CHAPTER 5: Volcanoes

Geothermal gradient:
  rate at which T increases with depth = 30 degrees C every km of depth

Increase of pressure with depth ~ 3 Kbars/10km
 
 
 

Melting at depths between 50-250 km - (Most in upper MANTLE)
 

•   Seds. and rocks at subduction zones  (if they have low melting point) => get molten

 
•   Melts (seds. + molten lithospheric rocks) density < asthenosphere density

        can melt surrounding rock                                                                         find a fracture to
        while ascending -                                                                                     the surface
        (compositional restriction)
 
 
 
 
 
 
 
 

                                                  Volcanic eruption
 
 
 
 

Magma source & extent of melting control the
      COMPOSITION of the melt

•  Magma =  relation SiO2, Fe, Mg

                                              seds + basalt             ==>       Andesite (Hybrid)
 

         SiO2                             Fe - Mg                     ==>         Rhyolite:                                              sluggish
                                                                                           similar to cont. crust                                 viscous
                                                                                                                                                            trap gases
                                                                                                                                                            explosive
Types and location of volcanic activity:
Fissure                                   Seafloor spreading ridges = submerged no danger
Eruptions                               Flood basalts                        Columbia Plateau.
                                              May represent aborted          Brazil
                                             cont. rifts                               India

•   Individual Volcanoes

•    Location      Subduction zone = along lines (Cascade Ranges)

•    Structures               1) Shield volcanoes (Mauna Loa)

Hazards related to Volcanoes
1) Lava usually not very fast advancing
                         destroys or buries property
                         do not build on volcano slopes

  hazardous but predictable => they go downhill
  measures = if possible   stop lava by quenching (Heimaey 1973)
                                         divert lava = explosives (Italy)

2)Pyroclastics: fragments of hot rock and spattering lava, suddenly erupted in the air, explosive, spread faster & farther
         bombs: large but fall close
         ash and dust: huge volume, transported far. St. Helens, Vesuvius, Tambora, Mazama

3) Lahars =     downhill mudflow of melt water and ash
                        choke streams =>  floods
    St. Helens, Nevado de Ruiz (Colombia - 20,000 deaths)

4) Nuées Ardentes = denser than air mixture of hot gas and fine ash that scorches everything along its path
                                hot: 1000oC interior                         fast: 100 km/h
                                St. Helens - Mount Pele

5) Toxic gases = CO2 odorless-colorless-dense. Lake Nyos, Cameroon
                           CO
                           SO2-SH2
                           HCl

6) Steam Explosions = phreatic explosions
       water reaches magma chamber. Risk: island, lakes
      Krakatoa + Tsunami

Secondary effect =  Climate changes + acid rain
                                 dust blocking oncoming sunlight
                                 sulphuric acid = acid rain
 

Volcanoes according to their activity
             active: erupted recently
             dormant: inactive presently - Potential to become active
             extinct: no recent volc. history - Eroded
 

•   Knowledge of eruptive history              periodicity

                                                                scale and type of eruption

                                               ignore extinct

• Monitor volc. activity             watch for dormants

                                               act accordingly with active ones

seismic activity

bulging or uplift of volcano slopes

change in composition of gases

temperature

(animal behavior)

       exact timing

       volume to be erupted

        length of eruptive stage

EARTH PROCESSES AND ENVIRONMENTS
VOLCANISM: REVIEW QUESTIONS AND TOPICS
1)  A complete definition of MAGMA. How are magmas generated and why do magmas ascend?
2)  Types of magmas: major chemical components, flow velocities, viscosity and eruptive styles. Rocks they form when cooled.
3)  Centers of volcanic activity. Relationship to plate tectonic settings. Examples.
4)  Types of volcanic structures: shield volcanoes, domes, cinder cones, composite volcanoes. Examples mentioned in class.
5)  Hazards related to volcanoes: all: lava flows, pyroclastics (ash, cinders bombs), nuees ardentes (ashflows), lahars (volcanic mudflows), toxic gases, phreatic explosions. Effects on climate and atmospheric chemistry. Preventive measures. Examples of the U.S. and the world.
6)  Classification of volcanoes according to their activity. How to regard each of the classes. Areas at risk in the U.S.
7)  Precursors to volcanic activity. The status of Prediction. What can be predicted and what cannot.
8)  The 1980 eruption of Mount St. Helens. Precursors measured, institutional and public attitude. Hazards. Outcome of the eruption. This material is covered during the lecture through comments and videos. Make sure you pay close attention and record the information accurately during the showing of the film.