Page 139 - Fundamentals of Geomorphology

122 STRUCTURE

Table 5.1 Pyroclastic flows and deposits Maars form in a similar way to scoria cones, but
in this case involving the interaction between magma
Pyroclastic flow Pyroclastic deposit and a water-bearing stratum – an aquifer. The result of

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