Week 9

Nov 22, 2015
3 min read

This week's classes covered several topics centering on structural geology. The lecture included the conceptual deformation of rock structures, the stress – strain relationship in rocks and rock layers, mega structures such as folds, faults and unconformities, as well as the relationship between structural geology and petroleum engineering.

Structural geology is the study of the processes and products of the deformation of rocks and minerals. It is a branch of geology.

The field of structural geology is vital to today’s economy, as the faults in the strata are often traps for the accumulation of oil and gas in the subsurface, and the veins found in faults contain various metals which are valuable for different applications, as well as being the source for learning the art of geological map interpretation.

How a rock behaves will depend on several factors:

• Temperature - At high temperature molecules and their bonds can stretch and move, thus materials will behave in more ductile manner. At low Temperature, materials are brittle.

• Confining Pressure - At high confining pressure materials are less likely to fracture because the pressure of the surroundings tends to hinder the formation of fractures. At low confining stress, material will be brittle and tend to fracture sooner.

• Strain rate -- At high strain rates material tends to fracture. At low strain rates more time is available for individual atoms to move and therefore ductile behaviour is favoured.

• Composition -- Some minerals, like quartz, olivine, and feldspars are very brittle. Others, like clay minerals, micas, and calcite are more ductile This is due to the chemical bond types that hold them together. Thus, the mineralogical composition of the rock will be a factor in determining the deformational behaviour of the rock. Another aspect is presence or absence of water. Water appears to weaken the chemical bonds and forms films around mineral grains along which slippage can take place. Thus wet rock tends to behave in ductile manner, while dry rocks tend to behave in brittle manner.

The kinds of rock behavior are: brittle, ductile, and elastic.

The main types of forces that rock structures experience are:

Tension

Tension stress (or tensile stress) occurs when two forces pull on an object in opposite directions so as to stretch it and make it longer and thinner.

Compression

Compression pushes or presses an object so as to make it shorter and thicker.

Shear

Shear stress is two forces acting parallel to each other but in opposite directions so that one part of the object is moved or displaced relative to another part.

Brittle deformation

As for the types of rock fractures, there are two of them, the first one joints and the other one is faults.

Examples:

Left-lateral faults in siltstone.

Vertical joints in granitic rock, Alabama Hills, southeastern California.

Conjugate normal faults, Black Mountains frontal fault zone, Death Valley, California

Ductile deformation

These include folding, crenulation as well as assimilation.

Examples:

Axial planar cleavage in sandstone

Anticline-Syncline pair in Devonian Old Red Sandstone, SW Wales, UK

Recumbent isoclinal fold in marble

Folds are a type of ductile deformation in rocks. They are permanent wavelike deformation in layered rock or sediment. There are three types of folds, which are: