Volcanoes
Lesson Objectives
As a result of this lesson and the reading, you should be able to:
 Define the following terms or phrases: Shield Volcano, Stratovolcano, Flood
Basalts, Lahar, Pyroclastics, Lava.
 Distinguish between the volcanism found over hot spots, subduction zones,
and spreading centers in terms of their rock composition, volcano type, magma
viscosity, and danger.
 List and discuss at least three of the hazards associated with volcanoes.
 Relate the benefits associated with volcanoes and volcanism.
 Discuss which areas of the United States are at most risk from volcanic
eruptions and why.
 Explain two of the techniques used to predict volcanic eruptions.
Volcanoes I
How form new igneous rocks? Rising magma
Magma originates at depths between 50 and 250 km (30-150 miles)
Igneous Rocks - What is the difference between Plutonic and Volcanic?
Difference in look of the rocks? crystal size correlates to cooling time
Where found?
Discuss pattern of distribution
5 eruptions/year from Alaska to Japan
Three main kinds of volcanoes we’ll discuss [recreate matrix on board]
Mafic
Tectonic
Setting
Composition
Volcano type
Viscosity
Danger
Felsic
Mafic
Divergent
Boundary
Convergent
Boundary
Hot Spots
Basalt
Shield Volcanoes
low
low
Andesite
Stratovolcanoes
high
High
Basalt
Shield Volcanoes
low
low
Composition of magma depends on the nature and proportions of materials
melted. Often just partial melting associated with subducting slab. Results
in less mafic magma.
Volcano types:
Good photos/diagrams in your book
Seafloor Spreading Ridges
X-sect. of Mid-Ocean Ridge
Fissure extending through lithosphere, low pressure invites magma
from below. Doesn’t spread out because chilled by seawater.
Flood Basalts
On continents. Mind-boggling flows in Pacific Northwest, India.
Shield Volcanoes
From vents/pipes. Over hot spots. Ex: Hawaii - Mauna Loa. Very
low slopes. Big piles of runny lava.
Stratovolcanoes
X-sect. of Subduction and strato-volcanoes. Structurally weak…
Caldera - enlarged volcanic crater formed by explosion or collapse
Volcanic Hazards
Longer interval between explosion, bigger potential that it will be
explosive and fatal
1. Lava
2. Pyroclastics - Fragments of rock and lava and ash
3. Nuee Ardente - dense mixture of hot gas and ash
Mt. Pelee on Martinique – 60-90mph
4. Lahars - volcanic mudflow
5. Toxic Gases
6. Lake Nyos CO2 release – 50 m thick cloud of CO2, denser than air
flowed down valleys and suffocated 2,000 people and countless
animals.
7. Acid rain – Hawaii’s Vog
8. Tsunamis (Krakatoa tsunami 1883 – 115’ above sea level at Java,
killed 36,000 people)
Volcanic Benefits
Far outweigh hazards - source atmosphere, hydrosphere, important anions
Produce fertile soils - rapid weathering of fine ash release nutrients.
Finest coffee in world grown on steep volcanic slopes in Columbia
But encourages high population density
Obsidian important for early man.
Geothermal power.
Use of cinder cone material for fill, gravel
Gold, diamonds
Climatic Effects
Cooling via aerosols (ash and small droplets block incoming solar
energy)
Residence time in atm – 1-2 years
SO2 coats particles white, making more reflective
Prediction of Eruptions