Geoscience Reference
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Oceanic trench
(subduction zone)
130° E
140° E
S.
KOREA
CHINA
S
JAPAN
A
KORE A
KORE A
K O
30° N
CH
NA
CH N
N
North
Pacific
Ocean
PHILIPPINE
SEA
20° N
Taiwan
Northern
Mariana
Islands
Guam
10° N
PHILIPPINES
Island arc
I a n n n d a r rc
Oceanic
crust
M
M ALAY
LA
L LA
Y
Palau
ALAY
M
Equator
Oceanic
crust
INDO NESIA
NDOO
S A
S
NEW
NE W
GUINEA
G UINE A
N
NEW
GUINEA
ND O
D
E
N
Active volcano
v v o c c a o
G
A A A A
A
A
A i v v
v
c c
c r u u t
A
Magma
M M M M M
M M M M M M g g g g g g g g g gm a
Partial
melting
Partia
m m ltin g
Subducting
lithosphere
S Su b b bd u uc ti n ng
tho o p p ere
i t
here
g
(a)
(b)
Figure 13.11 Collision of two plates of oceanic crust. (a) When two plates of oceanic crust collide, one of the plates will subduct
beneath the other despite the fact they both have similar densities. An oceanic trench forms along the subduction zone. An island
arc also develops due to volcanic activity along the plate boundary. (b) Guguan Island in the northern Marianas Islands in the Pacific
Ocean. This island is a volcano (note the cone in the middle of the island) that formed due to subduction along the Marianas Trench.
This trench is the deepest location on the surface of Earth's crust, with a maximum depth of 9636 m (31,614 ft) below sea level.
area. This kind of fold occurs when the amount of compres-
sion is relatively low. An excellent example of a monocline
occurs in southeastern Utah, near the town of Mexican Hat
(Figure  13.12c). More extensive compression causes anti-
clines and synclines to form (Figure 13.12a). An anticline is
a portion of the fold where the rock layers arc upward to form
a concave arch along the fold axis. One-half of such a fold is
called a limb . In contrast, a syncline is the portion of the fold
where rock layers dip downward to form a convex trough.
If the collision is especially intense, the rocks can be folded
so much that an overturned fold (or recumbent fold) results.
Still more compression results in an overthrust fold , which
occurs when one part of the rock mass is shoved up and over
the other.
mountain formation, as well as the shape of the Earth's surface
in general, falls within the subdiscipline of physical geography
called geomorphology . Geomorphology is the study of the for-
mation, shape, spatial distribution, and evolution of landforms
on Earth—thus, the name geo (Earth) morphology (shape). In
contrast to a landscape , which is the overall appearance of a
place in terms of its vegetation, topography, or human modifi-
cations, a landform is a distinct geographic feature, such as a
mountain, river valley, coastline, or sand dune, to name but a
very few. Each of these landforms evolved due to distinct geo-
morphic processes, which will be discussed in later chapters.
As you might imagine, the study of geomorphology is rooted
deeply in geology because it requires a thorough understanding
of how sediments are eroded, transported, and deposited. The
discipline is also geographically based because geomorpholo-
gists are interested in the way landforms change across space
and time.
Geomorphology and the Evolution of the Appala-
chian Mountains Large-scale compression of rocks is fre-
quently associated with the growth of mountains. The study of
Anticline A convex fold in rock in which rock layers are bent
upward into an arch.
Overthrust fold A structural feature where one part of the
rock mass is shoved up and over the other.
Syncline A concave fold in rock in which rock layers are bent
downward to form a trough.
Geomorphology The branch of physical geography that
investigates the form and evolution of the Earth's surface.
Overturned fold A structural feature in which the fold limb is
tilted beyond vertical, which results in both limbs inclined in the
same direction, but not at the same angle.
Landform A natural feature, such as a hill or valley, on the
surface of Earth.
 
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