Over millions of years, carbon
dioxide in the air combines
with rainwater to form weak
acids that very slowly dissolve
rocks. Rivers and
streams carry these minerals
to the oceans where they are
used by animals to form coral
reefs and shells. Over longer
time periods, organic carbon
(formed from the remains of
marine life) becomes stored
deep within the Earth’s crust
and forms fossil fuels, such
as oil and natural gas.
Some of this carbon will
be released back into the
atmosphere by volcanoes,
completing the cycle.