Most of the things humans call rocks emerge from the depths of geologic time–rocks are not objects but processes composed of and decomposing in the rock cycle. To be considered a rock, a fragment of the earth’s lithosphere must be manageable for human manipulation (which means ‘handful’)–smaller than a boulder (16″ or more in diameter) but not yet ground down to grit (less than 5mm in size).
Pierre Jardin is awestruck when he learns about the epic geologic histories of stones he collects, including these pert chert and polished jasper pebbles from Moonstone Beach, samples of which may be found in the Petriverse’s Little Free Rock Library. These rocks were formed in the Franciscan Formation, which itself was formed when, from about 200 to 80 million years ago, the North American Plate overrode the Farallon Plate, and an accretionary wedge of islands, deep-sea sediments and oceanic crust was scraped off the Farallon Plate as it descended under the North American Plate. This wedge, the Franciscan Formation, then became part of uplifting coastal mountains, where it was in turn exposed by weathering and erosion and subsequently broken off into pieces, which were gradually carried by streams to the sea, where these worn down stones were tumbled in the waves, and now wash up on the beach as polished pebbles. Considering his own brief life-story and its humble origins, Pierre Jardin is humbled by the stupendous life-path and complex origin story of these stones. As the authors of the informative book Geology Underfoot in Southern California observe, “If these pebbles were interrogated, what a yarn they would spin!”
In striking contrast, some rocks are formed instantaneously, in the heat of a moment. When Pierre Jardin found this curious piece in the Yuha desert, he was unsure what to make of it. Research revealed that it is fulgurite, popularly called “lightning glass,” because it is created when silica is vitrified (heated and melted) by a lightning strike. Jardin found it shocking to think of a rock formed by a lightning bolt. Pondering the power and product of this process electrified his imagination and nearly fried his brain.
Picturing the rock’s creation, Jardin realized that the rock formed in the brief, dreadful silence after a rumble of thunder when lightning flashes, in this case striking the desert sand. In tribute he mounted the fulgurite on a small rock found nearby, with a white mineral streak that neatly conjures a lightning bolt. The display is a diorama of a fiery desert drama!
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