Roaming Rocks

Humans have set foot on the moon, sent space machinery to distant planets, and investigated samples collected by the rovers from their operational zones. What is surprising in these triumphs of human history is that the attempts, and the success following them, to know the unknowns are aimed at knowing the space outside of our mother Earth. We have unearthed mysterious truths about how crusts form on the moon. But when it comes to literally unearthing our own ‘Earth’, we have, compared to space exploration, a far shorter list with a handful of historical events of digging downwards.

The immense high pressure and temperature—conditions that have, in fact, powered the major events of Earth since its birth—are the two main reasons behind our futile attempts to look beneath the surface. The inferences we draw about the core world of Earth are mainly based on the signs that rocks and sediments reveal to us.

Interestingly, a special category of the rocks is called ‘Metamorphic Rocks’, or the roaming rocks as the headline of the text says. Originated at the surface level, such as seashores, these rocks go beneath the surface when the tectonic plates go through massive displacements, experience the high pressure and temperature fueling the core of Earth, and return back to the surface level, but with signs of their experiences of thousands of years marked on them.

When rocks of early stages go beneath the surface, their molecules’ bonds weaken because of pressure and temperature, eventually making themselves free to move and change—a capability they didn’t have back in their hard and strong form on the surface. As they adjust to the new environment—at an extremely large time scale—colorful signs, the testaments of their journey to the center of the Earth, gradually become a permanent part of their initial body.

Despite pressure and temperature being the main factors, water—the exceptional element separating Earth from other planets and the ruler of chemical reactions on it—doesn’t play an inconsiderable role. In fact, as the author’s research in Norway shows, lack of water can halt these metamorphic reactions, thereby sometimes making the rocks not develop the signatures or colors that others do in the presence of water.

We might observe the signatures within the blink of our eyes—or maybe within years if we engage in methodological research on them as the author did—but the story dates back enough to go beyond our casual imagination. Roaming rocks have roamed across the core of our planet, and they have a lot to tell us humans about what they saw down there—phenomena and information we could never gather by going so deep into the Earth and returning to the surface as they did.

This summary is based on Marcia Bjornerud’s essay “Roaming Rocks” (Aeon), which explores how metamorphic rocks reveal the deep, hidden history of Earth's interior through their transformative journeys. Main essay link: https://aeon.co/essays/metamorphic-rocks-go-on-deep-journeys-we-never-can