Crack, cleft, fissure, crevasse, crevice, cranny, chink are comparable when meaning an opening, break, or discontinuity made by or as if by splitting or rupture.
Crack basically applies to substances or structures that are subjected to drying, slow disintegration, or shrinking or are fragile or brittle.
Cleft implies an opening or break wider and deeper than a crack and often in a natural structure; it may suggest a defect that is constitutional or an opening that is left by nature.
Fissure does not differ materially from cleft except that it usually suggests a narrow and deep opening and does not carry so strong an implication of inherent defect. The term may denote a normal structural feature or an abnormal condition.
In extended use it usually suggests something abnormal or undesirable.
Crevasse is applied generally and usually in its extended use to a fissure or cleft that is broad and deep, but is particularly applicable to a deep break in the surface of a glacier or a wide breach in a levee.
Crevice and cranny apply especially to a space made by a break or crack (as in a wall or a cliff) that forms a place for dirt to gather or for plants to root and grow.
Cranny often (crevice occasionally) conveys so strongly the notion of an obscure, remote, or hidden nook that it loses completely all suggestion of a mode of formation.
Chink implies a small break or hole sufficient for one to see through or for something to come through.