Granite and Grace by Unknown

Author:Unknown
Language: eng
Format: epub
ISBN: 9781948908177
Publisher: University of Nevada Press

Out of this different world, I want to ask, “How, when, and why did plate tectonics begin?” And one answer would seem to be, “It is unlikely that plate tectonics began on Earth as a single global ‘event’ at a distinct time, but rather it is probable that it began locally and progressively became more widespread from the early to the late Archean.”3 Sometime around 2.5 to 4 billion years ago, the earth had cooled enough for continents to form and so began the process of differentiating minerals on the earth’s crust that are presently available for study.

When trying to picture an Earth that did not behave as it has in the last few billion years, imagination is stretched. This world lacked free oxygen and water. One thing is sure: Any granite in that world was not produced by the same means as more recent plutons. By this reckoning of time and process, our granite is a relatively new inhabitant of Earth, born in an emerging physico-chemical environment. And there is every reason to believe that granite, in this sense, is neither original nor pure. It must be a result of what Brown calls “crustal reworking.” To put this case succinctly: On the one hand, “continental crust is the archive of Earth history,” yet on the other hand, “plate tectonics resulted in a biased preservational record.”4

Granite is neither pure, nor original, nor eternally present, and what can be observed now is itself a complicated and perhaps biased record or archive. To put this in the context of a mountaineer’s experience, just as there is no such thing as pure friction climbing, indeed there is no such thing as a pure experience of granite either. How much granite is derived from recycled crustal rocks is by no means clear, but certainly some substantial percentage.5

During the evolution of the lithosphere, everything changes: Petrology—the study of igneous, sedimentary, metamorphic rocks and their formation—becomes fluid. In this reality, granite is ultra-metamorphic. Below the crust, under the pressure of change, everything that rises to the surface of the earth has undergone shape-shifting and alteration: Everything is old and new. As Brown puts it, “Thus, metamorphic petrology is concerned with decoding the mineralogical and microstructural record of burial/heating and exhumation/cooling imprinted on preexisting sedimentary, igneous and metamorphic rocks by processes such as subduction, accretion, trench advance or retreat, collisional orogenesis and orogenic collapse.”6 Call these rocks avatars: concrete embodiments of something abstract, something that has ascended from within Earth. Call granite an avatar of a new world, a concrete manifestation of an abstract concept.7

Just as, in more prosaic terms, one might say that granite is not so much new as recycled, so too each human, as a cultural and biological being, is recycled from previous constituents of the Earth, in multiple and complex ways. To stretch this simile, I note that people have been treading on granite for quite a long time so that many of my experiences are recycled from innumerable traditions of mountain walking and mountain climbing.

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