Forces in Earth's Crust
Reading Preview
Key Concepts
• How doe m .... in the Ulirt
0\;1"9" fMth"l surface 1
How Does Stress Affect
• wt-e lire bults USUIIIy found,
- ' why do they form 1
1. Put on your goggla
2. Holding iI ~ stick at both ends,
slowly bend it into an ard!.
l. Rele.lse the prM§U~ on the ~
• ~ IMld fNIUfe5 result from
tho! IoKti of plolte 1l'lOIIemefl11
Key Terms
• urlMS • (er$on
• compteWon • Jhe"';"lI
• normal filUlt • M"lIing ....,,11
• lootw.llil •
f.ult
• nrike-olilll...1t • anticline
• ')'I'dine • IMINU
'_I.e
~T,.rget Reading Skill
BuIlding \Ioubo 11..,­
clefonition nates tN muning of
.. ""Of'd ... ph'as<'. AI you rud.
_Ole .. ~ of each Kt'y Teon
Earth's Crust1
stktt.1'ld observe what N1pptnS.
4. Repe.t Steps 1 and 2. This time, how~ kup bending the
fflds of the popside nick toward uc:h other. What happens
10 the wood?
ThInk It OYer
P',edia:ing Think of the
~
stid< as a model fOf
~rt
of
e..rth\ U\ISt. Wh.t do you think might event.... IIy"'~ as the
forces of pl.te movement bend the crust 1
A
in you' """" worm.
" ••n
Effects of Stress
~II_ in Earth's crust
cauwd the ground belleilth this
.thletic field in T.iwMl to ~nge
in Jhllpe.
... ,
The movcmc:m of Earth's plates crtates tt!ormous forces thaI
squec.re or pull the rock in the crust as if it were a candy 001'.
These (orces are examples oriilaa, a fol"Ct that aeu on rod 10
change its sha~ or volu~. (A rodt's volume is tM amount of
space the rock takes up.) Bec:aU5e stress is a force, it adds CIlctg)'
to the rock. Thc eneTgY is stomi in the rock until the rod
l:hangc$ sha~ or breaks.
If you try to break a caramel candy bar in two, it may only
bend and stretch at first. lJlte a candy bar, many types of rock
can bend or fold. But bqvnd a c:crIain limit, even these rocks
....iIl break.
C o _..IOf.
Compreouion
puloh~
rodtt~.
T.........
TIMSiOfI streldle rock.
-.
ShearIng un c.use
IMSSoeS
of rod 10 slip.
Types of Stress
Threedifferent kinds of stress an occur in the l:rust-tension,
l:ompression, and shearing. Tension, compression, and
shearing work o\'(r l11iUions of years to change the shape'
and volume of rode.. Thtsf' forces caUM' some rocks to become
brittle and snap. Other rocks knd slowly, like road tar soft­
ened by the sun. Figurt 2 shows how stress affectli the crust.
MDSt dlangc.s In the awot uocur ~ ~wty that th~ annot
~ ob5ervcd directly. But if you could speed up time so a billion
years passed by in minutes, you could see the crust bend,
stretch, break. tilt, fold, and 51ide. The slow shift of Earth'.
platC5 ClIIUSCS these changes.
..... ,
In E.. rth., Cnrst
SttlMS fon:es push, putt ()( twist
the rocks in E.nh'> Cry".
ReIatin9 ~se and Effect WhidI
Iype of SlRSS fet'>ds to sho'IM part
of lhe ctult?
S~s
Tension The stress force called tmsion pulli on the awol,
stretching rock 50 thai it becom~ thinner in the middle. The
effecl of lension on nxk is 5OIIIn\'hal like pulling apart a piece
of wum bubble gum. Tension occun where two platts are
moving apart.
Cornpt'esslon The 51ress fOf« caIkd rompftSSioD squecus
rod:: until it folds or hrtaks. One plate pushing.gainst another
can eumpre» rcx::k w..,. giant tra$h rompactor.
Shearing Stress that pumes a mass of rock in two opposite
directiora il; called lhearing. Shearing can caUM' ror.:k to break
and slip apart or 10 chan~ iUshape.
ONlptet" 1 F • •5
Kinds of Faults
When enough suess builds up in rock, the rod:: breaks, creating
a fault. R«all that a faull is a bruk in Ihe rock of the crust
whc~ rod:: surfa(~ slip PM! ~ other. The rocks on b€uh
sides of a fault can mow: up or down or si<kways. Mo!t faults
occur along plale boundaries. "'here the forces of plate
molion push or pull the ,rust so much that the crust breaks.
Then are thru main types of faults: normal faults. nvcrsc
faults, and strike-slip faults.
NOf'mal Fauns Tmsion in Earth's crus! pulls rock apart,
C2using normal fault$.! In a normal fault, the faull i:l; at an
angle. so onc block of rock lies above the fault while lhc other
block lies below the faull. The block of rock thai lies above is
called the hanpnS wall. The rock Ihal lies ~low is calltd the
footwall. Look at Figure 3 to Stt how the hanging wall lies
abow: the footwall. When movement occun along a normal
faull, the hanging wall slips downward. Nonnal {',lulls occur
where plaIa; diverge, or pull apan. I'm example. normal (aullS
are found along the Rio Grande rift valley in NI:W Maim,
whtre two pit«'S of Eanh's crust are under tmsion.
""""
Kinds of Faults
Thtrt are three ma;n kinds of
faults: normal fautts. re¥ef'W
fautts. .nd rtfiklHlip f~tts.
InfflTlng Which half of • ~J
fM would you ~ to fonn
the fIoo,r of. lIallty? Why?
..
......
,~
... ,
Mol
...
"I along
~,
NomYI filuh
In a normal fault. the tw.nging waH
.JiP5 down relative to the footwall.
wh~~
the rock of ttlt all'lt is
pushed together. comprusion causes r~ faults 10 fonn. /I.
,I'C'Yll:I"M falo&h; has the same struaure as a normal fault, but the
bIocb move in the oppositt direction. Look at Figu~ J to set
how tnc rods along ill ~ fault move. As in a normal fault,
oot' side of a l'eVC1'St' fault lits at an anglt abo\l' the other sidt'o
The ruck fOllning the h:>nging waU of a rrvnl(' fault s1idts up
and over tht footWilU. Mow:menl along rn~fSot' faulu produc~
part of Iht northern JW<:ky MountainJ in tht western United
Stales and Canada.
Reverse Faults In pbces
cGo'::,~e
•. >
- -liNKS.
for: UNs on iUts
~ -.xiI.i"hOl9
wetl Codot: KIl-l021
5trik..Slip Faults In placn whe:rt' platts moyt past each
olher, sharillg cre.ua strike-slip faulu..ln :I arib-dip fawl,
the rocks on e:ithc:r side of the (awt slip past each other side­
W;lys, WIth little: up or Jown motion. A nih-slip fault that
forms tht boundary betwt'c:ll IWO plates is called I tra~form
ooundary. The San Andreas fault in California iJ an l!J<ampic of
a ,"Tilt-slip fault that is a transform boundary.
'!';;';:'",)
What Isthedille<enUI be"",,". hanglng w~l and
..!:
• footwall1
ReverH fa..h
In. '''"'"''' fNlt, 1M Mnging wall
mQYe up relative
to the footwilil.
Strlk..sllp , ...h
Rods on cit'- '!ide of a strit..-.lip
fault dip PMt each other.
o..pt... 2 ' • •7
.......
EtfKts of folding
CompnulOfl and lold­
ing of the crust produce
anticlines. whidl ardl
vpwa'd, "nd "ynd;~
whic:h dip downw.I.d.
OYer millions 01 y@an.
folding can posh up
high mounain .anlJfl.
Pnodicti"l1 If 1M (olding
in tM d.am COl>'­
tirwed. whoIr l<ind of
favl! might fonn~
Changing Earth's Surface
The (orces produ<ed by the movement of Earth's plates can
fold. surleh. and uplift lhe l;TU$1. Over millions of years, the
n.,
fot"(t:S of plale movement can dlange a
plain inlo land­
forms sudl as anticlines and syncline$, roldw mountains,
fault-block mountains, and platEaUs.
Modeling Stress
You can mod@! the stresses
that create faults.
1. Knead II piec:e of plilstil:
putty until it is wft.
2. Push the ends of ~ putty
toward the middll!'.
3. Pull the ends apart.
4. Push MIf of the putty one
WilY and the other half in
the opposite dirKtion.
c»ssifying Which step In
this activity modelJ the type
of stress th.It would produc:e
antidines and sync:lioes1
....
Folding Earth"s Crust Somcti~ plate movement (1Iuses
the cruSllo fold. H.\'t you ever skidded on a rug that wrinkled
up as )'Our feet pushM it across lhe floor? Much as Ihe rug
wrinkks, rock stress«! by wmpression may ~nd without
breaking. Folds are bends in rock thaI fonn wh~n comp~ion
short~w and thickens part of Earth's crust. A fold can be only a
few c~ntimauucrossor hundrcclsofki]ometen wid~. You can
oft~n see small folds in the rock exposed where a highway has
been cut through a hillside.
Geologists use Ih~ terms antklin~ and syncline to dl'SCribe
upward and downward folds in rock. A fold in rock thaI bends
upward into an arch is an anticline, shown in Figure 4. A fold
in rock thaI bends downward to fonn a valley is a synCline.
Antidincs and synclines ar~ found in many placcs wh~re com~
pression forces have folded the crust. The untral Appalachian
Mountains in Pennsylvania are folded mountains made up of
anlicliJl('5 and $)'nclincs.
The collision of two platcs can cause compression and fold­
ing of the crust over a wide area.. Folding produced som~ ort~
world's largc:5t mounlain rangcs. The Himalayas in Asia and
lh~ Alps in Europe fonned when picccs of the crust folded dur~
ing the coUision of two plates.
Stnltchlng Earth's Crust When two normal faults cut
through a block of rock. iii fault-block mountain forms. You can
see iii diagram of this process in Figure S. How does this process
begin? Where two plates movt away from each other, tensKIO
forces create many normal faults. When two of tht'St normal
faults form parallel 10 each other, a block of rock is left lying
~n them. As the hanging wall of each normal fault slips
downward, the block in between moves upward, forming a
faull-block mountain.
If you tra~led by car from Salt lake City to los Angeles. you
would cross the Great Basin. This region contains many range;
offault·block mountains separated by broad valleys, or bums..
r~a.''b~';l",!;I~,~)
Whal ~ of plale movement ... ~ fiKJh..tllodl
_~
mount'lInstafarm?
.It.
..... ,
FiKJMllodt Mountains
IU tl!Mion foKes pull thoI <ruSt
apan. two p.ilralltl .-mal faults
can form' r,"'9'!' of fautt·bIock
rnountlil'll, like Ih" mountain
range in Idaho.
-~,
The
Kai~
f'l.ltuu
The "allal'd on tl>(> horizon is Ifw:o
K..ibab PliJtuu. whid'o forms Ifw:o
North Rim of the Grand Canyon in
NU:0I'\iI. Tht Kalbab Plateau is
pan of the CoIor.oo Plateau.
Uplifting Earth's Crust The forces that raise mountains
can <llso uplift, or raise, pb.I(2U$. A ~taU is a large area of flat
land elevated high above sea It'VC'1. Some plalC~aus form when
forces in Earth's cnm push up a large, fuu block of nx:k. Lik.: a
fancy S<lndwich, a plateau oonsist$ of many difTttenl flat layCI"lIi,
and is wider than it is lall.
Form; dr-forming the crust uplifted the Colorado Plateau in
the -Four Comer.;" region of Arizona, Utah, Colorado, and
Nt'W Mexico. Much of the Colorado Plateau lies moT(' than
1.500 meters abo\~ sea level. Figure 6 shows one put of thai
plateau in northern Arizona.
SecHon
1 Assessment
~Target Reading Skill IYllding VoabulMy
Ran- 10 your ckfinitions oftM Kq> T=tU to help
you mswa 1M following questions..
Reviewing Key Concepts
bWl',(N'"Ig WhaaruM thlft main I~ of
Sll't$$ in rod?
b. RNting c.~ and EH..:t How doe tension
change lbe wpe of Earth's crwl!
c. com~ring and ContraJtint ComlMl'e the:
way Ihal oomprusion a{feclS ~ trmIlO lhe
way thai tension a{fectli Inr CTUSI.
2. a. Dfl<rIbing Whal is. faultr
b. ~nlng Why do faulu oflrn occur along
pIale boundaries?
c. RelIIdng CauM and Effect Whal typeof
faull is formed wnrn plales diverge. or puJI
ap.an! \\'h3llype of fault is fuTllled ",-hen
1.
iI.
.
pIales are pushed ~
" ,
1.
LIlting Name five kinds oflandfomu
caused by plllle movnnenl.
b. Reliltlng CiluMi iIt\d!Effect Whal arr thrtt
landfomu produced by compression in lite
trmI? WhallandfOTlll is produced by
tmsion?
iI.
ModelIng FiIUlts To ~ Eilrth's crust, roll
modeling cliIy Into layers and then press the
layen together to form a reeungulilr blod<.
Use II pillst;( k.nife to lJi(e through the block
U lin lingle, forming II fllull. bplll;n wtlid'1
pam of your model represent the ..nd
wrf<KI!, the hanging Willi, .nd the footwall.
Then show the three WilY' In wtlid'1 the silk-s
of the fault <an 1'I'IOYe.