TEXTURE AND STRUCTURE OF IGNEOUS ROCK
- 3. • NITYA PARASHAR • NISHA CHOURASIYA • PRANAV TIWARI • PRINCE RAJ SINGH • SHOBHIT DAHIYA • VAISHNAVI UPADHYAY
- 5. IGNEOUS ROCKS ARE FORMED WHEN MOLTEN ROCK COOLS AND SOLIDIFIES. THEY ARE ONE OF THE THREE MAIN TYPES OF ROCKS, ALONG WITH SEDIMENTARY AND METAMORPHIC ROCKS. IGNEOUS ROCKS CAN FORM BENEATH THE EARTH'S SURFACE OR AT ITS SURFACE AS LAVA. THEY ARE DIVIDED INTO TWO GROUPS, INTRUSIVE OR EXTRUSIVE, DEPENDING ON WHERE THE MOLTEN ROCK SOLIDIFIES. E.G:- OBSIDIAN,BASALT,APLITE,ETC.
- 6. "TEXTURE" REFERS TO THE SIZE, SHAPE, AND ARRANGEMENT OF MINERAL GRAINS OR CRYSTALS WITHIN A ROCK. IT PROVIDES INFORMATION ABOUT THE CONDITIONS UNDER WHICH THE ROCK FORMED, PARTICULARLY THE RATE OF COOLING OF THE MOLTEN MATERIAL THAT LED TO ITS SOLIDIFICATION.
- 7. DEGREE OF CRYSTALLIZATION OR “CRYSTALLINITY” SIZE OF THE GRAIN OR “GRANULARITY” FABRIC
- 9. RATE OF COOLING OF MAGMA VISCOSITY OF MAGMA RAPID COOLING AND HIGH VISCOSITY FAVOR THE FORMATION OF GLASS. SLOW COOLING AND LOW VISCOSITY PROMOTES THE FORMATION OF CRYSTAL.
- 10. CRYSTALLINITY REFERS TO THE DEGREE OF CRYSTALLINE STRUCTURE PRESENT IN THE ROCK. TYPES OF CRYSTALLINITY HOLOCRYSTALLINE HEMICRYSTALLINE / MEROCRYSTALLINE HOLOHYALINE
- 11. A HOLOCRYSTALLINE ROCK IS COMPOSED ENTIRELY OF MINERALS THAT HAVE FORMED RECOGNIZABLE CRYSTALS DURING THE ROCK'S COOLING AND SOLIDIFICATION PROCESS. E.G:- GRANITE, GABBRO, SYENITE.
- 12. WHEN THE ROCK IS COMPOSED PARTLY OF CRYSTALS AND PARTLY OF GLASS THE TERM MERO/ HYPO/ HEMICRYSTALLINE IS USED. E.G. RHYOLITE, TRACAYTE.
- 13. THE ROCK WHICH IS ENTIRELY COMPOSED OF GLASSY MATERIALS OR GLASS IS KNOWN AS HOLOHYALINE E.G. OBSIDIAN, PITCHSTONE.
- 14. GRANULARITY DEALS WITH THE SIZE OF THE CRYSTALS IN IGNEOUS ROCKS FROM ALMOST SUBMICROSCOPIC DIMENSIONS TO CRYSTAL MEASURABLE IN YARDS. TYPES OF GRANULARITY:- a) PHANERIC OR PHANEROCRYSTALLINE b) APHANATIC
- 15. If the crystals are visible to the naked eye or by ordinary hand lens, the rock is said to be Phaneric or Phanerocrystalline. Phaneric or Phanerocrystalline is further classified by. Coarse Grain: > 5 mm Medium Grain: 5 mm to 1 mm.. Fine Grain: <1 mm fine grain Medium grain Coarsegrain
- 16. If the crystals are not visible to the naked eye or by ordinary hand lens, the rock is known to be Aphanitic. I. Microcrystalline: individual crystals are distinguishable with the help of petrographic microscope .II. Cryptocrystalline: individual crystals are not distinguishable with the help of petrographic microscope. It is classified by:-
- 17. The fabric or pattern of the rocks depend on the shapes, relative size and arrangement of the crystals. a. Shape of the crystal b. Mutual relationship of crystal
- 18. SUBHEDRAL: •DEFINITION: SUBHEDRAL CRYSTALS EXHIBIT SOME, BUT NOT ALL, WELL-FORMED CRYSTAL FACES. THEY ARE INTERMEDIATE BETWEEN EUHEDRAL AND ANHEDRAL. Crystals forms are described with references of the development of their faces. . Euhedral: •Definition: Euhedral crystals are well-formed crystals that display sharp, distinct crystal faces Anhedral: •Definition: Anhedral crystals are crystals that lack well-defined crystal faces. They form within a confined space or where other minerals or physical constraints prevent the
- 19. fabric of a rock is influenced not only by the shape of the crystals, but their relative size and their mutual arrangement.
- 20. In Equigranular rocks, there is a relatively uniform distribution of mineral grains, and they exhibit similar sizes throughout the rock mass. Panidiomorphic : Most of the crystals are euhedral Hypidiomorphic : Most of the crystals are Subhedral Allodiomorphic : Most of the crystals are anhedral panidiomorphic hypidiomorphic allodiomorphic
- 21. Porphyritic texture is a type of igneous rock texture that occurs in coarse, medium, and fine-grained rocks. It is defined by the presence of larger crystals, known as phenocrysts, surrounded by a matrix of considerably smaller crystals or igneous glass, called groundmass. phenocryst groun dmass Porphyritic texture
- 23. Poikilitic texture refers to igneous rocks where large later-formed less perfect crystals ('oikocrysts') surround smaller early-formed idiomorphic crystals ('chadacryst’s') of other minerals. A poikilitic texture is most easily observed in petrographic thin sections.
- 24. Ophitic texture is a variation of the poikilitic texture where random plagioclase laths are enclosed by pyroxene or olivine. It is an important variety of basalt texture where pyroxene forms larger crystals and typically contains numerous crystals of plagioclase. OPHITIC TEXTURE
- 25. When two minerals crystallize simultaneous, they may result intergrowth texture. Graphic intergrowth: intergrowth between orthoclase (Feldsper) and Quartz Myrmekite intergrowth: intergrowth between Plagioclase and Quartz. Perthitic intergrowth: intergrowth between Orthoclase and Albite.
- 26. Since magma is a fluid, it can flow. If elongate crystals, such as feldspar are present in the magma whilst it is flowing, these crystals may well align themselves in the direction of flow Directive texture
- 27. When triangular or polygonal interspaces between the crystals are entirely filled with granules of other minerals, the texture is said to be Intergranular. Generally, Basalt shows this type of texture and the filled granules are olivine, augite. basalt
- 31. When lavas heavily charged with gases and other volatiles are erupted on the surface, the gaseous constituent's escapes from the magma as there is a decrease in the pressure. Near the top of the flows in the material, empty cavities, bubbles, vesicles are formed, which may elliptical, spherical, cylindrical or irregular in shape. The individual openings are known as vesicles and the structure as a whole is known as vesicular structure . If however, the vesicles thus formed are subsequently filled in with some low- temperature secondary minerals, such as calcite, zeolite, chalcedony etc., these infillings are called "amygdales". Amygdaloidal
- 32. Since lavas of acidic composition, due to their high viscosity, do not flow to greater distances, after solidification they are found to offer a very rough surface. Such lava flows are known as block lava. It is also known as 'aa' structure. Lavas of basic composition are quite mobile because of their low viscosity and they can flow to greater distances and after solidification offers very smooth surface. Such lava flows are known as ropy lava and are also
- 33. A distinctive form of lava flow in which the solidified rock forms rounded masses with a glassy exterior. Pillow lava is found where lava flows into the ocean or a lake and quickly cools. The liquid lava forms a sphere when it enters the cold water as a result of surface tension. These hot balls of quickly solidifying lava settle to the ground quickly. As they settle, they flatten and become pillow like in form. As more and more pillows settle to the seafloor, the weight flattens them even more. Pillow lavas therefore form piles around the vent or entry point of a lava flow. They make up much of the
- 34. Joints are divisional planes which are found in all kinds of igneous rock impressed upon them mainly by force acting from outside. Joints plains may be vertical or horizontal. The horizontal joints planes are sometimes so closely spaced as to produce a sheet structure. The sheets are commonly thinner as the surface of the ground is approached and they usually show some degree of parallelism.
- 35. Columnar structure refers to the structure of a mineral aggregate made up of nearly parallel slender columns that is intermediate between an equant and acicular structure. It is found in some amphiboles.
- 36. Pegmatites are extreme igneous rock that form during the final stage of a magma's crystallization. They are extreme because they contain exceptionally large crystals and they sometimes contain minerals that are rarely found in other types of rocks. To be called a "pegmatite," a rock should be composed almost entirely of crystals that are at least one centimetre in diameter. Granite and Syenite more often show this type of structure.
- 37. Spherulites are typically two-mineral aggregates (mainly quartz and feldspar), formed by initial Spherulitic growth of one mineral and later crystallization of a second mineral from the liquid or glass between the fibres. Spherulitic aggregate (Spherulites) arrays of fibrous (needle-like, acicula) Are common in glassy felsic volcanic rock
- 38. Orbicular granite (also known as orbicular rock or orbiculite) is an uncommon plutonic rock type which is usually granitic in composition. These rocks have a unique appearance due to orbicules concentrically layered, spheroidal structures, probably formed through nucleation around a grain in a cooling magma chamber