1.5 Metamorphic processes.
These are the metamporhic rocks you need to know about:
Metamorphic rocks are the result of pressure on pre-existing rocks causing recrystallisation to form new minerals.
1.6 Geological Structures
The Rock record provides evidence of plate tectonics. For example, there are matching formations found in Eastern South Africa and Western Africa. This indicates that at one time, the continent was a single land mass.
Folding is caused by tectonic stress. Folding is when one or a stack of originally straight and flat surfaces are bent or curved.
Folds are caused at a collision margin where two plates collide and force eachother upwards.
Faulting is caused by tectonic stress.
Faults are along a convergent plate boundary where one plate is moving faster than the other.
Unconformities are breaks in the rock record. For example, there is a period of time that is not represented by any rock in the sequence. Unconformities are formed by a sequence of events including deformation, uplift, erosion and later deposition.
Geological events are dated and interpreted using:
- the principle of Uniformitarianism. This means that the present is the key to the past, so everything that happens now (such as a volcano errupting, or rain falling), would have happened in exactly the same way millions of years ago.
- the concept of original horizontality. This means that sediment was originally deposited horizontally.
- the principle of lateral continuity. This means that layers of sediment initially extend laterally.
- the law of superposition. this means that undisturbed strata in the lower layers must be older than the layers on top.
- the principle of cross-cutting relationships. this means that the intrusions must be younger than the rock it has intruded. it helps us age rocks because it shows which layers were deposited before or after the intrusion.
Rocks can be dated by using fossils.
Fossils such as cephalopods and graptolites have morphological changes with time (they have evolved through time) so they can help us date more precisely.
Cephalopods - the suture line differs and becomes more straight (less squigly).
Graptolites - less thecae and reduce to one stipe.
The decay of radioactive material helps give us an absolute age of when the rock was formed.
Weathering is the break down of rocks in situ by chemical, biological and physical weathering.
Physical weathering - freezethaw. this produces angular rock fragments (scree)
Chemical weathering - granite is chemically weathered to produce sand, clay and soluble material. Limestone is chemically weathered to produce soluble materials.
Biological weathering - by plants, seeds and organic acids.
Erosion involves the transportation of solid weathering products by water, wind, ice or gravity.
- water, ice and wind erode rocks (abrasion and attrition)
- they transport and deposit sediment (traction, saltation, suspension, solution)
- debris accumilates at the foot of slopes due to gravity (scree)
Valley shapes generally reflect the mode of…