GLG 110

Part 2

Chapter 8

Volcanic Activity

 

8.1 Introduction to Volcanic Hazards

            50 to 60 volcanoes erupt each year worldwide

            effects can be catastrophic if eruption occurs near a city (e.g. Mexico City, Seattle)

            over the last 100 years 100,000 people have been killed by volcanic eruptions

            Japan, Philippines, Indonesia are especially vulnetale; US west coast

 

8.2 Volcanism and Volcanoes

            volcanism is directly related to plate tectonics

            most active volcanoes are located near plate junctions

            magma (molten rock) is produced by spreading or sinking lithosphere

            80% of all volcanoes are located around the Ring of Fire (Fig. 8.3)

            Volcano Types

                        Type dependent on:

-         temperature

-         viscosity (silica content)

-         lava: molten rock which reaches earth surface

                        Shield Volcanoes

-         largest volcanoes

-         e.g. Hawaii

-         shield shaped (slopes of 3 to 5 degrees, sometimes up to 10 degrees)

-         non-explosive eruptions

-         basaltic compositions (lava tubes possible)

-         pyroclastic debris possible (forms cinder cones)

-         summit caldera (diameter of 10 km possible)

-         eruptions along fissures (rift zones) common

                        Composite Volcanoes (stratovolcanoes)

-         cone shaped

-         steep slopes (30 to 35 degrees)

-         andesitic compositions

-         composed of alternating layers of lava flows and pyroclastic debris

-         often explosive eruptions

-         responsible for most of the volcanic hazards worldwide

-         e.g. Mount St. Helens, Mt. Rainier

                        Volcanic Domes

-         very viscous compositions (rhyolitic)

-         mostly explosive

-         e.g. Mt. Lassen

            Volcano Origins (Fig. 8.9)

-         Mid-oceanic ridge volcanism produces basaltic rocks on sea floor (e.g. Island)

-         Hot spot volcanism occurs within a plate (e.g. Hawaii)

-         Subduction zone volcanism produces andesitic rocks

-         Caldera-forming volcanism is extremely violent (rhyolitic rocks)

            Volcanic Features

-         craters, calderas, vents:

-         craters: depressions on top of volcano, small size

-         calderas: very large depressions on top of volcano

-         vents: openings through which volcanic material is erupted

-         hot springs and geysers:

-         hydrologic features associated with some volcanic areas

-         water gets heated in subsurface and rises to surface

            Caldera Eruptions

- they are huge eruptions of pyroclastic material

- at least 10 eruptions within the last million years (3 in the North America)

- craters can be huge (several km in diameter; e.g. Long Valley Caldera, Yellowstone)

- duration of major eruption lasting days or weeks

- lesser-magnitude events can last for about 1 million years

8.3 Volcanic Hazards

            primary effects: e.g. lava flows, pyroclastic activity, gas release

            secondary effects: debris flows, mudflows, landslides, floods, fires

            Lava Flows:

-         basaltic, andesitic, rhyolitic flows most common

-         speed of flow depends mostly on viscosity

-         most flows move slower than people can run

-         control methods: bombing, chilling with water, wall construction

            Pyroclastic Hazards:

-         ash eruptions (ash falls) into atmosphere, can cover large areas

-         can affect vegetation, surface water bodies, damage buildings, health hazard

-         ash flows (pyroclastic flows) roar down mountain side at high speeds, are super-heated gases

            Poisonous Gases

-         water vapor, carbon dioxide, carbon monoxide, sulfur dioxide, hydrogen sulfide

-         other gases: e.g. argon

-         toxic gases endanger populations (e.g. Cameroon, 1986)

-         SO2 may become acid rain

-         Ash may absorb gases to form toxic debris falling on land surfaces (e.g. fluorine)

            Debris Flows and Mudflows

-         lahar: debris flows and mudflows

-         movement is slow to fast (e.g. Hawaii, slow; Nevado del Ruiz, fast); confined to valleys;

-         debris flows: small eruptions melt large amounts of snow and ice; fast moving mixture of sediment and water; travel long distances

-         mudflows: smaller grain sizes; travel long distances;

 

8.4 Some Case Histories (read in textbook)

            Mt. Unzen, Japan

            Nevado del Ruiz, Colombia

            Mt. Pinatubo, Philippines

            Mt. St. Helens, USA

 

8.5 Prediction of Volcanic Activity

                        predicting eruptions is not possible today (and near future)

                        methods of predicting eruptions include:

            Seismic Activity

-         monitoring quakes prior to eruptions

-         not all eruptions are preceded by tremors

            Geophysical Monitoring

-         changes in local magnetic, thermal, hydrologic, and geochemical conditions

-         injected magma is usually responsible for changes

            Topographic Monitoring

-         ground deformation studies

-         change in shape, size, and angles on volcanoes

            Monitoring of Volcanic Gases

-         changes in gas geochemistry prior to eruption

-         change in quantity and composition of gases emitted

Geologic History

            Process in Prediction of Eruptions

            Volcanic Alert or Warning

 

8.6 Adjustment to and Prediction of the  Volcanic Hazard

 

Review questions are on page 231.