Answer for Week 12, “Water”
Question: Explain the interrelation between the rock cycle
and the water cycle. What role does carbon (e.g., carbon
dioxide, carbonic acid, calcium carbonate) play in this?
Answer:
The water cycle is the means by which the upper part
(above ground) of the rock cycle is carried out. Once
metamorphic, igneous, or sedimentary rocks are uplifted
above the surface and exposed, they begin to erode. Most
major erosion, transportation, and deposition processes are
linked to water in some way. These two related cycles also
intersect the carbon cycle with important consequences.
During precipitation, water is delivered to places where it
can weather the rock. It could fall as snow on a mountain
and become a glacier, which have tremendous erosive
power as they scour out huge U-shaped valleys. Also in
cold climates, rainwater can seep into cracks within the
rock. When it freezes, it expands and breaks open the rock.
This ice wedging is most prevalent at mid latitudes where
diurnal temperature swings can be great. If the rainwater
has combined with enough carbon dioxide, it will be acidic
and can chemically dissolve the rock. This chemical
weathering process is extremely important in wet climates
and in cities where smog makes acid rain stronger. The
continuous pounding of water on rocks at shorelines can
quickly erode protruding sections of shores inland.
Transportation and deposition are mostly carried out by
water as well. Glaciers have already been mentioned as
erosive agents, but they are also transport rock long
distances. Stream and river systems make up the main
form of transportation on the surface. They can carry
particles of rock long distances before slowing enough to
deposit them. Eventually, they reach an ocean and drop out
the sediment that is left. These deposited sediments
eventually become sedimentary rocks.
Groundwater can also be a strong erosive agent, especially
in carbonate-enriched areas (limestone). Any slight acidity
in water percolating through the rock has the potential to
dissolve it. Carbonic acid in water is especially good at
dissolving the calcium carbonate of limestone. This is how
huge networks of caves can form. The dissolved minerals
can be transported to oceans via underwater streams or
remain in underground water tables.
Another interaction of the water cycle with the rock cycle is
at subduction zones. Some water is brought down into the
upper mantle with the subducting plate. This water lowers
the melting point of the rock around it, causing magma to
form. When the magma rises and erupts, it creates igneous
rocks, which can then undergo erosion, transportation, and
deposition at the surface.
The role of carbon has already been seen with carbonic acid
dissolving rock and with carbonate rock being especially
susceptible to such dissolution. In a broader sense, carbon
dioxide (CO2) interacts with the rock and water cycles in
some important ways. Plants (which need water to live)
take in CO2 and release oxygen. Over time, they have
helped take most of the CO2 out of our atmosphere. Much
of the CO2 is stored as carbon within the plants and their
remains (such as oil and natural gas deposits). The other
major sink of CO2 is the carbonate limestone rock forming
on the ocean floor. CO2 can be re-released into the
atmosphere from these two sinks when fossil fuels are
burned, plants are killed (so less CO2 is removed from the
air), or when certain worms churn the sediments on the
ocean floor. Another major way that CO2 is released is
through volcanic eruptions. The amount of carbon dioxide
in the atmosphere can affect climate through the
greenhouse effect and change the erosion regimes both
regionally and worldwide as temperatures rise and fall.
NOTE: The above answer is considerably more detailed
than you would need to get full credit.