Origins of Mountains - Mountain Geography: Physical and Human Dimensions - page 44

FIGURE 2.15 Diagram illustrating dynamics of a tectonic aneurysm, shown at a mature state. The fea-
ture begins with a capture or diversion of a major river, producing a deep gorge that weakens the
crust and draws advective flow of crustal rock at depth toward the topographic gap. This builds
relief, leading to further rapid erosion rates, which further weaken the crust, allowing updoming
of the brittle-ductile transition, as well as a steepening of the thermal gradient. Hot crustal rock
can experience decompression melting and intrusion (anatexis), as well as low-pressure/high-tem-
perature metamorphism as it moves rapidly toward the surface. (After Zeitler et al. 2001b.)
Principal Mountain Types
Individual mountains or small mountain ranges are also formed in a variety of ways.
Turning to Fairbridge's (1968) pre-plate tectonic, generic classification of mountains,
and leaving aside the most minor types and those discussed above, we have various
categories of structural, tectonic, or constructional forms: volcanic mountains, fold and
nappe types, fault-block mountains, and dome mountains, all with various degrees of
erosion.
Volcanic Mountains
Volcanoes result from both molten and solid material that is ejected from the Earth to
accumulate on the surface in various sizes and forms (Decker and Decker 2006). Vol-
canic mountains are classified into four types by differing eruptive habits and constitu-
ent materials (Fig. 2.16). Volcanic activity commonly begins as gas-rich magma moves
forcefully to the surface through a circular conduit pipe, and more rarely through a lin-
ear fissure in the Earth. Successive eruptions of different lavas or of semisolid or sol-
id particles known as pyroclastics (Greek for “fire-broken”), or a combination of these,
commonly separated by long periods of inactivity, build the structure of a volcano.
Small, steep-sided volcanic mountains include ash and cinder cones and silica-rich
rhyolite lava domes. Ash and cinder cones result from explosive eruptions of pyro-
clastics. Volcanic domes are built of slow-flowing, cooler lava, which, because of high
viscosity, also tend to plug volcanic conduits and lead to later explosions. Among the
largest of volcanoes are shield volcanoes, gentle-sided, massive accumulations of fluid