EOS

A fracture's pattern of tiny cracks in brittle hydrogel — In a Harvard University lab, researchers injected pressurized fluid into a block of clear hydrogel, where it spread in much the same way that fractures travel through rocks or ice in the field, creating the fractures seen here. They found that a hydrogel with few flaws, such as this one, forms long, continuous cracks, creating a smooth fracture surface. (Credit: Will Steinhardt)

Online in December 2018; in print in the February 2019 issue

Our dynamic Earth is scattered with cracks. Earthquakes and petroleum fracking make cracks in rocks underground; melting ice forms cracks in glaciers. Freeze-thaw, roots pushing into weathered rock, the shatter from an impact—all these processes create cracks.

The details of the fracturing process remain a mystery. Scientists know that the roughness of a rock or ice surface can affect how fluids flow across it and how fractures travel through it. But what if you had a detailed 3-D movie of fractures in the act of forming, crack by crack?