Download Plate Tectonics

An earthquake occurs in city A. Recordings on a
seismograph in city B show only the presence of
P-waves. City A and B are on opposite sides of
Earth-180' apart. What does this information
allow you to infer about the structure of Earth's
interior?
How does the composition of the oceanic crust
compare with the composition of the continen-
tal crust?
(1) The oceanic crust is mainly limestone, while
the continental crust is mainly sandstone.
(2) The oceanic crust is mainly limestone, while
the continental crust is mainly granitic.
(3) The oceanic crust is mainly basaltic, while the
continental crust is mainly sandstone.
(4) The oceanic crust is mainly basaltic, while the
continental crust is mainly granitic.
37. As one travels from an ocean shore to the
interior of a continent, the thickness of Earth's
crust generally
１
２
The overall density of Earth is approximately
5.5 9/cm3. The average density of Earth's crust
is between 2.5 gy'cmtand 3.0 g/cm3. What does
this suggest about the density of Earth's core?
The composition of some meteorites supports
the inference that Eartht core is composed of
(1) aluminum and calcium
(2) iron and nickel
(3) silicon and oxygen
(4) magnesium and potassium
41. The temperature of rock located 1000 kilometers
below Earth's surface is approximately
１
２
００
３
decreases
increases
remains the same
How does thickness and density of the continental crust compare to that of the oceanic crust?
(1) The continental crust is thicker and less
dense than the oceanic crust.
(2) The continental crust is thicker and denser
than the oceanic crust.
(3) The continental crust is thinner and less
dense than the oceanic crust.
(4) The continental crust is thinner and denser
than the oceanic crust.
1000° C
2600° C
3300° C
4300° C
Plate tbctonics
People have always wondered about the origin of continents, mountain
ranges, volcanoes, earthquakes, and the multitudes of other features and
events. In the past, many legends, religious beliefs, and scientific theories
have tried to explain Earth's features and events. Sone of the older
Start Here
scientific theories include cooling and contraction of Earth, expansion
of Earth, and continental drift. None of these earlier theories seems
satisfactory for most of the scientific community today. Since the 1960s
many new discoveries from the studies of ocean drilling, Earth's
magnetism, satellite observations, and detailed analysis of rocks and fossils
have led to the plate tectonic theory. This theory has done for the earth
sciences what evolution and genetics have done for the biological sciences.
It has provided a unifying model to explain most, if not all, major features
and events of Earth's lithosphere.
The Plate Tectonic Theory
The basic concept of the plate tectonic theory is that Earth's lithosphere
is broken up into sections or pieces called plates-also called lithospheri<
plates and tectonic plates-and their movement and interaction produce
major changes in Earth's surface. ln this book the term "plates" will mostly
be used.
These plates move about Earth's surface at a rate of a few centimeters per
year (approximately the rate of fingemail growth). These plates can also
move up and down-usually at rates of only millimeters per year--due to
uplifting and sinking. The plates can move around and up and down
because they are floating on the asthenosphere-a plastic-like layer of the
mantle'
Topic 12: Earths Dynamic crust and
lnterior 259
Refer to Figure 12-12 and the Tectonic Plates rnap in the Earth Science
familiar with the names and locations of the
various plates. You will observe that the plates usually don't follow
continent or ocean boundaries. Note that the North Arnerican plate
includes parts of Asia and North America; the Pacific, Arctic, and the
Atlantic Oceans; and Greenland and part of Iceland. Westernmost
Reference Tables to become
‐
■shOull be understOod that l
‐
lthe contiⅢ ents dOnt d“
‐
ft
or m●ve on their o嘲 叫 they
move as ther assoclated
PlateS move.Contine,鑢
:
dnft was a term used before‐
。mm● ￨“ e口 ance● ‐
口ate
￨こ
tectonic theOry ConthF由
:
i drtt means the continents
lbythemselves att m"ng
llrOund Eartht so:u sttrface
F● ●re 12‐ 12 and訥 IInfered
詢 輛 On of Farthヽ
Lndmmes
l鰈 棚
Mヽ 世
show that the continents
@
Califomia and Mexico are part of the Pacific plate. Most plates have
continental and oceanic crust at their tops. A small number of plates are
oceanic-that is, they have only oceanic crust at their tops, like the Nazca
and Philippine plates.
At least three times, if not more, most of the large landmasses (continents
and larger islands) have come together, forming supercontinents.
Figure L2-12 shows how the supercontinent Rodinia formed approximately
million years ago and how the continents have split and formed since
then. Note that diagrams G and H indicate the projected positions of
landmasses in the future, based on the rates and directions the plates are
moving in now.
800
The Three Types
of Plate Boundaries
As the plates move, they interact in three ways-they can separate, collide,
and slide by each other. Most of the major events involving Earth's crustsuch as mountain building, earthquakes, and volcanic eruptions-occur at
the boundaries where the plates interact. Each of the three types of plate
boundaries has its own set of unique events and features, which are
illustrated in Figure 12-13.
Divergent Plate Boundaries Where two plates separate, or diverge, the
boundary is called a divergent plate boundary. At these locations the plates
move apart and magma rises from below to fill in the separation, resulting
in igneous intrusions and extrusions, such as lava flows and volcanoes.
(See Figure 12-138.) This divergence is sometimes called sea-floor
spreading.
This magma and resulting lava form igneous rock that creates new crust
and lithosphere, which are then split and divided in two by the divergence.
The rnagma that forms in these regions is the result of divergence that
lowers the confining pressure and melts rocks of the lower lithosphere and
asthenosphere. The divergence also results in rnany earthquakes, most of
which are shallow in depth. If the divergence is within the continental
crust, the result is a continental rift valley of mountains created by faulting
and much volcanic activity. (See Figure 12-13A.)
When the divergence is within oceanic crust, the faulting and volcanic
activity result in a mid-ocean ridge, a mountain range at the bottom of
the ocean that is composed mostly of volcanoes and lava flows. The
mid-ocean ridges often have a central depression, or rift valley, as shown
in Figure 12-138.
Convergent Plate Boundaries When two plates collide, or converge, the
result is a convelgent plate boundary. Much of the dramatic events and
features of Earth's crust are created at these convergent boundaries. There
are three varieties of convergent plate boundaries:
o both plates with
260
fopic 12: Earth's Dynamic Crust and lnterior
oceanic crust on top
Earth around 70o millibnt years ago as superontlnont Rodlnl6
br.aking up,
it
Eanh around 550 million year after the brlak up ot
a
5upercontlnont.
Earth arcund 135 millions yeart ago a5 Pangea continues
Earth around 250 million y€ar! ago when 3uper@ntinent Pangsa
is iust beginning to break up.
to breal up.
Earth arcund 10o million yea6 ago with furthcr br€ak up
Earth at
prtsent
of Pangoa,
Earth 100 mlllion years in future uring prefenlday plate motions
ar a guide. l{ote bigger Atlantic Ocean and how Africa hae split
and collld€d with Eu.ope.
Using present-day plat€ motionr as a gulde, ln apprcximately
million years, Ea.th may be clore to .nother
251,
sup€nco lnent
Flgur. 12.12. change3 ln pogltlons of Errth! landma$€r trom t(x, mllllon yea6 ago to 250 million yarrt into the frnura
be<auie of plate t€ctonlct '
Topic t2: Earths Dynamic Crust and
lnterior 261
Key
B
I
I
E
I
n
r
Sediments and sedimentary
Stripes of ocean crust with
normal and reversed polarity
Asthenosphere
Oceanic crust
Continental crust
Lithosphere
Earthquake foci
--.- Convection currents
@
Figure 12.t3. lyp€s of plate boundaries: (A) Diverging plate boundarie5
Regional metamorphism
in the (ontinental crust r€sult in a rift valley.
Diverging plate boundades in the ocean result in a mid.ocean ridgb, shallow-depth earthquak€r, igneoui intrusions, extrusions ol
lava tlows. vol(anoes, and passive margin basins in a down-warped part of crust. Plate 1 is moving weit and plate 2 is moving east.
(c) converging Plate boundades reiult in an oceanic trench, vol<ani( iiland ar<s, igneous int.urions, regional metamorphism, young
mountains, and subduction ot o(eani< plate, Earthquake lo<i exi5t at various depths. indicating subductions. plate 1 is moving ea5t
and plate 2 i5 moving west. (D) Ttansform plate boundaries rerult in many shallow-focus earthquakes and linle to no igneoua activity.
Plate 1 is moving north and plate 2 is moving south,
(B)
262
toptc tZ. Eanh's Dynamic Crust and tnterior
・ both plates with coninental crust on toP
・ one plate wi■ l oceanic crust and the o■ er with coninental crust on toP
Where hvo plates with ocearuc crust or ocea∬
c and conmtalcrustat
their edges converge′ the denser ofthe bvO plates siJG underthe other
礎
:雪
:轟
ヤ
露
場
L麓
ち
λ
揚協
礎 l器 雪 ∬ 霊ギ 裾
]鷺
〇
顎
subducuon of One oceanic Plate under anOther resultin ocean trenches and
volcanlcisland arcs The bendhg down ofthe subductt Plate warPs the
crust′ ProduChg a long′ steeP′ and narrow depression called an ocean
trench.Some ofthese trenches are deeper below sea levelthan the hghest
mountams are above sea level The subducthg plate also results in inatta
bodies.Some of he magコ a breaks through Earth′ s solid surface and forms
a seHes of volcanoes and volcttc islands an isiand arc.The subduchng
PrOCeSS also results h a large arnount of earttquake acivity followhg礎
slope ofthe subducing plate i■ to the bottom porions Ofthe lithosPhere.
Very decP earthquakes only occur wi■ in subducttg zones The heat and
PreSSure of the subduchon causes large areas of crustal rocks to become
metamorPhoSed h a processで alled″ reJonal metamorphsm.″
IfJhereis a conttt nearthc hvo conver8脚 ng Plates with oceaic crust′
the crust atthe lnargh ofthe continent may become bent down produchg
Cean basn thatis the site of much deposi● on of
a」 Ш 量 壼 型 亜 。
sedlrnents(See Figure 12‐ 13C)The Sedh℃ nts come from the erodhg
island arcs and the edge ofthe conunents.Many ofthe rocks of eastem
New York State fo...led h hstype ofdo― warped bash before he
fo...laion ofthe superconhnent Pangea
Where a plate、 vith oceajc crust on toP conVerges with a Plate With
con■ mental crust on toP′ the denser ocett crust subducts under the
conhntallithosPhere.An ocean mnch Off he coast ofa conhnt often
forlns from this event The inapa thatfo...ls from the subduclon doesn′ t
create an island arc because the contmentis above the region ofrnagma
fo...lalon.ヽ hatforms instead are relaively new mountam ranges called
∼
young rnountains.Like the Cascadesin westem Oregon and Wattgton/
these mountam ranges may be mostly volcanlc.They may also be a
combhalon of volcaruc′ faulted′ and folded
When hvo Plates with continё nts at their edges converge′ the hvo plate
edges bunch up togethet causttt a great ttickenhg of■ ℃ crust and
hmsPhere hs bunctt uP createsthe nghest Ofyoung mountalrts
such as the Himalayas′ where lndia is cOШ dhg with Asia
Plate convergence that results in the grow■ ■of young lnountain ranges is
called org,圏 嘔y The terIIn is also used to hdicate the jコ ne when mountam
麗 :峨 I]げ 潔 鼻 1服 翻 簾 瀧 雹 ぶ 麗 滉 N蒻 況 甥 肝
and you will note the four orogeies(Gren宙 lle′ Tacoruan′ Acadian′ and
①
Alleghaian)that have resulted m mountams h New York State and
surrounding regions
Transform P:ate Boundaries When hvo plates co■ ide by sliding Past each
otheル the boundaヮ is called a transform plate boundary.The drag"g of
hu10sPheriC rOcks along the edges of the transfo...lbOunda五 es builds up
muCh Potential energy in rocks,this energy is eventua■ y released as k旋 uc
Topic 12: EarthS Dynamic Crust and
lnterior 263
2 million-year-
old volcano
Present active volcano
Plate position
at hot spot
2 million years ago
/
mechanical energy in the form of earthquakes. The
San Andreas fault system in Califomia is an
example of a transform plate boundary between the
North American and Pacific plates.
Driving Forces of Plate Tectonics
Plate
1
million yea6 ago
Rising magma or rock
(stays in same lo(ation)
聰憔濡 蜘…
Hawaiian lsland5
Necker (1'l million years)
(5m‖ ‖on years)
Nihau (7 million years)
Oahu (3 million years)
30° S
m‖ ‖
on years)
Hot spot today
on Hawaii
FIillrt ll-L.
Fot 3pots, ri3ing mrgmr, tnd pl.te
is believed that dsing magma from the
mantle m€lt5 through the plates of the lit'toiphere. forming
movamantr: lt
volcano€s at hot spots. As the plate moves, new hot spots and
volcanoer form, ai shown in diagram A. The Hawaiian lrlands,
Emparcr Ssamount (underwater volcanoes) Trall was cr€ated by
movemonts of the Padti( plate over an area ol ridng magma,
as rhown in diagram
ote how the PacifiG plate may hav€
(hang€d dl]lction some 25 million to 30 million yea6 ago or the
rBing hot matqrial changed its location.
8.
While most scientists accept the major part of the
plate tectonic theory, there is still much debate
about what makes the plates move. The most
accepted theory is that mantle convection cu(rents
drag or push the plates apart at places where plates
diverge. The exact location of these convection
currents is hotly debated. The energy source for
these convection currents is the heat of Earth's
interior, which causes hotteg, less dense parts of the
mantle to rise under diverging plates. Gravity pulls
down the cooler, more dense regions of the mantle,
causing falling convection currents in subduction
areas. (See Figures 12-11 and 12-13 for illustrations
of convection in the mantle.)
Hot Spots
One major aspect of lithosphere and crust that is
difficult to explain by the conventional plate
tectonic theory concems major regions of volcanic
activity in the interior parts of plates away from
plate boundaries. These regions are called hot spots.
Examples include the big island of Hawaii, the
region around Yellowstone National Park in
Wyoming and Montana, and maybe even the
Adirondack Mountains of New York State. See the
hot spots on Tectonic Plates in the Earth Science (D
Reference Tables.
Scientists have proposed that hot spots occur
where rising material frorn the lower mantle
remains stationary for millions of years. When a plate moves over the
;fi i3,Hff :"n'ffi1l*il"fl;:,,lffi
'
*ilr,'#::?"'#il;['ff;
can build volcanoes and lava flows and push up regions of the crust to
form mountains. Because the plate keeps moving over the rising materials,
a series of volcanic mountains form for thousands of miles. These trails of
hot-spot featurcs can be used to infer past movements of the plates that
moved over the hot spots. See Figure 12-14 to help you understand rising
materials and hot spots.
Effects of Plate Tectonics
fhe rnovements of the plates for 3 to 4 billion years in the past and for
untold millions of years in the future has had and will continue to have
many effects on Earth and its inhabitants. Many of these effects are
described in the sections that follow
254
Topic t Z: farth's Dynamic Crust and tnterior
Appe.rrance 01 (ontrnents The outlincs of the present-dav continents
appear to fit together like the preces of a jigsan puzzlc because
supercontinent Pangea split apart starting approximatelv 225 million
years ago, and a ferv pieces have rejoinecl one another. Tlris can be seen in
Figure 12-12 and in the Inferrt'd Positions of Earth's Landmasses section
oiihc Geologic History of Nerv York in the Earll Sci'rtcr' Rr'[ ri trcc' Inhle'. .!
ol Landmasses Similarities in minerals, rocks, fossils, age, and
structure features of mountain ranges are found at places where the
continents and other landmasses mav have fitted k)gcther in the past.
These similar feature s on different landmasses indicate a commonality in
age and origin. Todav the continents are separated, and their respective life
forms are often greatlv ditTercnt. The variation in life forms on the different
land areas is a result of evolution. Hor.vevet fossil eviclence short's that in
the past manv prlants and trnimals, such as thc Clttssopteris fossils of
Figure 12-15, n'ere the samc throughout the world. Such a rvide
distribution of the same plants and animals probably cor-rld not have
Features
Digging6u*"An interestinq theory related
to hot spots is that the impact
of an asteroid or comet on
Earth could cause weak spots
in the crust and mantle,
allowing material to rise and
causing hot-spot regions.
It has even been suggested
that these asteroid- or
comet-created areas of rising
materials could cause the
plates to begin moving by
cracking the Iithosphere and
starting the rise of convection
at new divergent plate
boundaries.
occurred unless the continents n ere connected. Figure 12-15 shows somc
oi the now,separated locations that have rocks, rnincrals, fossils, and
mountain structures in common. These similaritics suggest that the lancl
areas wcre together when thest'features n'ere formecl, and that these areas
have since separated as thc plates moved.
As the plates move portiorrs of Earth's surface to ncrv locations, rocks
are exposed to different climate conditions. The fincling of rocks near the
Earth Today
South po e
Fi9u7e 12''l!. Changing features ot landmasses due to plate movement3: Many feaiures of Earth'5 landmas5es in rocks more
than 250 million years old don't make senge unless the landmas5e5 were in different locations approximately 250 million years ago
when the supercontinent Pangea existed.
Top. 12 Eanh's Dynam
c
Crlst and ntet
or 265
equator with evidence of glacial erosion and deposition and rocks near
the poles with coal deposits of the same ages indicates a climate pattem
of hot near the poles and cold near the equator. The probable answer to
this dilemma is that plate rnovements have changed the locations of the
landmassps.
Age and Heat Patterns of Oceanic Rocks As the magma rises and forms
basaltic igneous rocks at the mid-ocean ridges, it spreads outwards away
from the ridges with divergence. This divergence of the plates is illustraied
by dating samples of the oceanic basaltic rocks. The farther the sample is
from the center of a rnid-ocean ridge, the older the igneous rock is. The
rocks are dated using the methods of radioactive-decay dating.
Another similar pattern is indicated from heat flow measurements taken
from the basaltic ocean crust. Measurements indicate that heat decreases
as distances from the mid-ocean ridges increase. This makes sense because
the hottest rocks should be closest to the magma and lava from which
they formed.
／
Magnetic Patterns of Oceanic Basaltic Rocks The divergence of the oceanic
lithosphere and crust at mid-ocean ridges is shown by patterns in
magnetism of the oceanic-basaltic rocks. The Earth's magnetic poles
flip-flop in polarity (north changes to south and south changes to north)
in periods of thousands of years in a process
called reversal of Earth's magnetic polarity. The
Ge"rathに norh pde
reasons for these magnetic reversals and the
question of whether the magnetic reversals are
cyclic or non-cyclic have not been answered.
Iiy'hat is known is that Earth's magnetic field has
reversed hundreds of times since the magnetic
field's origin, probably billions. of years ago.
(See Figure 12-16.)
／
graphic south pole
.////
Normal magnetic polarity of today
,'/
,/
/
Geogmphに notth pde
＼
ヽ
＼
ｅ ＼
ｐ
０
Geog'aphic sou/th pole
,"ny.nynry
Reversed magnetlc polarity
Flgurc tI-16. o.nal and r€veF€d polarity of Eartht
mrgnetirm; Eanh! magnetic lield fliFflops from nomal polarity
to r€ver5ed polarity in time spans of thougands of yea6.
266
fopt
12: Eanhs Dynamic Crust and tnterior
When basaltic rock crystallizes at the mid-ocean
ridges, its magnetic minerals are aligned;
therefore, they record the particular polarity when
the magma and lava solidi$'. Normal polarity is
when magnetic north is near the north geographic
pole and magnetic south is near the south
geographic pole-as is true today. Reversed
polarity is when magnetic north is near the south
geographic pole and magnetic south is near the
north geographic pole. It has been found that
there is a pattem of corresponding stripes of
basaltic rock on either side of the mid-ocean ridges
of normal and reversed polarity. This evidence
suggests that the corresponding stripes were
formed at similar times, and the sea-floor
spreading has separated them during divergence.
These stripes are illustrated on Figure 12-13 . Note
that these stripes are nbt a visual feature but a
pattem only indicated by magnetic instruments
used in airplanes or oqeanographic research ships.
Other Effects of Plate Tectonics The movement of the plates has had and
will continue to have other effects on Earth and its inhabitants:
o environmental hazards, such
.
.
.
.
.
as volcanic
eruptions, earthquakes, and
tsunamis
changes in Earth's climate and weather, such as altering the locations of
land and ocean areas and creatilrg mountains to change wind pattems
changes in the factors that cause our day-to-day weather by changing
the dishibution of land and ocean areas and other features, such as
mountains and large continental regions
the rock cycle
the major landscape features of Earth-continents, ocean basins,
mountains, plains, and plateaus
exposing rocks to weathering and erosion that carve the details of the
solid Earth surface and form sediments
Base your answers to questions 42 through tt8 on the following diagram, The diagram is an earlier tectonicplate model that represents one possible interpretation of the movements of Earth's rock surfaces according
to the theory of plate tectonics. According to this interpretation, Earth's lithosphere consists of several large
plates that are moving in relationship to one another. The arrows in the diagram show some of this relative
motion of the plates. The diagram also shows the age of formation of the igneous rocks that make up the
oceanic crust of the northern section of the Pacific plate.
Age of rocl6 of Pacitic plate
A lurassic
B Early Cretaceous
C Middle Cretaceous
D
Late Cretaceous
E Eocene
１
According to the Earth Science Reference Tables,
during which geologic time period were the
continents of North America, South America,
and Africa closest together?
(3) Triassic
Tertiary
(4) Ordovician
Cretaceous
２
According to the theory of plate teqtonics, the
distance between two continents on opposite
sides of a mid-oceanic ridge will generally
１
２
３
decrease
increase
44.
Which statement is best supported by the
relative movement shown by the arrows in
the diagram?
(1) North America and South America are
moving toward each other.
(2) The lndian-Australian plate is moving away
from the Eurasian plate.
(3) The African plate and Eurasian plate are
moving away from the North American
plate.
(4) The Antarctic plate
is moving away from the
North American plate.
remain the same
Topic 12: Farths Dynamic Crust and
lnterior 267
The boundaries between all of these plates are
best described as the sites of
(l)
53. Evidence
of subduction exists at the boundary
between the
(1) African and South American plates
(2) lndian-Australian and Antarctic plates
(3) Pacific and Antarctic plates
(4) Nazca and South American plates
frequent crustal activity
(2) deep ocean depths
(3) continental boundaries
(4) magnetic age pattern
Which geologic structure is represented by the
double line separating the North American
Which of the following cross-sectional diagrams
best represents a model for the movement of
rock material below the <rust along the midAtlantic ridge?
●●０“
plate from the African and Eurasian plates?
thick continental crust
thick layers of sediment
mid-ocean ridge
granitic igneous rock
47. Which provides the best explanation of the
mechanism that causes these plates to move
“２０“
across Earth's surface?
48.
convection currents in the mantle
faulting of the lithosphere
the spin of Earth on its axis
prevailing wind belts of the troposphere
The age of formation of the igneous rocks A, B,
C, D, and E that make up the oceanic crust of
the northern half of the Pacific plate suggests
that this section of the Pacific plate is generally
moving in which direction?
(1) from north to south (3) from westto east
(2) from 5outh to north (4) from east to west
49.
55.
The following drawing represents the ocean
floor between North America and Africa.
Scientists theorize that Africa and South America
were once part of the same large continent. Cite
two pieces of evidence that support this theory.
50. Which statement
Describe two expected similarities between rock
samples found equal distances from and on
opposite sides of the Mid-Atlantic Ridge.
52. Which feature is commonly formed at a plate
boundary where oceanic crust converges with
１
continental crust?
a mid-ocean ridge (3) a transform fault
an oceanic trench (4) new oceanic crust
２
268
fopt l2:
Eanhb Dynamic crust and tnterior
otd
-l
Young
ｄネ
︱
︲
明
０ 割﹁
も
51.
Which graph best represents the age of the
bedrock in the ocean floor along line AB?
ｄネ
︱
︲
呵
０ 瀾刊
お
best supports the concept that
continents have shifted position?
(1) Basaltic rock is found to be progressively
younger at increasing distances from a midocean ridge.
(2) Marine fossils are often found in deep-well
drill cores.
(3) The present continents appear to fit
together as pieces of a larger landmass.
(4) Areas of shallow-water seas tend to
accumulate sediment, which gradually sinks
56,
lgneous materials found along mid-ocean ridges
contain magnetic particles that show reversal of
magnetic orientation. This is evidence that
(1) volcanic activity has occurred constantly
throughout history
(2) Earth's magnetic poles have exchanged
their positions
(3) igneous materials are always formed
beneath oceans
(4) Earth's crust does not move
Base your answenr to questions 57 thrcugh 51 on the following diagram. The diagram shows the magnetic
orientation of igneous rock on the sea floor on the east side of a mid-ocean ridge, The pattern on the west
(left) side of the ridge has been omitted. The age of the igneous rock and its distance from the ridge center
are shown.
Ridge Center
I
Magnetic minerals in these rock indicate magnetic nonh
as it i5 today.
01234
I
Magnetic minerals in this rock indiaate magnetic nonh
to be where magnetic south is today.
0
Age (millionr ef years)
s0
r00
150
Distance (km)
57. Which of the following diagrams best represents 59. Which inference can best be made from the
the pattern of magnetic oribntation in the sea
diagram?
floor on the west (left) side of the ocean ridge?
(1) The orientation of Earth's magnetic field
(2)
has
reversed with time.
The size of the continents has changed
with time.
(3) The elevation of sea level has changed
with time.
(4) The amount of fossil material preserved in
the igneous rock has changed with time.
①②００
50. The crustal material on both sides of the ridge
indicates that the tectonic plates are
diverging
converging
not moving
moving parallel to the ridge
58. According to the diagram. what
is the
approximate rate of sea-floor spreading?
(1) 1 km/million years
(2) 2 km/million years
(3) 40 km/million years
(4) 50 km/million years
“ ２０“
61. As the distance from the center of the ridge
increases, the age of the rocks
decreases
increases
remainS the same
increases and decreases in a cyclic pattern
Topic 12: EarthS Dyoamic Crust and
lnterior
269
Base your answers
to questions 62 through 66 on the following diagrams. Diagram I is a map showing the
location and bedrock age of some of the Hawaiian lslands. Diagram ll is a cross section of an area of Earth
that illustrates a stationary magma sourre (rising magma and hot spot) and the process that could have
formed the islands.
Ｎ ４ １ １
Kauai
56to38m‖ lion
years
Oahu
2.5
to 2.2 million years
宵tua6
聰
Southeast
Pacincocean
Hawaii
than
L0 million
Less
0
50
l`
Magma source
￨
lf each island formed as the tectonic plate
moved over the magma source in the mantle,
as shown in diagram ll, where would the next
volcanic island form?
(1) northwest of Kauai
(2) northeast of Hawaii
(3) southeast of Hawaii
(4) between Hawaii and Maui
Which of the following graphs best represents
the ages of the Hawaiian lslands, comparing
them from point A to point B?
olヒ
A―
① ００ ０
Hawaiian lslands is probably
thinner and similar in composition
thinner and different in composition
thicker and similar in composition
thicker and different in composition
Volcanic activity like that which produced
the Hawaiian lslands is usually closely correlated with
(l)
nearness to the center o{ a large ocean
(2) sudden changes in Earth's magnetic field
(3) frequent major changes in climate
(4) frequent earthquake activity
Which of the Hawaiian lslands has the greatest
probability of a volcanic eruption?
３
１
４
２
Oahu
Maui
Hawa11
奎IL
●B
A―― ― ―― ‐ B
(1)
Compared to the continental crust of North
America, the oceanic crust in the area of the
Kauai
lll
years
100 150km
￨
Mantle
曹
￨[〕 ≧
1≧ :≧
A―
B
(3)
曼lh
A―
(2)
6フ .
B
(4)
What do mid-ocean ridges and hot spots have in
common?
(1) They are associated with rising magma.
(2) They are always associated with present-day
plate boundaries.
(3) They commonly are associated with
earthquakes of great depth.
(4) Neither is associated with plate motions.
Which observation provides the strongest
evidence for the inference that convection
cells exist within Earth's mantle?
(1) Sea level has varied in the past.
(2) Marine fossils are found at elevations high
Stop Here
above sea level.
(3) Displaced rock strata are usually
accompanied by earthquakes and volcanoes.
(4) Heat-flow readings vary at different
locations in Earth's crust.
27Q
toptc l2: Earths Dynamrc Crust and lnterior