Hornfels

In short

Hornfels is a hard, fine-grained metamorphic rock that forms where hot magma, most often granite, pushes into the crust and “bakes” the surrounding rocks without melting them. Because pressure plays no part, it shows no foliation; its minerals are arranged at random. The name is German for “horn rock”, after its hardness and texture, which recall animal horn.

Photos

How to recognise it

1. 1

   It is very hard: a steel knife or a coin will not scratch it.

2. 2

   It is a dark grey, black or brown, dense and very fine-grained stone; the broken surface is smooth and curved like a shell.

3. 3

   It rings like porcelain when struck; in South Africa it is known as “ring-stone” for this reason.

4. 4

   It shows no foliation and does not split into sheets; when broken it tends to give cube-shaped pieces.

5. 5

   In some specimens pinkish crystals of andalusite or cordierite show as spots on the dark background.

6. 6

   It does not fizz in vinegar or dilute acid.

Myths and misconceptions

• MythHornfels is a rock formed from lava, like basalt.

  FactIts dark, fine-grained look makes it easy to mistake for basalt; but hornfels does not form from lava. It forms when the rock beside hot magma is transformed by heat.

• MythMagma melts the rock it touches, and hornfels is that melt frozen solid.

  FactIn hornfels the rock does not melt; the heat only makes its minerals recrystallise and new minerals (such as andalusite and cordierite) grow. So the rock can still carry traces of the original rock, for example old bedding.

Did you know?

Where is it found?

• Skiddaw, Lake District, Cumbria, England · Known localityAround the Skiddaw granite the rocks of the Skiddaw Group have been turned to hornfels. Keswick's famous “Musical Stones of Skiddaw” were made from hornfels of this area.
• Dartmoor, Devon, England · Known localityThe emplacement of the Dartmoor granite created a contact metamorphic aureole of hornfelsed sedimentary rocks around it.
• Land's End, Cornwall, England · Known localityIn the country rocks around the Land's End granite are hornfelses derived from basaltic pillow lavas and greenstones.
• Henry Mountains, Utah, USA · Known localityContact metamorphic aureoles are seen in the sedimentary rocks around the laccoliths that make up the mountains.
• Harz Mountains, Germany · Known localityOne of the known hornfels areas in Germany, used locally as crushed stone.
• Whangārei Heads, Northland, New Zealand · Known localityOne of the places in New Zealand where hornfels occurs next to igneous rocks.
• Beypazarı, Ankara, Türkiye · Known localityAlong the contact of the Beypazarı granite with its wall rocks of phyllite, schist and marble, hornfelses 3–10 m thick are seen.
• Around the Gümüşhane granite pluton (Kurdoğlu zone), Gümüşhane, Türkiye · Known localityThe clay-rich rocks around the Gümüşhane granite pluton reached the hornblende-hornfels facies when the granite was emplaced. The point marks the city of Gümüşhane.

Locations are approximate to protect sites. Red dots mark places in Türkiye. Open in the big map →