Environmental Engineering Reference
In-Depth Information
rejuvenated elements of Proterozoic terranes up to 2 Ba old in the central Andes. The
type cordilleran Andes were formed by 'head-on' motion of the Nazca plate after
c
. 190
Ma ago. Older, late Panthalassic Ocean marine sediments and associated rhyolite-granite
intrusions were compressed and thrustfaulted with older terranes to form the eastern
Andes. Active subduction, as the Atlantic opened after 135 Ma, formed the western
Andes by progressive arc-continent collision with the South American plate and
associated landward intrusive and volcanic activity (Figure 25.1).
Molasse
sediments,
from vigorous syn-formational erosion, and ignimbrite deposits from eruptive volcanoes
THE CHARACTER OF MOUNTAINS
key concepts
In
topographical
terms, landsystems exceeding 600 m in altitude with typically steep and
often rocky slopes are considered to be mountains. The term is also applied to other high-
altitude regions above 2 km with more subdued relief which share other
montane
characteristics such as extreme climatic and geomorphic systems. The Tibetan plateau
(reaching elevations over 5 km) and the Bolivian Altiplano (over 4 km) meet these
criteria. Less distinct elevated landscapes with less extreme environments are termed
uplands
. The Hispanic word
sierra
describes saw-toothed peaks over 2 km high.
Mountains usually occur in linear chains, ranges or cordillera but also embrace single,
isolated high peaks worthy of a distinct name. The latter may be constructional, in the
case of stratovolcanoes, or erosional remnants of a former plateau in the case of
monadnocks
. In
structural
terms mountains are large-scale, elevated crustal disturbances
characterized by intense folding, metamorphism and granitic intrusion - the very essence
of morphotectonics.
Climatic
character shares extremes of temperature and precipitation
of the mountain climate with extreme hydrological and geomorphic processes on steep
slopes.
Ecological
definition emphasizes the presence of one or more
montane forest-
timberline-alpine
elements or their
ecotones
. A proviso that these may be contemporary
or Pleistocene in age acknowledges the impact of Holocene climate change, biotic
refugia
(survival habitats) and relict landforms. The terms 'mountain' and 'alpine'
environments often become entangled. Although they are not synonymous, confusion is
understandable, given the number of related terms derived from the Latin name alpes for
the European snow-covered mountains bordering northern Italy - the type Alps.
collected in intercordilleran and marginal basins (Plate 25.1). These are best developed in
sedimentary rocks of the Altiplano, in southern Peru and Bolivia, and Puna plateaux
ignimbrites of northern Chile and Argentina.
The Andes rise steeply from a narrow Pacific coastal plain, nowhere more than 125
km wide, and extend the entire length of South America. Four divergent lower cordillera,
emerging from the Central American- Caribbean orogens, fuse into the Cordillera
Oriental (east) and Occidental (west) of southern Colombia and Ecuador, over 200 km
wide and rising to peaks above 5 km. The Andes widen steadily through Peru to a
maximum of 750 km in northern Chile and Bolivia, where the main east and west chains
(5·5-7·0 km high) diverge around the Altiplano. The system narrows again to less than
250 km and falls to 2·5-4·0 km in southern Chile, south of the highest peak, Nevos Ojos
del Salado (7084 m) on the Chile-Argentina border. The Andes now run out in Tierra del
