Geology Reference
In-Depth Information
greatly enhancing the physical and chemical dissolution of basalt and granite. Wherever
life on land grows on these rocks, it uses a variety of physical and chemical means to
extract the rich lode of nutrients they bear. In so doing life accelerates and enhances the
chemical entanglement of water, carbon dioxide and calcium into calcium bicarbonate,
a liquid trap for carbon dioxide that is eventually deposited at the bottom of the sea by
the chalk-forming sea creatures, and it also speeds up the release of silica that ends up
in the glassy bodies of the diatoms, sponges and radiolarians.
How does life on land accomplish this enhanced weathering? You are a calcium ion
locked away in the crystal lattice of a huge granite dome, and you have been sitting here,
some ten metres from the surface, for tens of millions of years. As you look through the
lattice you see a great deal of empty space, but you spy other calcium ions, and indeed
other chemical beings such as oxygen, silicon and aluminium, all in regular crystalline
arrays. It's like putting your head between two parallel mirrors in a Chinese restaur-
ant where there are a few white lanterns above you. Look into either mirror and you
see serried ranks of lanterns curving off to seeming infinity on either side. The white
lanterns are your fellow calcium ions. You would need to wait for a very long time to
escape if you had only chemical rock weathering to rescue you. But the rock surface
ten metres above you is in fact teeming with countless living beings in a rich, dark soil
which supports fungi, microbes and large plants such as trees and shrubs. A massive tree
root snakes its way towards you through a natural joint in the granite. The root phys-
ically splits open the rock, and in its wake comes rich, black, moist soil, full of micro-
bial life. Some of these microbes are bacteria which secrete a complex sugar molecule
that swells when wet, splitting off little grains of granite from the main rock surfaces of
the tree root tunnel. These little granite fragments provide a massively increased surface
area for the weathering, and are bathed in the gaseous exhalations of billions of teem-
ing microbes that populate the soil. These microbes, like us, use oxygen from the air to
extract energy from food molecules and breathe out carbon dioxide. As a result the soil
is far richer in this gas than is the air above. The tree roots themselves exhale carbon
dioxide as they use oxygen to burn sugars made by photosynthesis in leaves way up in
the churning atmosphere. Furthermore, the soil is made wonderfully porous thanks to
the actions of many small creatures such as woodlice, millipedes and earthworms, who
turn it over like so many gardeners, allowing rainwater to easily percolate down to com-
plete the chemical marriages between calcium and carbon dioxide on the tiny fragments
of rock.
Life on land is a great rock-crushing, rock-dissolving being. Weathering can be con-
siderably enhanced by even relatively simple beings such as lichens and bacteria grow-
ing on the rock surface, but trees and shrubs can reach deeper into the rock, making the
whole process happen very much more quickly. Thus in warm wet tropical conditions,
