Visitors to Armenia can hardly fail to be aware of two key geological features of the country: the Lesser Caucasus mountain range that projects into the country, with the dormant volcano of Mount Aragats being the highest peak at 4,090m; and the frequency of earthquakes. The two are both accounted for by the theory of plate tectonics.
Under this theory, which has been generally accepted internationally since the 1960s, the outermost shell of the earth or lithosphere is formed of a dozen or so large, rigid slabs of rock together with many smaller ones. These slabs, called tectonic plates, are around 75km thick and comprise the thin outer crust plus the solid part of the mantle. The plates float on the liquid part of the mantle or asthenosphere, which is several hundred kilometres thick, and the slow movement of the plates is due to convection currents in the asthenosphere caused by heat escaping from the earth's core. As the enormous plates move, they grind against each other and stress builds up until there is a sudden movement of one plate against another resulting in an earthquake when rocks break along fault lines.
Armenia is on the line where the Arabian plate, moving at about 2.5cm per annum, is colliding with the larger Eurasian plate and it is consequently very prone to earthquakes. About 25 million years ago the Caucasus Mountains themselves were formed as a consequence of this collision. Quite young as mountain ranges go, they are largely volcanic rocks such as basalt, andesite and tuff, all three of which have been used as building materials in Armenia. Basalt and andesite are magma (liquid material from the asthenosphere) which escaped to the surface during the collision of the plates and then solidified. The difference between the two is in the relative proportions of silica, iron and magnesium. Tuff by contrast is formed when small rock fragments (less than 2mm across) which have spewed out from a volcano become fused together on the ground. Tuff has long been the building material of choice in Armenia when it is available and it is highly characteristic of the country. Today the biggest working tuff quarry is close to the town of Artik in Shirak province. Another consequence of the volcanic past is that the semiprecious stone obsidian can be found here. Obsidian, which occurs in a range of colours and is used to make jewellery, is a glassy rock formed through the very rapid solidification of lava. In Armenia it occurs most commonly in the Hrazdan region. Apart from these volcanic rocks, Armenia also has substantial deposits of the sedimentary rock limestone and in some of these are extensive though little-known cave systems. The most important mineral deposits are of copper (7.4 million tonnes, of which 4.5 million tonnes are at Kajaran), molybdenum (711,000 tonnes, of which 600,000 tonnes are at Kajaran) and gold (268 tonnes, of which 97 tonnes are at Zod).
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