Getting sphere-ious about spherulites
If you ever have the chance to visit the silica-rich Hot Creek rhyolite lava flow in the Long Valley Caldera, you might get the sense that you're being watched. The hills here don't have eyes, but outcrops of the Hot Creek flow do have another striking feature - spherulites!
The word "spherulite" comes from the Greek "sphaira" (ball or orb) and "lithos" (rock or stone). These distinctive rounded masses of radiating needle-like crystals (first photo) form by devitrification, a process where glassy, non-crystalline substances restructure into crystalline ones.
Spherulites are often formed from minerals like quartz or feldspar, and can range from microscopic to tens of cm in size. At the Hot Creek flow, CalVO scientists have documented spherulites as large as 20 cm (~8 in), some of which can be seen in the second photo. In some cases, when a spheruloid develops one or more void spaces during its growth, it is known as a lithophysa, from Greek "lithos" and "physan" (to blow).
Spherulites can give us insights into the cooling history of a lava flow, since they correspond to the rate at which lava cools. In general, their presence means the flow cooled quickly, but smaller spherulites can indicate faster cooling. The growth patterns of specific minerals also correspond to temperature conditions during cooling. Some spherulites nucleate on crystals or bubbles already in the lava, which can reveal the geochemistry and gas chemistry of the lava at the time of eruption.
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