Geo 110 Volcanoes

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 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.

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 km 2 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.

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.

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

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 - Nu é es 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 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.

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 5.31 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.