fissure, crack, hole
罅 centers on a narrow opening or break in a surface: a fissure, crack, or hole that disrupts solidity.
罅 combines 缶 (earthenware jar) with 虖, which likely contributes the sound and the idea of a crack or fissure, as jars often develop cracks.
A clay jar (缶) develops a crack (虖 suggests the sound and the flaw). Picture a fissure spreading across the surface of an old pot.
For カ, imagine a crack in a car windshield: car -> カ, and the fissure spreads like a crack in a jar.