The main type of energy that is used to convert metamorphic rocks into igneous rocks is heat.
Metamorphic rock can only be converted into igneous rock by being melted and, thereby, made into magma. The main type of energy that is used to melt a substance is, of course, heat.
If we were talking about changing igneous rocks into metamorphic rocks, then the answer would be different. To do that, you need both heat and pressure. But to melt metamorphic rocks into igneous rock, heat is what is needed.
The energy necessary to convert metamorphic rock into igneous rock is heat caused by fire. The word igneous means "born of fire." The rocks can be formed underground or above ground. The ones formed underground occur when magma becomes trapped in pockets. As the magma cools igneous rocks are formed.
Above ground the igneous rocks are formed when magma oozes to the surface, cools and then leaves behind the igneous rock. A volcanic eruption often creates many igneous rocks around the volcano opening. However, magma also escapes in some locations under the ocean. Islands have developed from the formation of igneous rock in the ocean.