EESA06H3 Lecture 8: Lecture 8
Lecture 8: Geologic Time – How do we date the past?
Terms to Remember:
• Strata (layers of rock) and stratigraphy (study of age of rocks)
o Can date these strata (book has a story → use of hapters ad page uers → to
date strata).
• Relative age vs. absolute age
• Accommodation: i order to get roks presered, e eed areas of the rust thats susidig to
accumulate sediments – use of basins).
o Uplift helps in non-accommodation and erosion.
• Conformable succession: the layers of rock (old layer bottom, new layer on top) sit on top of
each other with no breaks (continuous record of data).
• Unconformable succession: most common situation → ajor tie reaks ithi the suessio
(more time unrecorded than record rocks)
o Record big events: separated by time of nothing big happening
• Index fossils and faunal succession: help record evolution of time and organisms → to help date
the iteral of tie i Earths histor.
o Can be quite specific in where ancient organisms lived in (index fossils)
• Brittle failure of rocks to form faults: like snapping a ruler in two (happens quickly). Usually at
shallo depths i Earths depths.
o At deeper → begin to bend them and fold them (plastic deformation of rocks to form
folds)
• Usually believe that oldest rocks are at deep levels soe area opletel opposite → oug
rocks at deep levels due to folding)
• Isotope dating using stable radioactive isotopes: rate of which isotopes break down (daughter
produts → deteried ratio etee paret isotope ad daughter produts to gie absolute
age (or radiometric dating methods).
o E.g., U238 (uranium-238) and daughter product (e.g., Pb-lead)
o Use of half-life
• The geologic timescale → ot epeted to reeer ook that deals ith this
• Unconformity: the layers on top sitting uncomformably on layers on bottom (a major gap is in
etee → ajor gap i geologial tiesale.
Relative Age of Rock Layers:
• Dip: tilted rock layers (old) due to
tectonic activity
o Rocks deposited horizontally
and then tilted and been
intruded by plutons (bodies
of igneous rocks intruded in).
Top of it cleaned off by fault.
o Then, after fault → laers of
deposited rock
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