Page 449 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Mono Lake
413
Inyo Craters volcanic chain
▼Figure 13.25 Long Valley Caldera and forest death from CO2 emission, California.
[Bobbé Christopherson.]
(b) The effects of CO2 gassing from subsurface magma and related volcanic activity have killed one person to date.
Explosive Eruptions
Explosive eruptions are violent explosions of magma, gas, and pyroclastics driven by the buildup of pressure in a magma conduit. This buildup occurs because magma produced by the melting of subducted oceanic plate and other materials is thicker (more viscous) than magma that forms effusive volcanoes. Consequently, it tends to block the magma conduit by forming a plug near the sur- face. The blockage traps and compresses gases (so much so that they remain liquefied) until their pressure is great enough to cause an explosive eruption.
Such an explosion is equivalent to megatons of TNT blasting the top and sides off the mountain. This type of eruption produces much less lava than effusive eruptions, but larger amounts of pyroclastics, which include volca- nic ash (<2 mm, in diameter), dust, cinders, scoria (dark- coloured, cindery rock with holes from gas bubbles), pumice (lighter-coloured, less-dense rock with holes from gas bubbles), and aerial bombs (explosively ejected blobs of incandescent lava). A nuée ardente, French for “glow- ing cloud,” is an incandescent, hot, turbulent cloud of gas, ash, and pyroclastic that can jet across the landscape in these kinds of eruptions (Figure 13.29).
A mountain produced by a series of explosive erup- tions is a composite volcano, formed by multiple layers of lava, ash, rock, and pyroclastics. These landforms are sometimes called stratovolcanoes to describe the alternat- ing layers of ash, rock, and lava, but shield volcanoes also can exhibit a stratified structure, so composite is the pre- ferred term. Composite volcanoes tend to have steep sides and a distinct conical shape, and therefore are also known as composite cones. If a single summit vent erupts repeat- edly, a remarkable symmetry may develop as the mountain
CALIFORNIA
Long Valley Caldera
Crowley Lake
(a) The Long Valley Caldera in eastern California formed over 700 000 years ago.
Flood basalts cover several large regions on Earth, sometimes referred to as igneous provinces (Figure 13.28). The Deccan Traps is an igneous province in west-central India that is more than double the size of the Colum- bian Plateau. Trap is Dutch for “staircase,” referring to the typical steplike form of the eroded flood basalts. The Siberian Traps is more than twice the area of the igneous province in India and is exceeded in size only by the On- tong Java Plateau, which covers an extensive area of the seafloor in the Pacific. None of the presently active sites forming flood basalts (such as on Mauna Loa) come close in size to the largest of the extinct igneous provinces, some of which formed more than 200 million years ago.
▲Figure 13.26 Kı¯lauea landscape. The newest land on the planet is produced by the massive flows of basaltic lavas from the Kı¯lauea volcano, Hawai‘i Volcanoes National Park. In the distance, ocean water meets 1200°C lava, producing steam and hydrochloride mist. [Bobbé Christopherson.]