GEOLOGY (PETROLOGY)

ROCK OR STONE

 is a natural substance, a solid aggregate of one or more minerals combined together in an orderly manner.

PETROGRAPHY

deals with descriptive part of rocks

PETROGENY

deals with the mode of formation of rocks.

GEOLOGICAL CLASSIFICATIONS OF ROCKS

1. Igneous Rocks

• Intrusive Rocks(Hypabasal, Plutonic)

• Extrusive Rocks (Volcanic)

2. Sedimentary rocks

• Clastic Deposited Rock (Rudaceous, Arenaceous, Argillaceous)

• Chemical Depositeed Rocks

• Organic Deposited Rocks

3. Metamorphic rocks

• Foliated rocks

• Non-foliated rocks

IGNEOUS ROCKS

Are the primary rocks which are formed due to cooling and solidification of magma.

MAGMA

• is a hot viscous, siliceous melt, containing water vapor and gasses.

INTRUSIVE IGNEOUS ROCKS

• are formed from magma that cools down and solidifies within the crust of a planet, surrounded by pre-existing rocks. 

• The magma cools slowly and as result, these rocks are coarse-grained. The mineral grains in rocks can generally be identified with the naked eye. 

HYPABASAL ROCK

• These rocks are formed at intermediate depth generally up to 2 km below the earth surface and exhibit mixed characteristics of volcanic and plutonic. 

• Diorite and Granite

PLUTONIC ROCK

• are formed at considerable depth generally up to 7-10 km below the earth surface. Because of the very slow rate of cooling at these depths coarse grained rocks are formed. (Ex. Gabbro, Pegmatite)

EXTRUSIVE IGNEOUS ROCK

• It is also called volcanic rocks. These rocks are formed due to cooling and solidification of magma at the crust surface. 

• The magma which is brought to the surface through fissures or volcanic eruptions, solidifies at a faster rate. Hence, such rocks are smooth, crystalline, and fine-grained.

VOLCANIC ROCK

• These rocks are formed due to cooling and solidification of lava erupted from volcanoes, since lava cools down at a very fast rate, the grain size of the crystal formed in these rocks is fine. (Obsidian, Basalt)

SEDIMENTARY ROCKS

• Are those which are formed by the accumulation, compaction and consolidation (cementation) of sediments. The sediments are the particles produced from the disintegration of pre-existing rocks (Igneous/Metamorphic Rocks) through the process of transportation and deposition by various natural agencies like wind, water, and glaciers.

• Sandstone, Conglomerate, Shale and Limestone

CLASTIC DEPOSITED ROCKS

• Are mechanically formed rocks due to the process of weathering, erosion, transportation and deposition of pre-existing rocks.

RUDACEOUS ROCKS

grains size is more than 2 mm in diameter (Conglomerate, Breccia)

ARENACEOUS

grain size is between 1 to 2 mm (Quartz Sandstone, Grit)

ARGILLACEOUS

the size of the particle is <1 mm in diameter (Shale and Mudstone)

CHEMICAL DEPOSIT ROCKS

• These are formed by precipitation, evaporation or crystallization from natural aqueous solutions.

• When water is rich in dissolved salt it evaporates and leaves behind minerals (Halite).

• Limestone can form when calcite minerals dissolved in lakes, seas, and underground water come out of solution and form crystals. (Limestone and Halite)

ORGANIC DEPOSIT ROCKS

• These are formed exclusively from remains of organisms like plants/animals deposited in thick layers. 

• Peat -> Lignite -> Coal

• Coal and Lignite

METAMORPHIC ROCK

•  are those rocks that are formed as a result of transformation that takes place in the pre-existing rocks (igneous/sedimentary rocks).When the pre-existing rocks are subjected to higher temperature, pressure and chemically active liquids and gasses, the minerals present in the original rocks changes to new environmental condition. 

• Lignite, Coal and Gneiss

META : ____

MORPH: __________

CHANGE

FORM

FOLIATED

• When pressure squeezes the flat or elongated minerals within a rock so they become aligned. These rocks develop a platy or sheet-like structure that reflects the direction that pressure is applied in. Slate, Schist, and gneiss are all foliated metamorphic rocks. 

• Slate, Schist

NON-FOLIATED

• Non-foliated metamorphic rocks are formed around igneous intrusions where the temperature is high but the pressures are relatively low and equal in all directions. 

• Marble

PHYSICAL CLASSIFICATION OF ROCKS

1. Stratified rocks

2. Unstratified rocks

3. Foliated rocks

STRATIFIED ROCKS

• These rocks have layered structures. They possess planes of stratification or cleavage. They can be easily split along these planes.

• Sandstone, Limestone, and Slate

UNSTRATIFIED ROCKS

• Not stratified. They possess crystalline and compact grains. They cannot be split into thin slabs. 

• Granite, Trap, Marble

FOLIATED ROCKS

• These rocks have a tendency to split along a definite direction only. The directions need not to be parallel to each other as in case of stratified rocks. This type of structure is common in case of metamorphic rocks. 

CHEMICAL CLASSIFICATIONS

1. Siliceous

2. Argillaceous

3. Calcareous

SILICEOUS ROCKS

• These rocks have silicas as the main constituent.

• The silica in the free state is called sand and in the combined state is silicate.

• Granite, Quartzite, and Sandstone.

ARGILLACEOUS ROCKS

• Clay or Alumina as the main Constituent

• Kaolin, Laterite, and Slate

CALCAREOUS ROCKS

• Calcium carbonate or Lime as leading constituent. These rocks are readily acted upon by even dilute HCI.

• Limestone and Marble

FORMS OF IGNEOUS ROCKS

1. CONCORDANT INTRUSIVE BODIES

2. DISCORDANT INTRUSICE BODIES

CONCORDANT INTRUSIVE BODIES

The magma intrusion is parallel to the structure of the country rock, they are called Concordant intrusive bodies.

SILL

• A concordant intrusive igneous body. Sills are formed due to the penetration of magma planes of enclosing sedimentary rock. Their spreading capacity mainly depends upon viscosity of magma, its temperature and the weight of overlying rocks. The thickness varies from a few centimeters to several kilometers.

PHACOLITH

• When thick sedimentary strata are folded, along the crust and tough, some empty space occurs. These spaces are readily occupied if magmatic intrusion gets access to them. On solidification these appear as lenses shaped across the axial plane. 

LOPOLITH

• Basin or saucer shaped concordant intrusive igneous body of enormous size. Its top is nearly flat and bottom is convex downward bowl-like bodies, which are saggy downwards due to the weight of the intruded magma. 

LACCOLITH

• Mushroom-like concordant intrusive body. It is nearly flat bottom but it is convex upward like dome-shaped. When viscous magma is injected along the bedding plane, as it cannot spread easily, it pushes up the overlying rocks and piles more at the place. 

BATHOLITH

• Largest known concordant intrusive igneous bodies mainly occurring in mountain ranges. Batholiths have side sloping away from each other which makes them larger and larger downwards and they extend to very great depth covering several kilometers. 

DISCORDANT INTRUSIVE BODIES

Magma intrusion is not parallel to the structure of country rock.

DYKE

• Are discordant igneous bodies of more or less tabular shape and exhibit a cross cutting relationship with the country rocks they occur commonly in the forms of wall like masses of exactly or nearly vertical altitude. If the rocks constituting the dyke are hard and compact, they can resist weathering and erosion. 

VOLCANIC NECK

• The vents of quiet volcanoes have become sealed with the igneous intrusion and are called volcanic neck/plugs. These forms may be circular, semi-circular or irregular and in varying diameter.

STRUCTURE

• are physical features associated with the rocks. They are primary in nature and occur along the formation of rocks. 

• It helps contribute to the strength and weakness of the rock.

• It helps to distinguish features of rock groups.

• It reveals the mode of formation. 

STRUCTURES OF IGNEOUS ROCKS

1. VESICULAR

2. AMYDOLOIDAL

3. COLUMNAR

4. SHEET

5. FLOW

6. PILLOW

VESICULAR STRUCTURE

• Porous in nature

• Commonly observed in volcanic rocks

• Magma is a mixture of rock melts and volatiles (gasses)

• The gasses present in the magma are lighter and escape into the atmosphere.

• Empty cavities of various sizes and spares are formed on surfaces.

VESICLES

empty cavities in vesicular structures are called?

AMYGDOLOIDAL STRUCTURE

• The empty vesicles are filled by surface water or underground water or sediments.

• The infilled cavity is called amygdales.

COLUMNAR STRUCTURE

• Appears to be made up of numerous parallel polygon prismatic columns bundled together. This results in the contraction of lava during cooling. 

SHEET STRUCTURE

• Appears to be made up of a number of sheets because of the development of nearby horizontal cracks.

• Plutonic rocks are formed at a great depth which means under the great pressures. When erosion takes place in the overlying strata gradually disappear, ultimately exposing plutonic rock in the form sheet. 

FLOW STRUCTURE

• Are linear and nearly parallel features occurring in volcanic rocks which develop as a consequence of the flow of lava.

PILLOW STRUCTURE

• Appears to be a pile of numerous overlapping pillows.

• Only occurs in soda rich basalt rock.

• The pillows are generally interconnected vesicular and glassy tops.

TEXTURE OF IGNEOUS ROCKS

1. EQUIGRANULAR

2. IN-EQUIGRANULAR

3. PORPHYRITIC

4. POIKILITIC

5. OPHITIC

6. PEGMATITIC

7. VESICULAR

EQUIGRANULAR TEXTURE

• The mineral grains are all of approximately the same size. This is because all the minerals simultaneously get consolidated. 

• Granite and Basalt

IN-EQUIGRANULAR TEXTURE

• The mineral grains show marked differences in their grain size. Different mineral grains consolidate at different sizes and hence different minerals exhibit different sizes.

• Syenite

PORPHORYTIC TEXTURE

• Type of in-equigranular texture, where tabular or large sized minerals called PHENOCRYSTS are fully embedded within the fine grained mineral known as matrix. 

POIKILITIC

• Converse of porphyritic texture, which is characterized by the presence of fine grained crystals within the body of large sized crystals.

• Peridotite.

OPHITIC TEXTURE

• Similar to porphyritic structure, which shows phenocrysts are partially embedded within the matrix. This is observed in dolerite rock whose lath shaped mineral Augite enclose within small Plagioclase mineral. 

• Dolerite

PEGMATITIC TEXTURE

• Two or more minerals crystallize simultaneously  in a limited space. Here one crystal intrudes another. An alternate bands of dark and light colored minerals are seen (Quartz and Feldspar)

• Pegmatite

VESICULAR TEXTURE

• The escape of these gasses leads to the formation of different sizes/shapes of holes in the cooled volcanic rocks.

STRUCTURE OF SEDIMENTARY ROCKS

1. STRATIFICATION AND LAMINATION

2. GRADED BEDDING

3. CURRENT BEDDING

4. RIPPLE MARKS

5. MUD CRACKS

6. TRACKS AND TRAILS, RAIL PRINTS

STRATIFICATION AND LAMINATION

• Are bedded in nature. A bed is called stratum and a number of beds are called strata. 

• A bed is generally homogeneous in composition, texture and color. These are called bedding planes. The different layers may vary in grain size, mineral composition, color, texture etc., depending on the environment and formation. This feature is called stratification. 

GRADED BEDDING

• There is a gradual decrease in the size of the particles from bottom to top.

CURRENT BEDDING

• enerally, sedimentary rocks are parallel to one another. Sometimes the beds are deposited slightly inclined to the major bedding plane because of change in the velocity and direction of flow of stream. 

RIPPLE MARKS

• A minor structure in sedimentary rocks is formed due to mechanical origin. They are the undulations structure formed on the surface of loose sediments due to the action of wind in a shallow water body. It is also called wave marks. If the ripple marks are formed by stagnant water then the feature will be symmetrical and if they are formed by moving water then they are asymmetrical. 

MUD CRACKS

• Common structural features of fine grained sedimentary rocks. The development of mud cracks is because of the dying of huge masses of fine grained sediments under the suv-aerial condition. It is also called sun cracks since they are formed due to the effect of solar heat. 

TRACK AND TRAILS

The movement of organisms on the surface of loose sediments develops markings or impressions

RAIL PRINTS

• are formed on the top surface of loose sediments due to impact of drops of rainwater.

TEXTURE OF SEDIMENTARY ROCKS

1. CLASTIC

2. MICRCLASTIC

3. BIOCLASTIC

4. CRYSTALLINE

5. MICROCRYSTALLINE

CLASTIC TEXTURE

• Grains do not interlock but rather are piled together and cemented. Boundaries of individual grains may be another grain, cement or empty pore space. Overall, rock is generally porous and not very dense.

MICROCLASTIC

• Texture is the same as clastic but the clasts are not visible to the eye. Because the grains are invisible, examining the ease in which grains (silt or clay) may be knocked off is the best to perform.

BIOCLASTIC

• Similar to clastic texture except that all of the clasts or grains are fossils. 

CRYSTALLINE STRUCTURE

Crystals are visible and form an interlocking network.

MICROCRYSTALLINE STRUCURE

• No crystals are visible but rock is composed of interlocking microscopic crystals. Dense rocks and typically nonporous. Breaks with characteristics of conchoidal fracture. Broken surface maybe smooth concentric line resembling the inside of an oyster shell or broken glass.

STRUCTURE OF METAMORPHIC ROCKS

1. PORPHYROCLASTIC

2. MACULOSE

3. SCHISTOSE

4. GRANULOSE

5. GNEISSOSE

PORPHYROCLASTIC STRUCTURE

• Produced under stress and in absence of high temperature, whereby rocks are subjected to shearing and fragmentation. Only the durable minerals partly survive the crushing force and the less durable ones are powdered. Thus, when resistant minerals and rock fragments stand out in a pseudo porphyritic manner in the dinner material. 

MACULOSE STRUCTURE

• Produced by thermal metamorphism of argillaceous rocks like shale. Here, larger crystals of cordierite and biotite are sometimes well developed giving a spotted appearance to the rocks. The well developed crystals are known as porphyroblasts with increasing degree of metamorphism, the spotted slates pass into extremely fine grained granular rock known as hornfels. 

SCHISTOSE STRUCTURE

• Platy or flaky minerals like the mica and other equidimensional minerals show a preferred orientation along parallel planes, under the effect of the stress dominating during metamorphism. The longer directions are parallel to the direction of maximum stress.

SCHISTOSITY

•  the property or tendency of a foliated rock, whereby it can be readily split along the foliation plane. 

GNEISSOSE

Banded structure due to alternation of schistose (dark colored) and granulose (light colored) bands and is produced by highest grade of metamorphism, typically by regional metamorphism. The bands differ from each other in color, texture, and mineral composition. Gneisses typically show this type of structure, hence the name.