Rocks Suffering from Stress!

What the Syllabus says, and the Details...

Interpreting Geological Events

Rocks are expected to fit the Principle of Uniformitarianism (the present is the key to the past). This basically says that if you can't imagine it happening now, it probably didn't happen in the past either. For example - this (below) doesn't normally happen - it takes some very big forces.

The concepts that we test the likelihood of arrangements of rocks against are...

Original horizontality - rocks are most likely to have been laid down flat.

Lateral continuity - rocks would normally be deposited over quite a large area at the same time.

Superposition of strata - younger rocks are laid down on top of older ones.

If the rocks are not in a 'pattern' that fits with these principles than hey have probably been deformed by either faulting of folding.

Folding is caused by compressional tectonic stress.

You need to be able to interpret features of folding in rock exposures, diagrams, simplified maps and block diagrams.

The rock will have started as horizontal beds, but compression will squash it into folds. These folds will have dipping beds along their flanks. The folded beds will form anticlines (folds which have their beds pushed up)...

Notice that the top surface of the block diagram (which is basically a map) show older rocks in the middle of an anticline with younger rocks appearing symmetrically each side.

and synclines (where they are pushed down). 

The 'map' on the top of the block diagram show the younger rocks outcropping in the middle of the syncline with the older rocks occurring symmetrically on the limbs of the fold.

You need to be able to identify the axial plane trace as the line of symmetry along the centre of the fold. The diagram below also shows how we can expect things to be squashed (deformed) if they were in the rocks when they were folded. This 'squashing into layers' is called foliation.

Faulting is caused by tectonic stress [compressional, tensional, shear]

You need to be able to interpret features of rock deformation by faulting in rock exposures, diagrams, simplified maps and block diagrams.

To do this you need to be able to recognise and explain the formation of...

Normal faults - faults that happen when the rock is under tension, a break happens and one side falls down the break.

Reverse/Thrust faults - faults that happen when the rock is under compression, a break happens and one side 'rides up' over the other.

Strike-slip faults - faults that happen at conservative plate margins where one plate is forced to move sidewards past another.

You need to be able to estimate fault displacement - the amount by which a fault has moved. For instance, if we assume the person on the cliff in the picture to be about 1.75m tall (average height) then this fault has been displaced by about 4m.

Unconformities are breaks in the rock record. Angular unconformities are formed by a sequence of events; deposition of rock, deformation of that rock, uplift of the area containing that rock so that it becomes tilted, erosion of the locality to leave the tilted beds exposed and later deposition of another rock on top. The picture below shows an angular unconformity in the Grand Canyon - note the angle of bedding of the rocks at the bottom of the photograph compared to those at the top.

If you are asked to identify an unconformity on a map it will have one rock type 'juxtaposed against' (next to) two or more other types along the same side - as shown by the arrowed unconformity on the map below.

You need to be able to identify unconformities on photographs, diagrams, maps and cross sections. You should understand that unconformities show that there have been two separate periods of deposition of sedimentary rocks separated by a break that could be anything up to hundreds of millions of years long.