18.2 - Eruptions
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