other way round. Igneous rocks are changed into metamorphic by heat and pressure. Granite becomes Gneiss. Sedimentary rocks can also become metamorphic by the same heat and pressure. Limestone becomes marble; shale to slate

They can't. Sedimentary or igneous rocks can be converted into metamorphic, but it doesn't work the other way round. EDIT Apparently I'm wrong. Glenn Blaylock is a geologist, so please see his answer.

To convert metamorphic rock into igneous rock it must first be melted in order to produce magma. When this magma cools and crystallizes, it will become an igneous rock.

sounds like a job for eddy van halen

"Igneous rocks (etymology from Latin ignis, fire) are rocks formed by solidification of cooled magma (molten rock), with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. This magma can be derived from partial melts of pre-existing rocks in either the Earth's mantle or crust. Typically, the melting is caused by one or more of the following processes — an increase in temperature, a decrease in pressure, or a change in composition. Over 700 types of igneous rocks have been described, most of them formed beneath the surface of the Earth's crust. These have diverse properties, depending on their composition and how they were formed." "Magma origination: The continental crust is composed primarily of sedimentary rocks resting on crystalline basement formed of a great variety of metamorphic and igneous rocks including granulite and granite. Oceanic crust is composed primarily of basalt and gabbro. Both continental and oceanic crust rest on peridotite of the mantle. Rocks may melt in response to a decrease in pressure, to a change in composition such as an addition of water, to an increase in temperature, or to a combination of these processes." Source and further information: http://en.wikipedia.org/wiki/Igneous_rock Further information: http://en.wikipedia.org/wiki/Magma http://en.wikipedia.org/wiki/Metamorphic_rock