Geology Reference
In-Depth Information
water
(hydrothermal)
vents deep in the ocean that spew water rich in dissolved minerals and lakes
super-saturated with salts such as Mono Lake in California.
In recent decades, scientists have discovered that life does indeed exist in places previously thought
too extreme to support life. Various species of bacteria and archaea thrive in these environments. As
a group, these bacteria are called
extremophiles.
Not only can extremophiles exist in these environ-
ments, but also each species has evolved to be specifically adapted for its extreme environment.
Perhaps the most fascinating thing about the discovery of extremophiles on earth is how it redefined
the conditions needed to support life. This means that scientists looking for signs of life outside of the
earth, or
extraterrestrial
life, can expand their search to include environments they previously assumed
could not support any type of life.
Scientists conclude that the formation of BIFs indicates that oxygen levels at the end of
the Archean eon and into the Proterozoic eon were fluctuating — increasing and de-
creasing and increasing again. When oxygen levels in the atmosphere (and therefore
also the oceans) are low, iron minerals dissolve into the seawater. As the oxygen levels
rise (most likely due to increased photosynthetic activity of algae), the iron combines
with oxygen to form minerals that precipitate to the seafloor, depositing layers that
eventually become rocks.
Today, banded iron formations are found in the Pilbara region of Australia and the Mes-
abi mountain range in Minnesota.
The conditions to form BIFs are limited to between 2 and 2.5 billion years ago.
After that time, oxygen levels in the atmosphere appear to have been steadily in-
creasing. Evidence for this increase exists in rock formations called continental
red beds.
Continental red beds
are sedimentary rocks of sandstone or shale,
colored red due to the iron minerals such as hematite (Fe
2
O
3
).
Snowball earth
Can you imagine earth so cold that the oceans freeze over and ice covers all the continents? The rel-
atively recent snowball earth hypothesis proposed in the early 1990s suggests that this may have oc-
curred more than once during the Proterozoic eon. The hypothesis states that the entire earth was
covered at least once, possibly many more times, with ice and snow. There are multiple lines of geolo-
gic evidence suggesting extensive glaciations, but they are not solid enough for the scientific commu-
nity to form a consensus concerning the reality of, or the extent of, snowball earth conditions.
