Answer:
The asnwer is by the modification of existing rocks under the earth's crust because Heat, pressure and active chemical substances.
Step-by-step explanation:
The main way in which metamorphic rocks are formed is by the modification of existing rocks under the earth's crust, hence the term metamorphic. This modification is presented by high temperatures, pressure or also by active chemical substances that produce the transformation of the rocks, changing their structural and mineralogical properties.