Laurence C. Smith - Rivers of Power: How a Natural Force Raised Kingdoms, Destroyed Civilizations, and Shapes Our World

When the first rains came, the world changed forever.

They would have come sooner, by 100 million years or so, if not for the collision with another planet. It was roughly the size of Mars. The crash was so massive that our young Earth was engulfed in fire and mostly melted. A giant piece sheared off and most likely became the Moon. A magma ocean churned and raged on the surface of the wrecked planet.

Then, the primordial surface began to cool. A crust of iron-rich rocks hardened on the Earths magma ocean. A lighter crust formed too, floating like slag in a smelter. A smattering of zircons, best known today for their use as inexpensive gemstones, began to crystallize. Trace remnants of them can still be found today in ancient rocks of Australia, Canada, and Greenland.

Australias zircons have been dated as far back as 4.4 billion years. This signifies that continental crust began forming on Earth much earlier than previously thought, perhaps just 200 million years after our planet first congealed out of a swirling disk of cosmic dust and gas some 4.6 billion years ago. The chemical composition of these crystals tells us that at least trace amounts of liquid water were already present, despite Earths extreme volcanism and the inferno caused by its collision with the other young planet. Like miniature time machines, zircons offer a glimpse into the Earths earliest eons, the Hadean (named after Hades, the Greek god of the underworld) and the Archean (derived from the Greek word arkh , meaning beginning). From their chemistry, weve learned that our worlds early magma ocean cooled quickly and that continents and water came soon after.

By four billion years ago, if not sooner, rain began falling from the young sky. Water pooled in lakes and seeped into the ground. Water flowed overland into rivulets, streams, and rivers to the newly filling seas. Water evaporated into the poisonous air, condensed into clouds, and rained down again to complete the cycle. Water began eroding Earths infantile, thickening continental crust, opening an eternal war against the continents.

Bit by bit, the rains broke down the high ground and filled in the low. They dissolved rock and loosened minerals. They weathered mountains and nudged their detritus downhill. Trickles found one another, came together, and grew stronger. They joined together, over and over, until millions combined into a powerful forcerivers.

The rivers had one job to do: Move it all downhill. Down, down to the sea.

Where tectonic collisions raised mountains, water and gravity allied to grind them down. Where plates wrenched open new seas, rivers toiled to fill them. Muddied with silt, their waters coalesced like roots toward a stem. Gravel jostled and rolled down flowing branches to some final destination.

At the end of their run, the rivers died in seas and lakes. Spent, they dumped their sediments and evaporated like spirits, rising aloft, back to the high ground to attack, flatten, carry, and dump again. Mountains are tough, but even the mightiest spire is doomed to fall to this tireless enemy. The water cycle outlasts them all.

By at least 3.7 billion years ago, rivers were steadily depositing sediments into the worlds oceans. A couple hundred million years later, blue-green cyanobacteriathe worlds earliest photosynthesizersbegan generating whiffs of oxygenated air. Around 2.1 billion years ago this oxygen production surged. Pyrite (fools gold) and other readily oxidized minerals disappeared from the riverbeds. The worlds iron-rich soils reddened like rust.

Another billion-plus years passed. Then, between 800 and 550 million years ago, the oceans exhalation of oxygen surged again. Sponges, flatworms, and other strange new marine life-forms appeared. In the eons to follow, these early organisms would persevere, advance, and eventually populate our world in strange and marvelous ways.

Meanwhile, the continents thickened and crashed about. New mountain ranges swelled and were beaten down. But their rocky substance was transmuted, not lost. The relentless rivers spread their debris across the lowlands, building wide, flat valley plains. Deep stratigraphic sequences were laid down, layer upon layer, slowly infilling the basins and seas. River deltas pushed out fingers of new land far offshore into the oceans.

Rivers are literally universal. From orbiting spacecraft, we see them on other worlds. Mars once had abundant liquid water, and its surface is now scarred with the dry channels, deltas, and layered sedimentary deposits of ancient rivers. At this very moment there are rivers flowing actively on Titan, a cold distant moon of Saturn. Their fluid is liquid methane and the bedrock they carve is believed to be made of ice, but the valleys, deltas, and seas they are busily creating are eerily Earthlike in pattern and form.

Oceans opened and closed. Continents crashed and bulged. Some of the rivers sediments were dragged deep down into the Earths mantle on the backs of subsiding tectonic plates, where they were ferociously squeezed and heated. The cooked remains further thickened the continents and rose, like heated wax in a lava lamp, to cool into the hardened roots of new mountain ranges. Eventually some of this same material was exhumed, pulverized, and carried off by rivers once again, for yet another journey back to the sea.

Our worlds destructive construction project never ends. Mountain ranges rise, then are pounded into sand. Their debris fans out across river valleys, deltas, and offshore continental shelves. Every earthquake, every landslide, every raging flood, marks just another little rumble in this ceaseless war between two ancient forcesplate tectonics and waterthat are locked in combat for the shape of our worlds surface. Their war will continue for at least another 2.8 billion years or so, until our dying, expanding Sun boils away every last drop into steam.