Answer:
As the slab gets to a certain depth, it gets hot enough that it start to melt again. As it melts, it undergoes differentiation and also gets less dense. Low density means that it will start to rise back up again, usually, as granitic magma.
Explanation:
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One of the factors that contributed to the rise of the kingdom of Kush was the brief fall of the kingdom of Egypt. With the Hyksos' conquer of Lower Egypt and their subsequent rule over the same, the kingdom of Kush became the superpower in Upper Egypt.
