Metamorphic rocks come from earlier-formed rocks that have undergone a
change or a _metamorphosis_. All igneous and sedimentary rocks, and
earlier-formed metamorphic rocks too, can be changed, without being
moved to some other place, into new and different rocks. As they are
changed, they may become harder, new minerals may form, and they may
look entirely different. For example, granite, an igneous rock, can be
changed into the metamorphic rock known as _gneiss_; limestone, a
sedimentary rock, can be changed into _marble_; shale, a sedimentary
rock, can be changed into _slate_. These changes occur because the earth
is a big and complex chemical system. The agents that bring about these
changes, which always occur below the surface of the earth, are heat,
pressure, and fluids—both liquids and gases. Several different kinds of
change or metamorphism can take place.