Geology Reference
In-Depth Information
commonly they are parts of linear associations of peaks and
ridges known as
mountain ranges
that are related in age and
origin. A
mountain system
, a complex linear zone of defor-
mation and crustal thickening, on the other hand, consists
of several or many mountain ranges. The Teton Range in
Wyoming is one of many ranges in the Rocky Mountains
(Figure 10.17). The Appalachian Mountains of the eastern
United States and Canada is another complex mountain
system made up of many ranges, such as the Great Smoky
Mountains of North Carolina and Tennessee, the Adiron-
dack Mountains of New York, and the Green Mountains of
Vermont.
Image not available due to copyright restrictions
Mountains form in several ways, some involving little or
no deformation. For example, differential weathering and
erosion have yielded high areas with adjacent lowlands
in the southwestern United States, but these erosional
remnants are flat topped or pinnacle-shaped and go by
the names
mesa
and
butte
, and most are less than 300 m
high (see Chapter 15).
Block-faulting
is another way that
mountains form, but this is caused by deformation of the
crust. It involves movement on normal faults so that one
or more blocks are elevated relative to adjacent blocks
(
Figure 10.18). A classic example is the Basin and Range
Province, which is centered on Nevada, but extends into
adjacent areas (see Chapter 23). Differential movement on
faults has produced uplifted blocks called
horsts
and down-
dropped blocks called
grabens.
Erosion of the horsts has
yielded mountainous topography.
Volcanic outpourings form chains of volcanic moun-
tains such as the Hawaiian Islands, where a plate moves over
a hot spot (see Figure 2.22). Some mountains such as the
Cascade Range of the Pacifi c Northwest are made up almost
entirely of volcanic rocks (
◗
Figure 10.19), and the mid-
ocean ridges are also mountains (see Figure 2.11). However,
most mountains on land are composed of all rocks types
and show clear evidence of deformation by compression.
◗
Image not available due to copyright restrictions
Geologists defi ne the term
orogeny
as an episode of moun-
tain building during which intense deformation takes place,
generally accompanied by metamorphism, the emplacement
of plutons, especially batholiths, and thickening of Earth's
crust. The processes responsible for an orogeny are still not
fully understood, but it is known that mountain building is
related to plate movements.
Any theory that accounts for mountain building must
adequately explain the characteristics of mountain ranges,
such as their geometry and location; they tend to be long and
narrow and at or near plate margins. Mountains also show
intense deformation, especially compression-induced over-
turned and recumbent folds, as well as reverse and thrust
faults. Furthermore, granitic plutons and regional meta-
morphism characterize the interiors or cores of mountain
ranges. Another feature is sedimentary rocks now far above
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