Critical magic

When spells are cast, magic is called to the world, and once the spell has ended, it remains in the area for some time, slowly returning from where it was called from. However, magic tends to gather. This means that spells cast near each other tend to heap together, and slowly the amount of magic will decrease until it disappears. However, the magical attraction becomes stronger when more magic is gathered, as a result, the rate at which the magic disappears slows down. When enough magic is gathered in one point the magic will no longer leave the area, causing a permanent magical area called a critical magic zone.
Critical magic zones are permanently magical, and often have many magical plants spontaneously growing in it. not every area where magical plants grow naturally are critical magical zones since any magic can attract the plants. Since magical plants naturally release a small amount of magic, the magic in an area can sustain itself without being a critical magic zone.
The zones can be destroyed by gathering the magic until there is no longer enough to sustain it, allowing the magic to disappear. This does require long-term monitoring of the area since magical plants and animals can fill the area with more magic. This can give the area enough magic to reach critical magic again.