METAMORPHIC ROCKS by Richard (Dick) Gibson 

Any kind of pre-existing rock can be metamorphosed. From Greek words meaning “change form,” metamorphism results when rocks are heated to great temperatures and put under great pressure.  At temperatures greater than about 1000° C (1850° F) most rocks begin to melt.  But long before that, they can become plastic enough to flow, producing convoluted folds and swirls in bands of minerals. And even before that, heat and pressure allow the chemicals in rocks to reorganize themselves into different minerals.  Micas like muscovite and biotite, garnets, and more unusual minerals like kyanite and staurolite are stable at relatively high temperatures and pressures, and may indicate that metamorphism has occurred.  

How metamorphic minerals form 

Most rocks—both sedimentary and igneous—are impure.  They may contain a lot of quartz (silica, the most common constituent of sand) and feldspar (another common mineral, made of potassium, calcium, aluminum, and silicon) and some clays (aluminum silicates with iron and other elements mixed in).  On the earth’s surface, quartz and feldspar are relatively stable and can exist in rocks for millions of years.  But under heat and pressure, the elements recombine.  Garnet is iron+aluminum+silicon, the same common elements found in the original rocks, but arranged more compactly to be stable at high pressure and temperatures.  Once formed, such metamorphic minerals usually last a long time—even at the earth’s surface.  The cores of the Tobacco Root and Ruby Ranges consist of very old—2.9 billion years—metamorphic rocks. Their great age means that they have undergone repeated episodes of metamorphism. The rocks up Mill Creek east of Sheridan are metamorphic rocks.  Metamorphism can occur when rocks are buried or dragged down to great depth, where things are hotter and pressures are greater.  This happens during subduction, when dense rocks plunge beneath lighter rocks, eventually melting.  Metamorphism takes place before the rocks melt, and often enough, those rocks never melt and are preserved.  

Metamorphic Rock Types  Metamorphism forms new minerals in rocks and rearranges them into new layers.  If a metamorphic rock ends up with a lot of mica, this platy mineral will typically create planes in the rock along which the layers can separate. Such rocks are called schists. Other layered metamorphic rocks, containing less mica, are called gneiss.  Marble is a special kind of metamorphic rock, the result of metamorphosing limestone.

Folded Gneiss

Contact metamorphism

Just being close to molten rocks such as granite magmas or lava flowing on the surface is enough to change the older, solid rocks with which the hot rocks come in contact. Contact metamorphism can occur even at the earth’s surface.