Yosemite Granite Tells a Different Story About Earth’s Geology Granites solidified from volcanic processes, forming igneous rocks mostly comprised of the minerals quartz and feldspar. “Granites are the ultimate product of the processes by which our planet separated into layers, and they are key to understanding the formation of the continental crust,” Ackerson said. “Minerals from granites record almost all of our planet’s history, from 4.4 billion years ago to today.” So understanding the conditions under which granites form is important to geoscientists trying to unravel the processes that have shaped the Earth. Until now, the prevailing wisdom was that the minerals that comprise granite crystallize as the molten rock cools to temperatures between about 1200 and 1300°F (650 and 700°C). Below these temperatures, the granites were thought to be completely crystallized. It was previously known that under certain conditions some of granites’ minerals could solidify at lower temperatures. So the team, which also included Nicholas Tailby of the American Museum of Natural History and Bruce Watson of Rensselaer Polytechnic Institute, used lab analysis to determine the temperatures of granite crystallization from Yosemite National Park granites. The team employed a technique called titanium-in-quartz thermometry. By measuring the amount of titanium dissolved in the quartz crystals, the team determined the temperatures at which it crystallized within the Earth when the granites formed 90 million years ago. They demonstrated that quartz crystals in samples of a body of granite body called the Tuolumne Intrusive Suite in Yosemite crystallized at temperatures between 885 and 1042°F (474 and 561°C), up to 200° cooler than previously thought possible for granite. “These granites tell a different story,” Ackerson added. “And it could rewrite what we think we understand about how Earth’s continents form.” These findings could influence our understanding of the conditions in which the Earth’s crust first formed during the Hadean and Archean Eons. They could also explain some recent observations about the temperature at which volcanic magmas exist before eruption and the mechanisms through which economically important ore deposits form.  Yosemite’s half dome is a spectacular display of granite. Image courtesy National Park Service/Don Wood, www.nature.nps.gov/geology/geologic_wonders/index.cfm Lead author Michael Ackerson (above) was a Carnegie postdoctoral fellow from 2015 to November 2017. He is now at Rensselaer Polytechnic Institute. Carnegie staff scientist Bjørn Mysen (top) was also part of the team. Images courtesy Michael Ackerson and Bjørn Mysen SUPPORT: NASA Astrobiology Institute supported this work. A team of scientists including Carnegie’s Michael Ackerson, now at Rensselaer Polytechnic Institute, and Bjørn Mysen revealed that granites from Yosemite National Park contain minerals that crystallized at much lower temperatures than previously thought possible. This finding upends scientific understanding of how granites form and what they can teach us about our planet’s geologic history. Nature published the work. This is one quartz sample analyzed. Image courtesy Michael Ackerson 13