They are formed when sedimentary or igneous rocks are exposed to very high pressure and heat underground.
(They; are formed; T:when sedimentary or igneous rocks are exposed to very high pressure and heat underground) (sedimentary or igneous rocks; are exposed; to very high pressure and heat underground)
They sedimentary or igneous rocks
are formed are exposed
T:when sedimentary or igneous rocks are exposed to very high pressure and heat underground
to very high pressure and heat underground