Plate tectonics, fold, fault, joint, fracture
 
 

DEFINITIONS

fracture: is a general term that means any breaks in rocks. Sometimes the

 breaks seen are the original breaks, other times it's erosion that exaggerates the break.

 Plate Tectonics

fault: is a fracture across which there has been a noticeable displacement

fold: a bend within a rock, curved deformations of geological features.

joints: are a type of fracture with little or no movement across them.

lithosphere:

asthenosphere:

READINGS
 
 

LECTURE
 
 

Plate Tectonics
 
 

Structural Geology isthe description and interpretation of deformational structures. The key structures you will see are three:
 
 

fractures, and folds.
 
 

What's the difference between these two?

Why do we get them?
 
 

Insert o'heads of joints and o'heads of faults
 
 

Arock can bend or break depending on its composition, confining pressure (depth), temperature, fluid content and time.
 
 

Plate Tectonics is the largest scale of Structural Geology you can do. Let's look at this first. Plate Tectonics can explain why there are earthquakes in certain places at certaindepths, why there are volcanoes in some places and not others, where and why oil exists and where it doesn't.
 
 

Plate Tectonics is the name given to a model. It's a big group of hypotheses that explains how and why rocks are deformed in certain ways and in certain places.
 
 

Because we are interested in deformation we would like to know when a rock is going to break (fault, joint) and where a rock is going to deform under stress but not break (fold).

If we look at the earth and map out regions that fault on a short time scale (e.g., our lives) we find that there are belts that subdivide the earth into blocks that essentially do not deform internally ona small time scale (these blocks are known as plates).
 
 

When these blocks are mapped out we find there are about 5 or 6 major plates.

 In the vertical we find that the earth is divided into regions that flow and other regions that are rigid on short time scales:
 
 

(1) the Plate is also known as the lithosphere:

composition: crust and mantle

mechanically: strong

thickness: 0-150 km
 
 

(2) below the plate lies a zone of very plastic material that can flow a few centimeters per year. This region goes down to about 400 km. If you take a load off the overlying lithosphere, it is thought that the asthenosphere will flow up to take the place, e.g. when glacialice melted off Scandinavia the lithopshere rebounded ata foot a year! The asthenoshpere is solid nevertheless and composed of peridotite ( a heavy rock composed mainly of olivine)
 
 

(3) Mesosphere: again, below the asthenosphere, lies a region that's less rigid again than the lithosphere but more so than the asthenoshpere. In this region however, the rock is still able to flow or convect.
 
 

Why are these plates deforming at their edges?
 
 

What controls the thickness/rigidity of a plate ?