Tectonic cracks are usually associated with pressure ridges. Whenever a ridge terminates in the middle of a sheet there is a tectonic crack continuing onward. They also occur by themselves. The have a characteristic irregularity about their shape. If they are refrozen there is usually several vertical white freeze lines from previous openings. They are also one form of what is called wet cracks (wet cracks also includes contraction cracks).
They range in width from an inch to over a foot. Often they are reasonably easy to see from a moderate distance but sometimes they are not. They are a hazard to iceboaters for catching runners and, possibly, causing a crash. They are generally not a problem for skaters who can step across them fairly easily.
In the spring they may take a different character. They take over the role of accommodating the expansion and contraction in the ice sheet (See Spring Tectonic Cracks)