Geology Reference
In-Depth Information
Figure 23: Carbon's longest journey depicted as a set of feedbacks that have kept Gaia's temper-
ature within habitable bounds over geological time.
There are seven interlinked negative feedbacks involved in this great set
(Figure 23)
Volcanoes have the important job of supplying the air with fresh legions of carbon diox-
ide molecules from the fusion of chalk and silica deep in the Earth. Note, however, that
these great conical mountains of lava are free to behave as they wish because there are
no couplings from the rest of the dance to control their tempes- tuous eruptions. Thus
the entirety of the surface world must adapt and respond to the immense churnings of
semi-molten rock deep in Earth's interior, of which volcanoes are one expression.
Let us journey around one of these feedbacks. First of all, look back at Figure 8 to
remind yourself of what the two kinds of arrow (solid and dashed) mean. Now imagine
that great volcanic eruptions have spewed vast amounts of carbon dioxide into the at-
mosphere. The whole of Gaia warms because of the increased greenhouse effect, and
so more water evaporates from the oceans into the air, eventually condensing as rain-
bearing clouds. Some of this rain falls on land where vegetation grows on granite or
basalt. The life-giving rainwater percolates down through the soil to be absorbed by the
plants, which grow better in the moister conditions. More rock is crumbled and ground
up by roots, fungi and bacteria, which breathe large quantities of carbon dioxide onto
the vastly increased surface area provided by a myriad of rocky fragments. This life-en-
hanced weathering of granite and basalt sucks carbon dioxide out of the atmosphere and
