Origins of Mountains - Mountain Geography: Physical and Human Dimensions - page 47

and overwhelmed the town of Armero (Wright and Pierson 1992; Decker and Decker
2006). Phreatic explosions occur when underground water comes in contact with rising
magma, flashes into steam, and bursts out of the ground. The crater mountain of Dia-
mond Head in Honolulu, Hawai'i, is the result of a phreatic eruption. In the year 79
C.E., the people of ancient Pompeii, at the foot of the mountain now called Vesuvius near
Naples, Italy, discovered to their horror that catastrophic ash falls and pyroclastic flows
cannot be escaped if one lives in the direct vicinity. Failure to heed the warnings of an
impending eruption—common ground tremors and precursory minor eruptions—too of-
ten has led to major loss of life in eruptions.
The explosive eruption of the stratovolcano Mount St. Helens in the Cascade Range
of Washington State, USA, in 1980 (Lipman and Mullineaux 1981; Decker and Decker
2006) occurred when new magma moved slowly up into the volcano after several cen-
turies of dormancy. A bulge formed on the north side of the mountain, which grew at
a rate of ∼1.5 m/day (∼5 ft) until after an intermediate-sized earthquake it collapsed
abruptly in a massive landslide. This released the pressure on the gas-and steam-filled
magma, which then burst forth in a superheated (500°C/932°F), supersonic lateral
blast, devastating a zone 20 km (13 mi) outward from the volcano and 30 km (19 mi)
wide. Within seconds a large vertical eruption sent a cloud of volcanic ash to an altitude
of about 18 km (11 mi). More than 60 people were killed. Study of the volcanic land-
slide that triggered the eruption enabled recognition of many prior events elsewhere
involving volcanoes that had collapsed and sent out large rock and debris avalanches.
This newly recognized hazard is now a part of the disaster planning for the monitoring
and warning systems that exist around many volcanic mountains in the developed parts
of the world.
Faulted and Folded Mountains
The major mountain ranges of the world are eroded out of rocks that were broken and
crumpled by the forces of plate motion. Most geologists differentiate between intense
deformational forces that cause faulting and folding, and the more general uplift that el-
evates landscapes. Many deformation structures are formed deep within the Earth and
then uplifted to produce mountains. In most cases, the geologic structures and the land-
forms eroded from them are separate and distinct. In some regions, however, faults and
folds are forced up out of the ground in active deformation, and the structure and the
landform are commonly the same. Such neotectonic activity generally produces a dis-
tinctive suite of landforms that enable interpretation of the sequence of structural and
geomorphic events that produced them. It is important to distinguish between previ-
ously deeply buried, old rock structures that have later been exposed and fashioned into
landforms by erosion, and those structures that are landforms by virtue of their recent
deformation at the surface.