CHAPTER 3: THE STUDY OF ROCKS

CHAPTER 3: THE STUDY OF ROCKS

INTRODUCTION Rock is defined as a mixtures formed of aggregates of one or more minerals (aggregate of minerals). Rocks can be formed by many different processes such as: (1) Igneous - Crystallization of a melts magma (intrusive) and lava (extrusive) (2) Sedimentary - Solidifying sediments like sand or clay (3) Metamorphic - Re-crystallizing previously formed rocks in the solid state (4) Hydrothermal - Some are formed by crystallization from hot aqueous fluids

Cont d Civil engineers have to deal with rock and soils during various stages in the process of construction, build a road, a tunnel, a slope or a dam. From the stage of planning to the execution of a construction project, the engineer must have a basic appreciation of the engineering behavior of rocks and soils under various conditions. It becomes imperative for engineers to have some basic geological appreciation of rocks and soils in order to understand the engineering limits to which these materials can be subjected to Suitable background to the further study of soil mechanics and foundation engineering.

CHAPTER 3.1: IGNEOUS ROCKS

IGNEOUS ROCKS Defined as rocks which are normally crystalline in nature having solidified from an original molten state or magma that exists for long period of time beneath the surface of earth. Igneous rocks can be derived from the cooling of molten magma or of lava from volcanic eruption. Frequently regarded as the parent material because they are the first product to be formed from the cooling of magma. MAGMA is molten rock material generated in the certain zones deep inside the earth's crust and possible in the upper zones of the mantle. Magma moves from deeper zones to higher zones of the crust through forceful injections into fractures and faults in the adjacent rocks.

Igneous Rock

What Is Magma? Magma is hot molten mobile rock. Igneous rocks form when magma cools and solidifies. Magmas come out of active volcanoes as lavas. The most abundant magma is a melt of silicate composition and this can carry suspended crystals and gases which bubble out in air. It is a mixture of liquid rock, crystals, and gas. Magmas are less dense than surrounding rocks, and will therefore move upward. If magma makes it to the surface it will erupt and later crystallize to form an extrusive or volcanic rock. If it crystallizes before it reaches the surface it will form an igneous rock at depth called a plutonic or intrusive igneous rock.

Eruption of Magma When magmas reach the surface of the Earth they erupt from a vent and it may erupt explosively or non-explosively. Non-explosive eruptions are favored by low gas content and low viscosity magmas (basaltic to andesitic magmas). Usually begin with fire fountains due to release of dissolved gases. Produce lava flows on surface. Produce Pillow lavas if erupted beneath water.

Cont d Explosive eruptions are favored by high gas content and high viscosity (andesitic to rhyolitic magmas). Expansion of gas bubbles is resisted by high viscosity of magma - results in building of pressure. High pressure in gas bubbles causes the bubbles to burst when reaching the low pressure at the Earth's surface. Bursting of bubbles fragments the magma into pyroclasts and tephra (ash). Cloud of gas and tephra rises above volcano to produce an eruption column that can rise up to 45 km into the atmosphere.

Tephra that falls from the eruption column produces a tephra fall deposit

If eruption column collapses, a pyroclastic flow may occur, wherein gas and tephra rush down the flanks of the volcano at high speed. This is the most dangerous type of volcanic eruption. The deposits that are produced are called ignimbrites

Grain size Three types of rock can be identified based on predominant grain size that reflects the depth at which molten rocks form within the Earth: a) Volcanic rocks (extrusive) solidify close to the Earth's surface. Because cool quickly they have a finer-grained matrix called groundmass. They may contain some larger crystals that formed earlier further down called phenocrysts. b) Plutonic rocks (intrusive) form deeper within the Earth and the slower cooling allows them to crystallize as coarse-grained rocks. c) Hypabyssal or sub volcanic rocks form at intermediate depths generally as dykes and sills and so tend to be medium-grained.

Classification of Igneous Rocks There are various ways of classifying igneous rocks. The most significant are (1) Mineralogical and chemical composition and (2) Rock texture (geological environment). Igneous rocks are either formed as Intrusive or Extrusive Rocks.

Types of Igneous Rocks Scientists have divided igneous rocks into two broad categories based on where the molten rock solidified. Volcanic or extrusive igneous rocks form when the magma cools and crystallizes on the surface of the Earth. Plutonic or Intrusive igneous rocks where in the magma crystallizes at depth in the Earth.

Intrusive and extrusive igneous rock bodies

Minerals of igneous rocks To correctly classify many igneous rocks it is first necessary to identify the constituent minerals that make up the rock. Piece of cake you say, I saw most of these minerals when I did the Minerals Exercise or I have them in my mineral collection. Well, its not quite that easy. The mineral grains in rocks often look a bit different than the larger mineral specimens you see in lab or museum collections. The following section is meant to assist you in recognizing common rockforming minerals in igneous rocks. Refer back to it often as you attempt to classify your rock specimens.

Plagioclase Plagioclase is the most common mineral in igneous rocks. The illustration given shows a large chalky white grain of plagioclase. The chalky appearance is a result of weathering of plagioclase to clay and this can often be used to aid in identification. Most plagioclase appears frosty white to gray-white in igneous rocks, but in gabbro it can be dark gray to blue-gray. If you examine plagioclase with a hand lens or binocular microscope you can often see the stair-step like cleavage and possibly striations (parallel grooves) on some cleavage faces. Some potassium feldspar is white like plagioclase, but is usually a safe bet to identify any frosty white grains in igneous rocks as plagioclase. Expect to find plagioclase in most phaneritic igneous rocks and often as phenocryts in aphanitic rocks.

Quartz Quartz is also a very common mineral in some igneous rocks. It can be difficult to recognize since it doesn't look like the beautiful, clear hexagonal-shaped mineral we see in mineral collections or for sale in rock shops. In igneous rocks it is often medium to dark gray and has a rather amorphous shape. If look it with a hand lens you will notice the glassy appearance and lack of any smooth cleavage surfaces. You will also find quartz grains resist scratching with a nail or pocket knife, you can expect to find abundant quartz in granite and as phenocryts in the volcanic rock rhyolite. In some other common igneous rocks you may find a few scattered grains of quartz, but it is often conspicuous by its absence. Once recognized, quartz is rarely confused with any other common rock-forming mineral.

Potassium Feldspar Think pink is the motto for potassium feldspar. The image given shows several large grains of the potassium feldspar, orthoclase; note the pinkish cast. As orthoclase is a feldspar, you should also see the stair-step cleavage characteristic of feldspars. Unfortunately, all potassium feldspar is not pink, microcline is usually white. How does one distinguish white potassium feldspar from plagioclase? The answer is that in hand samples it is nearly impossible. Sometimes striations on cleavage faces allow you to differentiate the two. Plagioclase has striations, potassium feldspar does not. But in most cases any white feldspar is identified as plagioclase and any pink feldspar as orthoclase. Expect to find orthoclase as a common constituent of granite and matrix material in rhyolite. In the latter rock the orthoclase is too fine-grained to be seen even with a binocular microscope, but its presence gives most rhyolites a distinct pinkish cast.

Muscovite Muscovite is not a common mineral in igneous rocks, but rather an accessory that occurs in small amounts. It is shiny and silvery, but oxidizes to look almost golden. In fact, more prospectors probably confused muscovite in their pans for gold than they did pyrite (fool's gold). Muscovite has excellent cleavage and will scratch easily. If you suspect muscovite is present, try taking a nail to it. It should flake off the rock. Muscovite occurs in some granite and occasionally in diorite. Unlike, its close cousin, biotite, it rarely occurs as phenocrysts in volcanic rocks.

Biotite Biotite occurs in small amounts in many igneous rocks. It is black, shiny and often occurs in small hexagonal (6-sided) books. It is often confused with amphibole and pyroxene. Like muscovite, it is soft and has good cleavage. Try scratching the black grains with a nail or knife. Biotite will flake off easily. Biotite is differentiated from amphibole by shape of the crystals (hexagonal for biotite and elongated or needle-like for amphibole) and by hardness (biotite is soft, amphibole is hard). It is differentiated from pyroxene by hardness, color (biotite is black and pyroxene dark green) and occurrence (biotite is found in light-colored igneous rocks like granites, diorites and rhyolites while pyroxene occurs in dark-colored rocks like gabbro and basalt). Expect to find biotite as a common accessory in granite, and as phenocrysts in some rhyolites.

Amphibole Amphibole is a rather common mineral in all igneous rocks, however, it is only abundant in the intermediate igneous rocks. Occurs as slender needle-like crystals. It has good cleavage in 2 directions and hence has a stair-step appearance under a binocular microscope. It is often confused with biotite and pyroxene. Biotite is softer and the needle-like crystals differentiate it from pyroxene. One caution, most students believe that all amphibole crystals must have the pencil-like appearance. Remember the orientation of grains in an igneous rock is random. What would your pencil look like if you looked at it down the eraser? Not all grains of amphibole will be oriented so you can see the elongation of the crystals. It s a good guess that if you see a few crystals that have the "classic" amphibole shape, the other black grains are also amphibole. Biotite and amphibole do occur together in igneous rocks, but the association is not all that common. Amphibole is very common in diorite, less so in granite or gabbro. It also is a common and diagnostic phenocryst in andesite.

Pyroxene Pyroxene is common only in mafic igneous rocks. Occurs as short, stubby, dark green crystals. It has poor cleavage in 2 directions and cleavage surfaces are often hard to see with even a binocular microscope. It is often confused with biotite and amphibole. Biotite is softer, darker and occurs in predominantly light-colored rocks Amphibole is also darker and occurs in needle-like crystals rather than the stubby shape of pyroxene. Association is the best guide for the identification of pyroexene. It is usually restricted to dark-colored rocks (the image below is of pyroxene is a very rare lightcolored rock called shonkenite) such as gabbro or basalt.

Olivine Olivine is common only in ultramafic igneous rocks like dunite and peridotite. Occurs as small, light green, glassy crystals (see image below). It has no cleavage. The texture of olivine in igneous rocks is often termed sugary. Run your fingers over the grains, do they feel like sandpaper? The mineral is most probably olivine. Although olivine occurs in gabbro and basalt, it is far more common in peridotite and dunite. Because of the light green color and sugary texture it is rarely confuded with other rock-forming minerals.

Rock Texture The most important distinction in igneous rocks is texture, which is related to the size and shape of the constituent crystallite grains. Igneous rocks have distinctive textures, characterized mostly by the interlocking grains that grow from cooling magma. In Igneous rocks, the cooling history and environment is the function of the formation of textures. Magmas located deep within the Earth's crust cools slowly and thus the individual minerals grains may grow. In contrast, lava extruded at the Earth's surface cools rapidly, where mineral grains do not have time to grow, therefore cannot be seen without the aid of a microscope. The rocks appear massive and structureless.

Phaneritic texture Individual grains are large enough and visible to naked eye. (Figure 3.2) Grains approximately equal in size, form interlocking mosaic and very coarse. Developed from magmas that cool slowly and common in intrusive bodies.

Examples of phaneritic rocks; phaneritic texture, consists of large grains and can be seen unaided

Aphanetic texture Individual crystals are so small and cannot be seen unaided. Rocks are massive and experienced rapid cooling that there was no sufficient time for the growth of large crystals. Characteristic of volcanic rock and some intrusive rocks which lost its heat to the surrounding country rock.

Aphanetic texture consists of grains too small to be seen without a microscope

Glassy texture Similar to ordinary glass. Crystals cannot be discerned in a glassy texture, even when the specimen is viewed under high magnification e.g. obsidian.

A glassy texture develops when molten rock material cools so rapidly

Porphyritic texture Larger earlier formed crystals are enclosed by a ground mass of smaller crystals. Cooling history of magma may begin slowly initially which developed coarse crystals and then while partly crystallized the magma may move to another environment in which the cooling is more rapid which precipitate fine crystals around the earlier coarse crystals.

A hand sample and a thin section of porphyritic aphanitic textured rocks. The porphyritic phaneritic texture results from two stages of cooling

Vesicular Texture This term refers to vesicles (holes, pores, or cavities) within the igneous rock. Vesicles are the result of gas expansion (bubbles), which often occurs during volcanic eruptions. Pumice and scoria are common types of vesicular rocks. The image below shows a basalt with vesicles, hence the name "vesicular basalt".

Vesicular rocks

Chemical and Mineralogical Composition The chemical and mineralogical composition of igneous rocks is a reflection of the composition of magma from which the rocks crystallized. Magma is variable in composition, most importantly in the amount of silica (Si0 2 ) that they contain (Table 3.0). The silica content ranges from less than 45% to more than 66%. Rocks that are rich in silica are called silicic or felsic, rocks and those that are low in silica content are called mafic rocks. Fortunately, colour provides a valuable clue for identification igneous rocks because the silicic rocks are mainly composed of lightly coloured minerals like quartz and feldspar, whereas the mafic rocks are dark coloured because of the abundance ferromagnesian minerals. The dark coloured ferromagnesian minerals are rich in iron and magnesium, include olivine, pyroxene and hornblende. The major igneous rock types fall into categories of high, intermediate and low silica content.

Cont d Silica content (SiO 2 ) which also controls the minerals that crystallize is used to further classify igneous rocks as follows: Acid: usually above 63% silica mostly feldspar minerals and quartz, for example granite. Basic: 45 to 55% silica mostly dark minerals plus plagioclase feldspar and/or feldspathoid minerals, for example basalt. Ultra basic: usually less than 45% silica mostly dark minerals such as olivine and pyroxene, for example peridotite.

Classification of Igneous Rock ACID INTERMEDIATE BASIC ULTRA BASIC Crystalline Texture Feldspar Orthoclase - Plagioclase Extrusive (Usual Occurrence) Intrusive Fine Medium Coarse Rhyolite Microgranite Granite Trachyte Microsyenite Syenite Andesite Microdiorite Diorite Basalt Dolerite Gabbro Ultrabasic lavas Peridotite porphyry Peridotite

Formation of Igneous Rocks (a) Intrusive Processes: Intrusive rocks which cool and solidify under pressure and at great depths are usually wholly crystalline in texture, since the conditions of cooling are conducive to crystal formation. Such rocks occur in masses of great extent, often going to unknown depths. Although originally formed deep underground, intrusive rocks are now widely exposed because of earth movement and erosion processes. Intrusion refers to the movement of magma from a magma chamber to a different subsurface location. Bodies of rock formed by the intrusive magma are called plutons. Rocks that make up plutons usually have phaneritic texture because the cooling time was sufficient to allow the formation of large crystals.

Types of Plutons Plutons differ in terms of size, shape and relationship to the rocks that were intruded by the magma, which are older rocks known as country rocks. A major group of plutons (Figure 3.0) is classified as tabular because they are thin in one dimension as compared with the other two dimensions. Common ones are: (a) Dykes (b) Sills (c) Laccoliths (d) Batholiths

Dykes Tabular or wall like mass. Results from magma injected into cracks and joints in rocks. Vary in width from a few cm to a few meters but not more than 3 meters wide. Largest known dyke in Zimbabwe, Africa which is 600 km long and average width of 10 km.

Sills Rising magma follows path of least resistance such as bedding plane, which separates layers of sedimentary rock. Magma injected between the layers form tabular intrusive body parallel to layering. Sills range from few centimeters to hundreds of meters thick and can extend to several kilometers.

Laccoliths Viscous magma injected between layers of sedimentary rock, tend to uparched the overlying strata forming mushroom shaped. Usually thicker in center and thinner near margin and may give rise to dome shaped hill. Can be several kilometers in diameter and thousands of meters thick and typically porphyritic.

Batholiths Largest rock bodies in the Earth's crust, generally granitic composition. Cover several thousand square kilometers and may be 60 km thick. Typically form in the deeper zones of mountain belts and are exposed only after considerable uplift and erosion.

Types of plutons

Extrusive Processes Extrusive rocks are formed from the violent eruption of volcanoes, fissures or cracks in the earth's cracks. Some materials will be emitted with gaseous extrusions into the atmosphere, where they will cool quickly and eventually fall to the earth's surface as volcanic ash and dust. The main product of volcanic action is a lava flow emitted from within the earth as a molten stream which flows over surface of the existing ground until it solidifies. Extrusive rocks are generally distinguished by their usual finegrained texture.

A Grouping of Igneous Rocks Mode of formation Rock Types Rock Textures EXTRUSIVE Lavas Glassy or fine-grained INTRUSIVE Minor Intrusions Major Intrusions Fine to moderately coarse-texture Coarsely crystalline

Granite Granite characterized by a granular texture, has feldspar and quartz (at least 20%) as its two most abundant minerals. In consequence most granite is light-coloured, Biotite or hornblende or both are also present in most granite with accessory apatite, magnetite and sphene. Granites can be fine, medium or coarse-grained depending on grain sizes of the essential minerals and porphyritic or non porphyritic depending on the absence or presence of phenocrysts (usually alkali feldspar) and/or muscovite.

Basalt Basalt is dark coloured (black to medium grey), fine grained (aphanitic) igneous rock composed of plagioclase, feldspar, pyroxene and magnetite with or without olivine and contain more than 53% by weight of SiO 2. Most basalts are non porphyritic but some contain phenocrysts of plagioclase, olivine and pyroxene. Basalt is the world's most abundant lava and is very widespread.

Gabbro Gabbro is dark coloured, coarse-grained, granular basic igneous rock consisting of essential calcium rich plagioclase, feldspar (approximately 60% augite and orthopyroxene plus or, minus olivine with accessory magnetite or ilmenite. Gabbros result from slow crystallization of magmas of basaltic composition. Gabbro is widely distributed in both large and small masses. Dykes and thin sills of fine-grained gabbro are especially common. In most of these small intrusions, the mineral grains are so small that they are barely recognizable without aid of microscope. Such gabbros, intermediate in grain size between basalt and normal grabbro, are called dolerite.

Diorite Diorite - is an intermediate, coarse-grained, granular igneous rock with up to 10% quartz, plagioclase and lesser amount of ferromagnesian minerals. The most common ferromagnesian minerals are hornblende, biotite and pyroxene. In general, diorite masses are much smaller than those of granites or granodiorite.

Crystallization of Magma Crystallization of magma is not a simple process. An experiment done by N.L.Bowen (Figure 3.6) in early 1900s demonstrated that minerals crystallize sequentially as the temperature drops in a silicate magma and that solid crystals can react with the liquid phase of the magma to form new minerals during the crystallization process. To explain crystallization process, assuming that initially we have a basaltic composition at about 1500 C. As temperature is slightly lowered, crystals begin to separate from the liquid. There are two crystallization sequences that are observed as the melt cools.

First sequence - crystallization of plagioclase This is solid-solution series between calcium-rich and sodium rich compositions. The first plagioclase crystals to form are higher in calcium content than the calcium content of the liquid phase. As the mixture continues to cool, the crystals that form have progressively less calcium and more sodium than the original plagioclase crystals. The crystallization of plagioclase follows what is called a continuous reaction series, in which the liquid and the crystals continuously change in composition until no liquid remains.

Second sequence - crystallization of ferromagnesian mineral The ferromagnesian minerals follow a second type of crystallization sequence. In this series, olivine is the first ferromagnesian mineral to crystallize. As the temperature decreases, no change in the olivine crystals occurs until a critical temperature is reached. At this point, augite rather than olivine begins to crystallize and the early-formed olivine crystals react with the liquid to form augite. These reactions are different from the continuous reaction of plagioclase because entirely new minerals with different internal structures form at specific temperatures. For this reason the ferromagnesian crystallization sequence is called a discontinuous reaction series. The same type of reaction occurs between augite and liquid to form hornblende at a lower temperature. The entire sequence of mineral crystallization is known as Bowen's Reaction Series.

Engineering and Igneous Rocks Igneous rocks vary greatly in suitability for various types of engineering projects. An engineering site investigation must answer two questions: 1) What rock types are present and how are they disturbed? 2) How have the rocks been changed or altered since formation? The geologists and engineers working on the project must determine the origin of the igneous rock, its contacts with adjoining rock types and their conditions and the mineralogy of the rocks. Unaltered intrusive igneous rocks generally are very suitable for most types of engineering projects: 1) The interlocking of mineral crystals gives the rock great strength and thus can provide adequate support for building or dam foundations can remain stable at high angles in excavations and require minimal support in tunnels. 2) Because of the dense interlocking of crystals within the rock, very little water can flow through. Therefore, unaltered intrusive rocks are well suited for construction of reservoirs because of the low potential for leakage.

Cont d The engineering properties of extrusive igneous rocks are much less uniform. Extrusive rocks contains pyroclastic materials and lahar deposits, which are much weaker than crystalline rocks. These rocks may be susceptible to slope failures in excavations and also provide more variable and generally weaker foundation support. In general, the water bearing capacity of extrusive rocks is much greater than intrusive rocks. This property can render the rocks unsuitable or reservoir or tunnel construction. Weathering produces other changes in the rock as well as fracturing. Chemical reactions between the minerals within the rock and air and water gradually form new minerals. Clay minerals are a common product of these alteration processes. The result is a significant loss of strength as the feldspars and ferromagnesian minerals are converted to clay. In warm humid climates, igneous rock bodies may be mantled with tens of meters of weathered material. The engineering properties of this material are totally different from the properties of unaltered rock. It is sufficient to note that a network of fractures within a rock mass can greatly increase the potential for failures of natural or excavated slopes and also increase the construction problems of dams, tunnels and other structures.

End of the Chapter 3.1 Q & A