Some 20 kilometers west of Naples, the crater Avernus caves in the volcanic soil of Campania as, according to the classical myth, the gateway to the depths of the Earth, ruled by Pluto, the Roman god of Death. The eldest son of Saturn and Ops, Pluto was one of the eight main gods at the crowded Roman Pantheon, which, as it is widely known, had incorporated almost every Greek deity. The Roman Pluto is thus the equivalent to the Hellenic Hades, eldest brother of Zeus and Poseidon, along whom he fought and won the Titans in one of those fierce, intricate struggles so dear to the ancient world. After their deed, the three brothers gambled the possession of the Earth’s different realms, so that the seas fell to Poseidon (Neptune in Rome), Zeus (Jupiter) got the skies, while the underworld and everything earthly fell to Hades, also named Ploutón by the Greeks – the Romans were happy to use his original name.
Honouring this powerful, dark deity, in the late 18th century the former geologists agreed on classifying as plutonic one type of igneous rocks – the most important one, if their occurrence in our planet is to be considered. Not without reason did they so, as these rocks, also named intrusive, are formed in the depths of the earth, by the slow cooling of huge masses of magma. This origin makes them the counterpart of volcanic (or extrusive) igneous rocks, which come into being when the magma reaches the surface as lava, to quickly solidify in contact with the air or water masses – something that, luckily to us, happens to a much lesser extent.
Although the mythic gateway to Pluto’s domains is located in a region where volcanic rocks actually predominate, this does not alter the indisputable hegemony of plutonic rocks: these embody the whole of the Earth’s upper, rigid mantle (that is, roughly, from 30 to 650 kilometers in depth) and most of the crust above it. If compared to this colossal volume, the rest of the rocks that we know (sedimentary, metamorphic and volcanic, too) remain nothing but a thin layer covering only a part of our planet’s surface – in a proportion that could be compared to the dust on a cast iron bollard.
The forming of plutonic rocks takes place under conditions where the magma is confined at very high pressures – so high that they are measured in gigapascals, a unit of measurement amounting to some 10.000 atmospheres. The process results in rocks that are very compact, hard and resistant. Among a diversity of plutonic rocks (syenite, tonalite, diorite, gabbro…) the granite stands out, not only because of its abundance. As a matter of fact, beyond the purview of Geology the term ‘granite’ is also used to referring to some of his plutonic siblings, as it is the case of syenites, tonalites or the gabbro, more widely known as “black granite”. In forthcoming posts it will be possible to be more specific about their composition, so that the generic definition of granite as a “conglomerate of quartz, feldspar and mica” learnt in our boyhood may be left behind.
For the time being, it seems revealing to examine the etymologic origin of the word: ‘granite’ derives from the latin noun granum, “grain” followed by the ending –ite. The latter is a Greek suffix frequently used in the nomenclature of minerals and rocks (as it may have been noticed above) and it denotes a matter having the quality of the root. Such a quality in the granite is, of course, the typical grainy appearance resulting from its formative process: in combination with the aforementioned high pressures, the magma cools and hardens slowly (and it must be borne in mind that when geologists allude to slowness, they usually measure it in thousands of years), which favors the formation of macroscopic crystals of pure minerals, therefore giving rise to a heterogeneous texture where the grain is clearly visible.
However, etymology may not give an idea of other fundamental qualities present in the granite. One of them is compactness, with densities around 2.700 kg/m3 and a very low porosity. Related to the conditions of confinement within which granites are formed, another important property is their isotropy, that confers an extraordinary resistance to compression on the material, typically above 200 MPa – that is, more than 2 tons per square centimeter. To illustrate this feature, let’s imagine that we want to stack identical granite blocks until the one at the bottom collapses by the weight of the rest: well, in that case we would need to build a column some eight kilometers tall before the granite at the base would crack. From the Avernus, Pluto himself would no doubt contemplate in awe so imposing a monument, reaching the realm of Jupiter, his brother.
* Image: “Pluto and his dog Cerbero admire a column of granite blocks rising into the sky near the Vesuvius. Collage made by the author”