Caves and fashion models share one thing in common: They're notoriously hard to date. Now geologists have found a way to get coy caves, at least, to reveal their true ages. The new dating tool, described  in tomorrow's issue of Science, offers a way to gauge rainfall histories, tectonic uplift, and other geologic processes in regions with certain types of caverns.
Pinning down the time at which rocks dissolved away to form a void poses a stiff challenge. Usually, geologists establish a cave's minimum age from the ages of deposits, such as stalactites or sediments, that accumulate after the cave has formed. The new method makes use of a chalky mineral, called alunite, that forms as rock dissolves and then crystallizes on the cave's walls. Alunite forms only where ground water laced with sulfuric acid eats away at limestone from below--a process that creates less than 10% of known major caverns.
One cave system formed by this process is world famous: Carlsbad Caverns in New Mexico. Speleologist Victor Polyak of Texas Tech University in Lubbock collected alunite from Carlsbad and several other caves near the New Mexico-Texas border. His team measured the amount of argon gas trapped in the mineral's crystal lattice. The argon builds up when one of the mineral's major components, potassium, radioactively decays over millions of years--providing an accurate "clock" that reveals when the alunite formed. Earlier estimates for the age of Carlsbad Caverns ranged from 750,000 to 1.2 million years, but the alunite was 4 million years old. "That's a time when a lot of interesting geologic things were happening in this region," says Polyak, including tectonic uplift of the Guadalupe mountains that rise above the caves. Indeed, Polyak notes that older caves are at higher elevations in the mountain chain--a trend that probably reflects a gradual drop in the water table as the land rose.
"The implications of this work go beyond the philosophical pleasure of assigning an age to an empty space," says geologist Ira Sasowsky of the University of Akron in Ohio. For instance, precise cave dates throughout a region may constrain how climate has varied in the past; for example, faster rates of cave formation could be due to wetter periods. However, Polyak notes, researchers cannot yet date the most common type of cave--those born from carbonic acid-bearing water that trickles from above.