Week7: The Man and the Stone

Indeed the history of stone proceeds the history of man. It existed before animal life evolved and plant life vegetated. But what is stone? The term stone is easy to confuse with the rock. The latter is any naturally formed mass of material that essentially contributes to the earth's crust. Only after the extraction, with the intent of constructional or industrial use, does the rock become a stone.

Armenian highland  holds enormous stocks of tuff, which is an easily cut resilient type of stone. Other stones in the country include basalt, travertine, felsite, granite, marble, perlite, andesite, limestone, syenite, pumice, diatomite and gypsum. 

 

Granite

Basalt

Perlite

Syenite

It was previously mentioned that the Caucasus region and Armenia in particular is prone to earthquakes, hence it is not surprising that tuff and other similar deposits are abundant in this area since most of these stones are an extrusive igneous rocks that forms from volcanic activity. A group of seven cinder and lava cones of Pleistocene-to-Holocene age is located in southern Armenia on the western slopes of the Vardenis volcanic ridge, south of Lake Sevan.

 

Aragats volcanic mount view from space

Depending on the mineral composition and the setting of the stock, the formation method of the rock can vary slightly. After a violent volcanic eruption, volcanic ash, frothy magma, dust and fine rock particles settle and build up on the land surface.

 

Dar Alages

The ejected particles form a rock with a soft, porous texture. The particles may eject from the main volcanic vent or may escape through cracks in the walls of the volcano, called fissures. These fissures are often smaller than the central vent but extend from the magma chambers toward the land surface. The magma and rock particles that settle from the volcanic eruptions can form tuff rocks in several ways. Some tuff rocks form when the particles become buried under layers of other rock and harden into stone through compaction. Alternatively, the particles can form a rock through cementation, typically when calcite or quartz in solutions precipitates and glues the particles together into a rock. Sometimes, a welded tuff is formed, as the temperature of the rock particles is very high as they erupt from the volcano.

Volcanic Tuff in Geghard

 

Unlike most rock, tuff can contain a variety of minerals such as augite, biotite, plagioclase and leucite, among others. The containing minerals in the rock can influence the  overall colour and appearances of the rock. Tuffs may be brown, grey, pink, green, brown etc. Most structures in Armenia from the early Christian period forward are made of the orange, red, golden and black varieties of soft tuff. The presence of dark minerals, such as augite and biotite, can provide a speckled appearance. Individual grains within the tuff rock can be ashy, glassy or crystalline and their size varies from sample to sample.
 

Tuff