Ch. 7 - Volcanoes - Trimble County Schools
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Transcript Ch. 7 - Volcanoes - Trimble County Schools
18.2 - Eruptions
Today: 12/9/15 Wednesday
Get
a copy of the guided notes off of the stool.
Today: 12/7/15
Get
a copy of the Dante’s Peak Facts sheet off of
the stool. Have it ready to start class. Don’t write
on this sheet. This is a class set.
Making Magma
Activity
depends on composition of magma
Lava can be thin & runny or thick & lumpy
Magma
Temperature
Most rocks melt between 800-1200 °C
In Crust & Upper Mantle
Pressure
Increase with depth because of weight of rocks
High pressure increases melting temperature
Composition of Magma
Amount of Gas & Silica
Increase gas dissolved = increased explosiveness
Dissolved gases = carbon dioxide, water vapor,
sulfur oxide, sulfuric acid
Water vapor determines where magma forms
Water vapor decreases melting temperature
Composition
Viscosity = Resistance to flow
Temperature & silica content affect viscosity
High viscosity = cool magma
High silica = thick & sticky
Traps
Low
gases, produces explosive eruptions
silica = thin & runny
Flow
easily, produce quiet, non-explosive eruptions
Types of Magma - Basaltic
Less
then 50% silica content
Low-viscosity magma
Gas easily escapes
Quiet eruptions
Ex: Kilauea & Mauna Loa
http://media-2.web.britannica.com/eb-media/31/20231-004-008EFA7E.jpg
Types of Magma - Andesitic
50-60%
silica content
Oceanic-continental subduction zones
Intermediate viscosity
Intermediate explosiveness
Send high volumes of ash & debris
Ex: Colima Volcano in Mexico
http://images.livescience.com/images/050531_colima_volcano_04.jpg
Types of Magma - Rhyolitic
More
than 60% silica content
Magma mixes with water and silica
High viscosity
Large volumes of gas trapped
Very explosive
Explosive Eruptions
If
lava is too viscous to flow, pressure builds up
until an explosion
Tephra = pieces of solidified lava or pieces of crust
Classified
by size
Ash = smallest tephra, diameter is less then 2 mm
Blocks = largest tephra
Ash
Can
rise into atmosphere
Tiny sulfuric acid droplets
remain in stratosphere and
block sun’s rays and decrease
global temperature
http://geology.com/articles/volcanic-ash/volcanic-ash-column.jpg
Pyroclastic Flows
Tephra
+ gases
VERY HOT
http://vulcan.wr.usgs.gov/Imgs/Jpg/MSH/Images/MSH80_pyroclastic_flow_from_st_helens_crater_08-07-80.jpg
In-Class Assignment
18.2
Study Guide WKT