Geology Reference
In-Depth Information
Geo-Focus
Some Remarkable Volcanic Necks
We mentioned that as an extinct
volcano weathers and erodes, a rem-
nant of the original mountain may
persist as a volcanic neck. The origin
of volcanic necks is well known, but
these isolated monoliths rising above
otherwise rather fl at land are scenic,
awe-inspiring, and the subject of leg-
ends. They are found in many areas
of recently active volcanism. A small
volcanic neck rising only 79 m above
the surface in the town of Le Puy,
France, is the site of the 11th-century
chapel of Saint Michel d'Aiguilhe
(
Figure 1). It is so steep that materi-
als and tools used in its construction
had to be hauled up in baskets.
Perhaps the most famous volcanic
neck in the United States is Shiprock,
New Mexico, which rises nearly
550 m above the surrounding plain
and is visible from 160 km away.
Radiating outward from this conical
structure are three vertical dikes that
stand like walls above the adjacent
countryside (
◗
◗
Figure 1
This volcanic neck in Le Puy,
France, rises 79 m above the surface of the
town. Workers on the Chapel of Saint Michel
d'Aiguilhe had to haul building materials and
tools up in baskets.
◗
Figure 2
Shiprock, a volcanic neck in
northwestern New Mexico, rises nearly
550 m above the surrounding plain. One of
the dikes radiating from Shiprock is in the
foreground.
Figure 2). According
to one legend, Shiprock, or
Tsae-bidahi
,
meaning “winged rock,” represents
a giant bird that brought the Navajo
◗
people from the north. The same
legend holds that the dikes are snakes
that turned to stone.
Chapter Summary
Magma is the term for molten rock below Earth's
surface, whereas the same material at the surface is
called lava.
Silica content distinguishes among mafi c (45-52%
silica), intermediate (53-65% silica), and felsic
(>65% silica) magmas.
Minerals crystallize from magma and lava when small
crystal nuclei form and grow.
Rapid cooling accounts for the aphanitic textures of vol-
canic rocks, whereas comparatively slow cooling yields
the phaneritic textures of plutonic rocks. Igneous rocks
with markedly different-sized minerals are porphyritic.
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Magma and lava viscosity depends mostly on temperature
and composition: The more silica, the greater the viscosity.
Igneous rock composition is determined mostly by the
composition of the parent magma, but magma
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