Geology Reference
In-Depth Information
brilliantredmineralcinnabar—tookmorethanabillionyearstoappear.Additionalspecies
followed in pulses: a dozen new minerals during the assembly of Kenorland; more than a
half-billionyearsofstasis;anotherhalf-dozenduringtheassemblyofColumbia.Evidently,
as continents collide, the resulting mountain-forming episodes unleash a flood of mineral-
izing fluids—processes that generate new minerals. However, the discovery that such min-
eralization was restricted to intervals of supercontinent formation was a big surprise.
And then an even bigger surprise: for an epic interval from 1.8 billion to 600 million
years ago—a gap exceeding even the span of the boring billion—nothing. Not even during
the time of Rodinia's assembly a billion years ago did a new mercury mineral appear. We
now suspect that the sulfide-laden intermediate ocean is to blame. Cinnabar, mercury sulf-
ide, is among the least soluble of all ores. Any mercury atoms that washed into the ancient
sulfidic seas would have immediately reacted with sulfur to form submicroscopic particles
ofcinnabar,toslowlysettletothebottom,tothwartfurthermercurymineralization.Onlyin
the last 600 million years, as the oceans became oxygen rich, as the lands became covered
with life, did the population of mercury minerals explode.
Mysteries
So, was the explosion of new minerals a consequence of the supercontinent cycle, one of
theboringbillion'ssignaturephenomena?Orwasitsimplyadelayedreactiontotheriseof
oxygen? And what of the element mercury: is the sulfur-rich ocean the whole story? And
what new and unexpected results will studies of the other fifty or so mineral-forming ele-
ments reveal? What's clear is that there's a lot more to learn, for we've only just recently
had our heads turned by the rich subtleties of that billion-year span.
This poorly documented interval, from 1.85 billion to 850 million years ago, shares the
inexorable processes of change that have characterized every stage of our planet's evolu-
tion. By 850 million years ago, Earth's near-surface environment had changed irreversibly.
The increasingly oxygenated ocean margins teemed with algae and other microorganisms,
including the stinky sulfur bacteria, while the lands were poised to burst with new life.
If nothing else, the mysterious, not-so-boring billion teaches us that Earth has the poten-
tial to settle into stasis, a benign balance of its many competing forces. Gravity and heat
flow, sulfur and oxygen, water and life can find and maintain a stable equilibrium for hun-
dreds of millions of years. But there's always a
but
. Give any one of these forces a nudge,
and Earth can become unbalanced, reaching a tipping point with consequences difficult
to anticipate—rapid changes that may disrupt the near-surface environment in a matter of
years.
And that is what happened next.
