Encyclopedia of Global Resources part 115 doc

Sedimentary processes, rocks, andmineral deposits Categories: Geological processes and formations; mineral and other nonliving resources Sedimentary processes occur only at the Earth’s

Sedimentary processes, rocks, and

mineral deposits

Categories: Geological processes and formations;

mineral and other nonliving resources

Sedimentary processes occur only at the Earth’s

sur-face, because they are driven by various components of

the hydrologic and biologic cycles Sedimentary

pro-cesses involve the breakdown, movement, and ultimate

deposition of broken rock fragments and chemicals in

solution These processes create many of the important

resources used by humans.

Background

Most sediments originate from the weathering of

ex-isting rocks Weathering is the breakdown of earth

material by physical and/or chemical processes

Phys-ical weathering only breaks the original material into

smaller sizes This is accomplished through

mechani-cal means such as freezing and thawing, plant-root

wedging, differential heating of rocks, and crystal

growth in rock cracks Chemical weathering, on the

other hand, actually changes the composition of the

original material into completely different

compo-nents through solution, oxidation, hydration, and/or

hydrolysis The by-products of physical and chemical

weathering provide the different types of sediment

and ion-bearing solutions that create sedimentary

rocks and minerals

Biological activity can also create sediments Many

invertebrates and some algae utilize calcium

carbon-ate in seawcarbon-ater to make shells Organisms, such as

coral, live in warm, shallow seas and construct reefs

In addition, both macro- and microscopic shells of

or-ganisms that do not live in reef communities become

sediments that blanket the seafloor after the

organ-isms’ deaths Also, some algae and bacteria, as well as

swamp vegetation, can become organic sediments

un-der certain conditions of oxygen deficiency, creating

valuable fossil fuels

Transportation

Once particles of rock are loosened and broken free

by weathering processes, those particles can be

car-ried off by water, wind, or glaciers Ions released

through chemical activity are also free to travel in

moving water, but they will never settle out of the

water unless a specific chemical reaction occurs that

causes the ions to precipitate as solid particles, or until shell-bearing organisms use the ions to build shells that later become sediments

The amount of energy available in a transporting medium determines which rock particles are picked

up (eroded) and moved along For example, more energy is usually required to move large (or more dense) grains than small (less dense) grains If a trans-porting medium loses energy, the sediments being carried will drop out in order of relative size, the larg-est ones first Sediment sizes range from boulders to clay Each has a specific size determination for the name: boulder (more than 256 millimeters), cobble (256 to 64 millimeters), pebble (64 to 4 millimeters), granules (4 to 2 millimeters), sand (2 to 0.062 milli-meter), silt (0.062 to 0.0039 millimilli-meter), and clay (less than 0.0039 millimeter) This separation of grains ac-cording to size by the transporting medium is called sorting A well-sorted sediment is one that contains nearly all the same size grains The farther sediments travel, the better sorted they become Thus, a poorly sorted sediment probably is still fairly close to its source area

Transport of material also tends to make grains more rounded in shape Rounding occurs as particles bump into one another or into objects in the environ-ment, causing abrasion of their edges The longer and farther a grain is moved, the smaller and rounder it becomes Transportation tends to winnow material either by sorting or by continued weathering of the particles Some minerals are more resistant to break-down than other minerals Quartz is more resistant, while minerals such as feldspar, mica, pyroxene, and amphibole are less resistant The longer and farther the sediments are transported, the more likely the weaker minerals are to disappear, which would in-crease the percentage of quartz present Therefore, rocks composed of nearly all quartz can be inter-preted as having traveled a great distance from their source of weathering These rocks also exhibit good sorting and are well rounded

Most sediments are carried by running water Run-ning water begins as rainfall, flows into increasingly larger streams, and eventually runs into the oceans Running water is present nearly everywhere on the Earth, including in arid regions (where, although un-usual, it is particularly effective as an agent of erosion because of the lack of vegetation for ground cover and because much of the rain in arid lands occurs in the form of downpours)

Waves and currents along shorelines also move

large amounts of sediment, as can some currents out

in deeper waters Energy is usually greater nearer a

shoreline, so, if grains accumulate, beaches form

De-pending on the energy level, the beaches can be

com-posed of sand, pebbles, cobbles, or even boulders

The energy usually decreases offshore, so

finer-grained silt and clay (mud) often accumulate in the

deeper, quieter water

Groundwater can carry dissolved minerals

Some-times when the rock is dissolved, minerals of

eco-nomic importance, such as copper, aluminum, or

iron, can be left behind In some limestone areas,

groundwater can dissolve enough limestone to form

caves If conditions are right in a cave, the

ground-water can redeposit some minerals in the forms of

dripstone and flowstone, features admired by tourists

Wind can move sediment grains as well, but

nei-ther in the quantity nor in sizes as large as does water

Fine sand is usually the upper size limit that wind can

transport Grains carried by wind do not have the

cushioning effect that water provides, so grains are

rapidly abraded as they hit each other during

trans-port Quartz sand grains become frosted in this

manner

Sediments are also transported by glaciers

Move-ment of grains in glacial ice will break the grains,

mak-ing them smaller, but they remain fairly angular

be-cause they do not tumble around next to each other as

they would in water or wind and, thus, do not undergo

much rounding Because ice will not let particles

readily settle, as water or wind will, the particles do not

become sorted at all Glacial deposits are probably the

most poorly sorted of all sediments However, some

sediments do not end their journeys under a glacier;

they may move away from the ice in glacial meltwater

When this happens, the sediments are transported by

running water and take on the characteristics of any

sediment carried in running water

Deposition (Sedimentation)

All particles eventually are carried to specific

environ-ments where the transporting energy decreases to the

point of no longer being able to carry the material,

and the grains are deposited If not destroyed by

fur-ther erosion, they may harden into various

sedimen-tary rocks There are three main depositional systems:

marine, transitional, and terrestrial

Marine environments are important mainly, but by

no means exclusively, for deposition of chemical

sedi-ments Marine environments include shallow-marine environments (from the shore to the edge of the con-tinental shelf ), reefs, and deep marine environments Transitional and terrestrial depositional environments are important for deposition of mainly clastic sedi-ments Transitional environments include beaches, deltas, barrier islands, lagoons, and tidal marshes Terrestrial environments include rivers, lakes, alluvial fans, glacial environments, and windy areas such as deserts

Diagenesis Diagenesis is any postdepositional alteration of sedi-ments or sedimentary rocks Diagenetic processes usually take place after sediments are buried by newer sediments An important diagenetic process is lithification, the process by which loose sediments are hardened into rock There are two main processes in-volved in lithifying sediments: cementation and com-paction Cementation occurs when pore spaces be-tween the grains are large enough for mineral-rich water to seep around the grains, depositing either silica or calcite crystals that grow and eventually co-alesce and hold the grains together Many sediments typically lithify by physical compaction As more and more sediments collect, the weight of overlying sedi-ments compresses the lower sedisedi-ments, squeezing out much of the water and pressing the grains close enough together that they become a harder mass of material Lithification processes can occur below water; an ocean basin does not have to dry up before the sediments it contains can lithify into rock Sedimentary Rocks and Minerals

Sedimentary rocks are the most abundant type of rocks found at the surface of the Earth All sedimen-tary rocks and minerals can be put into one of two cat-egories: clastic, composed of rock and mineral frag-ments that were weathered from preexisting rock materials and lithified in new combinations to create

a sedimentary rock, and nonclastic, made of precipi-tated chemicals or of organically derived material such as shells or plants and animals Identification of sedimentary rocks begins with the determination of whether a rock is clastic or nonclastic Once such tex-tural characteristics have been determined, the grain sizes and composition of the rock are used to com-plete the identification process

Clastic rocks are fairly easy to identify because indi-vidual particles in the rock can usually be seen The

rock is identified mainly on the sizes and shapes of the

grains it contains Rocks made of large particles such

as boulders, cobbles, and/or pebbles in a matrix of

sand are called conglomerate if the grains are mostly

rounded or breccia if the grains are angular The

sand-stone family contains many varieties of rocks

depend-ing on the composition of grains present Sandstone

feels like sandpaper, and the purer variety can be white,

tan, or pink and is composed almost entirely of quartz

grains More commonly sandstone is gray, indicating

that it contains particles other than quartz Arkose is a

sandstone that contains fairly large, angular granules

of pink feldspar Sedimentary rocks composed of silt

are called siltstone, and shale is composed of grains of

clay that are so small they cannot be seen, even with a

microscope (Because the grains are so small, shale

can easily be misidentified as a nonclastic rock.)

The nonclastic rocks are dominated by limestone

and dolostone (or dolomite) Limestone is calcium

carbonate that formed in warm, shallow seas Most

limestone originated as accumulations of shells either

in reefs or in beds on the seafloor Some limestone can also form by chemical precipitation Dolostone,

a close relative of limestone, is thought to form diagenetically from limestone deposits when mag-nesium ions in the environment replace some of the calcium in the limestone Microscopic shell accumu-lations from calcareous algae have created thick layers

of chalk, while similar accumulations from siliceous plankton create diatomaceous earth and chert (flint) Plants that die in swamps and do not completely rot away can evolve into peat and eventually into coal, and microscopic algae and bacteria are the source

of petroleum deposits Although petroleum is not a rock, it is a resource that originates in a sedimentary setting

Some resource minerals form under evaporative conditions Ions in solution travel with running water and eventually end up in the oceans or in lakes In shallow embayments where seawater can flush into

Sedimentary Rock Types

Clastic Rocks that consist of

fragments of other rocks

Conglomerates Grains are boulder-,

cobble-, or gravel-sized

Breccia

Sandstone Grains are sand-sized Quartz sandstone,

arkose, graywacke Mudstone Grains are silt- or

clay-sized

Mudstone, shale

Precipitates Chemically precipitated

or replaced; inorganic

in origin

Evaporites Solids have precipitated

after evaporation of water in which they were dissolved

Salt, gypsum, anhydrite, borax, potash

or magnesium

Calcite (inorganic limestone), dolomite Siliceous Chemically precipitated

silicas

Chert: flint, jasper

Organic

(biochemical)

Remains of plant or animal organisms

limestone

the bay and not be diluted with fresh stream water,

salts can accumulate if the water evaporates Likewise,

in arid regions where streams enter landlocked lakes

(playa lakes), evaporative conditions cause salts to

ac-cumulate There are a great variety of salts (gypsum,

halite, magnesium sulfate, and potassium salts, to

name a few), and each one forms in turn as the

chemi-cal concentrations change as more and more of the

water evaporates

Economic Importance of Selected Sedimentary

Rocks and Minerals

Sedimentary resources are classified into four main

groups: sedimentary metallic ore deposits,

sedimen-tary nonmetallic deposits, evaporites, and energy

re-sources The sedimentary ore deposits contain some

of the world’s most valuable mineral resources Many

of these deposits were formed in depositional

envi-ronments where large amounts of dissolved metals

collected For example, the iron ores of the famous

Me-sabi Range in Minnesota originated when the Earth’s

early atmosphere was poor in oxygen This permitted

an abundance of iron in its soluble (ferrous) form to

be leached from large areas of the Earth’s surface and

transported in solution to vast, shallow marine

envi-ronments, where it oxidized to its insoluble (ferric)

form and precipitated in thin layers

Although gold originates from igneous and

hydro-thermal processes, once it weathers out of its original

rock setting, it becomes influenced by the

sedimen-tary processes of transportation by running water and

ultimate deposition in streambeds This is called a

placer deposit Placer deposits are not limited to gold

Diamonds, tin, chromite, platinum, and magnetite

can undergo similar histories Placers can sometimes

be traced upstream to find the source rock, which

can then be mined

The nonmetallic deposits are also of great

eco-nomic importance Limestone is quite extensive and

has a variety of uses It is obtained by quarrying; the

rock can be either cut into large blocks or blasted into

fragments The blocks, used for building stone, are

taken to mills and cut to order for particular

build-ings; decorative carvings can also be made Limestone

that has been blasted is usually ground into lime for

either agricultural purposes or the manufacture of

cement Larger blocks may be used as riprap along

shorelines or rivers to protect those areas from

exces-sive erosion

Pure quartz sandstone, which usually originates

from beach deposits, is quarried for use in glassmaking and fiber-optic cables Coarser sand and gravel, which originates from glacial deposits or channel deposits in rapidly moving rivers, is quarried for construction purposes Clays of high purity, often formed in coal swamps or in areas of prolonged weathering, are used for both craft and industrial ceramics Phosphatic rocks, usually marine shales and limestones that have been chemically enriched in phosphate in deep ma-rine environments, are an ingredient in agricultural fertilizers

Evaporite deposits have created vast and varied salt resources Halite is used both as table salt and as an ice melter for road clearance Gypsum is used in the mak-ing of plaster as well as the writmak-ing utensils used on modern “chalk” boards, contrary to popular belief that these writing implements are really chalk Potas-sium salts can be used as table salt and in fertilizer Epsom salts (magnesium sulfate) have health bene-fits, and borates (such as borax and boron) have uses that range from manufacturing of enamel to additives

to soap and gasoline

The fossil fuels, the major energy resources in use

in the twentieth century and early twenty-first cen-tury, all have biogenic origins in depositional environ-ments Coal forms from vegetation that grew in an-cient swamps Coal is obtained from both strip mining and underground mining Petroleum products are buried microscopic planktonic life-forms that lived in seas of the past Although these organics collect on small scales, the sedimentary rocks with which they are associated permit the migration and eventual ac-cumulation of great enough volumes of oil and gas that they can be extracted for human use

Diann S Kiesel

Further Reading

Boggs, Sam, Jr Petrology of Sedimentary Rocks 2d ed.

New York: Cambridge University Press, 2009

_ Principles of Sedimentology and Stratigraphy 4th

ed Upper Saddle River, N.J.: Pearson Prentice Hall, 2006

Chernicoff, Stanley, and Donna Whitney Geology: An Introduction to Physical Geology 4th ed Upper Saddle

River, N.J.: Pearson Prentice Hall, 2007

Davis, Richard A Depositional Systems: An Introduction to Sedimentology and Stratigraphy 2d ed Englewood

Cliffs, N.J.: Prentice Hall, 1992

Grotzinger, John P., et al Understanding Earth 5th ed.

New York: W H Freeman, 2007

Nichols, Gary Sedimentology and Stratigraphy 2d ed.

Hoboken, N.J.: Wiley-Blackwell, 2009

Pettijohn, F J Sedimentary Rocks 3d ed New York:

Harper & Row, 1975

Tennissen, Anthony C Nature of Earth Materials 2d ed.

Englewood Cliffs, N.J.: Prentice-Hall, 1983

Tucker, Maurice E Sedimentary Petrology: An

Introduc-tion to the Origin of Sedimentary Rocks 3d ed Malden,

Mass.: Blackwell Science, 2001

Web Site

U.S Geological Survey

Sedimentary Rocks

http://vulcan.wr.usgs.gov/LivingWith/

VolcanicPast/Notes/sedimentary_rocks.html

See also: Cement and concrete; Ceramics; Coal;

Evaporites; Fertilizers; Iron; Limestone; Oil and

natu-ral gas formation; Shale; Weathering

Seed Savers Exchange

Category: Organizations, agencies, and programs

Date: Established 1975

Since its founding, the Seed Savers Exchange has

helped preserve the genetic material of more than

twenty-five thousand plant species Modern agriculture

practices tend to focus narrowly on an increasingly small number of crops, resulting in the endangerment

or extinction of thousands of plant species However, the Seed Savers Exchange helps maintain genetic di-versity over time, which is critical to combating the fur-ther loss of species from disease, pestilence, and ofur-ther environmental factors.

Background The Seed Savers Exchange was founded in 1975 in Decorah, Iowa, by then husband and wife Kent Whealy and Diane Ott Whealy The couple had been given the seeds of two garden plants that Diane’s grandfather had brought to the United States from Bavaria in the 1870’s, a gift that made them recognize the value of preserving not only the genetic but also the cultural and historical heritage of plants Over time, the nonprofit organization has grown to several full-time employees and occupies 360 hectares, where

it maintains more than twenty-five thousand varieties

of vegetable, fruit, flower, and herb seeds as well as a small number of endangered cows and poultry Impact on Resource Use

The Seed Savers Exchange is a permanent repository

of thousands of seeds, including those of many plant species that have otherwise virtually disappeared Un-like many seed banks, the Seed Savers Exchange not only stores seeds but also actively plants approxi-mately 10 percent of its inventory each year in

rota-tion, allowing seeds to be distributed among members or sold The wide variety of plants the Seed Savers Ex-change grows each year helps com-bat the existence of monocultures,

or the covering of hundreds or thou-sands of hectares with a single crop While large commercial food grow-ers routinely deal in monocultures

in order to maximize profit, the prac-tice risks crop annihilation if that particular strain is attacked by a dis-ease or pest In addition, the Seed Savers Exchange’s planting activity keeps those species in contact with the larger natural environment, and thus better equipped to survive in the future, rather than “frozen” in storage and unable to react to chang-ing environmental conditions

Seeds are stockpiled in the Svalbard Global Seed Vault on the island of Svalbard near the

North Pole The Seed Savers Exchange’s initial contribution to the vault was five

hun-dred seeds (AFP/Getty Images)

The Seed Savers Exchange also promotes the

sav-ing and exchange of seeds among members, thus

cre-ating community, spreading the impact of its work,

and promoting long-term survival of plant species It

also sells seeds via print and online catalogs, which has

helped promote the organic farming industry,

be-cause commercially available seeds are far more

lim-ited in variety In addition, many commercial seeds

are either hybrids that do not reproduce reliably or

genetically modified, which is not permitted in the

or-ganic food trade

The Seed Savers Exchange considers education

and outreach to be important parts of its mission In

addition to providing seed-saving guidance in its

newsletters and on its Web site, the Seed Savers

Ex-change houses a visitors’ center that offers guided

tours to individuals and groups The organization also

participates in the global seed preservation

commu-nity, most notably by contributing almost five

hun-dred seeds for the opening of the Svalbard Global

Seed Vault in Norway in 2008 The organization has

additional global donations planned The donations

help ensure that some of the Seed Savers Exchange’s

seed stock will be protected in the event of local

disas-ter in Iowa

Amy Sisson

Web Site

Seed Savers Exchange

http://www.seedsavers.org/

See also: Agricultural products; Agriculture

indus-try; Agronomy; Jackson, Wes; Land Institute; Plant

do-mestication and breeding; Plants as a medical

re-source; Svalbard Global Seed Vault

Seismographic technology and

resource exploitation

Categories: Obtaining and using resources;

scientific disciplines

Knowledge of the Earth’s interior has been greatly

en-hanced by seismic data Without this knowledge, it

would be impossible to obtain the oil, natural gas, coal,

metals, and other earth resources that are a foundation

of industrial society.

Background The deepest mines and drill holes penetrate only a fraction of 1 percent of the thickness of the Earth Al-though such samples of the interior provide impor-tant information, they are neither distributed evenly enough over the surface nor numerous enough to provide a complete picture Therefore, information must come mostly from indirect evidence provided by instruments that probe the interior without actually going there Modern electronics and computer tech-nology have greatly improved the quality of instru-ments now used in resource exploration and exploita-tion The most widely used instrument to discover hidden resources in the Earth is the seismograph Seismographs

A seismograph detects and records the seismic (sound) waves traveling beneath the Earth’s surface These waves are of two principal types: compressional waves and shear waves In compressional waves, rock particles are vibrated parallel to the direction of wave propagation, whereas in shear waves, the motion is perpendicular to the direction of wave travel Seismo-graphs can be manufactured to record both wave types

Generally, a seismograph consists of a sensor (seis-mometer or seismic detector), an electronic ampli-fier, filters, and a recording system In its wide range of uses, from resource exploration and exploitation to earthquake studies, a seismograph may be required

to measure ground movements from as small as one millionth of a meter to as large as several meters, a range of more than ten orders of magnitude The seis-mic detector (sensor) consists of a weight suspended from a frame by a delicate spring The frame moves with the ground, but because of its inertia (mass), the weight tends to remain stationary Attached to the weight is a coil of electrical wire Ground motion moves the coil in a magnetic field created by a magnet attached to the frame of the seismograph The rela-tive motion between the coil and the magnet converts the mechanical ground motion into an electrical sig-nal that passes through an amplifier The amplified voltage controls a recording device that marks the ground motion on a moving sheet of paper The re-corded information is called a seismogram

Sources of Seismic Waves Our knowledge of the Earth’s deep interior has been mainly conveyed by earthquakes, most of which are

caused by the sudden movement of rock masses.

As these rocks grind together, energy is released

that produces both compressional and shear

waves These waves spread throughout the Earth

like the ripples made by a pebble tossed into a

quiet pond of water As the waves spread

out-ward, some are reflected and some are refracted

The reflected waves travel downward to

bound-aries between rock layers, where they reflect

(bounce or echo) back to the surface, while the

refracted waves follow paths that bend at each

rock boundary Eventually, these seismic waves

reach the Earth’s surface, where they are

de-tected by a seismograph

Earthquakes release the large amounts of

en-ergy needed to probe the deep layers (mantle

and core) of the Earth Other methods can

pro-duce seismic waves that can be focused on the

geologic features closer to the Earth’s surface

These waves can be generated by artificial

explo-sions, as of a charge of dynamite, or by dropping

a weight or pounding the ground with a sledge

hammer To eliminate the environmental risks

associated with using explosives, seismologists may

use a system called vibroseis (pronounced

VI-broh-size) In this system, a huge vibrator mounted on a

special truck repeatedly strikes the Earth to produce

sound waves

Geophysics

The branch of Earth science dealing with the analysis

of seismographic data is geophysics, the science that

applies physics to the study of the Earth and its

envi-ronment Geophysicists can use the speed of seismic

waves recorded by a seismograph to determine the

depth and structure of many rock formations, since

the speed varies according to the physical properties

of the rock through which the wave travels

Seismol-ogy is the field of geophysics that deals with the study

of seismic waves produced by earthquakes or other

sources, such as vibroseis These studies have helped

determine the location of many natural resources

in the Earth and have led to a better understanding

of earthquakes and other processes that shape the

Earth

Seismic Exploration with the Seismic

Reflection Method

In a seismic survey, geophysicists typically arrange

seismic detectors along a straight line (profile) and

then generate sound waves by vibroseis or an explo-sion A seismograph records how long it takes the sound waves to travel to a rock layer, reflect, and re-turn to the surface The equipment is then moved

a short distance along the line, and the experiment

is repeated This procedure is known as the seismic reflection profiling method Beginning in the mid-1980’s, the advent of high-resolution seismic detec-tors and digital engineering seismographs led to ap-plications of reflection seismology to environmental, groundwater, and engineering problems, as well as to oil, gas, and mineral exploration and exploitation Using a variety of computer programs, researchers process recorded data (seismograms) to generate cross-sectional images the Earth to a depth of 3 kilo-meters or greater Such cross sections present an image of the rock layers beneath the seismic line Based on the characteristic geometries for oil and gas traps and mineral ore deposits, these images outlin-ing rock structures are used to predict where oil, natu-ral gas, coal, and other resources, such as ground-water and mineral deposits, are most likely to exist in the subsurface Geophysicists and geologists cannot tell whether oil or other resources will be found for certain, but using processed seismic data as a basis for deciding where to drill makes it much more likely that the resource will be found

Lithosphere 100 km (including crust 5-40 km) Asthenosphere

700 km

Mantle 2,885 km

Outer core 2,270 km

Inner core 1,216 km

Earth’s Interior

By using highly sensitive seismographs,

geophysi-cists can detect the changes that occur in the

ampli-tude (height) of the recorded sound waves Sound

waves change in amplitude when they are reflected

from rocks that contain gas and other fluids Such

changes appear as irregularities, called bright spots,

on the recorded sound wave patterns, and they

indi-cate the presence of fluids in underground and

un-derwater rock formations In addition, with carefully

planned seismic surveys that collect both

compres-sional and shear wave data, the fine details of seismic

records can be used to infer the types of rocks in the

subsurface

Exploitation of Oil and Other Resources

Historically, most applications of seismic technology

have been limited to exploration However, seismic

reflection data can be used not only to explore for

new oil and gas reservoirs and other resources, but

also to exploit existing reservoirs and resource

depos-its by more extensively mapping these locations in

or-der to yield optimum production Many companies

now use teams of geophysicists, geologists, and

engi-neers to plan the acquisition of data from the best

sources and analyze and integrate all the information

into a consistent description of the reservoir and/or

deposit This team approach requires that each

mem-ber understand the technology involved in obtaining

reliable, accurate data so that the best possible

infor-mation is used to estimate reservoir and/or deposit

properties

The geologic detail needed to develop most

hydro-carbon reservoirs substantially exceeds the detail

re-quired to find them For effective planning and

drill-ing, a complete understanding of the lateral extent,

thickness, and depth of the reservoir is absolutely

es-sential This understanding can be achieved with only

detailed seismic interpretation of three-dimensional

seismic surveys In three-dimensional seismic

reflec-tion surveying a common practice is to place seismic

detectors at equal intervals and collect data from a

grid of profiles (lines) covering the area of interest

As more wells are drilled in the area, the

three-dimensional data evolve into a continuously utilized

and updated management tool that affects reservoir

planning and evaluation for years after the original

seismic survey

Ultimately, knowledge of subsurface geology comes

only from drilling the targets that have been

deter-mined to be most likely to contain the resources

sought Since drilling deep drill holes is very expen-sive, applied seismic technology is a key to cost-effective exploration and exploitation of oil, natural gas, and many other natural resources

Alvin K Benson

Further Reading Burger, H Robert, Anne F Sheehan, and Craig H

Jones Introduction to Applied Geophysics: Exploring the Shallow Subsurface New York: W W Norton, 2006 Clay, Clarence S Elementary Exploration Seismology

En-glewood Cliffs, N.J.: Prentice Hall, 1990

Gadallah, Mamdouh R., and Ray L Fisher Applied Seis-mology: A Comprehensive Guide to Seismic Theory and Application Tulsa, Okla.: PennWell, 2005.

Geldart, Lloyd P., and Robert E Sheriff Problems in Ex-ploration Seismology and Their Solutions Tulsa, Okla.:

Society of Exploration Geophysicists, 2004

Kearey, Philip, Michael Brooks, and Ian Hill An Intro-duction to Geophysical Exploration 3d ed Malden,

Mass.: Blackwell Science, 2002

Robinson, Edwin S., and Cahit Çoruh Basic Explora-tion Geophysics New York: Wiley, 1988.

Shearer, Peter M Introduction to Seismology 2d ed New

York: Cambridge University Press, 2009

Sheriff, Robert E., ed Reservoir Geophysics Tulsa, Okla.:

Society of Exploration Geophysicists, 1992

Sheriff, Robert E., and L P Geldart Exploration Seis-mology 2d ed New York: Cambridge University

Press, 1995

Stein, Seth, and Michael Wysession An Introduction to Seismology, Earthquakes, and Earth Structure Malden,

Mass.: Blackwell, 2003

Web Sites Enviroscan, Inc

Seismic Refraction Versus Reflection http://www.enviroscan.com/html/

seismic_refraction_versus_refl.html University of Calgary, Lithoprobe Seismic Processing Facility

The Seismic Reflection Method http://www.litho.ucalgary.ca/atlas/seismic.html See also: Coal; Earthquakes; Geology; Landsat satel-lites and satellite technologies; Oil and natural gas drilling and wells; Oil and natural gas exploration; Oil and natural gas reservoirs

Category: Mineral and other nonliving resources

Where Found

Selenium is widely distributed in the Earth’s crust but

does not occur in ore deposits of sufficient

concentra-tion to permit direct mining In nature, selenium is

principally found with metal sulfides Seleniferous

soils—soils with high selenium concentrations—are

found in Canada, the United States, Mexico,

Colom-bia, and Ireland

Primary Uses

Selenium’s most common industrial application is in

the glass industry It is also used as a nutritional

sup-plement in domesticated animals such as poultry,

cat-tle, and swine

Technical Definition

Selenium (abbreviated Se), atomic number 34,

be-longs to Group VI of the periodic table of the elements

and resembles sulfur in its chemical and physical

prop-erties It has six naturally occurring isotopes and an

av-erage molecular weight of 78.96 Pure selenium has

gray, crystalline, and red forms Its density is 4.79 grams

per cubic centimeter; it has a melting point of 217°

Cel-sius and a boiling point of 685.4° CelCel-sius

Description, Distribution, and Forms

Selenium is a widely distributed element of volcanic

origin that has chemical properties resembling sulfur

It occurs as inorganic oxides such as selenate and

sele-nite, as elemental selenium, and as selenide,

depend-ing on the alkalinity and aeration of the environment

Selenium also has a variety of soluble and volatile

or-ganic forms, such as dimethyl selenide and

selenome-thionine, an amino acid analog Most selenium is

com-bined with metals, as in ferroselite and challomenite,

or appears as a trace contaminant in metal sulfides

such as galena and pyrite, where it replaces sulfur

be-cause of their similarity in size Coal and oil deposits

also have appreciable selenium contents

Soluble selenium forms are used as nutritional

sup-plements for mammals, since selenium is an essential

trace element Selenium also plays a crucial role as an

antioxidant in the enzyme glutathione peroxidase,

and it contributes to the activity of vitamin E High

concentrations of selenium are toxic, however, and

some notable instances of widespread animal death have occurred when human activity made selenium more available for plants and animals to absorb On a commercial scale, about 1,600 metric tons of sele-nium are produced annually (1,560 in 2007, 1,590 in 2008), but this amount is dwarfed by the selenium re-leased into soil, air, and water that occurs as a result of general industrial activity

The highest average selenium concentrations oc-cur in organic deposits such as coal (3.4 milligrams

of selenium per kilogram) and oil shale (2.3 milli-grams of selenium per kilogram) However, selenium

is found in virtually all materials on Earth at low con-centrations Average concentrations of selenium in crustal material range from 0.05 to 0.14 part per mil-lion (milligrams per kilogram) Areas where selenium concentrations are low (for example, the Pacific Northwest or the northeastern United States) occur where the underlying sedimentary rock predates the Cretaceous period

Certain soils are called “seleniferous” because they formed in material with elevated selenium levels These soils typically developed from shale that was formed in the Cretaceous period In the United States, these soils were deposited primarily in South Dakota, Montana, Wyoming, Nebraska, Kansas, Utah, Colo-rado, and New Mexico Seleniferous soils tend to have selenium concentrations ranging from 1 to more than 80 milligrams of selenium per kilogram of soil, and it is in areas with these soils that selenium toxicity has historically occurred Seleniferous soils also occur

in Canada (in the provinces of Alberta, Manitoba, and Saskatchewan), Mexico, Hawaii, Colombia, China, and Ireland Wells in seleniferous soils can contain

as much as 1 milligram of selenium per liter

The major available forms of inorganic selenium found in well-aerated alkaline soils are selenate, sele-nite, and elemental selenium Selenate and selenite are water soluble and can leach out of soil Elemental selenium is relatively insoluble In poorly drained envi-ronments, selenium will be found as selenide, usually combined with some type of metal such as lead, cop-per, or iron These are relatively immobile forms of se-lenium When selenides are exposed to air, however, the selenium reoxidizes to form selenates and selen-ites, which are much more easily taken up by plants Some plants accumulate selenium Examples are

milk vetch (Astragalus), goldenweed (Haplopappus), prince’s plume (Stanleya), and woody aster (Xylor-hiza) The milk vetch can accumulate up to 20 grams

of selenium per kilogram of tissue The presence

of these plants in an area is sometimes an indication

that the soil may contain high selenium

concentra-tions In selenium-accumulating plants, the selenium

is found as a water-soluble compound such as selenium

methylselenosysteine In plants that do not

accumu-late selenium, the form is usually selenomethionine

Selenium can also take the place of sulfur in

sulfur-containing organic compounds

Selenium would not have an impact on other

natu-ral resources, except in localized areas, were it not for

human activity, such as disposing of fly ash from

coal-burning power plants and irrigating arid alkaline

soils The amount of selenium released each year by

industrial activity is approximately fifteen times more

than that which is released naturally

Selenium is usually a minor constituent of drinking water, appearing in concentrations ranging from less than 0.1 to 100 micrograms per liter The upper limit for the allowable selenium content of drinking water

in the United States was set at 10 micrograms per liter

by the 1974 Safe Drinking Water Act Wells from seleniferous soils in Colorado and Montana, however, can contain as much as 1 milligram of selenium per liter

Wildlife is a vital natural resource that can be ad-versely affected by selenium as a direct result of hu-man activity redistributing selenium to excess in the environment A notable example of this occurred in the Kesterson Reservoir in the San Joaquin Valley of California The Kesterson Reservoir was a series of twelve shallow ponds that were designed to be an

Data from the U.S Geological Survey, U.S Government Printing Office, 2009.

75 60

15

840 75

65

20

Withheld

120

Metric Tons of Selenium Content

900 750

600 450

300 150

United States

Peru

Japan

India

Finland

Chile

Philippines

Sweden

Other countries

200 120

Canada

Belgium

U.S data were withheld to avoid disclosure of company proprietary data.

Note:

Selenium: World Refinery Production, 2008