Stress cracks are tight cracks that are by far the most common crack in ice and the source of most of the noise ice makes. Their formation sounds especially dramatic when one runs between your legs and off to a distant part of the ice sheet.
Very new ice generally does not have many of them but by the time it is a few inches thick they are all over the place. Most of them are associated with thermal stress in the ice. This can take the form of general stress in the sheet or it can be stress created as the top of the sheet cools or heats faster than the bottom. The ratio of tensile strength divided by stiffness and the expansion coefficient is 4 ˚ F. This is the thermal shock resistance and is lower than most other materials. Geometry and thermal conductivity are other factors that come into play.
In black ice we often use stress cracks as a second way to verify that frozen over warm water holes are thick enough (the primary way is usually a test pole). Sometimes these cracks only go part way through the ice sheet.
It is instinctive for most people to pay attention to cracks that form as a result of their moving over the ice. Much of the time these cracks form as a result of a combination of preexisting stress and the added stress of your weight. That is why some parts of a uniformly thick ice sheet are noisy and others are quiet. Sometimes it is because you moved onto thin ice. If you hear cracking that appears to be related to your presence, always check the ice immediately to verify that it is thick enough. Vehicles often initiate stress cracks on thicker ice (see Vehicles and Ice).
Thawed and snow covered ice are more quiet than exposed, cold, hard ice.
As the ice gets thicker some of the stress cracks pull apart a little near the surface of the ice. These can catch skate blades if you happen to be going directly along the crack. A fall often results (one of several reasons why elbow and knee pads and a helmet are a good idea for skaters). If the ice is thick these gaps can be big enough to fit iceboat runners. Usually (but not always) the runner will climb back out before something breaks or gets torn off the boat.