Geology Reference
In-Depth Information
Photochemicaldissociation
occurs when rays of sunlight hit water molecules in the atmo-
sphere and break the molecular bonds. Once broken, the free oxygen atoms bond with
other oxygen atoms to create ozone (O
3
), as well as free oxygen molecules (O
2
). The lay-
er of ozone in the atmosphere protects the earth from the intense heat of the sun's radi-
ation.
While photochemical dissociation produces some free oxygen, after the
ozone layer builds up, it prevents further photochemical dissociation. This means
that photosynthesis must have played a larger role in increasing oxygen levels in
the atmosphere toward the end of the Precambrian time period.
You may be wondering how it is possible to know what gasses were in the atmosphere
billions of years. The answer is in the rocks!
A group of Precambrian sedimentary rocks called
banded iron formations
(BIFs) exist
that provide clues to the evolution of earth's atmosphere. BIFs have alternating layers of
iron minerals and
chert
(a chemical sedimentary rock made of silica; see Chapter 7).
Most likely, these rocks were created by the settling of particles and precipitation of
minerals out of shallow seawater.
BIFs present a puzzle to scientists studying the early earth because for these rocks to
form, iron must have been dissolved in seawater, which can happen only when almost
no oxygen is present in the water. After the iron is dissolved into the water, however, the
oxygen levels increase. When dissolved iron comes into contact with oxygen in the wa-
ter, it forms the minerals hematite (Fe
2
O
3
) and magnetite (Fe
3
O
4
) found in the BIF lay-
ers.
Going to extremes
For many decades, scientists seeking to understand the origins of life thought that only a limited range
of conditions and environments were suitable to support life. They defined the range according to the
needs of life as they knew it, which they understood to be carbon-based life. Carbon-based life requires
access to water, the presence of carbon and oxygen, and a fairly narrow range of temperature and
acidity. Imagine scientists' astonishment when they discovered life surviving in all the most extreme
environments on the planet!
Extreme environments include places that are really hot, like the hot springs at Yellowstone National
Park, or really cold, like the Antarctic ice sheet. Other environments considered extreme include hot
