ROTATIONAL STRAIN vs. NON-ROTATIONAL STRAIN

Calculating lithostatic stress:

At depth in the earth we always have three stresses.

In geology we usually attribute deformation to stress acting over a surface in a rock.

What controls the type of deformation?

(1) temperature

(2) rate of stress increase (strain rate= rate at which stress level is raised)

(3) composition

(4) confining pressure (pressure around the sample)

(5) differential stress

(6) presence of fluids

(7) time

If these are the factors affecting deformation, how do we examine them?

We can create those conditions in the laboratory with a deformation apparatus.

How is the stress created? Small pistons pushing on tapered anvils

How is the temperature created? Warm kerosene circulating around the sample

How is the sample isolated from the effects of the kerosene?

Copper jacket

How is the rate of stress increase modulated?

What can we learn about rock properties by examining their behavior under stress?

Let's plot differential stress (s1-s3) versus deformation (strain in %).

Rocks usually show that

stress is proportional to strain

Big E is Young's modulus and relates how much stress is required to achieve any given amount of elastic shortening.

Rocks that deform in a brittle manner are competent

Rocks that deform in a ductile manner are incompetent.

In general under a small amount of deformation rocks will behave elastically

At a certain point the same rocks will behave plastically, i.e. the curve starts to bend (yield strength) and then will break (rupture strength). Plasticity implies that the rock is permanently deformed; that the strain can not be recovered, or that the strain is irreversible.

What does a greater confining pressure do?

Makes the rock stronger, more elastic

What does greater temperature do?

decreases the field of elasticity, increases deformation

What does an increase in the strain rate do?

make the rock more pliable

What does time do?

Allow a rock to deform ductily. Given enough time, materials will blow. A rheid is a material which when below its melting point will deform 1000 times more that under elastic deformation for the same conditions.