Chapter 10

Structure

Deformation or strain (deformation caused by stress)

The original shape or volume or both have been altered by stress.

There are three kind of stress:

- Compressional stress

Rocks are squeezed or compressed by external forces directed toward one an other.

(Relative shortening)

After:http://www.educ.uvic.ca/Faculty/jtinney/earth%20science/Compr.html

-Tensional stress:

Forces acting in opposite direction along the same line

(Relative lengthening)

After:http://www.educ.uvic.ca/Faculty/jtinney/earth%20science/Compr.html

-Shear stress:

Forces acting parallel to each other but opposite direction resulted in displacement along closely spaced planes

After:http://www.educ.uvic.ca/Faculty/jtinney/earth%20science/Compr.html

As all together:

Strain can be:

-elastic

if the deformed rocks return to their original shape when stresses are relaxed (squeezing tennis ball)

-Rocks that are strained beyond their elastic limit are deformed, and can not recover their original shape.

Plastic or ductile (Folding) happens at high temperature, probably deep in the crust
Brittle (Faulting) happens at colder temperature, close to the surface

The type of strain depends on:

-kind of stress

-the amount of pressure

-temperature

-rock type

-length of time

STRIKE and DIP

Strike

Strike is the direction of a line formed by the intersection of a horizontal plain with an inclined plain such as a rock layer. The orientation of the strike line is defined by a compass to measure its angle in respect to North.

Dip

Dip is the measure of the maximum angular deviation of an inclined plane from the horizontal, so it must be measured perpendicular to the strike direction

Strike and Dip diagram

After: http://www.bhc.edu/academics/science/harwoodr/Geol101/study/structur.htm

FOLDS

Folds are plasticly streained deformation. It is usually heppening deeper in the crust, because rocks which are close to the surface are usually brittle.

Monocline (simple bend)

Anticline (up-arched fold)