Geology Reference
In-Depth Information
According to the hypothesis, after the ice covered large portions of the earth, the effect of
albedo,
where ice reflects the sun's energy rather than absorbing it, would result in rapid cooling of the earth's
atmosphere and increase the growth of glaciers and ice sheets. When more ice is present, the albedo
reflects even more energy back to space, decreasing the temperature further and allowing even more
ice to build up. So after the earth was covered in ice, it would stay that way for millions of years un-
til greenhouse gasses emitted from volcanoes (thanks to plate tectonics) built up in the atmosphere,
leading to global warming conditions that could melt the ice.
Currently scientists are working to test the snowball earth hypothesis and are coming up with many
questions that need to be answered before this hypothesis can be accepted as an explanatory theory.
One question that many researchers are asking is what happened to life during the snowball earth epis-
ode(s)? Some scientists suggest that hydrothermal vents deep in the ocean, or warm water surround-
ing active volcanoes, provided a refuge for life to exist while the rest of the planet was frozen. Other
researchers go so far as to suggest that the snowball earth event(s) played an important role in driving
the evolution of complex cells (eukaryotes), but they have yet to find any supporting evidence for this
assertion in the fossil record. And still others suggest that the earth could never have been completely
covered in ice, that regions near the equator likely remained ice-free, at least during part of the year.
As I explain in Chapter 7, sedimentary rocks such as sandstone or shale are the result of
rock particles on land being weathered and transported to the sea. Unlike the BIFs,
which are sedimentary rocks precipitated from seawater, continental red beds indicate
that sediment particles on land were exposed to high enough levels of oxygen in the at-
mosphere to form hematite minerals.
Continental red beds appear after BIFs become rare, around 1.8 billion years ago, and
are commonly found in Phanerozoic rocks created in the 542 million years since the end
of the Precambrian. (To find out about what the earth was doing during the Phanerozoic
eon, be sure to read Chapter 19.) Table 18-1 organizes the important things to remember
about BIFs and continental red beds.
