Phonolite

In short

Phonolite is a grey or greenish, fine-grained volcanic rock formed when lava poor in silica and rich in sodium and potassium cools. Its name comes from the Greek for “sounding stone”, because thin, unfractured plates give a metallic ring when struck with a hammer; in English it is also called “clinkstone”. It is a relatively uncommon rock found in only a few places. In Türkiye it occurs in the Kızılcaören complex in Eskişehir and in the volcanic fields of Isparta and Afyon.

Photos

How to recognise it

1. 1

   It is a grey, greenish grey or dark grey, fine-grained, compact rock; crystals of sanidine (feldspar) may show as white flecks in the groundmass.

2. 2

   A thin, unfractured plate gives a metallic ring when struck with a hammer; weathered or cracked pieces do not.

3. 3

   It contains no quartz; it resembles trachyte, but it holds feldspathoid minerals such as nepheline, and green aegirine may show as the dark mineral.

4. 4

   Feldspathoids such as nepheline are hard to recognise by eye; telling phonolite from trachyte for certain needs a thin section or chemical analysis.

Myths and misconceptions

• MythDevils Tower is the neck of an old volcano (a volcanic plug).

  FactDevils Tower is an intrusion: magma cooled underground without reaching the surface, and the rock was exposed when the sedimentary rocks around and above it eroded. Scientists debate whether it is a small stock or the remnant of a laccolith, but it is not considered to be the plug of a volcano that erupted at the surface.

• MythEvery piece of phonolite rings like a bell when struck.

  FactOnly unfractured, thin, plate-shaped pieces give a metallic ring. Cracked, weathered or thick pieces are silent; the name comes from this property.

Did you know?

Where is it found?

• Devils Tower, Wyoming, USA · Major depositA columnar rock of porphyritic phonolite emplaced in the Eocene, some 40-50 million years ago, rising 386 metres above the river. It was declared the first national monument in the United States in 1906.
• Mount Erebus, Ross Island, Antarctica · Major depositThe world's only presently erupting phonolite volcano; its 3,792-metre summit crater holds a persistent phonolitic lava lake that has existed since at least the early 1970s.
• Teide volcano, Tenerife, Spain · Known localityAt 3,715 metres the highest point in Spain and of any Atlantic island; the magma of the Las Cañadas eruptions differentiated over time from trachybasalts to phonolites. UNESCO World Heritage Site (2007).
• Mont Mézenc, Haute-Loire, France · Historic sourceA phonolite dome about 7 million years old and 1,753 metres high. From the Middle Ages to the 20th century phonolite was cut into slabs 2-3 cm thick and used as roofing, called “lauze”.
• Bořeň, Bílina, Czech Republic · Known localityA 539-metre hill in the Central Bohemian Uplands, described as the largest phonolite structure of its kind in Europe.
• Kızılcaören, Beylikova, Eskişehir, Türkiye · Known localityThe Kızılcaören complex, formed about 25-24 million years ago, began with stocks of phonolite and trachyte, followed by carbonatites; veins of the same complex form Türkiye's main rare earth deposit.

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