The Hadean Eon is over. Time to ring in the Archean! It's about 3.4 billion years ago. The huge meteoroids that once wandered the solar system have been cleaned out, often by smacking into planets. The moon is with us, a stable, tidally locked twin planet, cooling as the last of its lavas tint the marias black. The Earth has differentiated into a nickel-iron core, a plastic mantle, and a mostly solid crust with a hydrosphere and an atmosphere. Never again will the entire surface of the earth flash-melt, obliterating life and the rock record and everything else.

Not that this Earth is any idyllic garden. It's hot, hell-hot, hot enough that ultramafic komatiitic lava (a substance every bit as bizarre as its name, which freezes solid at temperatures below 1600°C) flows freely on the surface. The atmosphere is heavy with carbon dioxide, methane, and sulfurous volcanic gases. With no free oxygen in the atmosphere to react with, minerals such as iron-rich pyrites and uraninites accumulate. The iron-rich minerals form distinctive red-and-black-striped banded iron formations. The uraninites, frequently, collect in large enough amounts to go critical in massive spontaneous fission reactions, spreading life-giving nuclear radiation about the Earth.

The precursors of modern continents and oceans develop as less dense granitic rocks float above the basaltic floor, mini-microplates ranging in size from Hawaii to Madagascar, raw young land, all spires and cliffs, steep and rugged. The numberless protocontinents, combined, have nearly the same land area as modern Earth, and between every two mini-continents is a micro-ocean. The full-scale tectonic processes; orogeny, fluvial systems, mid-ocean ridges; are practiced on a small scale with all the exuberant fury of the young. These baby continents and oceans will later be the deep basement rock of the cratons, the great Precambrian continental shields. Twisted and metamorphosed over time, they'll become granitoid greenstone belts: huge blobs of gneisses, the former granitic protocontinents, twined with crumpled sheets of greenstone that were once ocean-floor basalts. As the eon draws on, microplates collide and join together, and small but full-scale continents develop, eventually complete with continental glaciers.

Life establishes itself in the Archean, spreads and flourishes, and from now on will be one of the major shapers of the Earth. Indeed, the story of the Archean is the great tragedy of life and progress.

Extremely simple prokaryotic cells are the first to form, probably in the hot, mineral-rich waters around hydrothermal vents in mid-ocean ridges. The tiny microplates mean that most of the ocean floor is mid-ocean ridge, so rift vents are both a friendly and easily available habitat. These earliest lifeforms get their nutrients directly from the waters around them; but as the population explodes, the food sources are depleted and best habitats overwhelmed. Competition for resources begins, and organisms learn to exploit two new energy sources.

The primitive nutrient-eating organisms are heterotrophs, literally meaning "eaters of others," and so, also, are the first of this new class of creature. But instead of absorbing ambient nutrients, they devour other life forms in order to live. The predator is born! Other organisms develop a different trick, less cruel on the surface; a clever innovation which allows them almost unlimited energy: Photosynthesis, making food from the light of the sun. With this new energy source the photosynthesizing autotrophs quickly become the dominant life form and a second population explosion occurs.

The tragedy is in the overlooked. Photosynthesis consumes carbon dioxide, and it produces oxygen as a waste gas. As life spreads it spews poison into the air. An ozone layer forms from the new oxygen, blocking ultraviolet radiation. Carbon dioxide is depleted; methane and acid-rain-forming gases precipitate; the waters become less acidic, heavy metals are no longer easily soluble, and organic compounds are used up. With the loss of carbon dioxide and the gain of ozone the Earth cools. The atmosphere becomes oxidizing rather than reducing. The Earth's first mass extinction occurs, as all the organisms which evolved under natural conditions can no longer survive in the polluted environment they've made for themselves.

It will be three billion years before life recovers to the point where we begin fixing this mess we've made, burning the waste oxygen, returning carbon dioxide to the atmosphere, rediscovering acid rain, eliminating that deadly ozone layer, reintroducing heavy metals and complex organics to our waters, even figuring out a method to once more bask in the glow of fission reactions--

But meanwhile, the Archean atmosphere has become oxygenated. The few surviving organisms begin to evolve forms and strategies for the altered, harsher environment. The continents are growing and have begun to exhibit entirely modern plate tectonics. It's a gradual process, but at 2.5 billion years ago we've passed the arbitrary end of the Archean Eon, and we move into the Proterozoic.

Sources: Levin, Harold. The Earth Through Time. John Wiley and Sons, Inc. :2003
Notes from Dr. Holtz's Geol120 lecture
Displays at the National Museum of Natural History