Geoscience Reference
In-Depth Information
basin-range systems (
Figure 24.8
).
Inland, the Rocky
Mountains are of Mesozoic origin, with older sediments
thrust eastward over continental cratons. They rise steeply
above the interior Canadian prairies and the High Plains
of the United States, 750 km east of the Pacific in Canada
and 1,500 km inland in Colorado, and widen from 250
km in northern British Columbia to 600 km in Wyoming.
Peaks rise steadily in the same direction from 2·3km to
3·9 km in Canada (highest peak, Mount Robson, 3,954 m)
to 4·2 km in the Wind River range (Wyoming) and 4·4 km
in the Park and Sangre de Cristo ranges (Colorado). The
Brooks range in Alaska is of similar age, and both systems
may contain A-subduction elements due to Atlantic
spreading. They formed a buttress against which Pacific
plate subduction established younger and still active
coastal cordillera comprising accret
ed
,accret
ing
and
oceanic arcs from Mexico to the Aleutian Islands.
This landsystem is complicated by massive
dispersion
tectonics
. Oblique subduction and transform plate
motion have displaced
cratonic
,
accretion prism
and
arc
terranes 10
2-3
km northwards along strike-slip faults. This
consumed the former Farallon plate and is now working
on the Cocos plate, creating a further complication.
Westward motion of the North American plate began
to override the East Pacific Rise in the Oligocene epoch
35 Ma ago, generating thermal epeirogenesis and basalt
effusion above hot spots 1,000 km inland. The resulting
crustal extension triggered large-scale asymmetrical rifting
rocks) formed in syntectonic basins among active granite-
gneiss fault blocks (distance) at Cajon Pass, southern
California. Although up to 1 Ga old, the granite and gneiss are
caught up in the active San Andreas fault system.
Photo: Ken Addison
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. Climaticcharacter shares extremes of temperature and precipitation of the mountain climate with
extreme hydrological and geomorphic processes on steep slopes. Ecologicaldefinition 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.
