Geology Reference
In-Depth Information
fort level, the thermostat responds by turning on the furnace, and the temperature rises.
Once the house warms up, the furnace shuts off. On hot summer days, the air conditioner
mirrors that response by tripping on if the inside temperature rises too high. Earth, too, op-
erates through many similar negative feedback loops that help the planet maintain its more
orlesssteadyconditionsoftemperature,humidity,andcompositionatandnearthesurface.
So, for example, warming oceans result in more clouds, which reflect sunlight back into
space and cool the oceans. Likewise, rising concentrations of atmospheric carbon dioxide
cause global warming, which accelerates rock weathering—a process that gradually con-
sumes excess carbon dioxide and leads in turn to cooling.
Housesalsooccasionallydisplayreinforcingor“positive”feedback,sometimeswithbad
consequences. If your heating system fails on a cold winter day, the pipes can freeze and
burst, causing cold water to flood your home, making the house even colder and less liv-
able. Many of the uncertainties regarding Earth's changing climate today focus on poten-
tial positive feedback loops and their tipping points. Rising sea levels will lead to coastal
flooding, which might result in more evaporation and rainfall, which causes more coastal
flooding. A warming ocean might cause the widespread melting of methane-rich ice at and
beneath the ocean floor, which could add the greenhouse gas methane to the atmosphere
and cause even more warming, which could release even more methane. We have only to
look at the runaway greenhouse effect of our sister planet Venus, with its thick carbon di-
oxide atmosphere and 900-degree Fahrenheit surface temperatures, to see the potentially
catastrophic effects of unchecked positive feedback.
The boring billion, to the extent that it really was boring, was a consequence of many
efficient negative feedbacks that held change in check. In spite of the globe-spanning mi-
grations of landmasses and the repeated assembly and breakup of supercontinents during
that lengthy interval, Earth's climate seems to have been fairly stable. There were no great
ice ages. The chemistry of the anoxic, sulfur-rich ocean didn't change much; nor did life
evolve in any striking new ways. Some new mineral varieties appeared, but no major tip-
ping points altered the air, land, or sea.
All that was about to change with the breakup of Rodinia.
Breakup
In sharp contrast to Earth's enigmatic quiet interval from 1.85 billion to 850 million years
ago, the next few hundred million years experienced some of the most remarkably rapid
and extreme near-surface fluctuations in our planet's history. About 850 million years ago,
most of Earth's many continental masses were still clumped together near the Equator in
thedryandutterlylifelessRodiniasupercontinent.TheimmenseoceanofMirovia,perhaps
punctuated by only a few isolated arcs of volcanic islands, surrounded this denuded, rusty-
