Origins of Mountains - Mountain Geography: Physical and Human Dimensions - page 45

basaltic lava, such as those of the Hawai'ian Islands, resembling a gigantic warrior's
shield emerging from the sea. Composite cones or stratovolcanoes are equally huge,
steep-sided accumulations of interspersed lava flows and pyroclastics that pile up into
a major mountain form.
Volcanic eruptions provide spectacular evidence of their ongoing construction. In
some cases they can develop with incredible speed. The Paracutin ash and cinder cone
in Mexico emerged from a small depression in a farmer's field in 1943 and grew to 40
m (130 ft) the first day and 140 m (460 ft) in a week. By the end of the first year it had
grown to a height of 325 m (1,065 ft). After nine years of sporadic pyroclastic eruptions
and nearly continuous discharge of lava from vents at its base, activity ceased and the
mountain went into dormancy or extinction (Anonymous 1982; Luhr and Simkin 1993;
Decker and Decker 2006).
FIGURE 2.16 Volcanic mountain features shown in schematic cross sections arranged according to
relative size and typical explosive-ness of eruption. (A) A cinder cone or small steep-sided volcano
formed by explosive eruption of pyroclastics. (B) A small, quiet effusion of high-viscosity, rhyolitic
lava forming a dome with steep sides. (C) A composite or stratovolcano formed from multiple, al-
ternating eruptions of lava and pyroclastic deposits, whose interlayering and imbrication provide
greater strength and result in a large, steep-sided cone. (D) A shield volcano, built from many
outpourings of low-viscosity basaltic lava which produce a large, gently arched surface. (Adapted
from various sources.)
Volcanoes occur on all continents and make up a substantial percentage of the
world's mountains. The number becomes even more impressive if we include those hid-
den by oceans—most islands are nothing more than the summits of undersea volcanoes.
If one includes the submerged as well as the visible part, the world's tallest and largest
peak is a volcano—Mauna Loa in Hawai'i. The above-water portion of this gently sloping
shield volcano, at 4,169 m (13,680 ft), is not particularly impressive, but it rises a total
of 9,100 m (30,030 ft) from its sea-floor base, which is over 225 km (∼140 mi) in dia-
meter. The highest volcano on land, at 6,267 m (20,556 ft), is Mount Chimborazo in the
Andes of Ecuador. The highest known mountain in the solar system is the giant shield
volcano of Olympus Mons on Mars. Its basal diameter is 600 km (375 mi), more than the