Lecture 4 Flashcards
Rocks near Earth’s surface are cold and strong
- what is an example of this where lithosphere flexure supports volcano
Hawaii island has deep troughs either side of the island where flexure due to elastic strength where strain is proportional to stress and is recoverable
What happens if you overcome elastic limit of these rocks
Get brittle and this is non-recoverable
- example of brittle is fault breaks after earthquakes
When can hot rocks be elastic
Short periods of time provided deformation (strain rate) is fast enough
What is brittle behaviour?
Where substances fracture without appreciable permanent deformation away from cracked surface
What is ductile substances
Substances deform appreciably by flow and is non-recoverable
- CREEP MECHANISMS
What is cataclastic flow?
- only in brittle rocks
- repeated fractures which progressively reduce the grain size
- the granules roll/slide over each other
- common in fault zones
What are creep mechanisms
Slow flow under constant load. There are three main types of mechanism
- VERY SENSITIVE TO TEMPERATURE
what is dislocation creep?
power-law creep
imperfections in atomic structure of crystals that can move through crystal
- dominant at relatively low homologous temperature and is relatively high stress—>rocks flow under this but need large stresses
Diffusion creep?
The sliding of crystals on grain boundaries and is important above θ=0.85
strength proportional to strain rate and constant of proportionality is viscosity
- dominant at relatively high homologous temperatures and is relatively low stress
- sensitive to grain size
Recrystallisation—>
disolving and regrowing crystals on grain boundaries
What is homologous temperature?
ratio of actual temperature to melting temperature (in kelvin) and symbol = θ
- when about 0.6 and above then rocks creep as close to melting temperature
Affect of temperature—> rocks creep more easily when hotter
- proportionality
strength is proportional to exp(1/T)
T= kelvin
The strength variation with depth in the Earth
GRAPH 6
temp increases with depth in earth so ways rocks deform changes. Brittle near surface to ductile flow at depth
What is strength in the context of rocks?
Resistance to flow
What is the Asthenosphere?
is the highly viscous, mechanically weak and ductile region of the upper mantle of the Earth. It lies below the lithosphere, at depths between approximately 100 and 200 km below the surface.
- very viscous fluid—>not a liquid
What is the lithosphere?
The top 100-125km of Earth and is relatively strong (its colder)
top part brittle
bottom is just warm enough to creep
Lithosphere-asthenosphere boundary
Main mechanical boundary in upper part of earth
- not a sharp boundary as based on temperature graduation
What is viscosity?
The ratio of shear stress to strain rate
pascal seconds
Why is Scandinavia increasing in height?
The ice in the ice age pushed and deformed the earth. The lithosphere is now going through elastic rebound. But this means that mantle moving to this spot and away from north france—> so this is reducing in level
What is the viscosity of the asthenosphere?
10^20- 10^21 Pa s
