EESA06H3 Lecture Notes - Lecture 8: Geologic Time Scale, Principle Of Faunal Succession, Radiometric Dating
Lecture 8: geological time: how we date the past?
• Strata: layers of rock.
• Stratigraphy: age of rocks, the study of how all the rocks are arranged in time.
• Relative age vs absolute age:
o Relative: age of a rock layer compared to other layers (determined by looking
at the position of rock layers)
o Absolute: determined using radiometric dating; age of a rock in years.
• Accommodation: the crust is subsiding (sinking). The surrounding rocks that need to
hold the evidence. Opposite of subsidence = uplift = non-accommodation = erosion =
removal of sediments and rocks. Allows to preserve sediment material
• Conformable succession: layers sit on each other without any major breaks,
continuous records of environments. Major time breaks within the succession.
• Unconformable succession: There is more time that is unrecorded by rocks that is
actually recorded (there are major gaps). The layers on top are sitting
unconformable on the layers underneath.
• Index fossils and faunal succession: fossils = remains of an organism; index –
organisms that evolved quickly. Can be specific about the time range they lived in.
Faunal succession is a tool of stratigraphy and comprises the basis for the geologic
time scale
• Brittle failure of rocks to form faults: faults occur when brittle rocks fracture and
there is an offset. Brittle failure is like breaking a ruler and it snaps; these processes
our at shallo depths of the earth’s rust
• Plastic deformation of rocks to form folds
• Dip and strike:
o Gives us data about the disposition of the rocks
o Rocks dip to the west
o If a rock is flat, its dip is 0 degrees. If vertical upright, 90 degrees
o Strike = the edge of any rock layer (north-south)
• (Parent) Isotope dating using unstable radioactive isotopes: the rate at which these
isotopes break down is a steady, constant rate. They produce daughter products we
can determine the amount of parent isotopes and daughter product in a rock using
scientific methods. Gives the idea of the absolute age. (radio metric dating, using the
steady decay of isotopes to get the time since that rock was formed).
• Parent isotope (U^ 238 and daughter product(Lead Pb)
• Half-life : time it takes for 50% of the radioactive (parent isotope) to decay into
daughter isotopes (i.e. uranium to lead)
• The geologic timescale: the living document, everything we need to know about
earth history.
• John Playfair: Concept of Deep Time 4.5 billion years.
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
The surrounding rocks that need to hold the evidence. Opposite of subsidence = uplift = non-accommodation = erosion = removal of sediments and rocks. Allows to preserve sediment material: conformable succession: layers sit on each other without any major breaks, continuous records of environments. Major time breaks within the succession: unconformable succession: there is more time that is unrecorded by rocks that is actually recorded (there are major gaps). The layers on top are sitting unconformable on the layers underneath. Index fossils and faunal succession: fossils = remains of an organism; index organisms that evolved quickly. Can be specific about the time range they lived in. Faunal succession is a tool of stratigraphy and comprises the basis for the geologic time scale: brittle failure of rocks to form faults: faults occur when brittle rocks fracture and there is an offset.
