Geology Reference
In-Depth Information
small proportion consist of pyroclastic material; these are described separately
below. The lava within plugs may be of various grain sizes and compositions;
it often shows alteration and deposition of secondary minerals as a result of
hydrothermal activity concentrated around the cooling volcanic vent. Most plug
rocks show features such as brecciation (that is, an unsorted mixture of angu-
lar fragments) as a result of the passage of gases and hydrothermal solutions
through them. Such brecciated rocks are termed
volcanic agglomerate
or
brec-
cia
. Many small pyroclastic volcanoes are underlain by plugs or pipes composed
of pyroclastic material. These are termed
diatremes
, and include
breccia-pipes
,
tuff pipes
and
kimberlites
. Diatremes are narrow, steep-sided cylindrical or
funnel-shaped pipes which have intruded through the crust, and consist of solid
fragments of crustal and mantle rocks with or without solid fragments of juvenile
rock, apparently emplaced at a low temperature. Diatremes may be composed
mainly of comminuted crustal rock or juvenile material. Some diatremes have
erupted at the Earth's surface as ash- or tuff cones, others can be shown not
to have reached the Earth's surface but to have been revealed subsequently by
erosion and are then termed 'blind pipes'. The crustal fragments in diatremes
range from fine ash to blocks several metres in size. Juvenile fragments are
generally glassy ash and pumice but may include crystalline material.
In natural exposures of diatremes, the walls of the vent are usually visible
over only 100 - 200 m; most diatremes seem to converge downwards like steep-
walled funnels. The rocks forming the diatreme may be bedded as a result of
infall of products of successive eruptions and frequently they can be matched
with rocks in the surrounding walls. Where diatremes were emplaced within a
well-defined stratigraphic sequence, matching of such fragments may indicate
their upward or downward movement. Commonly the fragments appear to have
subsided with the diatreme with layers of tuff. Such subsidence may be up
to several hundred metres (or, rarely, 1500 m) in vertical extent. The tuff and
breccias comprising such diatremes are termed
tuffisite
and
intrusive breccia
,
respectively, to distinguish them from extrusive rocks.
A distinctive rock that occurs in diatremes and occasional dykes is kimberlite
(Figure 6.12), which is of economic interest since many kimberlites con-
tain diamonds. Kimberlite is a serpentinised and carbonated mica-peridotite of
porphyritic texture, containing nodules (that is, xenoliths) of ultrabasic rock-
types characterised by high-pressure minerals such as pyrope (garnet) and
jadeitic diopside (pyroxene). They are dark-green to dark-blue grey, and are
often extremely brecciated with large crystal or rock fragments within a fine-
grained matrix. Kimberlite pipes include an extraordinarily varied assortment of
xenolithic inclusions; for example angular blocks of the country rock, rounded
blocks of high-grade metamorphic rocks, such as might form the lower con-
tinental crust, and rounded blocks of ultramafic rocks composed of varying
combinations of olivine, pyroxene, garnet (pyrope), spinel, Fe-Ti oxide and
