Physical Processes in Earth and Environmental Sciences Phần 2 ppt

20 Chapter 2

4 Magma has small but important fractions of pressurized

dissolved gases, including water vapor.

5 River water contains suspended solids, while the atmos￾phere carries dust particles and liquid aerosols.

6 Seawater has c.3 percent by weight of dissolved salts and

also suspensions of particulate organic matter.

Solid Earth substances may break or flow:

1 Ice fragments when struck, yet deformation of boreholes

drilled to the base of glaciers also shows that the ice there

flows, while cracking along crevasses at the surface.

2 Earth’s mantle imaged by rapidly transmitted seismic

waves behaves as a solid mass of crystalline silicate minerals.

Yet there is ample evidence that in the longer term

(103 years) it flows, convecting most of Earth’s internal

heat production as it does so. Even the rigid lower crust is

thought to flow at depth, given the right temperature and

water content.

2.1.5 Timescales of in situ reaction

The lesson from the last of the above examples is that we

must appreciate characteristic timescales of reaction of

Earth materials to imposed forces and be careful to relate

state behavior to the precise conditions of temperature and

pressure where the materials are found in situ.

2.2 Thermal matters

2.2.1 Heat and temperature

Heat is a more abstract and less commonsense notion than

temperature, the use of the two terms in everyday speech

being almost synonymous. We measure temperature with

some form of heat sensor or thermometer. It is a measure

of the energy resulting from random molecular motions in

any substance. It is directly proportional to the mean

kinetic energy, that is, mean product of mass times velocity

squared (Section 3.3), of molecules. Heat on the other

hand is a measure of the total thermal energy, depending

again on the kinetic energy of molecules, and also on the

number of molecules present in any substance.

It is through specific heat, c, that we can relate temperature

and heat of any substance. Specific heat is a finite capacity,

sometimes referred to as specific heat capacity, in that it is a

measure of how much heat is required to raise the temper￾ature of a unit mass (1 kg) of any substance by unit Kelvin

(K  C 273). It is thus also a storage indicator – since

only a certain amount of heat is required to raise tempera￾ture between given limits, it follows that only this amount

of heat can be stored. In Box 2.1, notice the extremely

high storage capacity of water, compared to the gaseous

atmosphere or rock.

Temperature change induces internal changes to

any substance and also external changes to surrounding

environments, for example,

1 Molten magma cools on eruption at Earth’s surface, turn￾ing into lava; this in turn slowly crystallizes into rock.

2 Glacier ice in icebergs takes in heat from contact with

the ocean, expands, and melts. The liquid sinks or floats

depending upon the density of surrounding seawater.

3 Water vapor in a descending air mass condenses and

heat is given out to the surrounding atmospheric flow.

In each case temperature change signifies internal energy

change. Changes of state between solid, liquid, and gas

require major energy transfers, expressed as latent heats

(Box 2.1). We shall further investigate the world of ther￾modynamics and its relation to mechanics later in this book

(Section 3.4).

Substances subject to changed temperature also change

volume, and therefore density; they exhibit the phenome￾non of thermal expansion or contraction (Box 2.1). This

arises as constituent atoms and molecules vibrate or travel

around more or less rapidly, and any free electrons flow

around more or less easily. If changes in volume affect only

discrete parts of a body, then thermal stresses are set up

that must be resisted by other stresses failing which a net

force results. Temperature change can thus induce motion

or change in the rate of motion. Stationary air or water

when heated or cooled may move. Molten rock may move

through solid rock. A substance already moving steadily

may accelerate or decelerate if its temperature is forced to

change. But we need to consider the complicating fact that

substances (particularly the flow of fluids) also change in

their resistance to motion, through the properties of vis￾cosity and turbulence, as their temperatures change. We

investigate the forces set up by contrasting densities later

in this book (e.g. Sections 2.17, 4.6, 4.12, and 4.20).

2.2.2 Where does heat energy come from?

There are two sources for the heat energy supplied to

Earth (Fig. 2.4). Both are ultimately due to nuclear reac￾tions. The external source is thermonuclear reactions in

the Sun. These produce an almost steady radiance of

shortwave energy (sunlight is the visible portion), the

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Matters of state and motion 21

Specific heat capacities, cp , units

of J kg–1

K–1

, at standard T and P.

Air 1,006

Water vapor (100°C) 2,020

Water 4,182

Seawater 3,900

Olive oil 1,970

Iron 106

Copper 385

Aluminum 913

Silica fiber 788

Carbon (graphite) 710

Mantle rock (olivine) 840

Limestone 880

Coefficients (multiply by 10–6

) of

linear thermal expansion, al

, units

of K–1 at standard T and P.

Iron 12

Copper 17

Aluminum 23

Silica fiber 0.4

Carbon (graphite) 7.9

Crustal rock (to 373 K) 7–10

Coefficients (multiply by 10–4) of

cubical thermal expansion, av, units

of K–1 at standard T and P.

Water 2.1

Olive Oil 7.0

Crustal rock 0.2–0.3

Thermal conductivity, l, units of

W m–1

K–1 at standard T and P.

Air 0.0241

Water 0.591

Olive oil 0.170

Iron 80

Copper 385

Aluminum 201

Silica fiber 9.2

Carbon (graphite) 5

Mantle rock (olivine) 3–4.5

Limestone 2–3.4

Heat flow required for fusion, Lf

,

units of kJ kg–1

. Sometimes termed latent

heat of fusion, more correctly it is the specific

enthalpy change on fusion (see Section 3.4).

Ice 335

Mg Olivine 871

Na Feldspar 216

Basalt 308

Heat flow required for vaporization, Lv

,

units of kJ kg–1

. Sometimes termed latent

heat of vaporization, more correctly it is the

specific enthalpy change on vaporization

(see Section 3.4).

water to water vapor

(and vice versa) 2,260

Heat flow produced by crystallization,

(multiply by 104

) units of J kg–1

.

Basalt magma

to basalt 40

Water to ice 32

Thermal Diffusivity, k, units m2

s–1 x 10–6

at

standard T and P.

Air 21.5

Water 0.143

Mantle rock 1.1

Thermal diffusivity indicates the rate of dissemination

of heat with time. It is the ratio of rate of passage of

heat energy (conductivity) to heat energy storage

capacity (specific heat per unit volume) of any material

Specific Heat Capacity , cp, cv, is the amount of heat

required to raise the temperature of 1 kg of substance

by 1 K . Subscripts refer to constant volume or pressure

Thermal Conductivity is the rate of flow of heat

through unit area in unit time

Box 2.1 Some thermal definitions and properties of earth materials

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22 Chapter 2

average magnitude of which on an imaginary unit surface

placed at the uppermost surface of Earth’s atmosphere

facing the sun is now approximately 1,367 W m2. This solar

constant is the result of a luminosity which varies by

>0.3 percent during sunspot cycles, possibly more during

mysterious periods of negligible sunspots like the Maunder

Minimum (300–370 years BP) coincident with the Little Ice

Age. At any point on Earth’s surface, seasonal variations in

received radiation occur due to planetary tilt and elliptical

orbit, with longer term variations up to 1 percent due to the

Croll–Milankovitch effect (Section 6.1).

Internal heat energy comes from two sources. A minority,

about 20 percent, comes from the “fossil” heat of the

molten outer core. The remainder comes from the radioac￾tive decay of elemental isotopes like 238U and 40K locked

up in rock minerals, especially low density granite-type

rocks of the Earth’s crust where such elements have been

concentrated over geological time. However, the total

mass of such isotopes has continued to decrease since the

origin of the Earth’s mantle and crust, so that the mean

internal outward heat flux has also decreased with time.

Today, the mean flux of heat issuing from interior Earth is

around 65 mW m2 (Fig. 2.4), though there are areas of

active volcanoes and geothermally active areas where the

flux is very much greater. The mean flux outward is thus

only some 4.8 105 of the solar constant. To make this

contrast readily apparent, the total output of internal heat

from the area enclosed by a 400 m circumference racetrack

would be about 1 kW, of the same order as that received

by only 1 m2 area of the outer atmosphere and equivalent

to the output of a small domestic electric bar heater. The

heat energy available to drive plates is thus minuscule

(though quite adequate for the purpose) by comparison

with that provided to drive external Earth processes like

life’s metabolism, hydrological cycling, oceanographic

circulations, and weather.

2.2.3 How does heat travel?

Radiative heat energy is felt from a hot object at a

distance, for example, when we sunbathe or bask in the glow

of a fire, in the latter case feeling less as we move further

away. The heat energy is being transported through space

and atmosphere at the speed of light as electromagnetic

waves.

Conductive heat energy is also felt as a transfer process

by directly touching a hot mass, like rock or water, because

the energy transmits or travels through the substance to

be detected by our nervous system. In liquids we feel the

effects of movement of free molecules possessing kinetic

energy, in metals the transfer of free electrons, and in the

solid or liquid state as the atoms transmit heat energy by

vibrations.

Convection is when heat energy is transferred in bulk

motion or flow of a fluid mass (gas or liquid) that has been

externally or internally heated in the first place by radiation

or conduction.

2.2.4 Temperature through Earth’s atmosphere

The mean air temperature close to the land surface at sea

level is about 15C. Commonsense might suggest that the

mean temperature increases the further we ascend in the

atmosphere: like Icarus, “flying too close to the sun,”

more radiant energy would be received. In the lower

atmosphere, this commonsense notion, like many, is soon

proved wrong (Fig. 2.5) either by direct experience of

temperatures at altitude or from airborne temperature

measurements. The “greenhouse” effect of the lower

atmosphere (Sections 3.4, 4.19, and 6.1) keeps the surface

warmer than the mean – 20C or so, which would result in

the absence of atmosphere. Although a little difficult to

compare exactly, since the Moon always faces the same way

toward the Sun, mean Moon surface temperature is

of about this order (varying from 130C on the sunlit

side to 158C on the dark side). Due to the declining

greenhouse effect, as Earth’s atmosphere thins, tempera￾ture declines upward to a minimum of about 55C above

Fig. 2.4 Heat energy available to drive plates is minuscule

when compared with that provided by solar sources for life, the

hydrological cycle, weather, etc.

GEOTHERMALHEAT

65 mW m–2

SOLAR HEAT

1,367 W m–2

HEAT ENERGY is required

for life, plate motion, water

cycling, weather, and

convectional circulations

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