Geology Reference
In-Depth Information
Maars
form in a similar way to scoria cones, but
in this case involving the interaction between magma
and a water-bearing stratum - an aquifer. The result of
this combination is explosive. In the simplest case, an
explosion occurs in the phreatic or groundwater zone
and blasts upwards to the surface creating a large hole
in the ground. Thirty craters about a kilometre across
were formed in this way in the Eifel region of Germany.
These craters are now filled by lakes known as maars,
which gave their name to the landforms. Some maars
are the surface expression of
diatremes
, that is, vertical
pipes blasted through basement rocks and that contain
rock fragments of all sorts and conditions. Diatremes are
common in the Swabian Alps region of Germany, where
more than 300 occur within an area of 1,600 km
2
. Being
some 15-20 million years old, the surface expression of
these particular diatremes is subdued, but some form
faint depressions.
Tuff rings
are produced by near-surface subterranean
explosions where magma and water mix, but instead of
being holes in the ground they are surface accumulations
of highly fragmented basaltic scoria (Figure 5.4b). A first-
rate example is Cerro Xico, which lies just 15 km from
the centre of Mexico City. It formed in the basin of shal-
low Lake Texcoco before the Spanish drained it in the
sixteenth century.
Tuff cones
are smaller and steeper ver-
sions of tuff rings (Figure 5.4c). An example is El Caldera,
which lies a few kilometres from Cerro Xico.
Table 5.1 Pyroclastic flows and deposits
Pyroclastic flow
Pyroclastic deposit
Column collapse
Pumice flow
Ignimbrite; pumice and
ash deposit
Scoria flow
Scoria and ash deposit
Semi-vesicular andesite flow
Semi-vesicular andesite
and ash deposit
Lava flow and dome collapse
(explosive and gravitational)
Block and ash flow; nuée
ardente
Block and ash deposit
Source:
Adapted from Wright
et al.
(1980)
Ignimbrites
(derived from two Latin words to mean 'fire
cloud rock') are deposits of pumice, which may cover
large areas in volcanic regions around the world. The
pumiceous pyroclastic flows that produce them may run
uphill, so that ignimbrite deposits often surmount topog-
raphy and fill valleys and hills alike, although valleys often
contain deposits tens of metres thick known as valley
pond ignimbrite, while hills bear an ignimbrite veneer
up to 5 m thick. A
nuée ardente
is a pyroclastic flow
or 'glowing avalanche' of volcanic blocks and ash derived
from dense rock.
Scoria cones
are mounds of scoria, seldom more
than 200-300 m high, with a crater in the middle
(Figure 5.4a). Young scoria scones have slopes of 33
◦
,
which is the angle of rest for loose scoria. Monogenetic
volcanoes - that is, volcanoes created by a solitary erup-
tive episode that may last hours or years - produce them
under dry conditions (i.e. there is no interaction between
the lava and water). They occur as elements of scoria
cone fields or as parasitic vents on the flanks of larger
volcanoes. Dozens sit on the flanks of Mount Etna. Once
the eruption ceases, solidification seals off the volcanic
vent and the volcano never erupts again. Monte Nuovo,
near Naples, is a scoria cone that grew 130 m in a few days
in 1538; San Benedicto, Mexico, grew 300 m in 1952.
Scoria mounds are like scoria cones but bear no appar-
ent crater. An example is the Anakies, Victoria, Australia.
Nested scoria cones
occur where one scoria cone grows
within another.
Mixed-eruption volcanoes
As their name suggests, a mixture of lava eruptions
and scoria deposits produces
mixed-eruption volcanoes
.
They are built of layers of lava and scoria and are
sometimes known as
strato-volcanoes
(Figure 5.5). The
simplest form of strato-volcano is a simple cone, which is
a scoria cone that carries on erupting. The result is a single
vent at the summit and a stunningly symmetrical cone,
as seen on Mount Mayon in the Philippines and Mount
Fuji, Japan. Lava flows often adorn the summit regions of
simple cones. Composite cones have experienced a more
complex evolutionary history, despite which they retain
a radial symmetry about a single locus of activity. In the
history of Mount Vesuvius, Italy, for instance, a former
