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Physical Properties of Rocks: Analysis of Density

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Introduction

Physical properties of rocks are of interest and utility in several fields of work , as well as geology, petrophysics, geophysics, materials science, geochemistry, and geotechnical engineering. the dimensions of investigation ranges from the molecular and crystalline up to terrestrial studies of the planet and alternative planetary bodies. Geologists have an interest within the radioactive age dating of rocks to reconstruct the origin of mineral deposits; seismologists formulate potential earthquake predictions the use of premonitory physical or chemical changes; crystallographers observe the synthesis of minerals with unique optical or physical properties; exploration geophysicists inspect the variation of physical properties of subsurface rocks to make possible detection of natural resources like oil and gas, geothermal energy, and ores of metals; geotechnical engineers study the character and behaviour of materials on, in, or of that such structures as buildings, dams, tunnels, bridges, and underground storage vaults are to be constructed; solid-state physicists study the magnetic, electrical, and mechanical properties of materials for electronic devices, pc parts, or superior ceramics; and petroleum reservoir engineers analyze the reaction measured on well logs or within the processes of deep drilling at elevated temperature and pressure.

Since rocks are aggregates of mineral grains or crystals, their properties are determined in massive part by the properties of their numerous constituent minerals. In a rock these general properties are measured by averaging the relative properties and occasionally orientations of the various grains or crystals. As a result, some properties that are anisotropic (i.e., differ with direction) on a submicroscopic or crystalline scale are quite isotropic for a great bulk of the rock. several properties also dependents on grain or crystal size, shape, and packing arrangement, the quantity and distribution of void area, the presence of natural cements in sedimentary rocks, the temperature and pressure, and the sort and quantity of contained fluids (e.g., water, petroleum, gases). Because many rocks display a huge range in these factors, the assignment of representative values for a specific property is overwhelmingly done by using a statistical variation.

Some properties varies considerably, relying on whether or not measured in situ (in place within the subsurface) or in the laboratory beneath simulated conditions. Electrical resistivity, for instance, is is distinctly dependent on the fluid content of the rock in situ and the temperature circumstance at the specific depth.

Density

Density varies significantly among dissimilar rock types because of contrastive in mineralogy and porosity. Awareness of the distribution of underground rock densities can help explaining subsurface geologic structure and rock type. In strict usage, density is defined as the mass of a substance per unit volume; however, in popular usage, it is the weight in air of a unit volume of a sample at a particular temperature. Weight is that the force that gravitation exerts on a body (and therefore varies with location), whereas mass (a measure of the matter in a body) is a constant fundamental property regardless of location. In habitual density measurements of rocks, the sample weights are considered to be equal to their masses, as a result of the discrepancy between weight and mass would end in less error on the computed density than the experimental errors introduced within the measure of volume. Thus, density is often decided the usage of weight instead of mass. of mass. Density ought to properly be reportable in kilograms per cubic metre (kg/m3), however remains usually given in grams per cubic centimetre (g/cm3). Another property closely associated with density is specific gravity. which is defined above, as the ratio of the weight or mass in air of a unit volume of material at a re-mentioned temperature to the weight or mass in air of a unit volume of distilled water at the same temperature. Specific gravity is dimensionless (i.e., has no units). The bulk density of a rock is ρB = WG/VB, where WG is the weight of grains (sedimentary rocks) or crystals (igneous and metamorphic rocks) and natural cements, and VB is the overall volume of the grains or crystals with the addition of the void area. The density can be dry if the pore area is empty, or it can be saturated if the pores are filled with fluid (e.g., water), which is more typical of the subsurface (in situ) situation.

If there is pore fluid existing, where Wfl is the weight of pore fluid.

In terms of total porosity, saturated density is and thus where ρfl is the density of the pore fluid.

Density measurements for a presented specimen include the determination of the following quantities: pore volume, bulk volume, or grain volume, along with the weight. A beneficial way to assess the density of rocks is to make a histogram plot of the statistical domain of a group of data. The representative value and its variation can be expressed as:

  1. mean, the average value of the data group
  2. mode, the most common value in the data group (i.e., the peak of the distribution curve)
  3. median, the value of the middle sample of the data group (i.e., the value at which half of the samples are less and half are more)
  4. standard deviation, a statistical measure of the spread of the data (plus and minus one standard deviation from the mean value includes about two-thirds of the data).

A compilation of dry quantity densities for different rock types located in the upper crust of the Earth is listed in Table 1. A histogram plot of these data, giving the percent of the samples as a function of density is shown in Figure 1. The parameters given contain:

  1. sample division, the range of density in a single data column (e.g., 0.036 g/cm3 for Figure 1).
  2. number of samples.
  3. standard deviation.

The little inset plot is the percentage of samples (on the vertical axis) that lie within the interual of the “mode – w” to the “mode + w”, where w is the horizontal axis.

rock type number of samples mean (grams per cubic cm) standard deviation mode (grams per cubic cm) median (grams per cubic cm)

  • all rocks 1,647 2.73 0.26 2.65 2.86
  • andesite 197 2.65 0.13 2.58 2.66
  • basalt 323 2.74 0.47 2.88 2.87
  • diorite 68 2.86 0.12 2.89 2.87
  • dolerite (diabase) 224 2.89 0.13 2.96 2.90
  • gabbro 98 2.95 0.14 2.99 2.97
  • granite 334 2.66 0.06 2.66 2.66
  • quartz porphyry 76 2.62 0.06 2.60 2.62
  • rhyolite 94 2.51 0.13 2.60 2.49
  • syenite93 2.70 0.10 2.67 2.68
  • trachyte 71 2.57 0.10 2.62 2.57
  • sandstone 107 2.22 0.23 2.22 2.22

Table 1: Dry bulk densities for various rock types

Fig.1 Dry bulk densities (distribution with density) for all rocks given in table 1.

In Figure 1, The most trending (modal) value of the distribution falls at 2.63 g/cm3, almost the density of quartz, an abundant rock forming mineral. Few density values for these upper crustal rocks above 3.3 g/cm3. A few fall below the mode, on occasion even under 1 g/cm3. Figure 2 shows the cause for this, which illustrates the density distributions for granite, basalt, and sandstone. Granite is an intrusive igneous rock with low porosity and a well-defined chemical (mineral) composition; its domain of densities is narrow. Basalt is, in most cases, an extrusive igneous rock that can display a large variation in porosity (because entrained gases leave voids named vesicles), Subsequently some highly porous samples can have low densities. Sandstone is a clastic sedimentary rock that can have a wide domain of porosities counting on the degree of sorting, compaction, packing arrangement of grains, and cementation. The bulk density varies 171386552514500accordingly.

Fig.2 the density distributions for granite, basalt, and sandstone.

Table 2 lists typical ranges of dry bulk densities for a variety of other rock types as all set by the American geologists Gordon R. Johnson and Gary R. Olhoeft.

When the rocks are progressively buried, the density of clastic sedimentary rocks increases. This is because of the increase of overburden pressure, which gives rise to compaction, and the progressive cementation with age. the porosity decreases as the compaction and cementation increase.Representative densities for common rock-forming minerals (i.e., ρG) and rocks (i.e., ρB) are listed in Table 3.

The bulk densities for sedimentary rocks, that typically have changing porosity, are given as ranges of both dry ρB and saturated ρB. The pore-filling fluid is ordinarily salty water, oftentimes indicative of the presence of seawater when the rock was being deposited or lithified. It should be noted that the bulk density is less than the grain density of the constituent mineral (or mineral assemblage), depending on the porosity. For instance, sandstone (characteristically quartzose) has a typical dry bulk density of 2.0–2.6 g/cm3, with a porosity that can vary from low to more than %30. The density of quartz itself is 2.65 g/cm3. If porosity equaled zero, the bulk density would equal the grain density. Saturated bulk density is higher than dry bulk density, owing to the added presence of pore-filling fluid. Table 3 also lists representative values for density of seawater, oil, and methane gas at a subsurface condition—pressure of 200 bars (one bar = 0.987 atmosphere, or 29.53 inches of 247650183197500mercury) and a temperature of about 80° C (176° F).

Table 3: Typical densities

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  • rock type density (grams per cubic cm)
  • amphibolite 2.79–3.14
  • andesite glass 2.40–2.57
  • anhydrite 2.82–2.93
  • anorthosite2.64–2.92
  • basalt glass 2.70–2.85
  • chalk 2.23
  • dolomite 2.72–2.84
  • dunite2.98–3.76
  • eclogite3.32–3.45
  • gneiss 2.59–2.84
  • granodiorite2.67–2.78
  • limestone 1.55–2.75
  • marble 2.67–2.75
  • norite2.72–3.02
  • peridotite3.15–3.28
  • quartzite 2.65
  • rock salt 2.10–2.20
  • schist 2.73–3.19
  • shale 2.06–2.67
  • slate 2.72–2.84

Table 2: Typical density ranges for some other rock types

Secondary rock structures

Secondary sedimentary structures shape after primary deposition happen, but in some cases, during the diagenesis of a sedimentary rock. Common secondary structures contain any shape of bioturbation, soft-sediment deformation, teepee structures, root-traces, and soil mottling. Liesegang rings, cone-in-cone structures, raindrop impressions, and vegetation-induced sedimentary structures also could be considered secondary structures.The structures are most readily noted if the rocks have obvious primary structures, like layering formed by successive episodes of deposition. Primary depositional layering is almost permanently horizontal: it parallels the general configuration of surface on which deposition takes place, such as a floodplain or the floor of a lake or ocean. Subsequently, when layers located and found out are not horizontal, the geologist assumes that some force has been exerted upon them that has destroyed their original horizontality.

Secondary structures:

Brittle structures: shaped owing to brittle failure (i.e., breaking of bonds).

Deals with fracturing or granulation.

Pieces ideally can be put back together after brittle deformation because the pieces doesn’t change it shape (not internally deformed).

creates angular fragments, breccia.

Faults and joints.

Plastic structures: shaped fundamentally owing to the distortion of bonds, deals with material flow.

Ductile shear zones and folds.

Figure 10 shows series of layers that depart from the horizontal by about 50 degrees. The geologist assumes that the tilt is a secondary structure, one imposed on primarily horizontal layers at a later time to the forming of the layers.

Fig.10 Series of layers

In Figure 11 The layers have been folded into a down-arch.

In Figure 12 The layers have been folded into an up-arch.

Fig.11 Down-arch folded layers Fig.12 Up-arch folded layers

In Figure 13 The rock has responded to deformational forces in a completely different way. Instead of tilting or folding, it has been broken and shifted. Geologists call this behavior ‘faulting’. Note how the layers aren’t continuous anymore; instead, they are displaced laterally. The line on the earth’s surface along which the rocks have shifted is called a ‘fault line’. In the figure, the yellow line is the ‘fault line’. The arrows show the direction of relative movement.

Fig.13 Faulting Fig.14 Faulting

In Figure 14, faulting has substituted the light-colored intrusive body. The yellow line is the ‘fault line’. The arrows Indicates the direction of relative movement. This apparent random swirl of lines in Figure 15 can be resolved into a ‘folded fold’.

Fig.15 Folded fold

References

  1. https://www.sciencedirect.com/https://www.britannica.com/After data from H.S. Washington (1917) and R.J. Piersol, L.E. Workman, and M.C. Watson (1940) as compiled by Gary R. Olhoeft and Gordon R. Johnson in Robert S. Carmichael (ed.), Handbook of Physical Properties of Rocks, vol. III, CRC Press, Inc. (1984).
  2. After data from R.A. Daly, G.E. Manger, and S.P. Clark, Jr. (1966); A.F. Birch (1966); F. Press (1966); and R.N. Schock, B.P. Bonner, and H. Louis (1974) in Robert S. Carmichael (ed.), Handbook of Physical Properties of Rocks, vol. III, CRC Press, Inc. (1984).
  3. *AILSA ALLABY and MICHAEL ALLABY. ‘sediment.’ A Dictionary of Earth Sciences. 1999. Encyclopedia.com. 8 Nov. 2010 .
  4. ^ Boggs, Sam jr, 2006 Principles of Sedimentology and Stratigraphy, Patrick Lynch, Principles of Sedimentology and Stratigraphy, Pearson Prentice Hall, Upper Saddle River, NJ. Ed 4, p. 83-84
  5. ^ AILSA ALLABY and MICHAEL ALLABY. ‘antidune.’ A Dictionary of Earth Sciences. 1999. Encyclopedia.com. 8 Nov. 2010 . B
  6. ^ http://jsedres.geoscienceworld.org/cgi/content/abstract/35/4/922

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Physical Properties of Rocks: Analysis of Density. (2022, September 27). Edubirdie. Retrieved January 29, 2023, from https://edubirdie.com/examples/physical-properties-of-rocks-analysis-of-density/
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Physical Properties of Rocks: Analysis of Density [Internet]. Edubirdie. 2022 Sept 27 [cited 2023 Jan 29]. Available from: https://edubirdie.com/examples/physical-properties-of-rocks-analysis-of-density/
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