Encyclopedia of Global Resources part 103 pdf

Plate Boundaries and Motion Geophysical data, geological observations, and theo-retical deductions support the existence of three ba-sic types of plate boundaries: divergent boundaries,

Plate tectonics

Category: Geological processes and formations

The theory of plate tectonics provides an explanation

for the present-day structure of the outer part of the

Earth It provides a framework for understanding the

global distribution of mountain building, earthquake

activity, and volcanism; the geology of ocean basins;

various associations of igneous, metamorphic, and

sedimentary rocks; and the formation and location of

mineral resources.

Background

Plate tectonic theory is based on a concept of the

Earth in which a rigid, outer shell, the lithosphere,

lies above a hotter, weaker, partially molten part of the

mantle known as the asthenosphere The thickness of

the lithosphere varies between 50 and 150 kilometers,

and it consists of crust and the underlying upper

man-tle The asthenosphere extends from the base of the

lithosphere to a depth of about 700 kilometers The

brittle lithosphere is broken into a pattern of

inter-nally rigid plates that move horizontally across the

Earth’s surface relative to each other Seven major

plates and a number of smaller ones have been

distin-guished, and they grind and scrape against one

an-other as they move independently, similar to chunks

of ice on water Most of the Earth’s dynamic activity,

including earthquakes and volcanism, occurs along

plate boundaries, and the global distribution of these

tectonic phenomena delineates the boundaries of the

plates

Plate Boundaries and Motion

Geophysical data, geological observations, and

theo-retical deductions support the existence of three

ba-sic types of plate boundaries: divergent boundaries,

where adjacent plates move apart (diverge) from each

other; convergent boundaries, where adjacent plates

move toward each other; and transform boundaries,

where plates slip past one another in a direction

paral-lel to their common boundary The velocity with

which plates move varies from plate to plate and

within portions of the same plate, ranging from two to

twenty centimeters per year This rate is determined

from radioactive dating estimates of the age of the

seafloor as a function of distance from mid-oceanic

ridge crests (seafloor spreading ridges)

Divergent Plate Boundaries

At mid-oceanic ridges, or divergent plate boundaries, new seafloor is created from molten basalt (magma) rising from the asthenosphere A great deal of volca-nic activity thus occurs at divergent boundaries Be-cause of the pulling apart (rifting) of the plates of lithosphere, earthquake activity will also occur along divergent boundaries, and since the rift is caused by magma rising from the mantle, the earthquakes will

be frequent, shallow, and mild

Plate Plate

Asthenosphere

Divergent Boundary

Plate

Plate Asthenosphere

Transform Fault Boundary

Plate Plate

Asthenosphere

Convergent Boundary

Plate Boundaries

An example of continental rifting (divergence) in

its embryonic stage is seen in the Red Sea, where the

Arabian plate has separated from the African plate,

creating a new oceanic ridge Another modern-day

example is the East African Rift system, which is the

site of active rifting If it continues, it will eventually

fragment Africa, and an ocean will separate the

result-ing pieces Through divergence, or riftresult-ing, large plates

are broken up into smaller ones

Convergent Plate Boundaries

Because the Earth is neither expanding nor

contract-ing, the increase in lithosphere created along

diver-gent boundaries must be compensated for by the

de-struction of lithosphere elsewhere Otherwise the

radius of the Earth would change At convergent plate

boundaries, plates are moving together, and three

scenarios are possible depending on whether the

crust of the lithosphere is oceanic or continental

If both converging plates are made of oceanic crust, one will inevitably be older, and thus cooler and denser than the other plate The denser plate will plunge (sub-duct) below the less-dense plate and descend down into the asthenosphere This type of plate boundary is called a subduction zone, and the boundary along the two interacting plates forms a trench The subducted plate is heated by the hot asthenosphere and, in time, becomes hot enough to melt Some of the melted ma-terial rises buoyantly through fissures and cracks to form volcanoes on the overlying plate, whereas other parts of the melted material will eventually migrate to and rise again at a divergent boundary (spreading ridge) Thus the oceanic lithosphere is constantly being recycled The volcanoes along the overriding plate may form a string of islands called island arcs Ja-pan, the Aleutians, and the Marianas are good exam-ples of island arcs resulting from subduction of two plates consisting of oceanic lithosphere

Types of Boundaries: Divergent Convergent Transform

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▲

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Plate

Pacific Plate

North American Plate

South American Plate Indo-Australian

Plate

Antarctic Plate

African Plate

Major Tectonic Plates and Mid-Ocean Ridges

If the leading edge of one of the two convergent

plates is oceanic crust but the other leading edge is

continental crust, the subduction differs from the

case above Since continental crust is less dense than

oceanic, the oceanic plate is always the one

sub-ducted A classical example of this case is the western

boundary of South America On the oceanic side of

the boundary, a trench is formed, where the oceanic

plate plunges underneath the continental plate On

the continental side, a fold mountain belt (the

An-des) is formed as the oceanic lithosphere pushes

against the continental lithosphere As the oceanic

plate descends into the mantle, some of the material

melts and works its way up through the fold mountain

belt to form violent volcanoes The boundary

be-tween the plates is a region of earthquake activity, with

the earthquakes ranging from shallow to relatively

deep, and some are quite severe

The last type of convergent plate boundary involves

the collision of two continental masses of lithosphere

When the plates collide, neither is dense enough to be

forced into the asthenosphere Thus the collision

com-presses and thickens the continental edges, twisting

and deforming the rocks and uplifting the land to form

unusually high fold mountain belts The prototype

ex-ample is the collision of India with Asia that resulted in

the formation of the Himalayas In this case the

earth-quakes are typically shallow, but frequent and severe

Transform Plate Boundaries

The actual structure of a seafloor spreading ridge is

more complex than a single straight crack Rather,

ridges consist of many short segments slightly offset

from one another The offsets are a special kind of

fault, or break in the lithosphere, known as a

trans-form fault, and their function is to connect segments

of a spreading ridge The opposite sides of a

trans-form fault belong to two different plates, and these

are moving apart in opposite directions The

trans-form faults are just boundaries along which the plates

move past one another The classic transform

bound-ary is the San Andreas fault that slices off a sliver of

western California that rides on the Pacific plate from

the rest of the state, which is on the North American

plate As the two plates scrape past each other, stress

builds up and is released in earthquakes

Why Plates Move

One mechanism that creates energy to move the huge

plates is convection currents that are driven by heat

from radioactive decay in the mantle These convec-tion currents in the Earth’s mantle carry magma up from the asthenosphere Some of this magma escapes

to form new lithosphere, but the rest spreads out side-ways beneath the lithosphere, slowly cooling in the process As it flows outward, it drags the overlying lithosphere outward with it, thus continuing to open the ridges When it cools, the flowing material be-comes dense enough to sink back deeper into the mantle at convergent boundaries A second plate-driving mechanism is the pull of the dense, cold, downward-moving slab of lithosphere in a subduction zone on the rest of the trailing plate, opening up the spreading ridges so magma can move upward Mineral Deposits

The theory of plate tectonics has greatly enhanced understanding of why many mineral deposits form where they do and has thus made mineral exploration more efficient During the evolution of new oceanic plates and mountain belts by plate tectonics, a large number of mineral deposits form, particularly in asso-ciation with the plate boundaries

Hot fluids (hydrothermal fluids) circulate at spread-ing ridges (divergent boundaries) and deposit miner-als For example, niobium deposits are found in the intrusions in the East African Rift zone, and iron and manganese are found in the sediments of the Red Sea Hydrothermal fluids also flow through the cracks and pores in rock along convergent boundaries and de-posit metals along these boundaries as they cool Good examples are the copper ore deposits associ-ated with the collisional boundary of the Himalayas and tin ores in southwestern England A general se-quence of minerals found when passing inland from a trench associated with subduction is iron, gold, cop-per, molybdenum, gold, lead, zinc, tin, tungsten, anti-mony, and mercury

Alvin K Benson

Further Reading

Brown, G C., and A E Mussett The Inaccessible Earth:

An Integrated Approach to Geophysics and Geochemistry.

2d ed New York: Chapman & Hall, 1993

Cox, Allan, and Robert Brian Hart Plate Tectonics: How It Works Palo Alto, Calif.: Blackwell Scientific,

1986

Erickson, Jon Plate Tectonics: Unraveling the Mysteries of the Earth Rev ed New York: Facts On File, 2001.

Hamblin, W Kenneth, and Eric H Christiansen

Earth’s Dynamic Systems 10th ed Upper Saddle

River, N.J.: Pearson/Prentice Hall, 2004

Kearey, Philip, Keith A Klepeis, and Frederick J Vine

Global Tectonics 3d ed Hoboken, N.J.:

Wiley-Blackwell, 2009

Keller, Edward A., and Nicholas Pinter Active

Tecton-ics: Earthquakes, Uplift, and Landscape 2d ed Upper

Saddle River, N.J.: Prentice Hall, 2002

Kusky, Timothy Earthquakes: Plate Tectonics and

Earth-quake Hazards New York: Facts On File, 2008.

Montgomery, Carla W Fundamentals of Geology 3d ed.

Dubuque, Iowa: Wm C Brown, 1997

Oreskes, Naomi, ed Plate Tectonics: An Insider’s History

of the Modern Theory of the Earth Boulder, Colo.:

Westview Press, 2001

Van der Pluijm, Ben A., and Stephen Marshak Earth

Structure: An Introduction to Structural Geology and

Tectonics 2d ed New York: W W Norton, 2003.

Web Site

U.S Geological Survey

This Dynamic Earth: The Story of Plate Tectonics

http://pubs.usgs.gov/gip/dynamic/dynamic.html

See also: Earth’s crust; Earthquakes; Geology;

Hy-drothermal solutions and mineralization;

Litho-sphere; Seafloor spreading; Volcanoes

Platinum and the platinum group

metals

Category: Mineral and other nonliving resources

Where Found

The platinum metals are extremely rare in the Earth’s

crust All occur together, with platinum and

palla-dium predominating The mineral sperrylite

(plati-num arsenide) is a major source in Canada

Signifi-cant deposits are also located in South Africa and the

former Soviet Union Smaller deposits have been

found in Colombia, Australia, and the United States,

chiefly in Alaska and Montana

Primary Uses

The most common application of the platinum

met-als is as catalysts for various industrial chemical

reac-tions They are also used to make a variety of alloys

and are frequently used in jewelry

Technical Definition Chemists generally refer to the block of six transition metals—ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), and platinum (Pt)—

as the platinum metals Their atomic numbers are, re-spectively, 44, 45, 46, 76, 77, and 78 Using the recom-mended group designations of the International Union of Pure and Applied Chemistry, ruthenium and osmium belong to Group 8, rhodium and iridium

to Group 9, and palladium and platinum to Group 10

In the older system of group numbering, the plati-num metals were placed in Group VIII

Description, Distribution, and Forms Ruthenium has seven naturally occurring isotopes with an average atomic mass of 101.07 It has another thirteen artificial (radioactive) isotopes Pure ruthe-nium is a hard metal and has a gray-white appearance Rhodium has only one natural isotope, with an atomic mass of 102.906 More than thirty artificial isotopes are known Rhodium has a silvery white metallic luster Palladium has six natural isotopes with an aver-age atomic mass of 106.42 It has eighteen artificial isotopes Palladium is a steel-white metal that does not tarnish in air Osmium has seven natural isotopes and

an average atomic mass of 190.2 It has nearly thirty ar-tificial isotopes The metal has a slight bluish color be-cause of a thin surface film of the oxide Iridium has only two natural isotopes, with an average atomic mass

of 192.2 It has nearly forty artificial isotopes The metal has a white appearance with a slight yellowish tinge and is hard and brittle Platinum has six natural isotopes and an average atomic mass of 195.08 It has thirty artificial isotopes It is a silvery-white metal with

a lustrous appearance Ruthenium, rhodium, and pal-ladium all have densities of about 12 grams per cubic centimeter (12.45, 12.41, and 12.02, respectively), while osmium, iridium, and platinum are about twice

as dense (22.61, 22.65, and 21.45 grams per cubic cen-timeter, respectively) The melting points increase

in the order: palladium, platinum, rhodium, ruthe-nium, iridium, and osmium—1,554°, 1,772°, 1,966°, 2,310°, 2,410°, and 3,054° Celsius, respectively The boiling points increase in the order: palladium, rho-dium, platinum, ruthenium, irirho-dium, and osmium— 3,140°, 3,727°, 3,827°, 3,900°, 4,130°, and 5,027° Cel-sius, respectively

The platinum metals are among the rarest of all nonradioactive elements in the Earth’s crust As a group, they are strong siderophiles (they tend to be

concentrated in the Earth’s metallic core)

Conse-quently, they are generally found in areas rich in other

transition metals such as nickel and copper In these

concentrated regions, the abundance of the platinum

metals can be more than a million times that of the

crustal average Most of the world’s platinum resources

come from sulfide ores of magmatic origin found in

large stratiform bodies of basaltic rocks The annual

world production of all the platinum metals totals only

about 500 metric tons By contrast, millions of metric

tons of copper are produced worldwide annually Since

the quantity of platinum metals mined is

compara-tively small, the environmental impact of these metals

is minimal None of the six metals has any significant

biological role in the plant or animal kingdoms

As a group, the platinum metals have the lowest

abundances of nearly all nonradioactive elements in

the Earth’s crust; only gold, rhenium, and bismuth

are metals with comparable low abundances Values

range from 0.0001 part per million (ppm) for

ruthe-nium to about 0.015 ppm for palladium In regions

where the platinum metals are concentrated

(Can-ada, South Africa, and the former Soviet Union),

lev-els of platinum reach 0.5 part to 20 parts per million

However, the platinum metals frequently occur in

ores that contain large quantities of other metals,

such as nickel, making recovery of the platinum

met-als commercially feasible The low crustal abundances

of the platinum metals and the fact that their minerals

usually occur as small inclusions (less than 1

millime-ter) in other minerals hindered the development of

platinum metals mineralogy With the development

of the electron microprobe in the 1960’s and its ability

to analyze mineral particles as small as 10

microme-ters, the mineralogy of the platinum metals was

greatly enhanced More than eighty clearly defined

minerals containing the platinum metals have been

identified, most of which contain palladium and

plati-num These minerals are generally compounds with

other elements such as sulfur, selenium, tellurium,

ar-senic, and antimony, or alloys with metals such as tin,

lead, and bismuth Several hundred less clearly

de-fined minerals have also been detected

In the Canadian deposits, platinum occurs in

copper-nickel sulfide ores that are associated with the

igneous rock norite The South African ores are

pre-dominantly pyroene as well as chromite and sulfides

of iron, copper, and nickel Platinum is also found in

native metallic form alloyed with iron or in mineral

form as the sulfide or arsenide Iridium, osmium,

ru-thenium, and rhodium generally occur uncombined

in nature; they can also be considered by-products of the transition metals mining industry Osmium and iridium occur alloyed as iridosmine (also known as os-miridium) This alloy also contains varying amounts

of platinum, ruthenium, and rhodium, depending on the location

Palladium is the most reactive of the platinum met-als, and it readily dissolves in acids At room tempera-ture it has the unusual property of absorbing up to nine hundred times its own volume of hydrogen Pal-ladium is highly malleable and can be beaten into sheets as thin as 0.000002 centimeter thick Iridium has the greatest resistance to corrosion of any metal It was used to make the old standard meter bar in Paris, which was an alloy of platinum (90 percent) and irid-ium (10 percent) Iridirid-ium levels in certain regions have been related to meteor impacts on Earth and have been used to study geological and biological pro-cesses such as extinction Levels of iridium in meteors are generally higher than levels found on Earth High terrestrial iridium levels in rocks from the Cretaceous-Tertiary boundary have provided evidence that exten-sive meteor impacts could have played a role in the Earth’s geologic history

History Hundreds of years before Europeans explored the Americas, the Indians of Colombia and Ecuador used platinum-gold alloys to make small artifacts by

Platinum and Paladium: World Mine

Production, 2008

Kilograms

Source: Data from the U.S Geological Survey, Mineral Commodity Summaries, 2009 U.S Government Printing

Office, 2009.

ing and hammering the alloy Because of platinum’s

high melting point, it was not possible for these

peo-ple to melt and work pure platinum In their

relent-less search for gold in the late seventeenth century,

the invading Spanish conquistadores discovered the

Indians’ platinum Being a white-colored metal it was

called platina, derived from the Spanish word for

sil-ver Initially it was considered a rather annoying

con-taminant rather than a precious metal By the

mid-eighteenth century, samples of platinum had reached

Europe In 1803, William Wollaston produced the

first pure samples of the metal after dissolving crude

platinum in aqua regia (a mixture of hydrochloric

and nitric acids)

In the same year, Wollaston’s studies with platinum

ores led to his discovery of two new metals, palladium

and rhodium, in the samples of crude platinum After

dissolving the ore in aqua regia and neutralizing it

with sodium hydroxide, he added ammonium

chlo-ride to remove the platinum as ammonium

chloro-platinate By then adding mercuric cyanide, he

re-moved the palladium as palladium cyanide Metallic

palladium was recovered by reduction of the

palla-dium cyanide compound After the pallapalla-dium

cya-nide was extracted, the residue was washed and dried;

it yielded a red compound of rhodium, which was

re-duced to the metal itself Because many of the

rho-dium compounds Wollaston prepared were pink to

red in color, he named the new metal from the Greek

word rhodon, meaning rose Palladium was named

af-ter the asaf-teroid Pallas

Both osmium and iridium were also discovered in

1803 In London, Smithson Tennant showed that the

black metallic substance remaining after reacting

platinum ores with aqua regia was actually a mixture

of two new metals He named one iridium (from the

Latin iris, meaning rainbow) because it formed many

colored compounds The other new metal he called

osmium (from the Latin osme, meaning odor) because

of its unpleasant smell

Ruthenium was the last platinum metal to be

dis-covered In 1808, the Polish chemist Jòdrzej Kniadecki

claimed to have discovered and extracted a new metal

from platinum ores Since others were unable to

re-produceKniadecki’s work, his discovery was soon

dis-missed In the mid-1820’s, extensive alluvial deposits

of platinum were discovered in the Russian Ural

Moun-tains Soon after, platinum coins were minted and

is-sued by the Russian government As a result of the

new mining industry, scientists began to examine the

insoluble residues that were produced from the plati-num refining In 1828, Gottfried Osann claimed to have discovered three new metals in these residues However, it was not until 1844, when Karl Ernst Klaus showed that there was only one new metal in the resi-dues, that ruthenium was actually isolated and shown

to be a new element Its name was taken from Ruthenia,

the Latin name for Russia

Obtaining Platinum Metals Because of platinum metals’ low natural abundances and the difficulty in extracting them, commercial pro-duction of the platinum metals is often viewed as a by-product of the mining of other metals such as nickel, copper, and silver For example, if not for the huge tonnage of nickel ore processed annually, the extrac-tion of the rarer platinum metals would not be eco-nomically feasible In general, the platinum metals are obtained by subjecting the ores to a series of com-plicated and costly chemical reactions Not surpris-ingly, the platinum metals are among the most expen-sive of all elements to produce Prices can fluctuate enormously depending on economic and environ-mental conditions The high cost and rarity of the platinum metals are also responsible for their exten-sive recycling

Platinum is obtained from crude ores by a process which eliminates other impurities: Magnetic metals such as iron and nickel are removed with powerful electromagnets; less dense impurities are removed by flotation methods in aqueous solution; volatile impu-rities are baked off at high temperatures; various acids dissolve away other metals Pure platinum is obtained through additional chemical processes The method for separating palladium from platinum is often de-termined by the type of ore being refined, but in gen-eral also involves a series of chemical processes to ob-tain the metal Like platinum, iridium is separated by treating the other accompanying platinum metals as impurities and removing them stepwise Treatment with molten lead, followed by aqua regia, and then baking at 2,000° Celsius concentrates iridium Ruthe-nium and osmium are converted into highly volatile (and toxic) tetroxide compounds that can easily be collected by distillation Reaction with base converts them to safer substances, such as sodium osmate, which are then reduced to the metal Rhodium is ob-tained from the residue remaining after the removal

of platinum, and the total world production of this rare metal is only a few metric tons annually

Uses of Platinum Metals

Of the six platinum metals, palladium and platinum

have the greatest economic importance Both are

fairly soft metals having a brilliant silvery appearance

and are therefore widely used in the jewelry trade

When alloyed with palladium, gold takes on a silvery

appearance (white gold) but will not tarnish as

jew-elry made from pure silver does Palladium is also

used in dentistry, surgical instruments, and electrical

contacts The mainsprings of many older wristwatches

were fashioned from palladium Powdered palladium

is a good catalyst and is used for hydrogenation and

dehydrogenation reactions Platinum is used to make

wires and vessels for laboratory use and as a coating on

missile nose cones and jets, which are subject to very

high temperatures It is also used to make medical

and dental alloys and electrical contacts Finely

di-vided platinum powder is an excellent catalyst that is

used in the production of sulfuric acid and in

petro-leum refining

The uses of the other four platinum metals are very

limited Their major use is in alloys, and most have

some catalytic activity All are fairly brittle metals and

therefore difficult to machine into shapes when pure

Ruthenium, rhodium, and iridium are all used as

hardening agents for softer platinum and palladium

Osmium is used to strengthen alloys where frictional

wear must be minimized as in electrical switch

con-tacts, ballpoint pen tips, phonograph needles, and

in-strument pivots Rhodium lends itself readily to

elec-troplating and has been used to protect silver objects

from tarnishing, on optical instruments, and on

high-grade reflectors for searchlights Because of its

resis-tance, iridium has been used for spark-plug

elec-trodes in aircraft engines When alloyed with other

metals, such as titanium, the presence of platinum

metals can enhance corrosion resistance Literally

thousands of chemical compounds that contain the

platinum metals have been prepared, and many play

important roles as industrial catalysts Considerable

research has revealed that some platinum compounds

can inhibit the growth of certain tumors and

there-fore have applications in chemotherapy

Nicholas C Thomas

Further Reading

Greenwood, N N., and A Earnshaw “Nickel,

Palla-dium, and Platinum.” In Chemistry of the Elements 2d

ed Boston: Butterworth-Heinemann, 1997

Heiserman, David L Exploring Chemical Elements and

Their Compounds Blue Ridge Summit, Pa.: Tab

Books, 1992

Lide, David R., ed CRC Handbook of Chemistry and Phys-ics: A Ready-Reference Book of Chemical and Physical Data 85th ed Boca Raton, Fla.: CRC Press, 2004 McDonald, Donald, and Leslie B Hunt A History of Platinum and Its Allied Metals London: Johnson

Matthey, 1982

Weeks, Mary Elvira Discovery of the Elements 7th ed.

New material added by Henry M Leicester Easton, Pa.: Journal of Chemical Education, 1968

Web Sites Natural Resources Canada Canadian Minerals Yearbook, Mineral and Metal Commodity Reviews

http://www.nrcan-rncan.gc.ca/mms-smm/busi-indu/cmy-amc/com-eng.htm

U.S Geological Survey Platinum-Group Metals: Statistics and Information http://minerals.usgs.gov/minerals/pubs/

commodity/platinum See also: Alloys; Canada; Metals and metallurgy; Na-tive elements; Nickel; Russia

Plutonic rocks and mineral deposits

Categories: Geological processes and formations; mineral and other nonliving resources

Plutonic rocks are formed by slow magma crystalliza-tion below the Earth’s surface Erosion can expose plutonic formations containing valuable mineral de-posits, including metallic resources vital to industry.

Background

Plutonic rocks (from Pluto, Roman god of the

under-world) form by slow crystallization of molten silicate magma intruded below the Earth’s surface Subse-quent erosion may expose “plutons” that have the area of a large house or huge granitic “batholiths” that encompass thousands of square kilometers Many plutons are sources of valuable industrial and metallic mineral deposits, including granite building stone, gold, copper, molybdenum, chromium, and other metals Plutonic bodies occur on all the major

nents of the Earth They are common in Precambrian

shield areas, the ancient cores of continents

com-posed of rocks formed billions of years ago, such as

the Canadian Shield of North America They also

oc-cur in younger mountain ranges such as the Rockies

and the Appalachian Mountains of North America

Most of the major metallic resources consumed

by modern industries—and thus many commercial

products—have their origin in plutonic rocks The

rocks themselves may be cut as building stones or used

for monuments and art objects Plutons are also an

important source of metal ores, including gold, silver,

platinum, chromium, copper, molybdenum, lithium,

beryllium, and nickel

Plutonic rocks crystallize from igneous intrusions

of molten magma that cool slowly beneath the Earth’s

surface Plutonic intrusions underlie many volcanic

areas where magma has made its way to the surface

Later erosion by water or glacial scouring removes

most or all traces of the overlying volcanic material

and other overburden to expose the pluton at the

sur-face Plutonic igneous rocks can be distinguished

from volcanic rocks by their grain size: The slowly

cooled plutonic rocks allow for relatively large grain

sizes (from a few millimeters to several centimeters in

diameter) compared with very fine-grained, quickly

cooled volcanic rocks

Types of Plutons and Plutonic Rocks

Major plutonic rock bodies in the world can be

di-vided into two major compositions based mainly on

silica (SiO2) content: “felsic-intermediate” rocks, in

which silica content ranges from about 52 to more

than 70 percent (examples include granite and

diorite), and “mafic-ultramafic” rocks, in which silica

is much lower, 52 down to about 45 percent SiO2

(ex-amples are gabbro and peridotite)

Light-colored felsic-intermediate, or “granitic,”

plutons are the most common type and include

rela-tively small “stocks” (surface exposures of tens of

square kilometers) up to immense “batholiths” that

cover hundreds or thousands of square kilometers

The largest of these in North America are the Idaho,

Boulder (Montana), Sierra Nevada (California), and

Southern California batholiths Many large stocks and

batholiths also occur in the Canadian Shield of Canada

and the northern states of Minnesota, Wisconsin, and

New York Granitic plutons also occur in every

conti-nental shield area (ancient core) of every continent

and in most of the world’s major mountain ranges

Less common are the iron-rich, dark-colored mafic-ultramafic plutons Most of these bodies consist of rel-atively iron-rich basaltic magma (like the dark lava flows in Hawaii) that, upon intrusion, crystallize dense minerals that settle to the bottom of the magma cham-ber to form mineralogically distinct layers These lay-ered rocks may contain economically important ore minerals Prominent examples are the Muskox intru-sion (Canada), the Skaergaard complex (Greenland), the Stillwater complex (Montana), and the metallic ore-rich intrusions in South Africa (Bushveld, Great Dyke)

Ore Deposits in Felsic-Intermediate Plutons Granitic plutons are the source of many valuable min-eral commodities, either directly from the rock itself

or indirectly as placer minerals (for example, gold or cassiterite in stream sediments) Some important sed-imentary ore deposits form by deposition of minerals leached from granitic plutons by groundwater (for example, many uranium ores) Ore minerals may also occur in hydrothermal (deposited by hot water) quartz veins as in the case of native gold These veins may occur within the plutons themselves or penetrate adjacent “country rocks.” Most North American cop-per mines (principally in New Mexico, Arizona, and Utah) obtain their ore from so-called porphyry cop-per deposits These are low-grade deposits (about 0.65 percent copper) of widely disseminated copper sulfide grains within the granitic pluton or in adjacent rocks mined from large open-pit mines Typical of these mines is the Kennecott Utah Copper mine at Bingham, Utah, the largest copper mine in the world Perhaps the richest source of valuable minerals in granitic plutons is pegmatite deposits, generally small exposures of large crystals, the largest measuring many meters in diameter Pegmatites are the source of many rare metals (beryllium, lithium, zirconium, bo-ron, tantalum, and niobium) and some valuable gem-stones

Ore Deposits in Mafic-Ultramafic Plutons Because the magma associated with these plutons is low in viscosity (“thin”) compared with felsic-interme-diate bodies, many important ore deposits in mafic-ultramafic intrusions arise by gravity settling of miner-als South African chromite deposits, for example, form as thick-layered accumulations of crystallized chromite grains that have settled on the floor of the plutonlike sand falling through thick motor oil Such

ore-rich layers are called “cumulates” (from

“accumu-lation”), and include platinum-palladium deposits

commonly associated with chromite layers

Similar layered ore bodies involve the formation of

large blobs of dense sulfide liquids that separate from

the silicate liquid and accumulate on the pluton floor

These “segregation” deposits include some of the

richest nickel and copper mines in the world,

includ-ing the nickel mine at Sudbury, Ontario, the Duluth

complex in Minnesota, and the Bushveld of South

Af-rica Gold, silver, and other valuable metals are also

mined from these rich deposits Some iron and

tita-nium deposits were created this way as well but involve

the segregation of titanium or iron-rich fluids in mafic

plutons, which eventually crystallize the mineral

mag-netite (Fe3O4; iron ore) or ilmenite (FeTiO3; titanium

ore) An example of this type of iron deposit is the

Kiruna district in Sweden Allard Lake, Quebec, is a

good example of a segregation titanium mine

John L Berkley

Further Reading

Best, Myron G Igneous and Metamorphic Petrology 2d

ed Malden, Mass.: Blackwell, 2003

Best, Myron G., and Eric H Christiansen Igneous

Pe-trology Malden, Mass.: Blackwell Science, 2001.

Jensen, Mead L., and Alan M Bateman Economic

Min-eral Deposits 3d ed New York: Wiley, 1979.

McBirney, Alexander R Igneous Petrology 3d ed

Bos-ton: Jones and Bartlett, 2007

Philpotts, Anthony R., and Jay J Ague Principles of

Ig-neous and Metamorphic Petrology 2d ed New York:

Cambridge University Press, 2009

Young, Davis A Mind over Magma: The Story of Igneous

Petrology Princeton, N.J.: Princeton University

Press, 2003

Web Site

U.S Geological Survey

Volcanic Rocks

http://volcanoes.usgs.gov/images/pglossary/

VolRocks.php

See also: Chromium; Copper; Earth’s crust; Gold;

Granite; Hydrothermal solutions and mineralization;

Igneous processes, rocks, and mineral deposits;

Lith-ium; Magma crystallization; Molybdenum; Nickel;

Pegmatites; Placer deposits; Platinum and the

plati-num group metals; Rare earth elements; Tantalum;

Tin; Titanium; Tungsten; Zirconium

Plutonium

Categories: Energy resources; mineral and other nonliving resources

Plutonium is both very useful and very dangerous It contributes a significant percentage of the power pro-duced in nuclear reactors, but it is also a radiological poison and a nuclear explosive.

Background Plutonium (abbreviated Pu), element number 94 in the periodic table, is a silvery-white metal that oxidizes readily Normally hard and brittle, it can be molded and machined if it is alloyed with gallium (0.9 percent

by weight) Plutonium has fifteen known isotopes, ranging from plutonium 232 to plutonium 246; all of them are radioactive Half-lives range from twenty-one minutes for plutonium 233 to eighty-twenty-one million years for plutonium 244 The most abundant isotope

is plutonium 239, which has a half-life of 24,390 years Minuscule amounts of plutonium 244 occur naturally

in uranium ore, but the only way to obtain usable amounts is to make plutonium in a nuclear reactor Plutonium is radiotoxic: It harms by radiation Plu-tonium primarily decays by emission of an alpha parti-cle (a helium 4 nuparti-cleus, a grouping of two neutrons and two protons) Nonetheless, a grape-sized pluto-nium sample could be safely held in the hand, even though it would feel warm because of its radioactivity Plutonium’s alpha particle radiation is easily blocked

by the outermost layers of a person’s skin Further-more, plutonium is not easily absorbed by the body If plutonium were ingested with food or water, almost all of it would be excreted However, 4 parts per 10,000 might be absorbed and eventually settle in the liver or bones, where the plutonium might produce cancer tens of years later The maximum long-term body burden of plutonium 239 believed to be safe is less than one microgram

The most toxic form of plutonium is thought to be fine (10 microns in diameter) airborne particles of plutonium oxide If inhaled, a significant fraction of such particles could be expected to lodge in the lungs Estimates based on animal studies suggest that 10 mil-ligrams of plutonium particles lodged in the lungs could cause death in about one month For compari-son, doses a thousand times smaller of anthrax spores, botulism, or coral snake venom will cause death within

a few hours or days With proper precautions,

pluto-nium can be handled safely More than fifty years of

monitoring plutonium workers at United States

nu-clear weapons plants has not found any workers who

have suffered serious consequences

Plutonium as a Fuel

A common type of nuclear power reactor contains

natural uranium that has been enriched in the

iso-tope uranium 235 When struck by a neutron, the rare

uranium 235 may fission to produce two daughter

nu-clei, a few neutrons, and energy Plutonium 239 is

formed when the more common uranium 238

iso-tope absorbs a neutron If left in the reactor,

pluto-nium 239 may also absorb a neutron and either fission

or become plutonium 240 Over half of the

pluto-nium produced in a power reactor does fission, and

this fission contributes about one-third of the total

en-ergy produced in the reactor

It takes nearly 3 million metric tons of coal to

pro-duce the same amount of energy as 1 metric ton of

plutonium 239 The world stock of civilian plutonium

is approximately 1,000 metric tons Eighty percent of

it is tied up in used reactor fuel elements

Plutonium in Nuclear Weapons The bomb dropped on Nagasaki, Japan, by the United States in World War II contained 6.1 kilograms of plu-tonium and had an explosive yield equal to almost 20,000 metric tons of dynamite The much greater yield of a hydrogen bomb is triggered by detonating a small plutonium bomb

The military organizations of the world possess about 250 metric tons of plutonium With the ending

of the Cold War, the U.S Department of Energy and Department of Defense declared that 38 metric tons

of weapons-grade plutonium (nearly one-half of its stockpile) was surplus plutonium The two ways that this plutonium is most likely to be disposed of are to use it as fuel in nuclear reactors or to mix it with radio-active waste and molten glass in a vitrification process Other Uses

Radioisotope thermoelectric generators (RTGs) use plutonium 238 (with a half-life of 86 years) to heat silicon-germanium junctions which then produce elec-tricity Having no moving parts, RTGs can be made

to be very sturdy and reliable RTGs are used to pro-vide electrical power for interplanetary spacecraft

such as Galileo and the Voyagers, since solar panels are too inefficient beyond the orbit of Mars RTGs may also be used

to power navigational beacons, remote weather stations, and even cardiac pace-makers Americium 241 formed by the de-cay of plutonium 241 is a vital constituent

of household smoke detectors

Nuclear Terrorism

It seems unlikely that terrorists would use plutonium as a radiological poison, be-cause its toxicity is relatively low Bernard

L Cohen has calculated that 0.45 kilo-gram of plutonium particles dispersed in a large city in the most effective way might produce twenty-seven fatalities ten to forty years later Chemical or biological weap-ons are probably easier for terrorists to ob-tain The nerve gas sarin, for example, was used by terrorists in the 1995 Tokyo sub-way attack in which thousands were imme-diately overcome; fifty-five hundred peo-ple were injured, and twelve died Following the breakup of the Soviet Union, there were several cases of

A ring of weapons-grade plutonium (United States Department of Energy/

Los Alamos National Laboratory)