Week 10: The geometries of the Nature

Located around 30 km  on the East of the Yerevan, a curious volcanic basalt patterns can be found around the Garni Gorge. These natural formations comprise of columns of various sizes which are mostly hexagonal in shape, even though squares, pentagons, heptagons, octagons and other geometrical shapes are common too. These formations are known in Armenia as the "Symphony of Stones", which is yet another semantic similar to Karahunge - Whispering stones, that integrates the materiality of the rock with their intangible component of the sound and vibration. 

Basaltic Symphony of Stones in Geghard

Both Symphony of the Stones and the more famous Giant's Causeway in Northern Ireland are estimated to be around 50 million years old. The Giant's Causeway offers a wide variety of shapes and structures and some were given different names depending on their shape or the morphology. Names such as the "Giant's Gate", "The Granny", "The Chimney" and most appropriately, the "Organ" or "Organ pipes" are among the most famous. As stated, in case of Garni Gorge, a well-accepted popular name is "Symphony of Stones". The columnar basalt of the Garni and the Giant’s causeway both are not the only such sites in the world. Columnar and polygon shaped exposed basalt formation can be found in other corners of the world, such as in the Yellowstone national park or the Parana river or elsewhere in Asia. In Garni, the cliff walls have been partially carved out by the Goght River. 

Goght River at Garni

The origins of these formations are of volcanic eruptions, but what differentiates these sites is the particular cooling and solidifying processes of the boiling lava, which gave these almost perfect geometric forms and hanging columns a distinct appearance. The rocks have multiple layers unveiling how the lava solidified, shrunk and formed these shapes over long periods of time. Volcanic basalt is not the hardest of rocks, which is ironic because "basalt" is ultimately derived from Late Latin basaltes, misspelling of L. basanites "very hard stone", hence millions of years have also resulted in erosion and weathering.

During the cooling of a thick lava flow, contractional joints or fractures form. If a flow cools relatively rapidly, significant contraction forces build up. While a flow can shrink in the vertical dimension without fracturing, it can't easily accommodate shrinking in the horizontal direction unless cracks form; the extensive fracture network that develops results in the formation of columns. The topology of the lateral shapes of these columns can broadly be classed as a random cellular network. These structures are predominantly hexagonal in cross-section, but polygons with three to twelve or more sides can be observed. The size of the columns depends loosely on the rate of cooling; very rapid cooling may result in very small (<1 cm diameter) columns, while slow cooling is more likely to produce large columns.

Volcanic lava flow

What I am most intrigued about is the potential correlation between the naturally formed stone patterns and the hand crafted stone carvings on the example of the Geghard monastery dome.

Garni natural formations and Geghard stone carvings

Basalt is a common extrusive igneous rock formed from the rapid cooling of basaltic lava exposed at or very near the surface of a planet or moon. It almost always has a fine-grained mineral texture due to the molten rock cooling too quickly for large mineral crystals to grow, although it can sometimes be porphyritic, containing the larger crystals formed prior to the extrusion that brought the lava to the surface, embedded in a finer-grained matrix. The mineralogy of basalt is characterized by a preponderance of calcic plagioclase feldspar and pyroxene. Olivine can also be a significant constituent. Accessory minerals present in relatively minor amounts include iron oxides and iron-titanium oxides, such as magnetite, ulvospinel, and ilmenite. Because of the presence of such oxide minerals, basalt can acquire strong magnetic signatures as it cools, and paleomagnetic studies have made extensive use of basalt.

Let's have a closer look at the three main minerals of Basalt.

1- Feldspars (KAlSi3O8– NaAlSi3O8 – CaAl2Si2O8) are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust. Feldspars crystallize from magma in both intrusive and extrusive igneous rocks, as veins, and are also present in many types of metamorphic rock.

2- Pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems. Pyroxenes have the general formula XY(Si,Al)2O6 (where X represents calcium, sodium, iron+2 and magnesium and more rarely zinc, manganese and lithium and Y represents ions of smaller size, such as chromium, aluminium, iron+3, magnesium, manganese, scandium, titanium, vanadium and even iron+2).

3- The mineral olivine (when of gem quality, it is also called peridot and chrysolite), is a magnesium iron silicate with the formula (Mg,Fe)2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface.

Feldspar and Olivine crystal structures

On the above image of the mineral crystals the same geometric patterns of squares, pentagons, hexagons etc. can be observed, similar to the natural formations of the basalt.

The geologist N.L. Bowen found that minerals tend to form in specific sequences in igneous rocks, and these sequences could be assembled into a composite sequence. No igneous rock ever displays the whole sequence, but rather they display a slice across the sequence. Basalt, for example, typically has olivine and calcium plagioclase forming first, followed by pyroxene and more sodium-rich plagioclase. In granite, sodium plagioclase and biotite typically form first, followed by muscovite, potassium feldspar, and last of all quartz. The sketch below turns the series on its side. It's actually a more realistic view since successive minerals often form simultaneously.

Basalt formation sequence

Again the same geometric clusters are present during and after formation of these crystals. These particular geometric shapes are the most efficient forms to hold the structure in an equilibrium, without impairing the geological processes.

The predominant shape in case of the Garni Gorge are the hexagonal columns and it is fair to say that hexagonal patterns are prevalent in nature due to their efficiency. In a hexagonal grid each line is as short as it can possibly be if a large area is to be filled with the fewest number of hexagons. The hexagonal packing arrangement is the most efficient system for packing circles on a flat plane. Since each cell naturally tends towards the most efficient enclosure of its space, they tend towards circularity, but because each cell has any number of neighbouring cells pressing up against it, the result is a quasi-hexagonal tiling. Hence when the basaltic lava was initially cooling, it organized itself into roughly cylindrical convection cells. As the cells cooled into solid rock, they naturally took on the hexagonal geometry of packed circles.