Fiordland National Park in New Zealand, the location of the study

What’s the News: Researchers have mapped out the detailed geological history of a 300-square-mile chunk of New Zealand, from 2.5 million years ago to the present day. The study showed how glaciers carved out the area’s distinctive valleys using a little-known technique called thermochronometry, which involves shooting proton beams onto rocks and making note of what happens—along with some impressive analytical skills.

How the Heck: Thermochronometry, as the name implies, is a way to measure both temperature and time. The general principle is that the deeper something is below the Earth’s surface, the warmer it is; thus, tracking a rock’s temperature over millions of years reveals how deep the rock was as it cooled.

This study used a particular version of the method called helium-4/helium-3 thermochronometry, first developed by a member of the research team in 2005. This technique lets you track the time and temperature for apatite, a mineral found in rocks like granite that solidify from liquid magma far beneath the surface. Two chemical elements in apatite—uranium and thorium—are radioactive. As they decay over millions of years, ...