Magma compositions vary in SiO 2 , iron, magnesium, and volatile gases
Mafic magma – low in SiO 2 (45-50 %) but high in iron, and magnesium
Felsic magma – high in SiO 2 (up to 75 %) but low in iron, and magnesium
Intermediate magma – intermediate range of SiO 2 (50-65 %), iron, and magnesium
Amount of volatile gases will affect explosive characteristics of eruptions
Figure 5.3 Common volcanic rock types (bottom labels) and their plutonic equivalents (top). The rock names reflect varying proportions of silica, iron, and magnesium, and thus of common silicate minerals. Rhyolite is the fine-grained, volcanic compositional equivalent of granite, and so on.
Shield like shape - larger area relative to height
Examples: Hawaiian Island chain
Composed of more viscous andesite or rhyolite
these lavas do not flow
Ooze out onto surface from a tube and pile up close to the vent
Compact, small, and steep sided
Various locations around Pacific Ring of Fire
Figures 5.8 Shield volcanoes and their characteristics (A) Schematic diagram of a shield volcano in cross section. (B) Very thin lava flows, like these of Kilauea in Hawaii, are characteristic of shield volcanoes. (C) Fluidity of Hawaiian lavas is evident even after they have solidified. This ropy-textured surface is termed pahoehoe (pronounded “pa-hoy-hoy).
Figures 5.9 Mauna Loa, an example of a shield volcano. (A) View from low altitudes, Note the gently sloping shape summit caldera has been enlarged by collapse. The peak of Mauna Kea rises at rear of photograph. (B) Bird’s-eye view of Hawaii, taken by Landsat satellite, shows its volcanic character more clearly. The large peak with abundant relatively fresh, dark lava flows surrounding it is Mauna Loa; the smaller one, above it, is Mauna Kea.
Figure 5.10 Volcanic dome formation. (A) Schematic of volcanic dome formation. (B) Novarupta dome, Katmai National Park, Alaska.
Size of pyroclastics range from ash (very fine), cinders, bombs, or blocks (very coarse)
Pyroclastics fall close to the vent creating a cone shaped volcano
Example: Particutin, Mexico
Figures 5.12 Paricutin (Mexico), a classic cinder cone. (A) Night view shows formation by accumulation of pyroclastics flung out of the vent. (B) Shape of the structure revealed by day is typical symmetric form of cinder cones.
Figures 5. 11 Include types of pyroclastics (which sometimes are produced even by the placid shield volcanoes). Bombs are molten, or at least hot enough to be plastic, when erupted, and may assume a streamlined shape in the air.. (A) Volcanic ash from Mount St. Helens (B) Bombs from Mauna Kea (C) Blocks from Kilauea (D) is volcanic breccia (at Mt. Lassen) formed of welded hot pyroclastics
Composite Volcanoes (Stratovolcanoes) are built up of layers of lava and pyroclastics
Mix of lavas and pyroclastic layers allows for a tall volcano to form
Usually associated with subduction zones
These tend to be violent and explosive
Example: Mount St. Helens, Cascade Range, Northwest U.S.A.
Figures 5.13 (A) Schematic cross section of a stratovolcano (composite volcano), formed of alternating layers of lava and pyroclastics. (B) Two composite volcanoes of the Cascade Range: Mount St. Helens (foreground) and Mt. Rainier (rear); photograph predates 1980 explosion of Mount St. Helens.
Secondary Effects; Climate and Atmospheric Chemistry
Figure 5.14 Formation of “lava trees” near Kilauea illustrates the effect of quenching lava. As hot lava hits cooler trees – and moisture in trees evaporates, absorbing more heat – lava is quenched and hardened. Main mass of fluid lava flows on, leaving the lava trees.
Figures 5.15 Impact of lava flows on Heimaey, Iceland. (A) May showing extent of lava filling the harbor of Heimaey after 1973 eruption. (B) Lava flow control efforts on Heimaey.
Figure 5.16 Aftermath of Mount St. Helens eruption, 18 May, 1980
Figure 5.17 Volume of pyroclastics ejected during major explosive eruptions. (numbers of casualties, where available, are given in parentheses).
Figure 5.18 The combination of large volumes of ash and heavy typhoon rains at Mount Pinatubo in 1991 proved too much weight for many buildings to bear, In fact, roof collapse was responsible for most of the casualties.
Figure 5.19 Town of Amero was destroyed by lahars from Nevado del Ruiz in November 1985; more than 23,000 people died.
Figure 5.22 St. Pierre, Martinique, West Indies, was destroyed by a nuee ardente ( pyroclastic flow ) from Mont Pelee, 1902
Figure 5.23 (A) by the time this ash cloud loomed over Plymouth on 27 July 1996, the town had been evacuated; the potential for pyroclastic eruptions of Soufriere Hills volcano was well recognized.
Figure 5.25 Ash and gas from Mount Pinatubo was shot into the stratosphere, and had an impact of climate and atmospheric chemistry worldwide. Eruption of 12 June 1991
Figure 5.26 Satellites tracked the path of the airborne sulfuric-acid mist formed by SO 2 from Mount Pinatubo; winds slowly spread it into a belt encircling the earth
Figure 5.27 Effect of 191 eruption of Pinatubo on near-surface (lower-atmosphere) air temperatures. Removal of ash and dust from the air was relatively rapid; sulfate aerosols persisted longer. The major explosive eruption occurred in mid-June 1991.
Present and Future Volcanic Hazards in the United States
Long Valley and Yellowstone Calderas
Figure 5.21 Pyroclastic flow from Mount St. Helens
Figure 5.31 The cascade Range volcanoes and their spatial relationship to the subduction zone and to major cities. Shaded area is covered by young volcanic deposits less than 2 million years old. Volcanic symbols on chart at right indicate dates of significant eruptions.
Figure 5.29 Restricted-access zones established by the Washington Department of Emergency Services before 18 May 1980 eruption of Mount St. Helens. Red zone : No access except by scientist, law-enforcement officials, and search-and-rescue personnel. Blue zone : Logging permitted, and residents with permits allowed access, but no overnight stays. Shaded area is national forest land. Casualties would have been still fewer if unauthorized people had not sneaked into the restricted areas.
Figure 5.32 Over the last 5000 to 6000 years, huge mudflows have poured down stream valleys and into low-lying areas, tens of miles from Mount Rainier’s summit. Even Tacoma or Seattle might be threatened by renewed activity on a similar scale.
Figures 5.33 a and b The Aleutians are a region of active volcanism – fortunately, a rather sparsely populated one. (A) Map of southwestern Alaska showing major volcanic peaks. Mount Spurr is showing signs of renewed activity as this is written. (B) Mount Veniaminof in eruption; note lahars formed when hot ash meets snow.
Figure 5.34 Map of Mammoth Lakes area, showing Long Valley Caldera, site of recent earthquakes, and area of recent uplift beneath which magma is rising.
Figures 5.35 a and b (A) Continuing thermal activity at Yellowstone National Park is extensive, (B) The size of the caldera and scale of past eruptions cause concern about the future of the region.