Lecture 4

Question 1

Rocks near Earth’s surface are cold and strong

- what is an example of this where lithosphere flexure supports volcano

Answer 1

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

Question 2

What happens if you overcome elastic limit of these rocks

Answer 2

Get brittle and this is non-recoverable

- example of brittle is fault breaks after earthquakes

Question 3

When can hot rocks be elastic

Answer 3

Short periods of time provided deformation (strain rate) is fast enough

Question 4

What is brittle behaviour?

Answer 4

Where substances fracture without appreciable permanent deformation away from cracked surface

Question 5

What is ductile substances

Answer 5

Substances deform appreciably by flow and is non-recoverable

- CREEP MECHANISMS

Question 6

What is cataclastic flow?

Answer 6

• only in brittle rocks
• repeated fractures which progressively reduce the grain size
• the granules roll/slide over each other
• common in fault zones

Question 7

What are creep mechanisms

Answer 7

Slow flow under constant load. There are three main types of mechanism
- VERY SENSITIVE TO TEMPERATURE

Question 8

what is dislocation creep?

power-law creep

Answer 8

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

Question 9

Diffusion creep?

Answer 9

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

Question 10

Recrystallisation—>

Answer 10

disolving and regrowing crystals on grain boundaries

Question 11

What is homologous temperature?

Answer 11

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

Question 12

Affect of temperature—> rocks creep more easily when hotter

- proportionality

Answer 12

strength is proportional to exp(1/T)

T= kelvin

Question 13

The strength variation with depth in the Earth

Answer 13

GRAPH 6

temp increases with depth in earth so ways rocks deform changes. Brittle near surface to ductile flow at depth

Question 14

What is strength in the context of rocks?

Answer 14

Resistance to flow

Question 15

What is the Asthenosphere?

Answer 15

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

Question 16

What is the lithosphere?

Answer 16

The top 100-125km of Earth and is relatively strong (its colder)
top part brittle
bottom is just warm enough to creep

Question 17

Lithosphere-asthenosphere boundary

Answer 17

Main mechanical boundary in upper part of earth

- not a sharp boundary as based on temperature graduation

Question 18

What is viscosity?

Answer 18

The ratio of shear stress to strain rate

pascal seconds

Question 19

Why is Scandinavia increasing in height?

Answer 19

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

Question 20

What is the viscosity of the asthenosphere?

Answer 20

10^20- 10^21 Pa s