Density of
Earth materials
Overview
• Density is a function of the mineral composition of Earth materials as well as their void volume and the material filling the voids
• Densities of rocks can be estimated
by considering their origin and the
processes that subsequently have
acted upon them
Introduction
• In actual practice it is not the density that is the controlling property of gravity anomaly fields, but rather the density contrast between the anomaly source and the laterally adjacent formations, often termed the country rocks, which are assumed to be constant and the norm
• The sign of the gravity anomaly is positive if the
anomalous mass exceeds that of the country rock and is negative for the reverse.
• Densities of rocks and other Earth materials are largely dependent on their major mineral
composition and void space and typically have a
limited range from 1000 to 3000 kg/m3 in SIu, or 1 to 3 g/cm3 in CGSu.
• Typically, crystalline and other well-lithified rocks have minimal void spaces, generally less than 1%.
Types of densities
• Void spaces within rocks,
regardless of their origin, tend to close up under increasing
lithostatic pressure.
• As a result, at depths of several kilometers, the void space
generally is assumed to be
negligible.
1. True density
• The mass of a unit volume of solid material where the volume excludes the voids in the rock is the true density, It is equivalent to the grain density of a rock
• True specific gravity is the ratio of the mass of a unit volume of material to the mass of the same volume of gas-free
distilled water at a temperature of 4
oC
• Specific gravity is numerically
equivalent to density, but this term is
seldom used in geophysical exploration.
2. Bulk density
• The bulk density which is
sometimes referred to as dry bulk density , is the density of a
thoroughly dry rock including
both the solid material and the
void space
3. Natural density
• The natural density which is sometimes also called the saturated or wet bulk density, is the density of a rock with all the pore space, both flow and diffusion, filled with water
• This is assumed to be the normal subsurface situation, but is not the case within the
unsaturated zone at the Earth’s surface and in pore volumes where gases are entrapped
• Gas-free distilled water is 1,000 kg/m3, sea
water has a density of 1,030 kg/m3, subsurface brines have densities of the order of 1,100
kg/m3 , and gases presumably have negligible densities.
Density of the Earth’s interior
• The density of surface rocks is of the order of 2,500 to 3,000 kg/m3, but the
calculated density of the entire Earth is
approximately 5,520 kg/m3 which provides unquestioned evidence that the Earth is not homogeneous
• The moment of inertia of the Earth is 8.07 kgm2, which is only 83% of the value it would have if it were homogeneous
• This confirms that the mass is
concentrated toward the center, and thus
the Earth’s density increases with depth.
Density of the Earth’s interior
• The change in density with depth in a homogeneous body may be determined from the change in compressional and shear wave velocities, Vp and Vs,
respectively, where
Rock densities
• The primary controls on the density of subsurface materials are mineral composition and void space, which are largely dependent on the lithology (rock type) and the chemical and physical effects of
secondary processes including rock fracturing, solutioning, and chemical alteration of minerals
• The vast majority of commonly occurring minerals range from 2,500 to 3,500 kg/m3, although ore minerals of metals are in the 4,000–6,000 kg/m3 range.
• For example, the density of minerals tends to
decrease with an increase in SiO2 content, and rock densities decrease with an increase in H2O content.
• Densities are affected by lithostatic pressure and temperature, both of which are primarily functions of depth within the Earth.
Lithology
• Earth materials are conveniently classified into crystalline rocks, sedimentary rocks, and
unconsolidated sediments
• Crystalline rocks include igneous
rocks, both plutonic and volcanic,
that originate from magma that
has solidified, respectively, within
the Earth and at the surface.
Crystalline rocks
• Unaltered plutonic igneous rocks that are formed deep within the crust or upper
mantle characteristically have minimal void space, generally less than 1%, and values seldom exceed 3%
• The decrease in density in fault zones is not only a result of the increase in volume during fracturing, but also the result of physiochemical alteration of the rocks to lower-density clay minerals
• The lowest-density minerals commonly
present in plutonic rocks are quartz and
orthoclase, which have densities around
2,600 kg/m3.
Crystalline rocks
• Rocks with relatively high proportions of these minerals are called felsic (or acidic) and have the lowest densities among
plutonic igneous rocks
• Rocks containing significant proportions of plagioclase feldspars, biotite, and
hornblende, and thus rich in calcium, magnesium, and iron, have greater
densities. These are informally termed mafic (or basic) rocks.
• The crystalline rocks of the continental crust in general become more mafic with depth, and thus densities increase with depth over and above the effect of
increasing lithostatic pressure
Crystalline rocks
• In a general sense, increasing metamorphic grade is evident with increasing depth,
supporting the view of increasing density with depth.
• The overall average of 2,670 kg/m3 reflects their general granitic (felsic) nature
• The increase in density with depth to values around 3,100 kg/m3 in the lower continental crust results in an average crustal density of roughly 2,830 kg/m3.
• The lower crust has an average chemical
composition equivalent to gabbro, but garnet becomes more abundant with depth.
• At the base of the crust, mafic garnet granulite is most abundant
Crystalline rocks
• The oceanic crust is apparently more laterally homogeneous in both composition and density than the continental crust and consists primarily of mafic
extrusive and intrusive rocks
beneath a layer of sediments and
sedimentary rocks *(p.84)
Crystalline rocks
• The rapid crystallization of igneous melts in this environment leads to a fine-grained texture in contrast to the coarse texture characteristic of plutonic rocks
• The fine-grained texture tends to lower the density despite the rock having a similar composition, but the effect generally is less than 10%
• Commonly the density of volcanic rocks is lowered by the presence of voids resulting from gas cavities frozen into the rock
during its rapid crystallization.
The Granite Family
• Light Color
• Low Density
• Felsic composition
• contain:
– Potassium Feldspar – Quartz
– Plagioclase Feldspar – Biotite
– Amphibole
The Gabbro Family
• Dark Color
• High Density
• Mafic Composition
• Contain:
– Plagioclase Feldspar – Biotite
– Amphibole
– Pyroxene
– Olivene
The Diorite Family
• Contain:
– Plagioclase Feldspar – Biotite
– Amphibole
– A tiny amount of Pyroxene
Metamorphic rocks
• The density of metamorphic rocks that make up the majority of the continental crust depends primarily on the original mineral composition of the rocks, but is also strongly influenced by the degree and type of metamorphism which largely reflects the
temperature and pressure to which the rocks have been subjected
• Densities increase with chemical compositions of rocks higher in iron, magnesium, and calcium.
• Mean densities of metamorphic rocks occur in the range 2,700 to 2,800 kg/m3 , but many metamorphic rocks derived from felsic igneous rocks and
metasedimentary rocks, such as granite gneiss, have densities in the range of 2,600 to 2,700 kg/m3. In contrast, intermediate to mafic metavolcanics rocks have significantly higher densities
Sedimentary rocks and unconsolidated sediments
• The densities of unconsolidated sediments and their lithified equivalents, sedimentary rocks, are
primarily controlled by their void space.
• Their mineral components are rather limited, and those that are present do not vary greatly in density.
• Clay minerals originating by chemical modification of feldspars and other silicate minerals which
dominate in clays and shales are another important component
• All of these mineral constituents, quartz, clays, and calcite, have densities in the range 2,600 to 2,700 kg/m3, whereas dolomite has a density 2,870
kg/m3 and salt deposits have densities in the range 2,000 to 2,200 kg/m3 .
• Methane hydrate, which may occur in deep oceanic sediments, has a density of the order of 900 kg/m3
Lithostatic pressure and void space
• Lithostatic pressure derived from the
weight of overlying rocks has a profound effect on the density of subsurface
materials, particularly sediments and sedimentary rocks.
• This is primarily due to the decrease in void space with increasing pressure.
• As explained above, both primary and secondary void space of crystalline rocks are minimized with increasing pressure leading to higher densities, with the process being especially effective in
igneous ash flow and lava flows and near
the surface.
Lithostatic pressure and void space
• Theoretically, if the grains are roughly spherical and of constant diameter, the pore space will vary from approximately 50 to 25% depending on the degree of compaction
• The degree of sorting or distribution of grain sizes is an important factor in determining the density of sediments
• An important factor in the effect of void space on the density of rocks is the composition of the pore content
• For example, a rock with 30% void volume has a difference in density of 300 kg/m3 depending on whether the pores are filled with air or water
• In contrast, the assumption of complete water saturation below the water table is incorrect where gas is entrapped within the sediments.
Lithostatic pressure and void space
• Clays at the surface which have densities of the order of 1,500 kg/m3 are transformed into
shales that exponentially reach the approximate density of clay minerals (2,600–2,700 kg/m3) at depths of the order of several kilometers
• Compaction has less effect on sandstones and chemical sedimentary rocks than on clay, with relatively consistent values reached at shallower depths than in the case of shale
• Faults, joints, bedding planes, and cooling cracks are also a source of voids in Earth materials.
• Their effect is normally at most a few percent of the rock volume, and thus their impact on
densities is minimal
Temperature
• Temperature has only a minor role in controlling the density of Earth materials because of the low volume thermal coefficient of expansion of rocks
• Most Earth materials have volume thermal
coefficients of expansion of roughly 20 to 40 × 10-6 /oC; thus a temperature differential of approximately 100oC is required to produce a density decrease of 100 kg/m3.
• Density decrease is also associated with partial rock melts where a magma chamber or a geologic unit consists of a mix of rock crystals and melted rock.
• The density of basaltic magma at the Earth’s surface is approximately 10% less than solidified basalt.
Basalt containing 20% melt by volume will be less dense by approximately 50 kg/m3.
Summary
• The distribution of the densities of granite is largely restricted to a tight normal
distribution around an average value of roughly 2,650 kg/m3
• A few lower densities extending from the central peak are probably associated with alteration of the feldspars and mafic
minerals to less dense minerals, and
perhaps with intergrain void. The existence of voids, probably largely gas cavities.
• Sandstone have densities between 1,800 and 2,600 kg/m3. The broad distribution primarily reflects the variation in the
interstitial pore volume, although minor
effects may be due to mineral content
Density measurements
• Densities are measured in three general ways: (1) laboratory measurements on samples; (2) gravity measurements; and (3) measurement of correlative properties
• Another method is to calculate densities knowing the mineralogical or chemical composition of
generic rock types
• Gravity measurements are particularly useful because they provide in situ values of relatively large volumes of rocks
• Correlative measurements, especially seismic velocity and gamma-ray attenuation, can provide more volume-restricted in situ density
determinations of the subsurface provided that
these measurements are available by virtue of other studies or can readily be made in drill holes.
1. Laboratory measurements
• Laboratory measurements of most consolidated rocks of low void space closely approximate in situ rock densities
• Generally, a minimum of 30
samples is desirable where
significant heterogeneity is
anticipated in the units
2. Gravity measurements
• Nettleton density profile method, the density of the material included within the topography can be determined by finding the density that will produce the minimum correlation between the Bouguer gravity anomaly and the topography.
• The method is normally implemented by taking a series of closely spaced observations over an
erosional feature that is not correlated with sub surface anomaly sources and primarily consists of a single geologic formation (Parasnis, 1952)
• The density which shows minimum correlation between the calculated gravity anomaly and the topography is the density of the surface material making up the topography
4. Correlative property measurements
• The gamma–gamma log, or the density log as it is commonly called, is useful in directly determining the in situ density of drilled formations, particularly
porosity in the hydro carbon exploration industry
• The basic principle of the gamma–
gamma log is that the gamma radiation from the source is attenuated in
proportion to the electron density of
the material making up the walls of the
borehole, and thus the saturated bulk
density of the formation
Density tabulations
• Neglecting the effect of gas cavities and other voids in volcanic rocks, the density of igneous rocks decreases with increasing felsic mineral content, but overlaps exist in the ranges.
• The broad ranges of density in these rock types are caused primarily by a wide variety of compositions from felsic to mafic
• Metamorphic rock classification is based more on texture reflecting the origin of the rock rather than upon the mineral composition as in the case of igneous rocks
• The density of metamorphic rocks increases with metamorphic grade – that is, increasing temperature and pressure lead to the highest densities where the minerals have been modified to high- temperature silicates of iron, calcium, and magnesium, while sedimentary rocks derived from clastic sediments are less dense than most igneous and metamorphic rocks
• Densities of less than 2,000 kg/m3 generally only occur in rocks with interstitial pore space that have undergone only limited compaction or lithification
Conclusion
• Three general measurement techniques are used: (1) laboratory measurements on samples extracted from the Earth; (2) gravity
measurements, both surface and underground;
and (3) measurement of correlative properties, such as seismic velocity and gamma ray
attenuation, using empirical relationships.
• Plutonic igneous rocks normally have minimal void space and densities ranging from roughly 2,650 to 3,000 kg/m3
• Densities increase from felsic to mafic rocks.
• Igneous rocks formed near the surface as a result of volcanic activity have similar mineral
composition to their plutonic equivalents, but they generally have a higher pore volume
resulting in lower densities.
Conclusion
• Sediments and sedimentary rocks primarily consist of minerals whose densities range from 2,600 to 2,700 kg/m3, but their densities are generally controlled by void space and the density of the void filling
• Compaction by lithostatic pressure is a primary
control on the density of these materials with density increasing exponentially with depth
• Sedimentary rocks formed by chemical precipitation are normally higher in density than other
sedimentary rocks except where they have been partly dissolved leading to increasing void space.
• Metamorphic rocks, higher densities are caused by the transformation of minerals to a high-density form as a result of a high pressure/temperature
environment and a decrease in the void volume due to pressure effects.