Researchers have discovered that the minerals comprising granite crystallize from a molten state at a temperature far lower than previously thought.
The new finding challenges geologists' long-held theories about the history of the Earth and how it formed into the crust, mantle, and core.
How Does Granite Form?
Granite is an igneous rock composed largely of quartz and feldspar. It forms when molten magma seeping through fissures in the crust or mantle solidifies when cooled under the surface of the Earth.
Granites are directly formed through the geologic processes that have separated the Earth into three layers: the crust, mantle, and core. For this reason, studying how granites are formed provides experts with deep insight into how the Earth itself took shape.
"Minerals from granites record almost all of our planet's history—from 4.4 billion years ago to today," explains Michael Ackerson of the Carnegie Institution for Science in Washington DC.
For more than 50 years, geologists have believed that granite forms at temperatures between 1,200 and 1,300 degrees Fahrenheit (650 and 700 degrees Celsius). A new study, however, turns the tables on this assumption by showing that granite crystallizes at temperatures 300 degrees lower.
Low-Temperature Crystallization For Granite
In a new paper published in Nature, a team of researchers explain how a laboratory analysis of granite samples collected at the Yosemite National Park crystallized at a temperature between 885 and 1,042 degrees Fahrenheit (474 and 561 degrees Celsius).
The team collected granite from the Tuolumne Intrusive Suite, a large mass of granite that forms part of the Sierra Nevada Mountains in California. They subjected the granite to a test that measures the amount of titanium in the quartz crystal, leading them to the conclusion that the granite crystallized at cooler temperatures.
Molten rock crystallizes into granite when atoms come together to form tightly connected bonds. Quartz crystals, which comprise majority of granite rocks, typically have four oxygen atoms bonded closely together around one silicon atom. However, under certain conditions, the silicon atom is replaced by one titanium atom. Typically, higher temperatures mean higher concentrations of titanium.
By measuring the amount of titanium present in the quartz crystals, the researchers found that the granite crystallized at lower temperatures than expected when it formed 90 million years ago.
Findings Supported By Computer Model
The findings from the lab analysis are consistent with the results of a computer model developed by Ackerson to check the veracity of their study.
The model looks into the changes in titanium concentrations that happen along with changes in temperature. By mapping titanium concentrations in a quartz crystal cross-section, Ackerson found changing concentrations in titanium as the crystal grew from a center point, in the same way that tree rings reveal the aging of a tree.
"Once you eliminate all the other possibilities, you're left with cold crystallization," says Ackerson. "And that is surprising."
Implications For Science
The discovery has various implications in the scientific and industrial fields. It practically upends what geologists know about the Earth's geothermal gradient, or the changes in temperatures according to how deep one goes into the Earth, and alters knowledge about where to find molten materials under the planet's surface.
Experts monitoring activity at magmatic centers such as the Yellowstone National Park will also have to incorporate their findings when interpreting their data.
Finally, many industrially significant minerals, including gold and copper, are found under granite. The new findings could help industries better understand where to find the ores for these minerals.
