Volcanoes

Figure 5.4 Volcanoes of the World

Magma Sources and Types

• Magma sources tend to be 50 to 250 km deep into the crust and upper mantle

• Temperatures increase as depth increases

• Volcanoes are generated at:– Divergent Plate Boundaries– Convergent Plate Boundaries– “Hot Spots”

Figure 5.2 Relationships of volcanic activity to plate tectonics.

Magma Sources and Types

• Magma compositions vary in SiO2 , iron, magnesium, and volatile gases

• Mafic magma – low in SiO2 (45-50 %) but high in iron, and magnesium

• Felsic magma – high in SiO2 (up to 75 %) but low in iron, and magnesium

• Intermediate magma – intermediate range of SiO2 (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.

Magma Sources and Types

• Mafic magmas produce basalt lavas– Intrusive equivalent is gabbro

• Intermediate magmas produce andesite lavas– Intrusive equivalent is diorite

• Felsic magmas produce rhyolite lavas– Intrusive equivalent is granite

Magma at Divergent Plate Boundaries• Magma produced at a Divergent Plate Boundary

is typically melted asthenosphere material• Asthenosphere is extremely rich in

ferromagnesian (ultramafic) and a melt from it is mafic (or ultramafic)

• Basalt is emplaced as new seafloor at the spreading ridge or a rift

• Rift systems in continental crust may melt granitic crust and produce andesite or rhyolite lavas– A bimodal suite of extrusive igneous rocks

characterize rift volcanoes

Magma at Convergent Plate Boundaries

• Magmatic activity at convergent boundaries is complex

• The composition of the subducted plate determines the composition of the lava– Subducted continental crust may melt and

produce rhyolite lava– Subducted oceanic crust may melt and

produce basalt or andesite lava– Subduction of sediments derived from the top

of the subducted slab may produce a variety of lavas

Magma at Hot Spots

• Magmas associated with a hot spot volcano in an ocean basin will produce a basalt lava

• Magmas associated with a hot spot volcano under continental crust generally will produce a felsic lava (and often an explosive one)

Figure 5.5 Selected prominent hot spots around the world. Some coincide with plate boundaries; most do not.

Figure 5.6 Schematic diagram of a fissure eruption. (At a spreading ridge, the magma has generally solidified before it can spread very far sideways at the surface, quenched by cold seawater.

Types and Locations of Volcanoes• Seafloor Spreading Ridges

– Most voluminous volcanic activity– About 50,000 km of ridges around the world– Mostly under the oceans - except at Iceland– Generally, harmless mafic fissure eruptions

• Continental fissure eruptions– Pour out of cracks in lithosphere– Result in large volume of “flood basalts”– Columbia Plateau (over 150,000 km2 and 1

km thick)– Other locations include India and Brazil

Figures 5.7 a and b Flood basalts (A) A real extent of Columbia River flood basalts. (B) Multiple lava flows, one atop another, can be seen in

an outcrop of these flows in Washington state.

Types and Locations of Volcanoes• Shield volcanoes

– Very large, flat, with abundant thin basalt flows– Basalt is less viscous than andesite or rhyolite– Shield like shape - larger area relative to height– Examples: Hawaiian Island chain

• Volcanic Domes– 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.

Types and Locations of Volcanoes• Cinder Cones

– Minor explosive volcano– Batches of lava shot into the air as pyroclastics– 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

Types and Locations of Volcanoes• 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.

Hazards Related to Volcanoes• Lava

• Pyroclastics

• Lahars

• Pyroclastic Flows - Nuées Ardentes

• Toxic Gases

• Steam Explosions

• 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 SO2 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.

Predicting Volcanic Eruptions

• Classification by activity– Active: erupted in recent history– Dormant: no historic erupts but not badly

eroded– Extinct: no historic eruptions and badly

eroded

• Volcanic Precursors– Seismic activity– Bulging, tilting or uplift– Monitoring gas emissions around volcano

Figure 5.28

Present and Future Volcanic Hazards in the United States

• Hawaii

• Cascade Range

• The Aleutians

• Long Valley and Yellowstone Calderas

Figure 5.21 Pyroclastic flow from Mount St. Helens

Figure 5.31The 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 5.29Restricted-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.