Whoever imagined that geology was a lifeless subject?

Rocks are still and lifeless things, and geologists are men with beards whose emotional bandwidth is taken up with an unnatural attachment to cherts and clasts and the chill beauty of the subducted lithosphere. Such is the stereotype. The academic geologist and New Yorker contributor Marcia Bjornerud has managed to go a fair distance towards dispelling it. In her previous book, Timefulness, she wrote for the general reader and with persuasive lyricism about readjusting our focus to thinking in geological time. 

Now, in Turning to Stone, she looks back over a lifetime of teaching geology in the American Midwest and fieldwork across the globe to argue for a more integrated understanding of the Earth’s long history. The book is at times intimate and confessional, but the overarching idea is vast and arresting. Bjornerund makes the case for challenging the barrier between the organic and inorganic, for seeing the slow but ever-shifting rocks of Earth’s crust as part of the planet’s ecology, just as much as plants and beasts.

Her argument is based in part on Earth’s unusual dynamism, on the prevalence of water and the ‘tectonic dance’ that is constantly recycling minerals. But she also applies to geology the principle that now drives so much of the natural sciences – universal connectivity. One fact, for me at least, erupted from her text: ‘More than 40 per cent of all mineral species on Earth are in some sense biogenic – produced either directly or indirectly through the action of lifeforms.’ So nearly half the world’s rock-types would not exist but for organic processes. Large-scale deposits of iron, for instance, only formed when atmospheric oxygen was circulated by micro-organisms in the Middle Proterozoic era. If it wasn’t for photosynthesising microbes, we would have no steel.