Module 12: Historical Geology

• Relative Dating - Deals with the sequence and chronology of the layers with respect to each other that are observed in the field and does not entail numerical ages.
• Absolute Dating - Determines the numerical ages of minerals, rocks, and fossils and is done in a laboratory through the use of radioactive isotopes to determine the numerical age of a sample.
• Radioactive - unstable nuclei that dissipates energy in the form of radiation.
• Isotopes - variants of the same atom but with different mass numbers
• Parent Isotope - It releases heat and loses energy and becomes one step closer to becoming more stable.
• How are radioactive isotopes used to determine numerical ages?

  Half-life
• Half-life - the length of time required for one-half of the nuclei of a radioactive isotope to decay
• What is the half life of Carbon-14 in years and its effective dating range?

  Half-life of Carbon-14: 5730 years Effective dating range: Up to 50,000 years
• Continental material is not consumed or renewed so it stays in the surface once it is created. The oldest materials on Earth are minerals from sedimentary rocks and rock fragments which have originated from outer space.
• The oldest rocks on Earth include Acasta Gneisses in northwestern Canada (4.03 Ga)

  Isua Supracrustal rocks in Greenland

  Rocks found in the Minnesota River Valley and northern Michigan

  Western Australia

  Faux-Amphibolites (older than the first)
• The oldest materials on Earth include:

  Zircon grains found in sedimentary rocks

  70- well dated meteorites

  Iron Meteorite (4.54 b.y)
• Faux-Amphibolites - part of the primordial crust
• Relative dating - putting rocks and events in their proper sequence of formation. Its dating rocks and rock units with the use of fossils and correlation of strata.
• Uniformitarianism - the idea that the natural ang geologic processes that are happening today are the same processes that have operated in the past.
• Steno's Laws: Law of Superposition - When examining an undisturbed sequence of stratified rocks, the oldest strata will be at the bottom and the youngest strata will be on the top of the sequence

  
• Steno's Laws: Law of Original Horizontality - During the period of deposition, most sediments/layers are deposited horizontally.

  
• Steno's Laws: Law of Lateral Continuity - Sediments would spread out until they thin out at the edge of the depositional basin, stop at a depositional barrier and it shows that even if the exposures are separated, it is possible that they are the same unit because of lateral continuity.

  
• Steno's Laws: Law of Cross-cutting relationships - When a fault or intrusion cuts through another rock, the fault or intrusion is younger that the rocks which it cuts

  
• Principle of Inclusions - It explains that the included rock fragments are older than the layer which contains it.

  So the sequence of formation of the units from oldest to youngest would be: schist, sandstone then intrusion of granite.

  
• Unconformities - Any significant break in time within a stratigraphic column and are gaps in the rock record representing a long period during which deposition ceased.

  
• The four types of Unconformity include:

  Angular unconformity

  Disconformity

  Paraconformity

  Nonconformity
• Angular unconformity - tilted or folded sedimentary rocks that are overlain by younger, more flat-lying strata and is the easiest to spot
• Disconformity - Strata on either side of the unconformity are essentially parallel with a distinctly recognizable surface and can form when the unit is uplifted or is exposed on the surface thus making it more susceptible to weathering and erosion.

  The surface in between the older layers and the newly deposited ones is the disconformity.

  
• Paraconformity - Beds above and below are parallel and the unconformity is identified by some evidence such as lack of certain diagnostic zone fossils in some horizon.

  A type of unconformity with no recognizable surface of erosion. Rather than being a site of erosion, this type of unconformity is a site of non-deposition. It means that there was a gap in deposition for a specific period of time.

  
• Nonconformity - Older metamorphic or igneous rocks are overlain by younger sedimentary strata. It is one of the easily recognized disconformities because of the difference in rock types.

  
• Principle of Faunal Succession - Fossil organisms succeed one another in a definite and determinable order. Thus, any time period can be recognized by its fossil content
• Index Fossils - If a fossil is short lived, it is a good indicator of a particular interval in time and are good age determinants of rock units.

  Can determine which rocks are older and which ones are younger (e.g. trilobites)

  
• Correlation - Used to demonstrate correspondence between geographically separated parts of a geologic unit and fossil records are required for chronology.
• Fossils - remains or traces of prehistoric life preserved in sedimentary rocks, naturally preserved and are older than 10000 years
• What are the three requirements for fossilization?

  Rapid burial, Protective Medium and Possession of Hard Parts
• Fossilization (preservation of unaltered parts) - the original composition of the body parts were not modified during the fossilization process.

  
• Chemical alteration of hard parts can be done by the following processes:

  Carbonization

  Replacement

  Recrystallization
• Carbonization - soft tissues preserved as thin carbon film
• Replacement - dissolution of original material and precipitation of new mineral (ex. pyritized ammonite)

  
• Recrystallization - conversion of a mineral polymorph to another (ex. aragonite → calcite usually occurs in corals)
• Permineralization - porous material filled with secondary materials (bone permineralized with silica)
• Petrifaction –replacement of wood (similar to petrifaction but refers specifically to wood)
• Preservation of the traces of the organisms - The remnant of the organism is no longer present however traces or behavior of the organism is preserved in the sedimentary rock
• Mold - dissolution of shell
• cast - filling of shell