precambrian's First Half: Hadean and Archean Time

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Precambrian rock

The Acasta Gneiss is 4 billion years old.

Andrew Alden photo

This article condenses 4 billion years of Earth history into two pages. It's a strange period, and during almost all of it the most complicated things alive were just strings of algae. If Earth is typical of inhabited planets, then the typical inhabited planet looks like Earth did during Precambrian time.

What's the Precambrian?

When geologists started cataloging the world, the rocks with fossils in them were the easy ones. Fossils are signposts of a rock's age, because they change consistently over geologic time as species evolved and went extinct. So early on, in the 1800s, geologists established a fossil-based framework of geologic age periods, marching back in time from the present Quaternary Period to the Cambrian Period.

Fossils seem to disappear in rocks older than Cambrian, thus all of these periods from the Cambrian forward are put in the Phanerozoic ("visible life") Eon. Anything older, for many years, was lumped together as "pre-Cambrian."

In the 20th century, we devised ways to tell a rock's age based on chemistry rather than fossils. From these we learned that while the Phanerozoic—the whole age of fossils—is 542 million years long, the Earth is about 4550 million years old (4550 Ma). That is, 90 percent of Earth's history is Precambrian.

Two Kinds of Earth History

Phanerozoic history is made for museums, with halls of fossils representing the geologic periods—the days of the trilobites, the amphibians, the dinosaurs and so on, with Man at the end the most interesting of all. And it wasn't just about the fossils: during the Phanerozoic the Earth itself went through significant changes as well—global climate, the composition of the air, the chemistry of the sea and the arrangement of the continents. It can be disconcerting to learn how strange the world has been at times during the Phanerozoic.

Precambrian history is much stranger, and strange in ways that museums can't show. It is a story of an Earth that is quite alien, that only very slowly became friendly to living things like ourselves. Compared to the age of fossils, the Precambrian's story is poorly connected and full of gaps and ambiguity. And there are no dinosaurs or other animated characters, just the dimly pictured face of the planet itself.

The Precambrian time scale has three large divisions: the Hadean, Archean and Proterozoic Eons.

The Hadean Eon (4550–4000 Ma)

The Hadean Eon is not yet a formally recognized time unit, but it essentially marks the era before we have any Earth rocks. The oldest rocks on Earth are just over 4 billion years old. What we know about this time is mostly based on modeling and conjecture. It basically covers the formation of the planet in the original cloud of gas and dust that the Sun was born in.

The most plausible story is that Earth was born a hot body as smaller rocky bodies slammed together (the other theory, now abandoned, is that Earth assembled in a cold state). The iron core separated out and the mantle around it was an ocean of magma. A large body given the name Theia, roughly the size of Mars, collided with the proto-Earth and splashed enough mantle material into orbit to assemble the Moon. All of this happened well before 4000 Ma.

A few microscopic zircon crystals, recycled from earlier rocks, have been dated from before 4000 Ma. It is argued that these indicate the presence of crustal rocks that formed continents, with corresponding oceans. Rocks from Greenland, dating from about 3980 Ma, show signs that living things were already processing carbon at this time.

The Archean Eon (3800–2500 Ma)

Archean rocks cover only about 5 percent of the continents. Most of them have been deeply changed by metamorphism since the time they formed, but they started out as the same kinds of sedimentary and igneous rocks we see around us today.

The Archean Earth had air that would kill us: nitrogen, methane and carbon dioxide. The Archean sky must have been red. Fossil microbes—photosynthetic bacteria—are known from rocks around 3500 Ma. These are anaerobic organisms that live not by oxidizing their food, as we do, but by reducing it, snatching carbon and hydrogen ions from their surroundings. One important class of these microbes is the cyanobacteria ("blue-green algae"), which make oxygen as a waste product.

Rocks remaining from that time are mostly small continental bodies of tonalite, a granitoid rock, surrounded by belts of greenstone, or altered basaltic rocks. What these rocks signify, and whether they participated in plate tectonics like today's, are disputed questions. The Earth's mantle was several hundred degrees hotter than today, producing magmas not found in today's volcanoes.

Bodies of Archean rocks form today's cratons, the cold, stiff hearts of the continents. The largest area of Archean rocks on Earth is in Canada, but all of today's continents have Archean cores.

Sedimentary rocks from shallow Archean seas are preserved in a few places, containing sediments that could not exist today because there was no oxygen in the air (or at least extremely low levels). Any oxygen produced by the cyanobacteria was snapped up by dissolved iron, which came out of solution and piled up in widespread areas of banded iron formation.

The end of the Archean is set by convention at 2500 Ma, but geologically the Earth's environment changed dramatically about 50 million years later. Perhaps some day the authorities will change the time boundaries to reflect geologic history when the Archean gave way to the Proterozoic Eon, described on the next page.

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