Petrology

IGNEOUS ROCKS are usually regarded as having crystallised from a molten state, although certain types may have arisen in other ways, e.g. by reaction between high temperature fluids of the right composition, and pre-existing rocks. Various forms have been described but the typical common forms are granite and basalt, which constitute 90-95% of all igneous rocks seen at the surface. "Granite" is coarse-grained, intrusive in other rocks, and contains the minerals quartz, potash or soda potash, and usually mica. "Basalt" is fine-grained, flows out as lava at the surface and contains the minerals augite, calcium feldspar and usually olivine.

These types of rock are not represented on the Maltese Islands. However according to C. Rizzo (1932), a farmer had dug up a peculiar boulder of dark, compact, fine-grained and heavy rock with an approximate spheroidal shape over a foot in diameter, and with a perfectly polished surface. This was found in a valley terrace high above the present bed at Tal-Qlejgha to the Northwest of the Imtarfa Plateau. This was identified by W. Campbell-Smith as "Dolerite". Rizzo suggests that this igneous rock was transported to Malta by running waters, however HPT Hyde (1955) prefers to think that this rock was a fragment of some antique ornament brought to the Island by human agency. Dolerite is similar in composition to basalt, differing in being medium-grained. The chief constituents of normal dolerite are labradorite and augite with much accessory iron oxide and often apatite. Olivine and quartz may also be interwoven in some types.

During excavations at the various neolithic sites, a selection of tools made from "Obsidian" were found. A specimen of obsidian was found by the author in the fields off Rinella (Malta). This dark vitreous volcanic rock much like bottleglass was very probably imported through the Straits of Messina from Lipari, again by human agency. Obsidian is usually composed of corroded crystals of quartz accompanied by sanidine, plagioclase, biotite and less often amphiboles and pyroxenes. Also owing its origin to human transportation, are specimens of "basalt" found by the author at Kalkara and Selmun (Malta). Very often one comes across small pieces of "Pumice" on the Maltese coast, particularly the sandy beaches of Mellieha Bay and Ghajn Tuffieha Bay in Malta and Ramla Bay in Gozo. Some of the specimens collected by the author had attached to them colonies of gooze-barnicles, indicating transportation by surface waves. Pumice is a light spongy piece of lava which floats over water.

DERIVATIVE ROCKS are of a very diverse origin, and show a great variety of composition, texture and structure. Some are dominantly siliceous, calcareous, argillaceous, ferruginous, or carbonaceous; others are mixtures of many kinds of material; while a few are composed of one mineral substance only. As regards to texture, these rocks vary from smoothly compact rocks to aggregates of the coarsest kind. So, likewise, they exhibit much variety of structure - the large majority consisting of fragmented (clastic) materials, while some are crystalline or sub-crystalline. All derivative rocks are of epigene origin, i.e. they have been produced at or near the earth's surface by the action of various denuding agents. Hence many have been formed mechanically; some are due to chemical action; while others are of organic origin. As a rule they are characterised by a more or less pronounced bedded arrangement, and thus are often termed collectively the "Stratified Rocks". All the derivative rocks are therefore composed of materials derived from the breaking up and disintegration of pre-existing minerals and rocks by epigene agents, and from the debris of plants and animals. Various systems of classification have been proposed, but the more convenient is the one based on the geological origin of the rock.

The process of formation of these rocks may be "mechanical" giving rise to subaerial and aeolian rocks, sedimentary rocks; and glacial rocks. Subaerial and aeolian rocks are those accumulations whose origins are the action of wind and "weathering". The most commonly met specimen of this group are the various types of soil and their sub-soils. These deposits are an unconsolidated heterogeneous aggregate of disintegrated rock material, with organic remains towards the surface. In the Maltese islands, overlying the coralline limestone, there is found a type of soil termed "Terra rossa soils". These soils are a red or brownish ferruginous earth, consisting of the insoluble residue derived from the dissolution of these rocks by atmospheric action. Analysis on the various types of Maltese rocks have been carried out by Murray (1890). The results are given below.
 

Mechanically formed rocks may also owe their origin to the action of water, hence being sedimentary. These rocks are usually arranged in layers or beds, and vary considerably in texture. They may take the form of coarse aggregates of boulders and shingle, or of sediments composed of the finest impalpable materials. Less sharply distinguished from each other, as a rule, than is the case with igneous rocks, sedimentary rocks of various kinds often merge into one another - coarse-grained grits and sandstones passing gradually into the finest argillaceous accumulations. The coarser-grained deposits are almost invariably of shallow-water origin, while the finest grained sediments have accumulated in still waters at some distance from the land. Sedimentary rocks very often yeild relics of animals and plants.

The Maltese Islands are composed of sedimentary rocks, of which the most typical of this mode of formation are the Greensand and Blue Clay horizons. "Greensand" is composed of cemented grit or sand with a high percentage of disseminated "glauconite". This mineral accounts for the occasional dull greenish colour of the deposit. Glauconite is found in fragments not exceeding 0.5mm. Even smaller fragments of other minerals are also found to a lesser extent. This shallow water deposit lies over the "Blue Clay" horizon, which was laid down in deeper waters. Clays are aggregates of very finely divided mineral matter, which become plastic when moistened. The clays of the Maltese Islands are considered to have a gradual transition from the Globigerina Limestone horizon, and have a high carbonate content. The Blue Clay horizon thus falls under the group of calcareous clays or "marls".

The arrangement of the porous Greensand overlying the impervious Blue Clay is essential to the development of the Upper water Table. Rainwater falling on the surface, percolates through the upper strata through fissures in the Upper Coralline Limestone and the porous Greensand to reach the Blue Clay strata where it collects. Greensands contain a large amount of clayey and calcareous material, and because of their soft nature weather badly. This rock has very limited industrial use, because of its physical properties. No useful building stone can be obtained from this horizon, though Fort Chambray in Gozo is partly built of this material. According to J. Murray, a fine variety of black sand obtained from this horizon was previously used as "blotting for writings on paper". Blue Clay has so far only a limited economical use. Mixed with sand and puddle, it was in the past used to build dams and other engineering purposes. Better materials are now available. The clay can also be used for pottery. The organic remains from the Blue Clay horizon are as a rule completely mineralised and replaced by peroxide of iron, thus making some fossils unrecognisable. Also common in this horizon are deposits of isolated masses of varying sizes and shape or in aggregations of small crystals of selenite or gypsum.

"Gypsum", hydrous calcium sulphate, crystallises in monoclinic forms - the crystals being often twinned ('swallow-tailed' twins). Its hardness (1.5-2.0) and specific gravity (2.3) are considerably less than those of another calcium sulphate from "anhydrite". the latter crystallises in the orthorhombic system and cleaves in three directions at right angles (pseudo-cubic); hardness 3.0-3.5, specific gravity 2.9-3.0. Gypsum may be variously coloured, but is usually transparent or white. Crystals, lenticular concretions, and interrupted layers of gypsum often occur in clays. Frequently is appears as granular and compact masses. Now and again it forms the cement or binding material of sandstone. Selenite" is the name given to crystallised gypsum; it shows perfect cleavage - the laminae being flexible but not elastic. The very fine-grained cryptocrystalline kinds are usually termed "Alabaster", and the fibrous varieties "Satin Spar".

Another Maltese deposit of sedimentary origin is the scattered Pliestonces deposits of "breccias" laid down by running waters. The composition of these deposits are variable in different localities and at different layers of the same locality. Fossil remains are often associated with these deposits, giving an intriguing picture of quaternary conditions on the Islands. These deposits have no economical value, further than the use of crushed material as soil substitutes.

Oligocene deposits under the Lower Coralline Limestone as determined by the Naxxar Borehole, showed these to be partly composed of subordinate shales. "Shales and Mudstones" are compact argillaceous rocks, the former dividing easily into thin layers or laminae, the latter being more massive with little or no trace of bedding. Shales vary greatly in composition. Some contain much sand ("arenaceous shale"), others much carbonaceous matter ("carbonaceous shale"). "Calcareous shales" are varieties more or less rich in carbonate of lime, either chemically precipitated or of organic origin.

Glacial rocks are the result of the mechanical action of ice. These forms of rock are not represented on the Maltese Islands, since low temperatures presently play a minor role in today's denudation processes. J. Borg (1936) believed that during the quaternary period, low temperatures played a more important role resulting in the decapping of the rock strata of the resistant Upper Coralline Limestone, thus exposing the more easily erodable underlying strata.

Water plays an important part in the erosive process of rock, particularly in a richly calcareous region as the Maltese Islands. redeposition of chemical precipitates from aqueous solution gives rise to "Chemically-formed Rocks". Very often water percolating through caves and fissures result in a crystalline precipitation to give "Stalactites and stalagmites". The stalactites grow downwards from the roof, while the stalagmites slowly accumulate on the floor. The colour of these deposits varies indefinitely depending on the amount of impurities present - they may be creamy-white, yellowish, brownish, or reddish, and are often mottled. They usually show a concentric, laminated structure, and the stalactites, in the early period of growth, are porous and readily crushed. Subsequently, however, their pores become filled up with calcium carbonate, and the structure thus gradually solidifies. Stalagmites are seldom or never porous, but exhibit a well-defined laminated structure. In course of time both stalactites and stalagmites, owing to molecular changes, tend to acquire a crystalline structure.

Another chemically formed calcareous rock formed by deposition from calcareous springs is "Tufa or Calc-sinter". This rock is porous and frequently very friable. The colour varies with creamy-white and yellow tints being the more common. Not infrequently the colour may be red or brown, while some forms are greenish or bluish. The rock is often mottled or marked with concentric bands of different colours. "Travertine" is the name given to compact varieties used for building stones. They frequently have a crystalline or sub-crystalline structure. Many interstratified common limestones, consisting of fine-grained crystalline aggregates of calcite, are of chemical origin. There are numerous varieties characterised by the presence of certain impurities and admixtures. Some forms belonging to the "Lower Coralline Limestone" horizon may be considered to belong to the chemically-formed rocks. These rocks are sometimes crystalline or semi-crystalline, and microscopic examination of wholly crystalline Lower Coralline Limestone shows no evidence of organisms. Some qualities are known as "Malta or Gozo Marble". This rock is not a real marble in the full petrological sense, but merely a polishable limestone, as the crystallisation of the calcite particles is not sufficiently coarse, and its hardness is below the average standard of a high class marble. This strata is quarried, along with the Upper Coralline limestone, to obtain first and second quality hardstone ("Zonqor"). First quality stone is now very little used for building purposes because of its high cost. It is now very often used as local low-grade marble. Both qualities of hard limestone have been used for metalling roads, and are crushed for use in building. "Marble" is a metamorphic rock which exhibits a granoblastic structure. If derived from a pure limestone, the structure consists of a mosaic of equidimensional grains of recrystallised calcite, all traces of clastic and organic structures having been obliterated. No metamorphic rocks are exemplified on the Maltese Islands.

Cretaceous and Eocene rocks from the Maltese Islands area obtained from the Naxxar borehole show these to consist of dolomitized limestones. "Dolomites and Dolomitic Limestones" are crystalline granular aggregates, the former composed of the mineral dolomite alone, the latter of calcite and dolomite. Ferrous carbonate and various impurities are more or less commonly present. When such is the case, the rock is usually yellowish; when impurities are sparingly present the rock is grey or white. Typical dolomite is easily distinguished from ordinary limestone by its superior hardness (3.5-4.5), its greater specific gravity (2.8-2.9), and its much less ready solubility in cold acid. Many dolomites appear to have been originality common limestones, which either at the time of their formation or subsequently, have by various chemical processes been converted into dolomitic limestones. Few limestones are without some proportion of magnesia, so that it is not possible to draw a hard-and-fast line between common limestone and magnesian limestone - a term applied to limestone free from dolomite but containing notable percentages of magnesium carbonate in solid solution in the calcite. Between this and common limestone which contain little or no magnesia, several gradations are possible.

Various authors record the presence of considerable quantities of siliceous concretions within the Lower Globigerina Limestone. These are either irregular shaped masses of 2 feet or more in length, and 6 - 12 inches thick, or spheroidal-shaped nodules with symmetrical outlines. The larger masses consist of pure "Flint or Silex"; and the smaller nodules of "Chert or Phtanite" which is an impure, calcareous variety of flint having a partly organic and partly chemical origin. The colour is grey, fawn, blue or black; microscopic studies show organic remains. Flint forms breaks with a marked conchoidal fracture. It may be translucent along the sharp cutting edges. Its precise mode of formation is not clear, but it would appear to be partly of organic, partly of chemical origin. Sponges and other organisms secrete soluble silica from seawater, when they die additional silica is deposited upon and within their skeletons and exuvia. Calcareous shells, and even the chalk in which these are embedded have often been partially or wholly replaced by silica. This type of formation may be encountered with on the Maltese Islands.

Mention has already been made to the mineralization of fossils in the Blue Clay strata. "Ironstones" are sometimes of chemical, sometimes or organic origin, or partly both. They may take the form of mineralization or organic remains by iron oxides. They may also be met with occupying fissures and irregular cavities. "Limonite", when approximately pure, is a compact fibrous or stalactitic aggregate, in which form it usually occurs in veins and cavities. When appearing as a bedded rock it is usually earthy and porous, and crowed with impurities. Iron oxide is a common constituent of Maltese rocks, particularly in the Blue Clay strata. It is however very finely mixed and forms no definite aggregate, except the mineralised fossils of Blue Clay and occasional deposits of reddish Globigerina limestone.

The majority of rock strata of the Maltese islands are highly fossiliferous, but the more important ORGANICALLY-DERIVED ROCK is the "Globigerina Limestone". This rock is composed of minute fragments of foraminifera shells, along with other fossil remains. J. Murray made 40 examinations of slides from different horizons and localities of Globigerina Limestone to show that the whole of the beds are principally made up of the shells of the pelagic Globigerinae and their broken down remains. The Globigerina horizon also exhibits interspersed layers of "Phosphatic Nodule Beds". Phosphatic rocks may form in warm seas with abundant life through the phosphatisation, usually in the form of nodule layers, of the bones, shells and other organic remains on the sea-floor. Analysis of these nodules carried out by Murray showed a much higher percentage of calcium phosphate (38.34%) than normally found in Globigerina (1.24-3.57%). Globigerina Limestone is a highly important horizon since it supplies most of Malta's building stone ("Franka"). Its soft nature enables this rock to be easily cut into large blocks and can be hewn into any shape. Sculpturing of this rock is also easily possible. Unfortunately some forms weather badly resulting in the honeycomb appearance of many old walls. The same rock is also used in the local production of lime.

In a number of localities in Malta and Gozo associated with the Scutella Transition Bed, can be found quantities of rounded boulders and angular fragments of "Black Limestone". These vary in size from a walnut to a medium-sized pumpkin and occur in the gorges beds, in the field soils, and in all localities where the Globigerina has been eroded away. A chemical examination of the rock showed traces of manganese, whilst the black matter was found to be of a bituminous nature. A thin section showed the limestone unaltered, but smudged and clouded by an impregnation of carbonaceous matter which was finely divided carbon rather than organic matter or hydrocarbons. Emmons test for petroleum bearing rocks carried out on a specimen of Black Limestone from Il-Mara (Malta) proved negative. The origins of the rock remain speculative. The prospects of petroleum exploration in the Maltese archipelago area have long been explored.

METAMORPHIC ROCKS are not known to be represented on the Maltese Islands, even though the Lower Coralline Limestone is sometimes referred to as Malta Marble. This is not a marble in the true petrological sense. These rocks are formed by the action of heat and/or pressure upon pre-existing rocks, producing considerable changes in mineralogy and texture. One form of metamorphic rock which may be present on the Islands in "Friction or Crush Breccia". This is an aggregate of angular and subangular fragments, varying in size up to one foot or even more in diameter. When there has been considerable movement of the rock debris and the fragments have been rolled over and are more or less rounded, the rock is termed a "friction-conglomerate". Although friction-breccias are best developed in regions where the rocks have been subjected to much compression - to folding and great dislocations and displacements - and where frequently metamorphism is more or less pronounced, they are nevertheless not confined to such regions. Faults traversing strata of all kinds are not infrequently accompanied by breccias. Sometimes these are confined to a line of fracture, filling up the space between the two walls of a fault; while in other cases the rocks forming one or both walls of a fault have been jumbled, shattered, and brecciated. The stones in such fault-breccias are often rubbed smooth and striated on one or more sides.
 

Related Topics

• Petroleum exploration
• Maltese Minerals

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