Engineering geology-petrology structure, forms, texture of various rocks and their classification

• Texture is defined as the size, shape and arrangement of the grains of crystals in a rock. 

Igneous rock can have different textures depending on different bases:

1. On the basis of degree of crystallization: 

    All of the minerals present in the rock may be distinctly crystallized and may be easily recognized by unaided eye or they may be very poorly crystallized or even glassy (i.e. non crystallized)

• Polycrystalline (Phaneric/ Phanerocrystalline): When all the minerals are distinctly crystallized. 

•  Holohyaline (Aphanitic): when the component minerals are very fine in size and glassy or non-crystalline in character.

• Merocrystalline: intermediate type of texture i.e. some minerals are crystallized and others are of glassy characters.

b) On the basis of granularity: Granularity defines the grain size of rocks taking into account the average dimensions of different component minerals. The rock is described as:

 

• Coarse grained: If the average size of the minerals of the rock is >5 mm. 

• Medium grained: If the average size of the mineral is in between 1-5 mm. 

• Fine grained: If the average grain size of the mineral is <1mm.

On the basis of fabric:  Fabric describes the relative grain size of different minerals within the same rock as well as the degree of perfection in the form of the crystals of the individual minerals.

• Equigranular: If all the minerals are of approximately equal dimensions e.g. Granite. 

•  Inequigranular: When some of the minerals in rock are exceptionally larger or smaller than the others.

• Inequigranular textures can be further divided into the following types 

 i. Porphyritic texture: If few conspicuously large sized crystals (phanerocrysts) are found in a fine-grained groundmass (Fig. 1a) e.g. Basalt, Diorite, Rhyolite. 

  ii. Poikilitic texture: if fine-grained crystals are embedded within the body of large sized crystals (Fig.1 b) e.g. Syenite, Peridotite etc.

iii. Ophitic texture: In the similar type of texture as in poikilitic texture, if the host mineral is Augite and the inclusion is of Plagioclase mineral (Fig. 1c) e.g. Dolerite.

 

      

 

 Fig: Inequigranular texture: a) Porphyritic b) Poikilitic c) Ophitic 

Similarly, on the basis of degree of perfection: of the individual minerals in the form of crystals as observed under microscope, the texture may be: 

• Panidiomorphic: if majority of the constituent minerals occur in perfect crystal shape (euhedral crystals) e.g. Syenite.

• Allotriomorphic: when most of the crystals are irregular in shape (anhedral crystals) e.g. Gabbro. 

• Hypidiomorphic: if the rock consist of mixture of anhedral, subhedral and euhedral crystals. eg. Granite

Forms of Igneous rock

• The size, shape and mode of formation of igneous rock is called form. 

• It is predominantly governed by temperature, composition, viscosity and mode of formation of magma as well as nature of rocks into which they are injected. 

• Similarly, chemical and physical properties, overburden load and structures of pre-existing rock are governing factors for different forms of igneous rocks. 

• The forms of igneous rocks can be broadly classified into two types namely concordant bodies and discordant bodies.

CONCORDANT BODIES 

• The intrusions in which magma is influenced by the structural features of the rocks into which it has been injected and solidified along the planes of weakness like bedding planes, foliation planes etc. are called concordant bodies. 

• Depending on the structure of the igneous intrusions and its effect to the host rocks, concordant bodies can be further divided into different types which are described below. 

i) Sills ( Figure 2 a) : Sills are concordant, tabular bodies that are emplaced essentially parallel to the foliation or bedding of the country rock. High degree of mobility is required to produce this sheet like form and basaltic magmas are more fluid than granitic ones, therefore most sills are basaltic in composition.

Laccoliths ( Figure 2 b) : They are concordant, commonly mushroom shaped intrusions that range in diameter from about 1 to 8 km with maximum thickness in the order of 1000 m. 

• They are created when magma rising upward through essentially horizontal layers in the earth’s crust encounter more resistant layer, magma spreads laterally under it forming a dome in overlying layers. 

 Lopoliths (Figure 2c) : It consists of a large, lenticular, centrally sunken, generally concordant, basin or funnel shaped intrusive mass. 

• The diameter ranges from tens to hundreds of kilometers and thickness up to thousands of meters. 

• They are usually mafic to ultramafic and few have upper siliceous layers.

iv. Phacoliths ( Figure 2 d) :Phacoliths are intrusive concordant bodies associated with folded rocks. 

• Within an anticline, they are convex upward, and in trough of a syncline they are convex downward. 

• It is assumed that it is passive i.e. magma fills and enlarges the open or potentially open areas that develop crests and trough during folding. 

Fig: Sill, Laccolith, Lopolith and Phacolith

DISCORDANT BODIES

• The intrusions in which magma is not influenced by the structural features of the host rocks and solidified by cross-cutting the rocks into which it has been injected are called discordant bodies (Fig. 3). 

• Depending on the structure and size of such igneous intrusions and its effect to the host rocks, discordant bodies can be further divided into different types which are described below.

 I). Dikes ( Figure 3 a):  Dikes are tabular discordant plutons that cut across the foliation or bedding of the country rock. 

• They are typically emplaced into preexisting joint system and may occur singly or in swarms. 

• Occasionally, vertical- or outward dipping ring dykes or inward dipping cone sheets may be found distributed in circular or oval pattern.

II) Batholiths ( Figure 3 b) : They are large intrusive plutons with steeply dipping walls. 

• They are often composed of silicic rocks, range in size from about hundred to several thousand square kilometers, they form the core of major mountain systems in the world.

• Although broadly concordant to the regional structure, are highly discordant when mapped in detail, may contain minor intrusions.

iii. Stocks or Bosses: Stocks are smaller bodies that are likely fed from deeper level batholiths. 

• Their maximum surface area is of 100 km2. 

• Stocks might be feeders for volcanic eruptions, but large amounts of erosion are required to expose a stock or batholiths, and therefore the associated volcanic rocks are rarely exposed. 

 

Fig: Discordant bodies a) Dike b) Batholith

 

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