The Principles of Petrology

by G.W.Tyrrell (1926)



Petrology is the science of rocks, that is, of the more or less definite units of which the Earth is built. In the nature of things the study is limited to the materials of the accessible crust, although we have in meteorites samples of rocks which must be identical with, or analogous to, those composing the interior of the Earth. The science deals with the modes of occurrence and origin of rocks, and their relations to geological processes and history. Petrology is thus a fundamental part of geological science, dealing, as it does, with the materials the history of which it is the task of geology to decipher. Rocks can be studied in two ways: as units of the Earth's crust, and therefore as documents of Earth history, or as specimens with intrinsic characters. The study of rocks as specimens may properly be designated petrography. Petrology is, however, a broader term which connotes the philosophical side of the study of rocks, and includes both petrography and petrogenesis, the study of origins. Petrography comprises the purely descriptive part of the science from the chemical mineralogical, and textual points of view. As we must gain an exact knowledge of the units we have to deal with before we can study their broader relations to geological processes and their origins, petrography is a necessary pre-requisite to petrology , and must be carried on as far as possible by quantitative methods, as in other physical and chemical sciences. The term lithology is nearly synonymous with petrology , but is now seldom used. Etymologically, it means the science of stones; and, accordingly, there is a tendency, which should be encouraged, to use the term to indicate the study of stones in engineering, architecture, and in other fields of applied geology. It would be equally appropriate and correct to speak of the Iithology of conglomerates and breccias, when dealing with the stones contained in these rocks. BroadIy speaking, petrology is the application of the principles of physical chemistry to the study of naturally- occurring Earth materiaIs, and may therefore be regarded as the natural history branch of physical chemistry. Viewed thus, petrology is again seen as a subdivision of geology. The too exclusive study of petrography sometimes tends to obscure this relationship, anti hides the fact that petrology is intimately connected with a host of fascinating geological problems.


Regarded as a whole, the Earth is a sphere of unknown material surrounded by a number of thin envelopes. The inaccessible heavy interior is known as the barysphere. This is followed outwardly by the lithosphere, the thin, rocky crust of the Earth; then by a more or less continuous skin of water, the hydrosphere; and finally by the outermost envelope of gas and vapour, the atmosphere. Other zones have been distinguished and named for special purposes. The zone of igneous activity and lava formation, situated between the lithosphere and the barysphere, is the pyrosphere; and water has distinguished the living envelope which permeates the outermost zones, as the biosphere A zone towards the base of the lithosphere which can sustain little or no stress has been called the asthenosphere (sphere of weakness) by Barrell; and the zone in which crustal movements originate has been named the tectonosphere by certain Continental geologists. The Barosphere - While the interior of the Earth, of course, is inaccessible to direct observation, many facts have been indirectly ascertained about its structure and composition. It must, for example be hot. Observations in wells, mines, and borings, show that there is a downward increase of temperature, which is variable in different parts of the Earth, and averages 1' C for every 31.7 metres of depth, or roughly 50' C per mile in Europe. In North America the temperature gradient is much gentler, only 1' C for every 41.8 metres of depth, or 38' C per mile. If these gradients continue indefinitely, enormous temperatures must prevail in the interior of the Earth, but it is probable that the rate of in- crease falls off with depth. Similarly, very great pressures must occur, even at moderate depths within the crust. The barysphere must also be composed of heavy materials. The density of the Earth as a whole is 5.6, but the average density of known rocks of the lithosphere is only 2.7. Hence the average density of the barysphere must be somewhat greater than 5.6. Several considerations serve to show that the barysphere must be rigid, with a rigidity greater than that of the finest tool-steel. In the early days of geology it was believed that the thin solid crust rested or floated on a molten interior; but when attention was given to geophysical matters it was soon shown that, under these circumstances, the thin crust would experience great distortion in response to the attraction of the moon, and, furthermore, owing to infernal friction, rotation would long be maintained. A strong confirmation of the rigidity of the barysphere is afforded by the study of earthquake vibrations. A heavy shock, say, in New Zealand, is recorded by seismometers in Britain about 21 minutes later, the vibrations having travelled by a more or less direct path through the barysphere. This speed of wave propagation is consistent only with high rigidity in most of the interior of the Earth.


The Earth has been called a projectile of nickel-steel covered with a slaggy crust. It is probable that all the planets and planetoids of the so1ar system have essentially the same composition; hence the wandering fragments of planetary matter known as meteorites or shooting stars, which the Earth sweeps up as it revolves in its orbit, are of particular interest in connection with the present topic. Meteorites fall upon the Earth's surface in masses which vary in size from the finest dust to huge blocks weighing many tons. Meteorites are divided into three main groups which pass gradually one into the other: 1. Siderites. The iron meteorites, consisting almost entirely of iron alloyed with nickel. 1. Siderolites. Mixtures of nickel-iron and heavy basic silicates, such as olivine and pyroxene. 3. Aerolites. The stony meteorites, consisting almost entirely of heavy basic silicates, oIivine and pyroxenes, and resembling some of the rarer and most basic types of terrestrial igneous rocks. There are small amounts of sulphur phosphorus, carbon, and other elements in meteorites, which, however, may be disregarded in the present connection. Professor J. W. Gregory shows that if all known meteorites are considered, the iron group far outweighs the stony group. The stony meteorites fall in greater abundance, but the siderites fall in such large masses that they bulk much greater than the aggregates of small aerolites Hence the relative masses of the different types of meteorites support the above-cited view of the composition of the Earth. From geophysical data based on the distribution of density, earthquake vibrations, etc., Williamson and Adams have arrived at the conception of Earth composition illustrated by Fig. I a. In this the Earth is shown to be built of four layers: (1) a thin surface crust of light silicates and silica; (2) a zone of heavy silicates (peridotite) which, of density 3'3 in its uppermost layers, is of density 4.35 in its lowest part at a depth of 1600 kms.; (3) increasing admixture of nickel-iron leads to a zone consisting of material similar to siderolites (pallasite) which, with a rapidly-mounting pro- portion of nickel-iron, passes into; (4) the purely metallic core. The actual composition of meteorites, as given above, supports this hypothesis. According to this view the Earth is conceived as the result of a gigantic metallurgical operation analogous to the smelting of iron, with the more or Jess complete separation of metal and silicate slag. Professor V. M. GoIdschmidt has put forward a view of Earth composition which differs from the above chiefly in the intercalation of a zone of metallic sulphides and oxides (Fig. 1 s) between the nickel-iron core and the shell of heavy compressed silicates. In this conception the analogy of copper smelting is kept in mind, involving the separation of metal. sulphide-matte, and slag.


The outer crust of the Earth down to a depth of 10 miles or so, consists of igneous rocks and metamorphic rocks, with a thin, interrupted mantle of sedimentary rocks resting on them. According to Clarke and Washington the 1ithosphere down to a depth of 10 miles is made up of igneous rocks, 95 per cent.; shale, 4 per cent.; sandstone, .75 per cent.; and limestone, 0.25 per cent. The metamorphic rocks, which are derived from igneous and sedimentary rocks by alteration under heat and pressure, are regarded as belonging to their initia1 types, and are neglected in the following calculation. The average chemical composition of each class of rock has been obtained by computation from a large number of existing analyzes; and if these are combined in the proportions given above the following figures are obtained (col. 1), representing the com- position of the lithosphere down to the 10-mile limit of- depth. Column 2 shows the composition as recast into the form of oxides: On the assumption that the 10-mile crust entirely is composed entirely of igneous rocks, the results of the computations only differ on the whole, from those given above, in the second figure of decimals. It thus appears that fifteen elements between them make up 99.75 per cent of the Earth's crust, and that the majority of the elements which are important in human affairs are included in the remainder, being present in the crust in amounts of the order of 1/100th of 1 per cent. It has been asserted that the Clarke-Washington method seriously over-estimates the acidity {amount of silica) of the crust, since no allowance is made for the relative amounts of the different kinds of igneous rock composing the average. Rocks of acid composition are largely restricted to the continents, and are probably underlain at shallow depths by basic rock (basalt, etc.). The floors of the oceans also probably consist of basalt. Hence, perhaps, the average crustal rock should be regarded as considerably more basic than is allowed by the Clarke method.


Rocks have already been defined as the more or less definite units constituting the Earth's crust, but in popular usage the term rock denotes any hard, solid material derived from the Earth. In geology, however, the term is often used without reference to the hardness, or state of cohesion, of the material; and sand, clay, or peat, are thus just as much rocks in the scientific sense as granite and limestone. When solid rocks are examined closely they are found to consist largely of fragments of simpler chemical composition, which are called minerals or strictly, mineral species. Hence we arrive at another definition of rocks as aggregates of mineral particles. It is important to distinguish the two uses of the term mineral It is used in a perfectly Legitimate sense by the ordinary man, the miner, prospector, quarry- master, lawyer, landowner, and the scientific mineralogist, to indicate materials, such as coal slate, clay, etc., which are won from the Earth's crust, but which, in the strict petrological sense, are rocks. We speak also of "mineral waters " and " mineral oil " in the same way. Natural glasses and other amorphous materials which occur as rocks and in rocks may also be regarded as minerals in this sense. The materials of which rocks are largely composed are, however, mineral species i.e. natural inorganic substances with a definite chemical composition, or a definite range of chemical composition, and a regular internal molecular structure, which manifests itself under favourable circumstances by the assumption of regular crystalline form, and the possession of definite optical and other measurable properties.


The table on page 6, column 2, shows the principal chemical elements present in the Earth's crust combined with oxygen, the most abundant element, to form oxides; and this, by the way is the most convenient and usual method of presenting the chemical analyzes of rocks. These elements however fall into various combinations from which the rock-forming minerals arise, in which oxides, as such, are only of secondary importance. Silicates are the most abundant compounds constituting the rock-forming mineral oxides come next; then carbonates, phosphates, sulphates, etc., in greatly diminished importance. Among the elements Listed on page 6, only iron {in the basalt of Disko, Greenland), carbon {as diamond and graphite), and sulphur {volcanic action; decomposition of sulphates and sulphides, occur native. It is probable that 99.9 per cent of the Earth's crust is composed of only about twenty minerals out of the thousand or so which have been described. These are the rock-forming minerals par excellence Referring first to the silicates, the felspars are by far the most abundant and important group, not only of the silicate but of the rock-forming minerals in general The chief members are orthoclase and microclines both silicates of potassium and aluminium and the various plagioclase which are mixtures in all proportions of the two end-members, albite, silicate of sodium and aluminium, and anorthite silicate of calcium and aluminium Allied to the feldspars but containing less silica in proportion to the bases present, are the felspathoid minerals, of which the most important are nepheline silicate of sodium and aluminium (corresponding to albite among the felspars), and leucite silicate of potassium and aluminium (corresponding to orthoclase among the felspars). The mineral analcite a silicate of sodium and aluminium with combined water, and property belonging ta the group of zeolites may nevertheless take its place as a rock-forming mineral with the felspathoids The mica group forms a link between the alkali-alumina Silicates above mentioned and the heavier and darker Ferro magnesia silicates to be described later Of micas proper, the two chief members are the white mica, muscovite silicate of potassium and aluminium with some hydroxyl, and the dark mica, biotite silicate of potassium, aluminium, magnesium, and iron, with hydroxyl Chlorite is the green hydrated silicate of magnesium and iron, of micaceous affinities, which is the most familiar alteration product of biotite and other ferro-magnesian minerals. Among the ferro-magnesian minerals proper there are the three main groups of the pyroxenes, amphiboles, and olivines. The pyroxenes are metasilicates of calcium, magnesium, and iron, of which the two chief members are the orthopyroxenes (enstatite and hypersthene), simple metasilicates of magnesium and iron; and augite, the monoclinic pyroxene, a complex metasilicate of calcium, magnesium, iron, and aluminium. The amphiboles form a parallel group to the pyroxenes, but with different crystal habit. The chief member is horn- blende, a mineral of composition similar to that of augite, but usually richer in calcium. The olivines are simple orthosilicates of magnesium and iron, and stand in the same relation to pyroxenes and amphiboles as the felspathoids do to the felspars. Serpentine is the hydrated alteration product of olivine and other ferro-magnesian minerals. Numerous other silicates occur as rock-forming minerals, but it is only necessary here to mention the garnets, a varied isomorphous series, chiefly silicates of iron, magnesium, calcium, and aluminium; epidote, silicate of calcium, iron, and aluminium, an abundant alteration product of lime-rich silicate minerals; andalusite, kyanite, and silliminite, all simple silicates of aluminium; cordierite, silicate of magnesium, iron, and aluminium; and staurolite silicate of iron and aluminium. The five last-named minerals are characteristic of the metamorphic group of rocks. Among the oxide minerals only four need be mentioned as prominent rock-formers. Quarts, the dioxide of silicon, is, perhaps, the most abundant mineral next to the felspars. Impure colloidal silica, especially the dark variety known as chert, also forms large rock masses. The oxides of iron come next: magnetite (Fe3O4) is very widely distributed in rocks in small quantities; hematite (Fe2O3) and limonite (Fe2O3), nH2O) form the universal red, brown, and yellow colouring matters of rocks. Ilmenite, the oxide of iron and titanium (FeTi)2O3, is, perhaps, even more widely distributed in rocks than magnetite. Of carbonates, the minerals calcite, the carbonate of calcium, and dolomite, the carbonate of calcium and magnesium, are by far the most abundant, and are the chief minerals of the important group of the limestone rocks. One phosphate, the mineral apatite, phosphate of calcium with some combined flourine or chlorine is universally distributed in small amounts; and one sulphide, the ubiquitous barytes, disulphide of iron, is a common rock-forming mineral. Two sulphates, gypsum sulphate of calcium with combined water, and barytes, sulphate of barium, occasionally form rock masses; as also one chloride common salt or halite the chloride of sodium. For information regarding the crystallographic optical and other properties of the rock-forming minerals the reader is referred to one of the numerous standard textbooks on that subject. The Classification of Rocks Whatever theory of Earth origin be held it is at Least certain that all parts of the original surface of the Earth passed through a molten stage, and that the first solid material which existed was derived from a melt or magma This original crust is nowhere exposed on the present surface, but all subsequently-formed rocks, in the first instance, have been produced either from this, or from later irruptions of molten matter. Rocks formed by the consolidating of molten magma are said to be Primary or Igneous After the solidification of the original crust, and the formation of the hydrosphere and atmosphere, the waters and the air, both probably of much greater chemical potency than now, began, to attack the primary rocks. Disintegrative action produced loose debris, and chemical action produced both debris and material in solution The loose fragments would be swept away by water and wind and would ultimately collect in the hollows of the crust, where also the waters and soluble matters would be found. The collected debris deposited from suspension in water or air would finally be cemented into hard rock, and would be thus added to the solid crust. Under suitable circumstances the soluble matter likewise would be precipitated, either directly, or indirectly through the agency of organisms, the latter, of course, in somewhat later geological times. The rocks thus produced would eventua1ly become solid and help to build up the crust. These processes have gone on through- out geological time, the newer increments of the crust under- going attack as well as the older parts. Hence it may be that some of the material has gone through many successive cycles of change. The rocks formed in these ways are called Secondary, because they are composed of second-hand or derived materials. They may be divided into Sedimentary, Chemical, or Organic according to the process by which they received their most distinctive characters. Finally, both Primary and Secondary rocks may be subjected to Earth movements which carry them down to dept is in the crust where they are acted upon by great heat and pressure. By these agencies the rocks are partly or wholly reconstituted; their original characters are partly or wholly obliterated, and new ones impressed upon them. Rocks thus more or less completely changed from their original condition are known as the Metamorphic rocks. We thus arrive at the time-honoured three-fold classification of rocks according to their modes of origin into Igneous, Secondary (Sedimentary), and Metamorphic. The Primary rocks are distinguished by the presence of crystalline minerals which interlock one with the other, or are set in a minutely- crystalline paste, or in a glass They show signs, as do present-day lavas of having cooled from a high temperature. They are usually massive, unstratified unfossiliferous and often occupy veins and fissures breaking across other rocks, which they have obviously heated, baked, and altered. The Secondary rocks are composed of clastic and precipitated materials, or of substances of organic character and origin. The materials are often loose and unconsolidated, or are welded together by pressure or by a cementing sub- stance into a solid rock. They are further distinguished by the frequent presence of bedding or stratification organic remains (fossils), and other marks indicative of deposition from water or air in the sea or on land. The Metamorphic rocks present characters which, in some respects, are intermediate between those of the Primary and Secondary rocks. Great heat and pressure cause recrystallisation; hence, like the Primary rocks, they often consist of interIocking crystals Furthermore, pressure causes the development of more or less regular layers, foliation or banding, in which the Metamorphic rocks resemble those of Secondary origin. Since the Metamorphic rocks are formed from pre- existing igneous or sedimentary rocks they of ten retain traces of their original structures. Mr. T. Crook has recently formulated a genetic classification, in which the rocks arranged according to a geological grouping of processes. He divides rocks into two great classes : 1. Endogenetic formed by processes of internal origin which operate deepseatedly or from within outwards (with respect to the crust). High temperature effects are predominant, and the water associated with the processes is partly of magmatic origin. 2. Exogenetic formed by pro- cesses of external origin, operating superficially or from without inwards. These rocks are formed at ordinary temperatures, and the associated water is of atmospheric origin. In the Endogenetic class are included the igneous rocks (along with certain pneumatolytic and hydrothermal types), and metamorphic rocks; and in the Exogenetic class come the rocks usually classed as sedimentary. The following gives a simplified statement of the classification l. Endogenetic Rocks. (1) Igneous Rocks. (2) Igneous Exudation Products (due to pneumatolysis, metasomatism, etc.). (3) Thermodynamically-altered Rocks {Metamorphic Rocks). 2. Exogenetic Rocks. (1) Weathering Residues. (2) Detrital Sediments. (3) Solution Deposits. (4) Organic Accumulations.