Geo-Mod4

• Petrology - formation of various types of rock, their mode of occurrence, composition, texture, and structure, geological and geographical distribution on the earth.
• Three Major Types of Rocks

  1. Igneous Rocks
  2. Sedimentary Rocks
  3. Metamorphic Rocks
• Igneous Rocks - rocks that have formed from an originally hot molten material through the process of cooling and crystalization.
• Magma - hot molten material occuring naturally below the surface of the earth.
• Lava - material that has poured out occasionally from volcanoes in many regions of the world again and again.
• Volcanic Rocks - formed on the surface of the Earth by cooling and crystallization of lava erupted from volcanoes.
• Plutonic Rocks - formed at considerate depths-generally between 7-10km below the surface of the earth
• Hypabyssal Rocks - formed at intermediate depths, generally up to 2km below the surface of the earth and exhibit mixed characteristics of volcanic and plutonic rocks.
• Degree of Crystallization - all the constituents minerals are distinctly crystallized forms and easily recognized by unaided eye.
• Holocrystalline - all the constituent minerals are distinctly crystallized.
• Holohyaline - when all the constituents are very fine in size and glassy or non-crystalline in nature.
• Granularity - the grain size of the various components of a rock are the average dimensions of different constituent minerals which are taken into account to describe the grain size of the rock as a whole.
• Coarse-grained - when the average grain size is 5mm
• Medium-grained - when average grain lies between 5mm to 1mm
• Fine-grained - when the average grain size is less than 1 mm
• Fabric - composite term expressing the relative grain size of different mineral constituents in a rock as well as the degree of perfection in the form of the crystals of the individual minerals
• Types of Textures

  1. Equigranular
  2. Inequigranular
  3. Directive
  4. Intergrowth
  5. Intergranular
• Equigranular Texture - crystal of rock are broadly equal in size
• Inequigranular - minerals show marked difference in their relative grain size
• Directive - indicate the result of flow of magma during the formation of rocks. It exhibits perfect or semi perfect parallelism of crystals
• Intergrowth - during the formation of the igneous rocks, sometimes two or more minerals may crystallize out simultaneously in a limited space so that the resulting crystals are mixed up
• Intergranular - crystals formed at earlier stages may get so arranged that polygonal or trigonal spaces are left in between.
• Concordant Bodies - all those intrusions in which the magma has been injected and cooled along or parallel to the structural planes of the hot rocks are grouped
• Sills - igneous intrusion that have been injected along or between the bedding planes or sedimentary sequence
• Types of Sills

  1. Simple Sills
  2. Multiple Sills
  3. Composite Sills
  4. Differentiated Sills
  5. Interformational Sheets
• Phacoliths - small sized intrusive that occupy positions in the troughs and crests of bends called folds
• Laccoliths - concordant intrusions due to which the invaded stata have been archer up or deformed into a dome
• Discordant Bodies - all those intrusive bodies that have been injected into the stata without being influenced by their structural disposition (dip and strike)
• Lipoliths - associated with structural basins, that are sedimentary beds inclined towards a common center
• Dykes - columnar bodied of igneous rocks that cut across the bedding plane or unconformities or cleavage planes and similar structures
• Special Types of Dykes

  1. Cone Sheet Dykes
  2. Ring Dykes
• Cone Sheet Dykes - defined as assemblages of dyke-like injections, which are generally inclined towards common centers
• Ring Dykes - typically arcuate, close and ring-shaped outcrops
• Volcanic Necks - vents of quiet volcanoes have become sealed with the intrusions
• Batholiths - huge bodies of igneous masses that show both concordant and discordant relations
• Important Igneous Rocks

  1. Granites
  2. Diorite
  3. Andesite
  4. Syenites
  5. Dolerites
  6. Basalts
  7. Pegmatites
  8. Aplite
  9. Lamprophyre
  10. Peridotites
• Granites - defined as plutonic light-colored igneous rocks
• Diorite - it is an intermediate type of igneous rock of pluto origin with silica percentage generally lying between 52-66 percent
• Andesite - these are volcanic rocks in which plagioclase felspars are the predominant constituents making the potash felspar only a subordinate member
• Syenites - are defined as igneous, plutonic, even-grained rocks in which alkali felspars are chief constituent minerals