Metamorphic Rocks

Metamorphic rocks are rocks that have been transformed by heat, pressure or chemical changes deep inside the Earth. The word "metamorphic" comes from Greek and means "changed in form". When older rocks (either igneous, sedimentary or even older metamorphic rocks) are buried deep underground where temperatures are high and pressures are enormous, their crystals re-grow and re-arrange themselves. Without ever melting, the rock changes into something completely different. Marble (changed from limestone), slate (from shale) and gneiss (from granite) are all famous metamorphic rocks.

  • Word originGreek "metamorph"Meaning "changed in form"
  • Caused byHeat + pressurePlus sometimes fluids
  • % of crust by volumeApproximately 27%
  • Without meltingAlwaysIf it melts, you get igneous instead
  • Famous: marbleChanged fromLimestone
  • Famous: slateChanged fromShale

How metamorphic rocks form

Metamorphism happens when rocks are subjected to:

  • Heat: from being buried deep, or from being near an igneous intrusion.
  • Pressure: from the weight of overlying rock, plus extra force from colliding tectonic plates.
  • Fluids: hot mineral-rich water can drive chemical changes.

Under these conditions, the minerals in the original rock recrystallise into different minerals (or the same minerals in different shapes). The whole rock changes texture and chemistry. Importantly, the rock does NOT melt: if it did, you would get magma and eventually an igneous rock instead.

Two main kinds of metamorphism

  • Regional metamorphism: huge areas of rock get cooked and squeezed at the same time, usually because they are deep underground at the heart of a collision between tectonic plates. This is how the cores of all major mountain ranges are made: enormous quantities of metamorphic rock, formed when continents collide.
  • Contact (or thermal) metamorphism: a much smaller patch of rock is baked by being close to a mass of igneous magma. Forms a "metamorphic halo" around the igneous intrusion.

Famous metamorphic rocks and their parents

  • Marble: comes from limestone. The original calcium carbonate recrystallises into larger interlocking crystals, often beautifully patterned with coloured veins. Used for sculpture (the Greeks and Romans loved it) and for stunning building floors.
  • Slate: comes from shale. Squashed flat, it splits naturally into thin, flat sheets, which is why slate has been used for roofing tiles for centuries.
  • Quartzite: comes from sandstone. The original quartz sand grains fuse together into one of the hardest natural materials known.
  • Gneiss (pronounced "nice"): comes from granite or other rocks. Banded appearance, with different minerals lined up in alternating layers.
  • Schist: comes from various rocks. Recognisable by its visible flakes of mica or other shiny minerals.
  • Anthracite: the hardest, most carbon-rich form of coal. The result of cooking ordinary coal under high pressure for a long time.

Why metamorphic rocks matter

  • Building and decoration: marble for statues and floors, slate for roof tiles and chalkboards, quartzite for tough kitchen surfaces.
  • Industrial use: graphite (a metamorphic mineral) for pencil leads, lubricants and batteries.
  • Gemstones: garnets, sapphires, rubies and many other gem minerals form during metamorphism.
  • History: metamorphic rocks reveal where past tectonic collisions happened. The presence of high-pressure minerals tells geologists that rocks have been deeply buried (and pushed back up) over hundreds of millions of years.
Fact The famous Carrara marble from Tuscany in Italy has been quarried for over 2,000 years. Romans used it for the Pantheon. Michelangelo used it for his David and Pieta. The marble forms in the Apuan Alps, where ancient limestone was metamorphosed by the same forces that built the Alps. Modern quarrying produces approximately 4 million tonnes a year: as much marble in a few decades as the Romans quarried in their entire empire.

Slate and the Welsh roofing industry

One of the best examples of how metamorphic rocks have shaped human life is Welsh slate. The mountains of north Wales are full of high-grade slate, formed when ancient ocean-floor mud was squeezed and cooked during the building of an ancient mountain range. By the 19th century, Wales was the world's biggest slate producer, with the famous Penrhyn Quarry employing approximately 3,000 men at its peak.

Almost every Victorian terrace in Britain (and many in cities as far as New York and Sydney) was originally roofed with Welsh slate. The industry shaped towns like Bethesda, Llanberis and Blaenau Ffestiniog so completely that the slate quarrying landscape of north Wales became a UNESCO World Heritage Site in 2021.

Did you know? Diamonds are technically a form of metamorphic rock too, formed deep in the Earth's mantle from pure carbon under extreme pressure and temperature. Most natural diamonds formed billions of years ago at depths of 150 km or more, then were carried up to the surface by deep volcanic eruptions. The diamonds you see in shops travelled an incredible journey.
Deeper dive: how the cores of every mountain range are metamorphic

Almost every great mountain range on Earth (the Himalayas, the Alps, the Andes, the Rocky Mountains, the Caledonian Mountains of Scotland and Norway) has a heart of metamorphic rock. The reason is simple: making mountains requires colliding tectonic plates, and colliding plates create exactly the conditions metamorphism needs.

When two continents collide, the rocks at the boundary get pushed deep underground (sometimes 30 to 50 km down) and squeezed under enormous pressure. They are also heated, partly by being deep, partly by friction. The original rocks recrystallise into metamorphic forms: marbles, schists, gneisses, slates. Whole regions become "metamorphic belts" stretching for thousands of kilometres.

Over tens of millions of years, the collision continues to push the entire mass of metamorphic rock upwards. The original sea-floor mud, river sand and ocean lava that were buried deep get raised back to the surface as the mountain range's peaks. Erosion strips the surface away, eventually exposing the deeply metamorphosed core.

This is why almost any walk in the Scottish Highlands, the Norwegian fjords, the Italian Alps or the foothills of the Himalayas is mostly through metamorphic rock. The rocks under your feet were once at the bottom of a sea, then crushed 30 km underground, then pushed back up to make a mountain. Looking carefully at the bands and crystals in those rocks tells geologists the whole story.

For more, see igneous rocks, sedimentary rocks and the rock cycle.