Tim Alexander, Born Under a Bad Sine (5.10-), Paradise Forks, Arizona

Not all cracks are defined by their size. Often, shape defines the most beautiful. Whether it’s the mesmerizing pattern of salt-and-pepper granite, towering cliffs of perfect hexagonal basalt, or iron veins fingering the tops of Wingate cliffs, nature has the ability to create fantastic patterns. The meaning of this photo, taken at the Sine Wall, lies in one such pattern.

Cracks 101

Rocks take a beating: the lavas that cool to form columnar basalt erupt at temperatures in excess of 1,200 C. Metamorphic rocks like schist and gneiss form deep in the Earth’s crust, where high temperatures combine with ultra-high pressure. And many of the sedimentary rocks on the surface were once buried beneath at least a few kilometers of younger rock.

Under just the right combination of temperature, pressure, and strain-rate, rocks will bend, but they more often break. Geologists call this brittle deformation, and the telltale signs are cracks. From hairline seams too tiny for even the smallest RURP, to gaping fissures too wide for full-body chimneying, it’s all brittle deformation and it all comes about due to stress.

Stress comes in a variety of styles. It can be transmitted as tectonic stress, traveling from one side of a continent to another — when plates slide past each other, sink beneath one another, or collide. Or stress can result from geologically rapid unloading, in which uplift and erosion remove a heavy blanket of covering rock. But cracks also form due to differential erosion. This occurs when layered rocks (sandstone, limestone, schist, and even volcanic ash) contain relatively weak layers that are easily eroded, leaving, say, the horizontal “letterboxes” in places like the Shawangunks, Eldorado Canyon, and American Fork Canyon.

Meanwhile, cracks that have some sort of organization, typically lining up in parallel or semiparallel fashion, are called joints. Joints form in all kinds of rock, but they are most pronounced in homogeneous (i.e., non-layered) rocks. Great examples of joints are the uniformly spaced cracks that split the Wingate sandstone in Indian Creek, the Navajo sandstone in Zion National Park, and the onionskin-like slabs of Sierra granite. Additionally, if one side of the crack gets moved relative to the other, then the crack is technically a fault (no matter how big or small).

What’s a crack to do? Because of weathering and erosion (most notably, things like dissolution and frost-wedging), once a crack forms, it’s likely just to grow bigger and bigger until one wall or the other collapses. So next time you’re below a cliff racking (and even taping!), think of all the cracks that have come and gone to reveal the route you’re about to climb.