Plate Tectonics Where the Action Is

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Skylab 4 photo with view nort h up the Rio G rande rift from altitude of 432 km over

central New Mexico. The rift crosses a wide zone with

a

strong northeast trending

grain. It passes through the Jem ez lineam ent one of the most active volcanic zones in

the

U.S.

for the past

1

million years. Note the Jemez caldera top left.

SCIENCE IDEA

Plate Tectonic

Where the

Action I

by

Bob

R ie c ke r

u r ing your l i f e t im e , Nor th

A m e r i c a a n d E u r o p e w ill

separate by the average height

of a person; the Pacific Ocean will

shrink by the average width of a single-

family home. Next year , Los Angeles

will move closer to San Fran cisco by the

length of your little finger. Last year,

some 100 cubic kilometers of new earth

crust were born at crests of ocean r idges.

In the last 76 million years, the north

magnetic pole has reversed its polarity at

least 171 times, and in the last 100

million years , a n a rea equal to the size of

the Pacif ic Oce an basin ha s disappeared

beneath surrounding continents.

Plate tectonics is the unifying concept

in geosciences. Th e Earth's surface is

divided into a mos aic of possibly 10 to

12 large (and many smaller) rigid, mov-

ing plates, varying from 10 to 100

kilometers thick and often many thou-

sand s of kilometers wide. The plates that

move apart on our globe must coll ide

somewhere, and when they do, ear th-

quakes and volcanism remind us of tur-

moil . In an ocean chain over 30,000

kilometers long, new earth crust forms

to add rock to continents. At other

places, mountains rise because of plate

collisions.

W e r i d e n o r t h w e s t w a r d o n o u r

namesake, the North American plate ,

which extends from the Pacific coast to

the middle Atlantic Ocean. We grind

against the Pacific plate along the San

Andreas Fault in California . Plate mo-

tion insures th at, in 5 0 million years, Lo s

Angeles will becom e a western su bu rb of

San Francisco. That tr ip is not easy

geologically, as m ajor earthqu akes attest

periodically.

Th e gurus of plate tectonics predicted

at the paradigm's inception, about 15

years ago , that the new theory w ould re-

quire rewriting of geology textbooks. In

fact, that's h app ened . But we are just

now be g inn ing to unde r s t a n d how

dynamic and changing the Earth 's sur-

face really is.

Problems remain. The simple model

of 100-kilo meter-thick , rigid plates, or

rafts, floating about on "greasy skids" of

the Earth's upper mantle looks naive.

Seismic research based on study of

propagation of elastic waves through the

Earth suggests that continents have

much deeper roots. Also, we don't really

know yet what mak es the plates go Cer-

tainly the Earth 's natural heat engine

drives the raf ts, but how, and over what

vertical dimensions? The Earth loses

heat, generated mainly by radioactive

decay, at a rate about f ive t imes more

than the rate at which man uses energy

( 2

1 12 calories/second). How long

have plates moved over the Earth 's su

face since the planet's birth? As t

Earth 's heat engine cools, are plate m

tions slowing, and is mountain buildi

c e a s i n g ? O f te n e a r t h q u a k e s a

sometimes volcanism appear in the

ter ior of plates, not only along the

edges as predicted by the theory.

Normally the hottest spots are whe

the a c t ion i s . Tha t ' s whe r e p l a t

originate or where they collide or a

consumed. At ocean spreading cente

new crust fo rms and enlarges the plat

One such hot zone cente rs a long t

Rio G ran de r if t. A r ift is a long, narro

usua l ly dow n-fau l ted va lley in t

Earth 's crust . The Rio Grande r if t e

tends from Leadville . C olora do, throu

New Mexico, and ac ross the border in

n o r t h e r n M e x i c o . A l o n g t h i s 8 0

kilometer rift, significant volcanis

earthquake activity, uplif t , and separ

tion have occurred during the last

million years.

F o r t h e p r e c e d i n g h u n d r e d s

mill ions of years, the entire zone alo

which the Rocky Mounta ins s tand h

been an active moun tain building regio

Then 30 million years ago, region

separation initiated rifting. The r

opened along a ser ies of northward a

no r the a s tw a r d f l a ws o r l i ne a m e n

which reach down through the overlyi

LOS ALAMOS SCIENCE Summer 1980


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SCIENCE

IDE

sedimentary cover. The series of deep

basins that eventually formed in the rift

now contain th ousa nds of meters of sedi-

ment and volcanic fill.

The deeply cutting flaws that cross

the rift are zones of weakness that

probably extend dee p into the Earth s

mantle. They leak vast quantities of

molten rock into and through the crust.

The Jemez lineament, which extends

across the Jemez caldera northeast , was

especially active volcanically during the

last 10 million yea rs as indicated by

large volunles of young volcanic rocks

blanketing the surface. Many of these

volcanics. silicic in composition, erupted

during violent explosions much larger

than modern man has seen. Layers of

fine ash from some of the eruptions

reached as far east as Iowa and Illinois

before falling from the atmosphere. The

giant scale of these events can be im-

agined as one rides past the Jemez

caldera in the center of the Jemez m oun-

tains. This grassy pastureland, more

than 15 kilometers in diameter, is the

remnant of a large volcano.

The scale of the Rio Grande rift is

also quite large. Separation across the

rift is as wide as 10 kilometers and uplift

exceeds many thousand meters . Does

this mean that the North American plate

is being torn asunder in the southwestern

Uni ted Sta tes? Some geoscien t i s t s

believe so, and in their jargon, the rift

represents an incipient spreading center

where new earth crust forms and shoves

aside older rock.

Major crustal stretching occurred not

only along the Rio Grande rift but over

the entire southwestern portion of the

United S tates. We see evidence of the

stretching in the patterns of faulting and

the appearance of volcanic rocks in the

Basin and Range provinces in Arizona

and Nevada to the west and in the High

Lava Plains of Oregon and California to

the north. The exact cause of the

stretching remains obscure and c

troversial. It must involve interacti

during the last fraction of geologic ti

am o n g t h e N o r t h A m er i can an d Paci

p la tes and th e

Faral lon plate, which

between them, bu t s ince has been co

sumed as the sou thwes tern par t

of

o

raft overrod e i t . These in terac t ions a

p ro d u ced an d n o w d r i v e t h e San A

d r e a ~ aul t sp l i t t ing Cal i fo r n ia .

There is no question that episo

rifting continues today in New Mexi

as evidenced by the abundant fa

scarps, earthquakes, high heat flo

m o d ern e l ev a t i o n ch an g es , r ec

volcanism, and the geophysical eviden

of an a noma lous thin cru st beneath

rift.

The earliest written record of ear

quake activity in New Mexico is of

ear thquake swarm descr ibed by an ar

surgeon cam ping near So co rro in l

1849

Most of the recent earthqua

have occu rred in a 15 0-kilom eter sect

between Belen and Socorro, New M

ico. During the last 20 years, seism

record ing suggests a very low level of

tivity. The level is surprising in view

major crustal m ovements in the past a

the presence of modern fault sca

along the edges of the rift . The rift n

appears quiet, but seismically pregna

Detailed seismic studies continue

New Mexico with measurements perf

med by LASL. the New Mexico Instit

of Mining and Technology, and

Albuquerque Seismological Laborato

of the U.S. Geological Survey. Ea

group installed seismometers for d

ferent reasons, but all cooperate clos

in trying to learn more about seismic

tivity in New Mexico. One motivat

for LA SL s seismic research has been

study natural earthquake act ivi ty n

the hot dry rock geothermal energy

periment at

Fenton Hill . so that m

made and natural events can be d

tinguished. Also construction of n

LOS ALAMOS SCIENCEISummcr 198



SCIENCE

IDEAS

The central Rio Grande rift. T he partly molten rock (dark grey) reaches within 35 km

of the Earth s surface below the Albuquerque volcanoes.

facilities in Los Alamos. such as the

hal f -mi le- long accelera tor , requ i red

knowledge of seismic risk. New Mexico

is still earthquake country even though

current activity is low.

J o i n t r e s e a r c h s h o w s t h a t

microseismic activity is concentrated in

two areas, one between

Beten and

Socorro and another 15 kilometers west

of Espanola. Near Socorro, seismic

analysis suggests the presence of a large,

c o m p l i c a t e d - s h a p e d m a g m a b o d y

molten rock) at a crustal depth o f about

19 kilometers, leaking into shallower

bodies. Most of the Socorro activity con-

sis ts of small-magnitude earthquake

swar ms apparent ly related t o movement

of molten rock at depth. The swarms

suggest an extension of the upper crust

produced by the intrusion of a layer of

mag ma at mid-crustal depths. Unusual ly

high heat flows occur over the inferred

m ag m as . Recen t s e i s m i c r e f l ec t

research by the Consort ium for C

tinental Reflection Profiling penetra

to depths of at least 35 ki lomet

Resul ts confirm the exis tence an d sh

of the molten rock mass.

West of Espanola, seismic activ

also suggests a possible underly

mag ma body. Act ivi ty consis ts of sm

magnitude earthquakes that recur at

tervals of several months. Seismome

LOS ALAMO S SC1ENCElSu1nrner



SCIENCE IDE

record a few swarm-like sequences. In-

terestingly. LASL researchers recently

noted a major re la t ive subs idence

nor thwes t o f Espanola associa ted

spat i a l ly with the ear t hqua ke belt .

Geodetic releveling analysis reveals a

maximum subsidence of about 5 cen-

timeters between surv eys perform ed in

1934

and 1939. Researchers suggest

that the sinking and the associate d ear th-

quakes originate in deflation of a shallow

magma body. Continued studies will

help to identify the processes responsible

for regional extension in the Southwest.

Plate movements not only create

earthquakes and produce volcanism;

they also determine where mineral

deposits form. In addition, most geother-

mal energy resources originate as a

result of plate interaction. The LA SL hot

dry rock experiment on Fenton Hill is

successful partly because of its location

on the edge of the R io Gra nd e rift. Plate

travels determine whether diamonds

form in Ark ansa s or gold appe ars in

California. Many ore bodies, for exam-

ple. occur at present or past boundaries

of crustal plates.

Geology is a young discipline with a

rapidly increasing pace of surprising dis-

coveries. Only astronomy shares with it

the complexities of scale and time. In 2

A. D. Senec a wrote, It is useful to be

assured that the heavings of the Earth

are not the work of angry deities. These

phenomena have causes all their own.

Tw o millennia later we ar e just beg in-

ning to understand what those causes

are. Moreov er, we are just beginning to

exploit our understanding through im-

proved exploration for mineral and

energy resources. Few theories have

made such an impact on their dis-

ciplines, and few disciplines, stimulated

by new theories, have had such an im-

pact on energy technologies as has plate

The Rio G rande Rift The crosshatched areas indicate the Rift itself and the stipp

tectonics on the geosciences.

areas indicate surface volcanics

LOS L MOS SCIENCEISummcr 198

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Plate Tectonics Where the Action Is