Earth Sciences 240A
Lecture 18 -
Volcanoes: Introduction
Introduction
The Best
Tourism and recreation
Rich soil for agriculture
Huge metal resources
Energy source
CO2 is climate moderator
The Worst
Violent: ash+aerosols+gas to atmosphere
Fast, hot, deadly ‘flows’
Quiet: loss of land, property
Heat requirement
“To heat 1 kg of rock by 1oC requires ~1200 joules energy”
“To heat 1 kg of water by 1oC requires ~4X(1200) joules energy”
Volume lava added to Earth yearly: Approx. 25 km3
Heat sources
Radioactivity: Long-lived; Short-lived (gone)
Accretional heat: ½ mv2
Compositional differentiation: Gravity/segregation motion
Dissipation of (Sun+Moon) Tides: Earth rotation; Earth-Moon distance
Lava Classifications
(from Prerequisite)
Basaltic lava
Most common
High temperature eruption
Pahoehoe and aa
Rhyolitic lava
Light colour
More viscous
Lower eruption temperature
Andesitic lava
Intermediate to basalt/rhyolite
Magma characteristics
Basic (chemical category)
Dark, heavy, dense
45-55% silica
Rich in Mg, Fe, Ca
Typical minerals: Olivine, Plagioclase, Pyroxene, Magnetite
Fluid and hot eruptions (basalt)
Silicic (include intermediate )
Paler colour, lighter weight, less dense
Andesite 55-66%
Dacite 65-70%
Rhyolite 70-80%
Higher Na, K, Al
Typical minerals: Quartz, muscovite, K-,Na-feldspar
Erupt cooler and more viscous
Magma viscosity
Polymerization (from prerequisite): Grouping together of clusters of molecules
Si – O bonds most significant
Develop within cooling magma
Simple tetrahedron; linked to chains/sheets
Hinder fluidity
Insignificant in basic magma
Easily 50% in silicic magma
Volatiles
Volatiles: Components: H2O, CO2, SO2, (others)
Water
~1% in basic magma
~5% in silicic magma
Can break polymers when dissolved
Exsolved bubbles increase viscosity
Exsolution with decreasing P
Propellant upward
Next
Eruptive environments