Isostasy

isotacy

1. GEO PHYSICS Topic ISOSTASY DENSITY,SUSCEPTIBILITY AND RESISTIVITY INSTITUTE OF GEOLOGY UNIVERSITY OF AZAD JAMMU & KASHMIR MUZAFFARABAD

2. WE PLACED FIRMLY EMBEDDED MOUNTAINS ON THE EARTH, SO IT WOULD NOT MOVE UNDER THEM (QUR'AN, 21:31)

3. ISOSTASY

4. ISOSTASY I t refers to the state of gravitational equilibrium between the earth's lithosphere and asthenosphere such that the tectonic plates "float" at an elevation which depends on their thickness and density.

5. Word attribution Isostasy  is derived from two Greek words ISO and STASIS ISO means “same” and STASIS mean “standstill”.  

6. history In 1735, expeditions over the Andes led by Pierre Bouguer, a French photometrist studied the isostasy for the first time. About a century later, similar discrepancies were observed by Sir George Everest, surveyor general of India, in surveys south of the Himalayas, indicating a lack of compensating mass beneath the visible mountain ranges . The general term 'isostasy' was coined in the year 1889 by the American geologist Clarence Dutton.

7. Isostastaic models T here are three principal models of isostasy : THE  AIRY MODEL THE PRATT MODEL THE VENING OR FLEXURAL ISOSTASY MODEL

8. THE AIRY MODEL Different topographic heights are accommodated by changes in crustal thickness, in which the crust has a constant density.   THE PRATT MODEL Different topographic heights are accommodated by lateral changes in rock density.   THE VENING OR FLEXURAL ISOSTASY MODEL Where the lithosphere acts as an elastic plate and its inherent rigidity distributes local topographic loads over a broad region by bending . Airy and Pratt isostasy are statements of buoyancy, while flexural isostasy is a statement of buoyancy while deflecting a sheet of finite elastic strength.

9. The main types of isostatic models. Each model implies a state of hydrostatic equilibrium such that the Earth’s outermost layers are in a state of flotation on their more fluid substrate. (a / 1) The Airy- Heiskanen Model (b / 2) - The Pratt- Hayford Model. (c) Vening Meinesz model.

10. Airy isostasy , in which a constant-density crust floats on a higher-density mantle, and topography is determined by the thickness of the crust.

11. Airy was mostly correct about what supports large (wide) mountains, but it took until the 1970’s to prove this with seismic work that measured the thickness of the crust and lithosphere beneath mountains. Pratt was correct in that the difference between the low standing ocean basins and the high standing continents is partially due to the fact that oceans have dense gabbroic composition crust whereas continents have lighter less dense ‘Andesitic’ composition crust.

12. the vening meinesz or flexural model This hypothesis was suggested to explain how large topographic loads such as  seamounts. A   seamount  is a  mountain rising from the  ocean seafloor that does not reach to the water's surface (sea level ) , and thus is not an island.(e.g . Hawaiian Islands) could be compensated by regional rather than local displacement of the lithosphere. This is the more general solution for lithospheric flexure as it approaches the locally-compensated models above as the load becomes much larger than a flexural wavelength or the flexural rigidity of the lithosphere approaches 0.

13. Regional Or Vening Isostasy - The Lithosphere Flexes Under Its Own Weight And Shields The Asthenosphere From The Difference In Pressures.

14. Which do you think would have the greater volume and mass? Why? 1 kg of feathers 1 kg of rock DENSITY

15. DENSITY Density is defined as mass per unit volume. It is a measure of how tightly packed and how heavy the molecules are in an object. Density is the amount of matter within a certain volume.

16. Units for density The  SI unit  of density is  kg /m 3 , g/cm 3. FORMULA M = D x V V = M / D D = M / V

17. Table showing average gravity of various sedimentary, metamorphic and igneous rocks Density is a property that is proportional to the composition of the rock. The higher the amount of silica (felsic) the less dense the rock will be. The less amount of silica in the rock the more dense the rock will be.

18. DENSITIES OF TYPICAL ROCK TYPES AND MINERALS

19. PUMICE Environment of formation =Extrusive (Volcanic ) Texture = Glassy, Vesicular Grain size =  Non-Crystalline Color =  Light Density =  Low (1.00 g/cm3 ) Composition = F elsic

20. VESICULAR BASALT Environment of formation =extrusive (volcanic) Texture = Glassy, vesicular Grain size = non-crystalline Color = dark Density =  medium ( 2.74 g/cm3) Composition = mafic

21. RHYOLITE Environment of formation =extrusive (volcanic) Texture = fine Grain size = less than 1 mm Color = light Density =  low ( 2.51 g/cm3) Composition = felsic

22. ANDESITE Environment of formation =extrusive (volcanic) Texture = fine Grain size = less than 1 mm Color = light Density =  medium ( 2.64 g/cm3) Composition = intermediate

23. BASALT Environment of formation =extrusive (volcanic) Texture = fine Grain size = less than 1 mm Color = dark Density =  high ( 2.99 g/cm3) Composition = mafic

24. GRANITE Environment of formation =intrusive (plutonic) Texture = coarse Grain size = 1 mm to 10mm Color = light Density =  low ( 2.667 g/cm3) Composition = felsic

25. GABBRO Environment of formation =intrusive (plutonic) Texture = coarse Grain size = 1 mm to 10mm Color = dark Density =  high ( 3.03 g/cm3) Composition = mafic

26. METAMORPHIC ROCKS SLATE ( 2.79 g/m3) PHYLLITE ( 2.18 and 3.3 g/m3)

27. SCHIST ( 2.64 g/cm3) GNEISS ( 2.80 g/cm3)

28. MARBLE ( 2.75 g/cm3) QUARTZITE ( 2.60 g/cm3)

29. Sedimentary rocks SANDSTONE ( 2.35 g/cm3)

30. ROCK SALT (2.17 g/cm3 ) SHALE (2.40 g/cm3)

31. GYPSUM ( 2.31 g/cm3) LIMESTONE (2.55 g/cm3)

32. IGNEOUS ROCKS Igneous rocks form when molten rock (magma) cools and solidifies, with or without crystallization, either below the surface as intrusive (plutonic) rocks or on The surface as extrusive (volcanic) rocks . IGNEOUS ROCK’S DENSITY > METAMORPHIC ROCK’S DENSITY > SEDIMENTARY ROCK’S DENSITY

33. Reason of high density of igneous rocks THEN metamorphic and sedimentary rocks Lack of pore pressure Due to mafic minerals Due to slow cooling Due to close packing or compaction Due to impermeable nature Number of atoms Impurities or Neighboring minerals involvement . Due to slow crystallization More pressure in the subsurface.

34. susceptibility

35. Susceptibility (K) The degree of magnetization in response to the external magnetic field is known as susceptibility . Or It is a measure of the ease with which the material can be magnetized. Mathematically K= I / H I= intensity of magnetization H = Magnetic Field Strength Definition

36. The values given here are for SI, International System Units.

37. value of the magnetic susceptibility The value of the magnetic susceptibility can either be POSITIVE NEGATIVE.

38. Positive value Positive value means that the induced magnetic field, I, is in the same direction as the inducing field, H.

39. NEGATIVE VALUE Negative value means that the induced magnetic field is in the opposite direction as the inducing field.

40. Remnant magnetization If the magnetic material has relatively large susceptibilities, or if the inducing field is strong, the magnetic material will retain a portion of its induced magnetization even after the induced field disappears. This remaining magnetization is called remnant magnetization. The total magnetic field is a sum of the main magnetic field produced in the Earth's core, and the remnant field within the material.

41. MAGNETIC PROPERTIES OF ROCKS All rocks contain magnetic properties. Sedimentary and metamorphic rocks have less magnetic properties as compared to igneous rocks. Because sedimentary and metamorphic rocks have low susceptibility and igneous rocks have high susceptibility. IGNEOUS ROCK’S SUSCEPTIBILITY > METAMORPHIC ROCK’S SUSCEPTIBILITY > SEDIMENTARY ROCK’S SUSCEPTIBILITY

42. Kinds of magnetic material Magnetic material are of different kinds. Three main types are as follows: Paramagnetic materials Diamagnetic materials Ferromagnetic materials

43. Paramagnetic materials The magnetic material which have weak positive susceptibility is called paramagnetic material. Grains of such material tends to line up with their long dimension in the direction of magnetic field. EXAMPLE Iron compounds Mica Biotite Garnet Amphibole

44. Examples of paramagnetic minerals O livine (Fe,Mg) 2 SiO 4 1.6 · 10 -3 Montmorillonite (clay) 0.34 ·10 -3 Siderite (Fe C O 3 ) 1.3-11.0 · 10 -3 Serpentinite 3.1-75.0 · 10 -3 ( Mg 3 Si 2 O 5 ( O H) 4 ) κ (SI) Mineral

45. Diamagnetic materials The magnetic material which have weak negative susceptibility is known as diamagnetic material. Grains of such material tends to line up with their long dimension across the direction of magnetic field. EXAMPLE Rock salt Gypsum Anhydrite Quartz

46. Quartz (SiO 2 ) - (13-17) · 10 -6 Calcite (CaCO 3 ) - (8-39) · 10 -6 Graphite (C) - (80-200) · 10 -6 Halite ( NaCl ) - (10-16) · 10 -6 Examples of diamagnetic minerals κ (SI) Mineral

47. Ferromagnetic materials Such material which have high susceptibility is called ferromagnetic material. Electron coupling is more stronger in these materials. Grains are aligned in the direction of magnetic field. EXAMPLE Iron Cobalt Nickel Hematite Magnetite

48. Main reason of magnetization of rocks The liquid portion in the outer core consist of iron, nickel and cobalt which are in continues motion because it is high density material and it wants to move from high density to low density, as a result convectional currents are produced in the outer core. These convectional currents are also produce in the upper mantle due to which plate moves. Due to these currents iron, nickel and cobalt are magnetized and earth behaves as a magnet as a whole. Lightening is another factor that magnetized the rocks when it pass through magnetosphere when the electron pass through magnetosphere current is produced which results in the magnetization of the earth and this is a rare case.

49. Susceptibility of various rocks

50. TABLE SHOWING SUSCEPTIBILITY OF FEW MATERIALS

51. Reason of high susceptibility of igneous rocks Susceptibility is higher in rocks having magnetic minerals and in igneous rocks ( mainly mafic and ultramafic ) have more ferromagnetic minerals, so they have more susceptibility then metamorphic and sedimentary rocks. Igneous rocks have high susceptibility because grains of igneous rocks align in the direction of external field, also electron coupling is stronger in these rocks. Magnetic susceptibility of rocks is principally controlled by the type and amount of magnetic minerals contained in a rock . The magnetic susceptibility depends also on temperature . Outer core’s material.

52. IGNEOUS ROCK’S SUSCEPTIBILITY > METAMORPHIC ROCK’S SUSCEPTIBILITY > SEDIMENTARY ROCK’S SUSCEPTIBILITY Volcanic rocks, particularly the young ones (called neo- volcanics ), are often strongly magnetic.

53. Resistivity

54. resistivity DEFINITION Resistivity is an intrinsic property that quantifies how strongly a given material opposes the flow of electric current. It Is also called electrical resistivity , specific electrical resistance, or volume resistivity. A low resistivity indicates a material that readily allows the movement of electric charge . Resistivity is commonly represented by the Greek letter ρ (rho ). The  SI unit of electrical resistivity is the  ohm meter ( Ω⋅m )

55. Basic physics of electric current flow SIMPLE RESISTOR IN CIRCUIT Ohm’s Law states that for a resistor, the resistance (in ohms), R is defined as R = IV V = voltage (volts); I = current flow (amps) ELECTRIC CURRENT FLOW IN A FINITE VOLUME Ohm’s Law as written above describes a resistor, which has no dimensions. In considering the flow of electric current in the Earth, we must consider the flow of electric current in a finite volume. Consider a cylinder of length L and cross section A that carries a current I .

56. where ρ is the electrical resistivity of the material (ohm-m). This is the resistance per unit volume and is an inherent property of the material. Resistivity is basically opposition to the flow of electron. R is the electrical resistance of a uniform specimen of the material (measured in ohms, Ω)  L is the length of the piece of material (measured in meters, m) A is the cross-sectional area of the specimen (measured in square meters, m 2 ).

57. How to calculate resistivity of rocks Pure materials are rarely found in the Earth and most rocks are a mixture of two or more phases (solid, liquid or gas). Thus to calculate the overall electrical resistivity of a rock, we must consider the individual resistivities and then compute the overall electrical resistivity.

58. Factors that will INCREASE the resistivity of a rock Factors that will INCREASE the resistivity of a rock (a) Minimum pore fluid. (b) Lower salinity of pore fluid (c) Compaction - less pathways for electric current flow. (d) Lithification - block pores by deposition of minerals. ( e)Fluid content constant, but decrease connection between pores.

59. Factors that will DECREASE the resistivity of a rock Factors that will DECREASE the resistivity of a rock : (a) M ore pore fluid (b) Increase the salinity of the pore fluid - more ions to conduct electricity (c) Fracture rock to create extra pathways for current flow (d) Add clay minerals (e) F luid content constant, but improve interconnection between pores.

60. Reason of high resistivity of igneous rocks Compacted rocks No pore spaces Presence of Magnetic minerals No fluids Pressure Temperature Fractures Composition

61. IGNEOUS ROCK’S resistivity > METAMORPHIC ROCK’S resistivity > SEDIMENTARY ROCK’S resistivity Igneous rocks have highest resistivity. Sedimentary rocks tend to be the most conductive due to their high fluid content Metamorphic rocks have intermediate but overlapping resistivity. Age of the rock is also important for the resistivity. For example: Young volcanic rock (Quaternary ) ≈10− 200 Ω m Old volcanic rock (Precambrian ) ≈ 100− 2000 Ω m

62. RESISTIVITY LEVELS OF VARIOUS ROCKS

63. Table showing resistivity of few rocks

64. Groundwater exploration. Mineral exploration , detection of cavities. Waste site exploration. Oil exploration Application of resistivity

65. References Archie, G.E., 1942: The electrical resistivity log as an aid in determining some reservoir characteristics . Tran. AIME, 146, 54-67 . Lowrie. Fundamentals of Geophysics. Cambridge University Press. pp. 254–.ISBN  978-1-139-46595-3 A.B. Watts, Isostasy and flexure of the lithosphere, Cambridge Univ. Press., 2001 Altschaeffel, A. G., and Harrison, W., 1959, Estimation of a minimum depth of burial for a Pennsylvanian under clay : Jour. Sed. Petrology, v. 29, p.178-185 Hrouda, F. & Rejl, L., 1982. Small-scale magnetic susceptibility distribution in some plutonic rocks and its geological implications. Věst. Ústř. Úst.geol ., 57-69. Prague . Smirnov, V., 1982. Geology of mineral deposits. Nedra. Moscow.