Metamorphic rocks are the changelings of geology. Any rock type will be
metamorphosed if it is subject to high temperatures and pressures.
Metamorphosis changes the minerals and texture of rock. If temperature and
pressure become too great, rock melts and, if cooled, becomes igneous.
Regional metamorphism (heat and pressure)
Pressure from tectonic plates converging can cause large regions of
rock to undergo metamorphism. In mountain ranges like the Andes and Himalayas,
rocks are both metamorphosed and folded by tectonic forces over many kilometres.
The heat and pressure of the collision rearrange minerals to align perpendicular
to the forces. This results in a striped, sheet-like structure known as
foliation.
At lower temperatures and pressures, fine-grained rocks such as shale
or mudstone are often metamorphosed to slate. Slate fractures easily along its
foliation and can be used to make blackboards, roof tiles and floor tiles. When
slate is subject to further heat and pressure, it develops progressively larger
crystals until it becomes gneiss (pronounced “nice”).
Increasing heat and pressure changes fine-grained slate into gneiss,
with its large, foliated crystals. (rock photos by J St John, Wikimedia
Creative Commons)
Contact metamorphism (heat)
Contact metamorphism often occurs around hot igneous intrusions. The
area affected is typically much smaller than that for regional metamorphism –
generally no more than a few hundred metres from the intrusion. The region of
affected rock is called a metamorphic aureole. This is commonly composed of hornfels,
a rock formed when clay-rich sedimentary rocks are heated by an igneous
intrusion.
This granite intrusion (light-coloured rock at top) has caused the
formation of hornfels (dark-coloured striped rock below). (Etienne 2011,
Wikimedia Creative Commons)
Which way did it form?
Limestone metamorphoses to marble in both regional and contact
metamorphism. The calcite crystals grow larger and interlock, but do not form
foliation layers. To determine the method of marble formation, you must examine
the surrounding area for igneous intrusions (contact metamorphism) or foliated metamorphic
rocks (regional metamorphism). Marble is a useful stone that is used for
buildings, statues and decorative items. It is even found in many toothpaste
formulas!
Marble has been used for sculpture for thousands of years.
Quartzite tools
Quartzite is metamorphosed sandstone. It is extremely tough and, when
broken, will fracture through quartz grains. It is also very dangerous to work
with because it is so hard. A hammer will often bounce off quartzite and shards
are sharp and dangerous. It takes great skill to make spear tips, cutting tools
and arrowheads from quartzite. People around the world have used quartzite for
tools for thousands of years. Aboriginal peoples across the north coast used
quartzite or silcrete to produce sharp spear tips and knives. Their distinctive
tools are known as leilira blades.
This quartzite handaxe found in Derbyshire, England, was made 800,000
– 40,000 years ago. (The Portable Antiquities Scheme/The Trustees of the
British Museum, Wikimedia Creative Commons)
Metasomatism and mineral deposits
Metamorphism rearranges existing mineral within a rock. Rocks may lose
water or carbon dioxide during metamorphism, but nothing is added. Metasomatism
is a different process – hot water moves through rock. The hydrothermal solution
acts as a solvent, transporting and adding minerals. This process is important for
mineral deposits because the fluids often concentrate economic minerals, such
as gold. Hydrothermal gold vein deposits are mined throughout Australia. One
example is the Eastern Goldfields area around Kalgoorlie.
This highly metamorphosed rock has metasomatic sulfide mineral
deposits.
Explore metamorphism
- Learn more about metamorphic rocks from BBC Bitesize.
- Try experiments at home as shown in this AusEarthEd video.