9 Metamorphic rocks

Formation

• Metamorphism is the change of a rock into a new state when the temperature and pressure increase, so that the rock re-equilibrates and recrystallizes.
• Any rock (igneous, sedimentary or metamorphic) may be metamorphosed.
• Signs of metamorphism: new minerals form, the texture becomes more crystalline, and minerals arrange themselves perpendicular to the direction of the applied pressure.

Massive metamorphic rocks

• Formed by high temperature near a mass of magma (contact metamorphism); the effect weakens with distance from the contact, and the crystals grow larger (massive granular texture).
• Quartzite: from sandstone (quartz) under very high temperature.
• Marble: from limestone; the calcite crystals are cemented and compacted, so marble is hard and cohesive and is used as an ornamental stone.

Foliated metamorphic rocks

• Temperature and pressure act together; crystals grow as flakes or sheets perpendicular to the pressure, forming foliation.
• Slate: from shale under high pressure and relatively low temperature (below $\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">20</span>}{\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">0</span>}}^{\circ }$C); used in construction.
• Schist (mica schist): mica crystals lie parallel in thin continuous sheets.
• Gneiss: from granite under heat and pressure; the minerals form parallel, discontinuous bands.

Point of comparison	Massive metamorphic	Foliated metamorphic
Main cause	High temperature (near magma)	Temperature and pressure together
Texture	Granular	Sheets or bands (foliation)
Examples	Quartzite, marble	Slate, schist, gneiss