How Is Heat Transferred From The Core To The Crust

Abstract

Distinguish betwixt the Earth'south compositional and concrete mechanical layers.

Key Chapter Objectives

• Distinguish betwixt the Earth's compositional and concrete mechanical layers.
• Identify the sources of Globe's internal heat.
• Compare and contrast conductive and convective rut flow.
• Recognize conductive and convective zones of estrus transfer from drill-pigsty temperature profiles.
• Explain the significance of heat flow maps and temperature-at-depth maps.

To more than completely understand geothermal resource and their distribution on the planet, a review of the Earth's compositional and concrete make-up is necessary. The World is compositionally inhomogeneous, consisting of an iron–nickel core, a dense rocky mantle, and a sparse, comparatively low-density rocky crust. This compositional diversification developed shortly after our planet formed when more than dense cloth sank to the middle and low-density material rose toward the surface. Furthermore, because of this compositional diverseness, differences in physical or mechanical properties exist (liquid or molten vs. solid; brittle vs. ductile deformation). Brittle behavior ways breaking or fracturing after a threshold level of stress is applied, such as what happens when a glass vase is dropped on a difficult surface. Ductile deformation, on the other paw, reflects bending without breaking after a material's yield strength is exceeded, such as bending a metal wire or molding dirt. Understanding both the compositional and physical characteristics of the Earth's interior lays the groundwork for the discussion about plate tectonics in Affiliate four; plate tectonics exerts a fundamental command on the distribution of Globe'southward mineral, fossil fuel, and geothermal resources.

Earth'due south Compositional and Rheological Layers

The Earth'south radius is merely under 6400 km. Extending outward from Earth's center, systematic changes occur in both composition and rheological beliefs (concrete or mechanical properties of a material, such as changes from solid to liquid or breakable to ductile). We will begin with compositional changes.

World's Compositional Layers

The area extending from the heart of the Earth to a depth of about 2900 km is known equally the Earth'due south core. The core consists of both solid and molten iron and nickel, and its temperature is comparable to the surface of the sun, or about 6000°C. Overlying the core is the curtain, which extends from a depth of almost 2900 km to less than 100 km. Volumetrically, the mantle makes up the largest part of Earth'due south interior. The mantle consists of dense atomic number 26- and magnesium-rich rock, whose temperature decreases progressively upwards from about 5000°C to less than 1500°C. The third and last layer is the World'due south chaff, which consists of a thin shell, varying from 70 to lxxx km thick under parts of continents to less than a few kilometers thick under parts of the ocean floor. A useful analogy of the compositional layers of the Globe is a peach. The size of the pit would exist proportional to the Earth'southward core, the lurid (the edible office) proportionally represents the mantle, and the fuzzy peel would have the proportional thickness of the crust. Globe's compositional layers are illustrated in Effigy 3.ane.

Effigy 3.1   Cross-sectional view of Globe'southward compositional layers. Notation that the crust consists of thin oceanic chaff and thick continental chaff. (Adapted from Visionlearning®, http://www.visionlearning.com/img/library/large_images/image_4859.gif.)

Dissimilar the more compositionally homogeneous core and drape, the crust consists of two types: oceanic and continental (Figure 3.1). Oceanic crust underlies the sea basins and consists of a dark-colored, moderately dumbo stone called basalt. It is relatively thin, reaching a maximum of seven km and a minimum of less than a kilometer beneath mid-sea ridges. Continental crust is comprised mainly of lower density, lighter colored igneous and metamorphic rocks, such as granite and gneiss (discussed in more item in Chapter iv). These igneous and metamorphic rocks of continental crusts are capped in places past a veneer of sedimentary rocks, including sandstone and limestone. Considering continental crust is less dumbo than oceanic crust, it sits higher compared to oceanic chaff, explaining why continents for the most role lie above sea level.

Earth's Rheological (Physical) Layers

In response to changes in pressure and temperature, a cloth's physical nature (known as rheology) can alter, such as from solid to liquid with rising temperature or the opposite with falling temperature or ascent force per unit area. The composition of the material, however, remains essentially unchanged despite changes in the physical state. Some other change in rheology would be the change from brittle breaking, forming fractures under low temperature and pressure, to ductile angle under loftier temperature and pressure prior to actual melting. In other words, a ductile substance is a solid that has the power to flow, and within the Earth ductile materials catamenia at rates of a few centimeters per year in response to force per unit area differences and convection. The Earth is comprised of five main rheological layers, moving from the surface downward: the lithosphere, asthenosphere, mesosphere, outer core, and inner core. The relationship betwixt Earth'south compositional and physical or mechanical layers is illustrated in Figure three.2.

Effigy 3.ii   Cross-sectional view of Earth's compositional and mechanical layers. For details, see text. (Adapted from Visionlearning®, http://www.visionlearning.com/img/library/large_images/image_4859.gif.)

Lithosphere

The lithosphere represents the stiff, relatively brittle outermost layer and averages about 100 km thick. It is compositionally diverse every bit it embraces both the chaff and uppermost mantle because both compositional layers conduct similarly from a rheological standpoint—relatively strong and brittle. The lithosphere will be discussed more in Chapter iv because it makes up the Earth'due south tectonic plates, great chunks of rock that are continually moving with respect to each other.

Asthenosphere

Underlying the lithosphere, between 100 km and about 300 km, is a weak zone of rock called the asthenosphere, which is part of the upper drapery. The rock in the asthenosphere is weak because it is shut to its melting point merely still mainly a solid (Figure 3.three). However, because of the high heat, the rock is mechanically weakened and has the ability to flow (ductile behavior) in response to thermal and pressure gradients. Motility in the asthenosphere contributes to movement of the overlying lithosphere or tectonic plates.

Mesosphere

Figure three.3   Depth and temperature plot showing the geothermal gradient (red line) and melting curve with depth of rock (bluish line). The melting point of rocks increases with depth because increasing pressure favors the denser, solid phase. Note that the rocks are close to their melting point in the asthenosphere and therefore mechanically weak. Equally the geotherm and melting betoken curves diverge below the asthenosphere, the rocks become less weak. The 1300°C marks the judge temperature at which basaltic rocks brainstorm to melt most the Earth'south surface.

Below the asthenosphere, the beliefs of the rest of the mantle, referred to as the mesosphere, is mechanically similar. The mesosphere consists of the lower and centre parts of the mantle. Considering of the added pressure with depth, the rocks are not as shut to where they would begin melting as in the asthenosphere and are therefore stronger and less ductile (Figure 3.3). Nonetheless, because of the increasing temperature with depth, rocks of the mesosphere are non as strong or brittle as in the lithosphere and still have the ability to flow but at a slower rate than in the asthenosphere.

Outer Cadre

At the base of the pall or mesosphere, temperature increases abruptly across the mesosphere–outer cadre boundary, reflecting the presence of molten iron and nickel. In response to gravitational and thermal gradients, the molten iron and nickel are convecting or circulating, promoting heat flow into the overlying mantle (resulting in abrupt temperature increases across the boundary). This circulation of molten iron in conjunction with World's rotation produces a geodynamo that gives ascension to the planet'southward magnetic field. The liquid nature of the outer core is deduced from seismic moving ridge data (see give-and-take beneath). Receiving stations on the contrary side of the planet from which an convulsion occurs will not receive whatsoever Southward-waves (as well known as shear or secondary waves), which are attenuated when they encounter liquid fabric.

Inner Core

The inner core is compositionally the same as the outer core but is a solid rather than a liquid even though the temperature has risen to virtually 6000°C (depending on the model used). The transition from liquid in the outer cadre to solid in the inner core results from the extreme pressure at these depths. The radius of the inner core is most 1300 km.

Evidence of Globe's Compositional and Rheological Layers

Our understanding of Globe's compositional and rheological layers is not known from drilling, as the deepest drill pigsty is about 12 km deep, which is a mere pinprick into the Earth's interior. Rather, our agreement comes from several sources, including meteorites, material erupted from volcanoes, and the nature of Earth'south rotation and precession (or wobble) of Globe's axis. Primarily, though, well-nigh of what we know of the Earth's internal compositional make-up stems from the study of seismic waves. These waves prototype the interior of the Earth, much like a computerized axial tomography (Cat) scan discloses internal components of the human torso. The speed, direction, and propagation of these waves modify in response to the density and composition of the material traversed. By collecting seismic wave data from receiving stations across the planet, the Earth'southward internal compositional layers can exist successfully modeled and imaged (Figure iii.4). Earthquakes generate two types of waves that travel through the interior of the Globe: P-waves, or principal (compressional) waves, and S-waves, or secondary (shear) waves. P-waves travel through solids, liquids, and gases, but S-waves travel only through solids, because liquids and gases have no elasticity to support shear stresses. Therefore, the liquid nature of the outer core is indicated because seismic receiving stations on the contrary side of the Earth from which an convulsion occurs receives no S-moving ridge signal, simply a P-wave response. The size of the resulting S-wave shadow zone is a directly indication of the diameter of the core (Figure three.4).

Effigy 3.4   Cross-sectional view of seismic waves every bit they traverse Globe'south interior. The size of the compositional layers tin be adamant by the refraction or attenuation of select seismic waves. For example, the liquid outer core is detected by the attenuation of seismic S- or shear waves that cannot travel through liquids, resulting in a shadow zone whose size reflects the diameter of the outer core. The size of the solid, inner core is determined by noting the location of received P- or compressional waves reflected off the sides of the inner core. (Adapted from Tarbuck, E.J. et al., Earth: An Introduction to Physical Geology, Prentice Hall, Upper Saddle River, NJ, 2005.)

Sources of Earth'south Heat

At that place are three main sources of World's internal estrus. Kickoff is remainder heat left over from the formation of the planet (primordial oestrus) about four.half-dozen billion years ago. This heat is a product of the commencement law of thermodynamics, which states that energy is conserved. Our planet formed by accretion of colliding meteorites or larger chunks of infinite debris chosen planetisimals.of movement was converted to heat energy after collision, resulting in a largely molten proto-Earth, leading to the eventual gravitational separation of heavy and light elements to form the core, mantle, and crust as described above. Because stone is a good insulator, the deep interior of our planet has stayed hot, with heat flowing outward toward the surface. This outward flow of heat, while fairly uniform at depth from the core through the drape, becomes irregularly distributed as it flows through the crust, being concentrated in select zones due to plate tectonics (discussed in Affiliate 4) and influencing the distribution of areas having loftier and low geothermal heat flow at the Globe'due south surface.

A 2d source of rut comes from the radioactivity of select elements, principally from uranium, thorium, rubidium, and potassium. These elements are largely full-bodied in the chaff because their large atomic radii are less compatible in mineral structures in the mantle due to the high pressures there, favoring dense mineral species. As a result, virtually 60% of the rut in continental crust is due to radioactive disuse of these 4 elements (Glassley, 2015). Nonetheless, these radioactive elements are present in the mantle, and even though their concentration is low there, the large volume of the mantle makes upward for the low concentration, indicating that a significant amount of heat coming from the curtain is due to radioactive decay. Recent studies of Earth's internal heat flow budget point that the proportion of primordial heat and radiogenic heat to full heat menses is most equal and in full amounts to virtually 47 terrawatts (TW) (Davies and Davies, 2010; Gando et al., 2011; Korenga, 2011). For comparing, the full installed world power capacity in 2012 was 5.55 TWe (EIA, 2016). The takeaway, conspicuously, is that World'south internal heat energy can provide a significant contribution toward supplying the energy needs of civilisation. Over l% of the total estrus flow is contributed by convection in Earth's mantle, with about 24% coming from the crust and supplied by a mixture of conduction, hydrothermal convection, and vertical and horizontal movement (advection) of localized zones of magma (Figure 3.v).

A third, admitting minor, source of rut is from gravitational pressure. When something is squeezed information technology heats up, and when expanded it cools. For gases, this behavior is described by Charles' law; a similar procedure happens with solids, except that the changes in volume are much smaller for a given increase in pressure. Again, considering rocks are good insulators, the escape of heat from Earth'south surface is less than the heat generated from internal gravitational attraction or squeezing of rock, so heat builds upwards with depth.

Other local sources of oestrus include frictional heating along earthquake faults. This frictional heating can exist sufficiently intense to actually partially melt the rock, producing what is called pseudotachylite. Indeed, a small corporeality of heat tapped by geothermal power plants located forth major active faults, such as the San Andreas fault in California or active faults in Nevada, probably comes from frictional heating every bit rocks grind past on either side of the fault.

Rut Transfer Mechanisms in the Earth

A flux of oestrus is emitted from every square meter on Earth's surface; in some places it is notably higher, particularly near the boundaries of the tectonic plates, than in other places. Overall, however, the average heat flux or flow for the Earth is about 87 milliwatts per square meter (mW/m^two). Multiplying this value by the total global surface surface area of 5.ii × 10¹¹ k² yields a total heat or ability output of almost 4.7 × 10^thirteen W or 47 TW (thermal) as noted to a higher place. The heat flow for continents averages 65 mW/m², and the average heat flux for oceanic crust is 101 mW/one thousandⁱⁱ. The difference reflects the thinner graphic symbol of oceanic chaff with hot drape rocks at comparatively shallow depths and the insulating nature of thicker continental crust. Indeed, if it were not for the body of water, whose depth averages about 3.7 km, much of the oceanic crust would accept the potential for harnessing geothermal energy. Simply then once again, without the oceans there would be a dearth of water, which is the primary vehicle for transferring heat energy from hot rocks at depth to the surface (see later discussion). Heat tin can be transferred past 3 principal mechanisms: conduction, convection/advection, and radiations. The first 2 are relevant for the solid Earth, as radiation applies mainly to the transfer of electromagnetic radiation through infinite, such as sensing rut from a campfire or the transfer of low-cal from the Sun.

Figure 3.5   Graph showing the modify in temperature (heavy solid line) of the Globe from its surface to the core (Earth's geothermal gradient). Also shown is the solidus, or the temperature at which rocks begin to melt. Notation that the geothermal gradient is highest near the surface, indicative of conductive heat flow, but becomes more gradual with depth, indicating a combination of convective and conductive oestrus flow. The highlighted xanthous layer marks the asthenosphere where the temperature of the solidus and that of the Earth are shut, resulting in rheologically weak stone. The layer in a higher place the asthenosphere is the lithosphere where the temperatures of the Earth and solidus are further apart, making for rheologically strong rock: (Adapted from Ammon, C.J., Earth's Origin and Limerick, SLU EAS-A193 Class Notes, Penn State Department of Geosciences, University Park, 2016, http://eqseis.geosc.psu.edu/~cammon/HTML/Classes/IntroQuakes/Notes/earth_origin_lecture.html.)

Conductive Oestrus Flow

Conduction is the transfer of heat by contact (transfer of energy from one atom to the side by side) and is an important ways of heat transfer within the Earth. The overall geothermal slope of Earth—the modify in temperature with depth—is largely governed by conductive estrus transfer. This gradient is high or changes rapidly virtually the surface just becomes more gradual at depth (Figure iii.5). This rapid alter in temperature with depth is indicative of conductive heat menses, considering, in the absenteeism of circulating fluids, rocks are good insulators. The geothermal slope averages well-nigh 25 to 30°C per km for the upper chaff (top 10 km or so), whereas in geothermal areas the geothermal gradient is nearly double to mayhap three times that of non-geothermal regions. In active volcanic regions, the geothermal gradient can exist every bit high as 150°C per km, such as at Yellowstone National Park, and the oestrus flux can exist 500 mW/m² or even more than.

Heat flux is governed by Fourier'southward law, which states that the flow of rut (Q) depends directly on the thermal electrical conductivity (chiliad, in units of watts per meter kelvin, or W/k·K) of the material and the geothermal gradient (∆T/∆x or ∇T). This gives us the equation Q = yard ×∇T. For case, if an exploration well is drilled in granite and encountered a temperature of 200°C at a depth of 1500 k, what is the heat flux at the site?

3.i

$$Q=\frac{k_{granite}\times (473\text{K}-298\text{K})}{1500\text{ m}}$$

Thermal conductivity itself is modestly sensitive to temperature and more often than not decreases as temperature increases for World materials (Clauser and Huenges, 1995). An average value of granite over this temperature range would be about 2.4 W/one thousand·Thou (Glassley, 2015). Substituting these values into the equation yields the following:

$$Q=\mathrm{ii.4}\text{ W/m}·\text{K}\times 175\text{ K}/1500\text{ yard}=0.280{\text{ Due west/m}}^2\text{ or}280{\text{ mW/thousand}}^2$$

which would be a very promising rut menstruation for developing geothermal free energy.

For continental chaff, the minerals feldspar and quartz are the most common, yet there is a significant difference in the thermal conductivity of quartz and feldspar (Glassley, 2015), such that the thermal electrical conductivity of quartz averages about twice that of alkali feldspar. Thus, the thermal conductivity of a stone will exist strongly dependent on the proportion of these two minerals, which in turn will directly influence the heat catamenia.

Related to thermal electrical conductivity is thermal diffusivity, which measures how speedily an object changes temperature in the presence of a thermal slope. Thermal diffusivity has the units of foursquare meters per second (yard²/s). Thermal diffusivity is divers by the ratio of thermal electrical conductivity to the heat capacity, past volume, of a material. Rut capacity measures how much heat is required to heighten the temperature of a unit of measurement volume of material by 1 K. Minerals have thermal diffusivity values of i × 10^–6 to 10 × ten^-vi mⁱⁱ/s, whereas most metals accept diffusivity values in the range of ane × 10^–4 to v × 10^–4 1000²/s, or about 100 times the diffusivity of minerals. Also affecting conductivity and diffusivity is the porosity or open space in rocks (porosity is discussed in more detail in Chapter 5). Pores in rocks can be filled with water or air or a mixture of the two. Because water conducts heat more than readily than air, the thermal conductivity of a h2o-saturated rock will exist 3 to 4 times that of its dry equivalent. Furthermore, conductivity is also dependent on pore size such that larger pores have a lower electrical conductivity for a given h2o content (Glassley, 2015).

Equally a result, role of the accurate characterization of the geothermal energy potential of a given region requires measuring and understanding the properties of the geological materials in which the system is developed. How is this of import for geothermal power production? Imagine a site having loftier heat flow but also characterized by quartz-rich rocks, which have relatively high thermal conductivity. Although estrus is transferred efficiently to water for product, the cooler injection water could unfavorably absurd the reservoir rocks, which would lower the system's enthalpy and ability/free energy potential. Thus, the rates of production and injection must be such that the organisation is non adversely perturbed, and determining the product and injection rates requires accurate characterization of the thermal backdrop of the geological materials.

Examples of conductive geothermal systems include some deep sedimentary basins and geopressured reservoirs, such as those found along the Gulf Coast of the United States. The Paris Basin is an example of a deep sedimentary aquifer whose geothermal fluids are used directly for space heating. The catamenia of water is slow enough that in that location is enough fourth dimension to be heated by the conductive estrus menses from the rocks. This happens because there is a general reduction in permeability (flow of water through stone, equally discussed in Chapter 5) with depth, which retards the fluid's ability to circulate easily. In geopressured reservoirs, permeable h2o-bearing horizons are deeply buried (more often than not >ii km) and are isolated by surrounding impermeable rock. These are cocky-contained systems in which the pore h2o was trapped with the sediments at the time of degradation. Because they are isolated from the surface, the pore water is nether the weight of the overlying rock (lithostatic) rather than a column of water (hydrostatic). The water is thus pretty much brackish and is heated conductively in response to the region's geothermal gradient of about 50°C/km.

A final example of conductively heated geothermal systems consists of engineered geothermal systems (EGSs) in which hot rocks be merely permeability or water content is sufficient to produce a circulating hydrothermal system. These conductive systems are existence explored in places to artificially produce convective systems (come across next department) through controlled fracturing of the rock. An EGS project, at Newberry Volcano in central Oregon, has proved encouraging with regard to developing improved permeability in hot rocks through the injection of cold h2o. (EGSs, deep sedimentary aquifers, and geopressured reservoirs are discussed in Chapter 11.)

Convective (Advective) Oestrus Menstruum

Technically, the movement of heat by majority fluid menstruum is advection; however, convection is the more widely used and general term and embraces both advection and conduction, significant that every bit rut is transferred by moving textile some heat is also transferred by conduction through contact with surrounding material. The slower the motility of the material, the greater the proportion of rut transferred by conduction. For simplicity, we volition use the more widely used term convection, agreement that the bulk of heat is transferred past move of material and a lesser corporeality by conduction. Because convection involves both move of textile (advection) and thermal improvidence (conduction), it is the near effective means of energy transfer within the World.

Convection develops in response to buoyancy forces in the presence of a gravitational field. As material is heated it becomes less dense and will begin to rise. To replace the rise material, cooler (and more dense) material sinks, where it also might be heated and also begin to rise, resulting in a convection bike. Without convection, a body of h2o, for case, can get thermally and density stratified, such that warm, less dense water lies near the surface and libation more dense h2o at depth. If fluids are convecting, however, they are mixing; thus, temperature changes little with depth over the convecting interval. Recognizing zones of convection from drilling can be an effective exploration tool for identifying prospective geothermal reservoirs (discussed in Chapter eight).

As established, the solid World is overall density stratified with a dense atomic number 26-rich cadre and a low-density, outer crust; withal, it is not static because the hot, liquid outer core is a potent source of energy. Although the overlying mantle is solid, it has the ability to menses slowly but significantly on the social club of the geologic time scale. The rate of flow is controlled in office past the force of the energy source merely besides in role on the viscosity of the material. Viscosity is a holding that measures the resistance to flow of a cloth when stressed. For example, molasses is more viscous than water. For most materials, viscosity is inversely related to temperature; as temperature increases, the viscosity decreases, similar to heating dear. Thermally disturbed portions of the lower mantle, perhaps situated above focused zones of upwelling in the underlying and convecting molten outer core, will be gravitationally unstable relative to overlying (and adjacent) libation and denser mantle and will brainstorm to rise buoyantly upwardly, producing a system of convection cells (Figure 3.6).

Rayleigh Number

Factors that promote convection are low viscosity, thermally induced expansion, a gravitational field to exert buoyancy forces, and low thermal conductivity to create a strong thermal gradient and drive buoyancy forces. Quantitatively, atmospheric condition that promote convection can be represented by the ratio between buoyant and viscous forces, or what is termed the Rayleigh number (Ra), which is represented quantitatively below:

3.ii

$$\text{Ra}=\left(\frac{\mathrm{thou}\times \alpha \times d^3}{v\times \kappa }\right)\times \Delta T$$

Effigy 3.vi   Cross-exclusive view of Earth's interior illustrating convection in the mantle and asthenosphere. Convection is induced by oestrus transfer and past convection in the liquid outer cadre. Note that motility of the lithosphere is in part due to convection in the underlying lithosphere. (Adapted from USGS, Some Unanswered Questions, U.Southward. Geological Survey Reston, VA, 1999, http://pubs.usgs.gov/gip/dynamic/unanswered.html.)

where

• chiliad = Acceleration of gravity (9.8 one thousand/s²).
• α = Coefficient of thermal expansion (1/K).
• d = Depth interval over which the temperature alter occurs (m).
• v = Kinematic viscosity (m²/s).
• κ = Thermal diffusivity (1000^two/s).
• ∆T = Vertical temperature change (Thou).

As a outcome, Ra is a dimensionless number that provides an indication of whether convection volition occur or not and therefore indicates whether the ascendant form of heat catamenia will be by conduction or convection. When Ra is >grand, convection is the dominant heat transfer mechanism; when Ra is <one thousand, conduction is the ascendant course of heat flow.

The Rayleigh number for the mantle ranges between 10^five and 10^vii, indicating that the mantle is mobile and that the main form of rut transfer to the Earth'southward surface is by convection. The movement of mantle material is a principal commuter for plate tectonics, which accounts for much of the distribution and types of geothermal resource across the planet (discussed in Chapter 4).

Convection in the Upper Crust

Currently exploited geothermal systems are typically less than 3 km deep and consist of convective hydrothermal systems, either liquid or vapor dominated. The fluid must reside in rock reservoirs that let fluids to circulate, which requires continued open space or permeability (discussed in Chapter v); otherwise, they would be mainly undeveloped conductive reservoirs. When a convective hydrothermal reservoir is intercepted by drilling, the geothermal gradient will decrease suddenly, reflecting the thermal mixing of fluid circulation. This is unlike than thermal stratification, which indicates conductive heat menstruation zones that unremarkably bound the tops and bottoms of geothermal reservoirs. In some cases, the geothermal gradient increases again below the convecting hydrothermal reservoir, whereas in other cases the gradient can decrease below the reservoir, reflecting lateral outflow of hydrothermal fluids above libation (and more dumbo) groundwater recharge zones. The Casa Diablo geothermal field in the Long Valley caldera in California is an case of the latter (Effigy iii.7). Most of the heat menstruum through the upper crust occurs past conduction (Figure 3.8), and convecting hydrothermal systems require special geologic characteristics. Such characteristics include a source of water, good permeability, properly positioned impermeable cap rocks, and a focused source of heat, such every bit a torso of magma in the upper chaff. These atmospheric condition are not met everywhere, and just because heat flow may exist loftier in a region does not indicate whether or non potentially developable convecting geothermal reservoirs are present.

Effigy 3.7   Temperature profiles of four drill holes into the Casa Diablo geothermal system in the Long Valley caldera, California. Note the areas of conductive estrus menstruum at shallow depths of all drill holes where in that location is a rapid modify of temperature with depth. The convective zones are characterized by lilliputian change in temperature with depth, reflecting circulation and thermal mixing. Note that in all holes, 1 or more temperature reversals occur, reflecting deeper, cooler aquifers. (Adapted from Glassley, West.East., Geothermal Energy: Renewable Energy and the Surroundings, 2d ed., CRC Press, Boca Raton, FL, 2015.)

Figure three.8   Pie slice through Earth'southward interior showing major compositional and rheological divisions and the relative proportion and type of heat flow from each sectionalization. Although conduction is the major form of rut catamenia in the crust, some heat catamenia occurs by convection in areas of circulating crustal fluids and past advection with the rise of magma below active volcanoes. (From Dye, S.T., Reviews of Geophysics, 50(3), RG3007, 2012.)

Heat Flow Maps

The Southern Methodist University Geothermal Laboratory (SMUGL) has been instrumental in compiling drill-hole data and generating and updating a series of maps showing heat flow in the United States. The squad of researchers there has also adult, from the compiled heat menstruum information, a series of temperature-at-depth maps from three.5 to 9.5 km. These maps aid illustrate prospective regions for developing geothermal energy for power and direct use, most of which are located in the western United States (Figures iii.9 and 3.ten). Researchers at SMUGL take likewise developed maps showing the potential for engineered geothermal systems (EGSs) for each of the states based on the oestrus flow data. For case, Nevada, which has an installed electric current geothermal ability capacity of 580 MWe (as of 2015) from conventional convective geothermal systems, could increase it geothermal power output to 41k MWe if simply ii% of its EGS potential is recovered. That number could keen to 288,000 MWe if just 14% of its EGS potential is realized (SMU, 2016). Although such potential is impressive, EGSs are hampered by their still experimental nature and associated high costs which must compete with currently inexpensive natural gas—the fossil fuel of choice for power generation because carbon emissions are almost one-half of those of coal (see Chapter nine). However, equally described past Allis et al. (2012), if sedimentary aquifers exist at these depths and are permeable, they might serve every bit a bridge to actual evolution of EGSs, offer large potential flow rates, and be cost competitive if current low free energy prices were to rising modestly.

Figure 3.9   Heat flow map of the The states for 2011. Ochreous orangish to more than securely red indicates heat flow values in excess of lxxx mW/m2. The highest value on the map is the pinkish crimson, which is >150 mW/m2, which would be at Yellowstone National Park. This and the following figure illustrate the big area of geothermal energy potential covering much of the western U.s.a.. (Adapted from Blackwell, D.M. et al., Geothermal Resources Council Transactions, 35, 1545–1550, 2011.)

Effigy three.10   Map showing temperatures at a depth of 4.5 km. Notice the large area of temperatures of 150°C and higher across much of the western United States. Almost all of northern Nevada has temperatures that are >150°C, including numerous scattered regions with temperatures between 175 and 200°C. This depth level is largely the realm of engineered geothermal systems (EGSs), which if adult (only in pocket-size function, ~10%) could greatly aggrandize (past one to two orders of magnitude) geothermal power production. However, accessing this potential energy resource would exist expensive, mainly due to the deep levels of drilling required, making it hard for this technology to compete economically with currently producing, more shallow geothermal reservoirs and natural gas-fired power plants. (Adapted from Blackwell, D.1000. et al., Geothermal Resources Council Transactions, 35, 1545–1550, 2011.)

Summary

The interior of the World is compositionally and rheologically partitioned into distinct layers. Compositionally, the Earth's interior consists of an iron-rich metallic core, a mantle, and a thin crust. The mantle makes up the largest volume of the Earth's interior and consists of dense, iron- and magnesium-rich rocks. The crust consists of two types: oceanic and continental. Oceanic crust consists of more dumbo basalt and is relatively sparse (<ane to 7 km thick). Continental crust is made of less dense granitic and metamorphic rocks and tin be every bit much as 70 km thick underneath mountain belts. This compositional division developed very early in the World'southward history, when the planet was all the same largely molten. Dumbo constituents settled to the center to class the core, and less dense elements rose toward the surface to grade the drape and chaff.

Due to changes in temperature and force per unit area within the Earth, the compositional layers develop dissimilar rheological (mechanical) properties, ranging from solid (brittle and strong), to weak and ductile, to molten. These different mechanical layers are the lithosphere, asthenosphere, mesosphere, outer cadre, and inner core. The lithosphere consists of the crust and uppermost mantle and is stiff rock that when stressed to a certain limit volition suspension (breakable beliefs) rather than bend (ductile). On boilerplate, the lithosphere is nearly 100 km thick and comprises the tectonic plates discussed further in Chapter 4. The underlying asthenosphere consists of weak stone near its melting point; however, it is not molten but notwithstanding largely solid. Because of the hot temperatures, the stone in the asthenosphere has the ability to flow ductilely. The asthenosphere is about 200 km thick, just its lower boundary with the underlying mesosphere is gradational. The mesosphere makes up the bulk of the mantle, and the rocks at that place are stronger due to the increasing force per unit area with depth, but they notwithstanding take the power to menstruum, although more slowly than in the asthenosphere. The outer cadre consists of liquid iron–nickel metal; the inner cadre is the same composition merely a solid due to the farthermost pressure.

The compositional and rheological nature of the Earth's interior is largely based on the study of seismic waves whose management and speed of propagation are based on compositional and physical backdrop of the material through which they laissez passer. For case, one type of seismic wave does not travel through liquids, and as such seismic receiving stations on the opposite side of the planet from where an convulsion occurs volition not discover that wave, creating a shadow zone. The size of the shadow zone is a direct reflection of the size of the liquid, outer core.

Earth'southward internal oestrus has ii primary sources. The commencement is heat left over from the tumultuous formation of the Globe, when kinetic energy of celestial collisions was transformed to oestrus energy (primordial rut). The second major source is radioactive decay of select elements, mainly uranium (U), thorium (Th), rubidium (Rb), and potassium (K). The contribution of each source is most equal. Rut flows from the Globe's interior toward the surface via two main mechanisms: conduction and convection. Conductive oestrus flow is transfer of energy by contact, also known as thermal improvidence. Conductive heat flow is mainly operative in the Earth's core and chaff. Convective oestrus flow is rut transferred by motion, with subsidiary contribution past conduction. Convective motion is induced by buoyant forces that arise from thermal gradients in a gravitational field. If cloth becomes hotter than its surroundings, its density is reduced, causing the heated material to rising. Conversely, the surrounding cooler material is more dense and sinks to replace the rising hotter cloth. Convective estrus transfer occurs in the liquid outer core and the rheologically ductile mesosphere and asthenosphere, where buoyant forces exceed viscous forces every bit measured by the Rayleigh number.

Geothermal heat flow and temperature-at-depth maps illustrate that much of geothermal resources adult and yet to exist developed occur in the western United States. For example, most of northern Nevada has a oestrus flow of >80 mW/ m², in places >100 mW/m^two. At a depth of 4.5 km, the temperature of crustal rocks in northern Nevada is >150°C and in places as high as 200°C. Although this environment (realm of engineered geothermal systems) represents a vast reservoir of heat and potential source of energy evolution, information technology is expensive to access and cannot compete economically with currently adult sources of geothermal free energy or natural gas-fired power plants. However, if sedimentary aquifers exist at these depths and are permeable, they could serve as major sources of available geothermal energy if energy prices ascent modestly.

Suggested Problems

1. Explain what factors control whether heat period will be conductive or convective? What blazon offers the greatest potential for geothermal free energy development and why?
2. Will the Rayleigh number of cloth impact the oestrus period measured at the surface? Why or why non?
3. Assume that a well is drilled in dry sand to a depth of 2500 1000 and the temperature measured at the bottom is 150°C. For simplicity, assume that the thermal conductivity of dry out sand is a abiding betwixt 10°C and 200°C. Is there likely to be a geothermal resource? Explicate why or why not.
4. Imagine y'all are a geologist and you have drilled hole RD08 whose temperature profile with depth is shown in Figure 3.vii. Using the temperature– depth profiles of the three other wells shown in Figure 3.7, should y'all proceed drilling deeper or end at the electric current depth? Justify your position.

References and Recommended Reading

Allis, R., Blackett, B., Gwynn, M. et al. (2012). Stratigraphic reservoirs in the Peachy Basin—the bridge to evolution of enhanced geothermal systems in the U.South. Geothermal Resources Council Transactions, 36: 351–357.

Arevalo, Jr., R., McDonough, W.F., and Luong, Grand. (2009). The G/U ratio of the silicate Globe: insights into curtain limerick, structure and thermal development. Earth and Planetary Scientific discipline Letters, 278(three–4): 361–369.

Blackwell, D.1000., Richards, Z.F., Batir, J., Ruzo, A., Dingwall, R., and Williams, M. (2011). Temperature at depth maps for the conterminous U.S. and geothermal resources estimates. Geothermal Resource Quango Transactions, 35: 1545–1550.

Clauser, C. and Huenges, E. (1995). Thermal conductivity of rocks and minerals. In: Rock Physics and Phase Relationships: A Handbook of Physical Constants (Ahrens, T.J., Ed.), pp. 105–126. Washington, DC: American Geophysical Spousal relationship.

Davies, J.H. and Davies, D.R. (2010). Earth's surface oestrus flux. Solid Earth, 1(ane): 5–24.

Dye, S.T. (2012). Geoneutrinos and the radioactive ability of the World. Reviews of Geophysics, l(3): RG3007.

Gando, A., Gando, Y., Ichimura, K. et al. (2011). Partial radiogenic heat model for Earth revealed by geoneutrino measurements. Nature Geoscience, iv(ix): 647–651.

Glassley, Westward.E. (2015). Geothermal Energy: Renewable Energy and the Environment, 2d ed. Boca Raton, FL: CRC Press.

Korenaga, J. (2011). Globe's heat budget: clairvoyant geoneutrinos. Nature Geoscience, 4(nine): 581–582.

Tarbuck, Eastward.J., Lutgens, F.M., and Tasa, D. (2005). Earth: An Introduction to Physical Geology. Upper Saddle River, NJ: Prentice Hall.

Source: https://www.routledgehandbooks.com/doi/10.1201/9781315371436-4

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