hierarchy
Laura Poppick (WW Norton)

The narrative of Earth is one of transformation. Over 4.5 billion years, our planet has evolved from a scorching mass of molten rock and poisonous gases into a temperate and vibrant habitat teeming with diverse life forms. This evolution was punctuated by pauses, restarts, and cataclysmic events, as the intricate biogeochemical processes shaped the most extraordinary phases in Earth’s history.

Our understanding of this vast tale is largely thanks to rocks. They preserve the chronology of events that contributed to the surface’s stratification through various deposits. This intricate ordering is understood through hierarchies, and the scientific discipline dedicated to interpreting them is known as stratigraphy.

In Strata: A Story from Deep Time, journalist Laura Poppick delves into this nuanced science of rock interpretation, offering insights on how planets respond to and recover from periods of upheaval. “Through these layers, we glimpse ancient versions of our planet, gaining contextual awareness as we traverse through the present,” she observes.

Poppick highlights several transformative periods in Earth’s history, selecting four key episodes. The first examines the development of oxygen-rich atmospheres, tracking the evolution of photosynthesizing microorganisms and significant oxidative events that led to mass extinctions around 2.4 billion years ago.

The second segment discusses “Snowball Earth,” a period approximately 720 million years ago when many regions are believed to have frozen over. Following this, she explores the advent of mud and the subsequent rise of vegetation. Finally, the Mesozoic era, dominated by dinosaurs, records atmospheric carbon dioxide levels much higher than today’s due to volcanic activity, offering a framework to understand planetary responses to climate shifts.

Throughout each episode, Poppick introduces geologists working to unravel the numerous unanswered questions regarding the timing and causes of these changes. She visits significant geological sites, from Newfoundland to the Australian Outback, where one can observe the strata that articulate these narratives.

The recurring theme emphasizes the importance of paying attention to rocks. To an untrained observer, they may appear ordinary; however, Poppick reminds us that “a trained eye discerns physical and chemical indicators—proxies—that reveal the characteristics of our planet during the formation of these rocks.” She underscores the value of geologists’ expertise.

This book is a remarkable attempt to make stratigraphy engaging. At times, it falters, and Poppick’s fragmented writing style led me to lose the thread of the narrative.

Her comparisons of geological transformations to human-centric changes sometimes felt uneasy. For instance, she likens the Mesozoic greenhouse climate to modern carbon emissions, though the historical era’s temperatures were so extreme that such analogies may be misleading, even at optimistic emission projections.

Another limitation lies in the currently incomplete nature of geoscience. Some of the pivotal questions raised by Poppick—including the true cause of Snowball Earth—remain unresolved or are subject to debate among different factions. By the end, I was left with a sense of uncertainty about what can be definitively stated. Yet, that unpredictability might be intrinsic to geology itself. “Nothing is immutable in stone as our understanding of geology continues to evolve, just like the rocks,” Poppick states.

Nevertheless, the book effectively captures the grandeur of the story embedded in rocks. It does so particularly well by showing how seemingly mundane observations about rocks can lead directly to profound insights into Earth’s history. Such revelations illuminate the stratigraphic process as Poppick examines overlooked outcrops, encouraging us to perceive the rocks in our surroundings with renewed appreciation.

“Hierarchies are, in many ways, love letters from a maturing Earth,” she argues. This book abundantly reveals the reasons to uncover the secrets they hold.

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