We have already said that there is not one, but rather several types of rocks that are commonly called granite.
Leaving this topic for other conversations, let's start now by saying that the term granite, in a strict sense, designates a plutonic rock (generated in depth, in the crust), granular, rich in silica (more than 70%), with essential quartz, expressed and abundant (20 to 40%), and alkaline feldspar (orthoclase, microcline, albite).
As a ferromagnesian mineral, it generally contains biotite, and granites with amphibole or pyroxene are rare. Among its accessory minerals, muscovite, apatite, zircon and magnetite stand out. This rock corresponds to what, in a more rigorous language, is called “alkaline granite”.
The term granite, attributed to the Italian Andrea Caesalpino, appeared in 1596, and is rooted in Latin big, which means grain.
Imagine the reader a landscape like that of northern Portugal, essentially formed by granites, clayey shales and greywackes, on the western margin of the Eurasian lithospheric plate, on the edge of an ocean (the Atlantic) that separates it from another plate (the American one) .
As is known, atmospheric agents (humidity, rainwater, oxygen and carbon dioxide in the air and temperature variations) alter (“rot”) the rocks and it is this alteration, or weathering, that generates the surface layer (regolith) that gives rise to the soil.
– And what are the materials of this alteration layer and its soil? - Ask if.
Restricting the answer to the place in question, to the main minerals of these rocks, and to the climate situation that exerts its influence here, we will say, in a very schematic way, but which points out the essence of the question, that:
(1) In granite, feldspar changes, transforming itself partially and, initially, superficially, into clay. By changing the feldspar, the remaining mineral grains detach from each other and the rock loses cohesion (it crumbles between the fingers).
The biotite grains (a mica containing iron) also change and this change results in their “rusty” appearance, which gives the exposed rock a yellowish-brown color, which contrasts with the color of the healthy, freshly cut rock. Quartz does not undergo any change, as does white mica (muscovite), which only breaks down into smaller and thinner reeds.
(2) In clayey shale, which, in addition to clay, has quartz in very fine, microscopic grains (at the level of dust), there is a loss of cohesion in these materials.
(3) In greywacke, the same happens, with the release of its sandy components (the same as in granite, but much finer).
We can now say that the regoliths and soils in this region of Portugal have a sandy fraction with abundant quartz, some feldspar, micas and a clayey or muddy fraction that makes the dust of the paths, in dry weather, and mud, in rainy weather. .
We can also say that, when it rains with a certain intensity, the runoff water drags these materials, with sufficient visibility in the suspended clay component. This is often seen in the floods, in the muddy waters of rivers and even in the sea, in front of the mouths of these rivers.
The stones (gravel) remain, in part, along the way, others reach the coast and do not go beyond it. Sand fills the beaches, dunes and rocky bottom of the continental shelf.
The finer sands and clays, incapable of depositing in shallow seas, constantly agitated by swells, progress towards the sea, going to the continental slope (where they remain in an unstable situation).
The much finer ones, essentially clayey ones, will immobilize further away, on the ocean floor. Whenever, for example, an earthquake shakes the region, the sediments in an unstable deposit situation on the slope detach themselves, going to decant over those already bedridden on the said bottom.
Let's imagine that this process (alteration of rocks, erosion, transport and accumulation in the sea) repeats itself over millions of years, resulting in a few thousand meters of thickness of this type of sediment. Let us also imagine that the same thing happens on the other side of the Atlantic.
Global tectonics teaches us that this ocean, like all others throughout Earth's history, will close. This will result in the shortening of the space covered by the said sediments which, like a paper we crumple between our hands, will suffer wrinkles, with "folds" that come up, forming new mountains, and others that go down, forming the “roots” of these mountains.
It is known that the Earth retains large amounts of heat in its interior and that temperature increases with depth, as does pressure (called lithostatic).
Thus, of the sediments involved in the aforementioned "roots", the most superficial will be subject to relatively low pressures and temperatures, undergoing a very slight transformation (ankymetamorphism), giving rise to rocks on the border between sedimentary and metamorphic ones, such as the clayey shale, the greywacke and, a little further down, the slate.
Continuing in depth, with increasing pressure and temperature, but always with transformations in the solid state, other frankly metamorphic rocks will be formed, of progressively higher degrees, expressed in the sequence: phyladiums or luminous schists (since the clay component turned into minerals that have characteristic, 'shiny' shines such as sericite, chlorite or talc), porphyroblastic shales, mica schists and, further below, gneisses (these representing the highest grade).
At depths in the order of 30 kilometers, the temperature can reach 800 ºC and the pressure exceeds 4000 atmospheres. In this environment and in the presence of water (all that contained in the composition of clays), the fusion of less refractory minerals (quartz and feldspars) will take place. Here one enters the domain of so-called ultrametamorphism and the process takes the name of anatexia (from the Greek “aná”, novo, and “teptikós”, to fuse), or palingenesis (from the Greek “pálin”, again, and “genesis” , generation), giving rise to migmatites.
Once the fusion is total, one enters the domain of magmatism, with the formation of a magma that, given the materials involved, can only be of composition granite, magma that, once cooled and solidified, will generate new granite.
The story we have just described in this kind of preview is the one we think we know how to tell in relation to what, just over 300 million years ago, gave rise to the Hercynian or Variscan orogeny and the granite, schist and greywacke that were generated in it and that mark the landscape of northern Portugal.
Likewise, this story tells the story of all related landscapes on the planet, from the oldest, with more than 4000 million years, to the most recent, with a few million.
Regarding granite, the most important magmatic rock that forms the “ossature” of the continents, we know that the former resulted from a slow and complex process of differentiation from a primitive crust, similar in nature to that of basalt. We also know that any generation of granite has, behind it, another granite and that, many millions and years later (400 to 500, on average), it will be reborn in a new generation of granite.
This story is, after all, the expression (recognizable at the level of Earth's landscapes) of the well-known Wilson Cycle (by Canadian geologist John Tuzo Wilson (1909-1993), concerning the successive openings and closings of the Earth's oceans.
Notes:
Gravesham – cohesive sandstone sedimentary rock, generated in the great seabeds, together with the clayey shales.
It mainly contains quartz (20 to 50%), feldspars and micas.
The term was introduced in the lithological nomenclature, in 1789, by Lasius, and is rooted in the German Grauwacke, which means gray stone.
migmatite – ultrametamorphic rock, generated by anatexia, which results in a granitoid composition, in which one part was molten and the other, more refractory, remained in a solid state.
It is located at the passage of metamorphic rocks from the catazona (such as gneiss) to the Frankish granite.
Below the gneiss zone, temperature and pressure allow the elements to melt.
Author Antonio M. Galopim de Carvalho
© 2018 – Science in the Regional Press / Ciência Viva
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