The term “rolling boil” is used to describe an especially vigorous state in a pot of boiling liquid, and it is sometimes referred to as a “roiling boil.” This type of boiling is often called for when it comes to blanching vegetables and flash-cooking other foods in water. Foods like pasta, which start cooking at the boil, sometimes also benefit from being dropping into a pot of water that has reached this type of boil.
Given that all water boils at the same temperature, 212°F (100°C), differentiating between different “types” of boil might seem odd. Water, in fact, behaves slightly differently depending on how long it has been at the boil, and whether or not the water is still being heated. When water reaches a rolling boil, it means that the boil is so aggressive that it cannot be disturbed or disrupted by stirring or by dropping ingredients into the water.
Cooks can tell when water is starting to boil because large bubbles begin to rise up from the bottom of the pot and burst open at the surface. Water at the early stages of the boil can easily be disrupted with a quick stir with a spoon or whisk. When water is at the rolling boil, the bubbles are much larger, and they bubble up so quickly that when the water is stirred, bubbles will continue to form. Clouds of steam will also roll from the top of the water, which can pose a burn hazard if cooks are not careful.
One reason many recipes call for a rolling boil is to ensure that water is really at the boil when ingredients are added. Water actually starts to bubble below the boiling stage, and when ingredients are added before water reaches the boil, the water does not get a chance to fully boil. When ingredients are added after the water is really boiling, the water is so hot and so agitated that it actually stays at that temperature unless the heat is turned down or a high volume of ingredients are added.
Incidentally, the reason many people recommend starting with cold water when bringing water to a boil is that many minerals are soluble in hot water. If pipes are lined with layers of mineral material and hot water runs through them, the heat will dissolve the minerals, and they will wind up in the cooking pot. Mineral deposits can alter the flavor of the food, and, depending on the minerals, they can also be harmful to a diner's health. These deposits can also accumulate in cooking pots. Cold water has significantly lower levels of dissolved minerals.