Lecture Notes 15 Tectonic volcanoes .doc

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11 Apr 2012
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Lecture 15
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Tectonics and Volcanoes
Volcanic eruption- Geological disasters
The Rock Cycle
Inner Core: solid iron:very dense
Outer Core: Liquid iron: very dense
Mantle: Iron and Magnesium: Dense
Crust: silicon and oxygen: low density
Crust
Thin, low-density crust rich in silicon and oxygen
Composition of the crust
As a result of planetary differentiation:
8 elements form 98% of the crust
Silicon and oxygen account for = 75%
Flements combine to form minerals
4000 minerals known, 25 common
Minerals are the building blocks of rocks
What is the composition of the crust?
45% Oxygen
27% Silicon
Minerals and rocks
Minerals: naturally occurring inorganic solid with ( building blocks or rock)
Orderly internal structure (atoms are bound together)
Individual physical characteristics (colour, crystal form)
Rock: solid aggregate of one or more minerals
limestone can be made up of one mineral
others are made up of more minerals Ex. metamorphic
Geochemistry
Biochemistry studies structures of carbon
geochemistry studies structures of silicon and oxygen
Basic building block is the silicon-oxygen tetrahedron molecule
Si-O tetrahedron molecule
Four oxygen ions surrounding a much smaller silicon ion
Negative charge, share one or more of the oxygens
Tetrahedrons combine to form rings, chains, sheets, and 3D structures
Ex. Single chain share individual oxygens, double chain, sheet,
Silicates
Silicates: group of minerals containing silicon and oxygen combined with one or
more other elements
Most abundant mineral group in the Earth’s crust
Silicon and oxygen account for = 75% of the Earth’s crust
What is the rock cycle?
describes processes by which older rocks are made into new rocks
Theoretical concept predating plate tectonics
Igneous rocks
form by cooling and solidification from a molten liquid called magma
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Crystallization: process of mineral formation in a cooling magma
Sedimentary rocks
Metamorphic rocks
Cooling at depth:
Intrusive igneous rocks
Slow cooling
Large crystals
Most magmas solidify at depth and are later exposed by erosion
Magma
What is Magma?
Ointment in ancient Greek: partially molten rock below the Earths surface
Lava: magma that reaches the surface
as it rises it crystalize because it looses heat
What are the three components of magma?
a liquid portion, called melt
Solid: minerals already crystallized from the melt
Volatiles: gases dissolved in the melt
What are the classification of igneous rocks?
Igneous rocks are classified according to:
Cooling environment ( crystal size)
intrusive vs. extrusive rocks
Mineral composition expressed as % SiO2
Properties closely related to:
Macroscopic features
Color
Macroscopic features
Color
texture: shape, size, and arrangement of minerals grains
magma behavior
What is viscosity?
internal resistance of a liquid to flow
The less viscous a fluid, the faster it flows
The most important property controlling magma behavior and, therefore, erup-
tive style
What controls magma viscosity?
1. % SiO2 magma with high % SiO2 has more silicate chains, sheets, and 3D
structures
More bounds between atoms increase viscosity
2. Temperature Atoms vibrate more vigorously at high temperature
Bounds between atoms tent to break, decreasing viscosity
3. Amount of solids in the melt ( how many crystals you have in your magma)
More solids, higher viscosity
What is low-viscosity magma?
Tends to reach the Earths surface
Erupts peacefully
characteristics:
basaltic magma
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