Dirt: The Erosion of Civilizations - Kootenay Local Agricultural Society

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eating into the ancient uplands from the side. Standing on the cliff at the
edge of the plateau—a small remnant of the original land surface—I
admired the wake of new rolling lowlands that fell away toward the
Atlantic Ocean.
The Carajás Plateau is made up of banded iron—almost pure iron ore
deposited by an anoxic sea long before Earth’s oxygen-rich atmosphere
evolved. Buried deep in the earth’s crust and eventually pushed back to the
surface to weather slowly, the iron-rich rock gradually lost nutrients and
impurities to seeping water, leaving behind a deeply weathered iron crust.
Aluminum and iron ore can form naturally through this slow weathering
process. Over geologic time, the ample rainfall and hot temperatures
of the tropics can concentrate aluminum and iron as chemical weathering
leaches away almost everything else from the original rock. Although it
may take a hundred million years, it is far more cost-effective to let geologic
processes do the work than to industrially concentrate the stuff.
Given time, this process can make a commercially viable ore—as long as
weathering outpaces erosion. If erosion occurs too rapidly, the weathered
material disappears long before it could become concentrated enough to
be worth mining.
On top of the Carajás Plateau, a gigantic pit opened a window into the
earth, extending hundreds of feet to the base of the deep red weathered
rock. Huge, three-story-tall trucks crawled up the terraced walls, dragging
tons of dirt along the road that snaked up from the bottom. Viewed from
the far side, the hundred-foot-tall trees left standing on the rim of the pit
looked like a fringe of mold. Gazing at this bizarre sight in the midday sun,
I realized how the thin film of soil and vegetation covering Earth’s surface
resembles a coating of lichen on a boulder.
Speeding off the plateau, we dropped down to the young rolling hills
made of rock that once lay beneath the now-eroded highlands. As we drove
through virgin rainforest, road cuts exposed soil one to several feet thick
on the dissected slopes leading down to the deforested lowland. Leaving
the jungle, we saw bare slopes that provided stark evidence that topsoil erosion
following forest clearing led to abandoned farms. Around villages on
the forest’s edge, squatters farmed freshly cleared tracts. Weathered rock
exposed along the road poked out of what had until recently been soilcovered
slopes. The story was transparently simple. Soon after forest clearing,
the soil eroded away and people moved deeper into the jungle to clear
new fields.
w estward hoe