18.2 - Eruptions
Bellringer 1/12/17 Thursday On a clean sheet of paper, Write 1 statement from each of the following -A, B, C, and D. A) Rhyolitic Rock (high silica content) –or- Basaltic Rock (low silica content) + B) Thin magma/lava thickness -or- Thick magma/lava thickness C) Mild Volcanic Eruptions –or- Very Explosive Eruptions D) Strato/Composite Volcano –or- Shield Volcano
Making Magma Volcanic 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 More gases 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 (how thick or thin it is) Temperature & silica content affect viscosity High viscosity = cool magma/thicker High silica = thick & sticky Traps gases, produces explosive eruptions Low silica = thin & runny/low viscosity Flow easily, produce quiet, non-explosive eruptions
Types of Magma - Basaltic Less then 50% silica content Low-viscosity magma/thinner Gas easily escapes Quiet/mild eruptions Shield volcano formed 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/medium thickness Intermediate explosiveness Strato/composite volcano produced 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/thick magma Large volumes of gas trapped Very explosive Forms Strato/Composite Volcanoes
Explosive Eruptions If lava is too viscous (thick) 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