What controls the distribution of volcanic activity?
= Plate Tectonics
=> Volcanic Activity = Tectonism
- What determines how explosive a volcanic eruption will be?
==> Driving force behind volcanic eruptions = gasses bubbling out of solution in the magma
A. Controls on Magma Behavior
-> One of the most important characteristics of magma = viscosity (resistance to flow)
-> Controls on viscosity =1) composition
1. Effect of Silica in Magma
=>more SiO2 -> increased complexity = high viscosity
=>less SiO2 -> less complexity = low viscosity
- fluid versus explosive…
Eruptions and Volcanoes
1. Fissure Eruptions *MOR's and Intraplate
=> very fluid lava = basalt = mild eruptions
=> flood basalts
=> lava plateau
EX.: Columbia River Plateau
2. Shield Volcanoes *MOR's and Intraplate
=> fluid lava
=> basaltic flows, few pyroclastics = mild eruptions
=> large, broad mounds, gentle slopes (< 15°)
=> the largest volcanoes.......
EX.: Mauna Loa
3. Cinder Cones *Intraplate & Subduction
=> all pyroclastics = violent eruptions
=> small (< 300 m) symmetrical cone w/steep slopes (~30°)
=> short lived eruptions, and volcano easily eroded
EX.: Paracutin (Mexico)
4. Composite Cones (stratovolcanos) * Subduction
=> andesitic lava flows and pyroclastics eruptions
= very violent eruptions
=> alternating layers of lava flows and pyroclastics
=> large steep cone that is resistant to erosion
=> can be destructive => ash flows and mud flows
EX.: Mt. St. Helens
5. Calderas *Intraplate and Subduction
=> regional volcanic collapse features
=> pyroclastic eruptions = extremely violent
a. Magma rises to near surface
b. Volatiles come out of solution
=> bulge - fracture - eruption - collapse
EX.: Yellowstone