Pressure builds in the frozen shell causing the comet to bulge and eventually crack.
The cracked surface allows outward geysers to shoot away from the comet. These geysers are referred to as jets. As the comet gets closer to the sun, more jets occur until a halo of gas forms around the nucleus called the coma. Coma - The coma is a fuzzy haze surrounding the nucleus which can grow several thousand kilometers in diameter depending on the comet's distance from the sun and the size of the nucleus. As the comet approaches the sun and the Sun's wind blows more strongly, the coma starts to flow from the Nucleus and form the long tail.