1. Mountains rise abruptly from the surrounding terrain, mountain ranges
group mountains, mountain belts are chains of mountain ranges with lengths
measured in thousands of kilometers but much narrower widths. Only about 1/8
of the crustal thickness of a mountain range is actually exposed.
2. Higher mountain belts are generally younger geologically than lower belts.
Episodes of uplift and erosion occur throughout the history of mountain ranges,
but ultimately they stabilize and become eroded into plains. The stable portion
of a continent is its craton. Basement rocks of the craton, exposed in the
Precambrian shield, are the roots of former mountains.
3. Mountains are formed from great thicknesses (>10 km) of marine sedimentary
and volcanic rocks deformed into fold and thrust belts produced by crustal
shortening. Metamorphism and plutonism accompany mountain-building and migmatites
may be formed in those portions of mountain belts once deeper in the crust.
Normal faulting is more common in the later stages of a mountain belt's history,
but some takes place simultaneously with compression in the active mountain
building stage.
4. Crust beneath mountains has the same density as the rest of the continent,
but is thicker. Crust is thicker under younger mountains than under older
ones.
5. Frequent earthquakes, active volcanoes, and parallel oceanic trenches
characterize young mountain belts.
6. Mountain building can be divided into three stages (accumulation, orogeny,
uplift and block-faulting), although there are no sharp time boundaries between
them. Accumulation in opening ocean basins is associated with passive continental
margins, and dominated by sedimentary rocks derived from continental sources,
with volcanics rare or absent. Accumulation along converging boundaries is
dominated by sandstone (as graywacke) and shale, usually associated with volcanics,
both of which are derived from magmatic arcs.
7. Orogeny is an episode of intense, mostly compressional folding and faulting,
metamorphism and igneous activity that actually forms mountains. Ocean-continent
convergence exhibits folding and faulting of an accretionary wedge "snowplowed"
off the subducting plate. Fold and thrust belts form on the craton side of
the mountain belt, with thrusting toward the craton.
8. Normal faulting may actually accompany the compressional folding and faulting
as a result of gravitational collapse and spreading (extension) of the central
portion of the mountain belt being formed. This flowing may also account for
the presence of deep-seated metamorphic rocks at the surface in mountain belts.
9. Arc-continent convergence welds the island arc to the continent and causes
the subduction zone direction to "flip" by developing subduction
on the oceanward side of the island arc.
10. Continent-continent convergence deforms sedimentary rocks deposited in
a closing ocean basin and accounts for mountain belts within continents (Urals,
Alps, Himalaya). The Appalachian Mountains along eastern North America reflect
this history through a Wilson Cycle of opening, closing, and reopening of
an ocean basin associated with Pangaea.
11. Uplift and block-faulting in response to isostatic adjustment follow
orogeny. Erosion lowers mountains and brings them into isostatic equilibrium.
Higher areas have thicker crust, although the crust under the Rocky Mountains
is no thicker than it is under Denver, immediately to the east. This discrepancy
may reflect hotter, less dense mantle beneath that part of the Rockies. Fault-block
mountains, like the Tetons and Sierra Nevada, may be formed during this stage
of extension and isostatic adjustment. Volcanism may also occur. Delamination
is the detachment of the mantle portion of the lithosphere beneath a mountain
belt. This process allows mantle rock to rise against continental crust, heating
and thinning it. This process may explain extensional features in mountain
belts, like the Basin and Range, and why continents break-up.
12. Continents grow larger by addition of accreted terranes along their margins
through orogeny. Suspect terranes are those that may not have formed at their
present site, while exotic terranes clearly did not form at their present
site. Suspect and exotic terranes can be explained by drift of fragments of
continents, microcontinents, and island arcs, ultimately colliding with other
landmasses. Virtually all continental margins exhibit some suspect and exotic
terranes.
