Topic 5 – Geologic Time
Adam Sedgwick – he defined Cambrian (named after people who lived their
after the Romans) aged rocks in Wales using Lithostratigraphy – define rock
layers on basis of composition and structure
Roderick Murchison – defined Silurian (named after the people who lived in
the time of the Romans) rocks in Wales by using
Biostratigraphy – define rock layers on basis of fossils they contain
William Lapworth – looked at unconformities (buried erosional surface
missing from rock record)
 Named missing rock layers after the Celtic tribe (ordovices)
Defined Ordovician
Strata – layers of rock
Stratigraphy – the study of rock layers in terms of time and space
Chronostratigraphy – define rock layers on basis of time (large to small)
Eon – Era – Period – Epoch …etc…
I. Dating of Rocks
A. Relative Age – is the age compared to the age of rocks next to them
B. Absolute Age – is the actual age of the rock in years, etc.
II. Relative Age:
A. Superposition – oldest rocks on the bottom, youngest on the top Except:
when layers have been overturned, faulted, folded, tilted
B. Law of Cross-cutting Relations – intrusions and extrusions are younger
than the rocks they cut across
intrusion – course textured igneous rocks
that do not reach the surface, from
magma
extrusion – fine textured igneous rocks that reach the surface, from
lava
C. Joint – crack in the rock
D. Vein – crack in the rock that has filled in
with a mineral deposit
E. Fault – crack in the rock with movement
F. Fold – bend in the rocks
G. Tilt – rocks are at an angle from uplift or
Subsidence
(B thru G are all younger than the rocks they are in)
III. Correlation – matching of rock layers from different areas
A. walk the outcrop – only good on a smallscale
B. Index fossil – fossil of an organism that
lived
over a large geographic area in a short
amount of geologic time
C. Volcanic Ash – deposited over a large area in a short amount of time
Uniformitarianism – the present is the key to the past
: the processes that shape the Earth now have done so for Earth’s entire
existence, so if you want to know and understand Earth history, look at the present
processes
IV. Absolute Age
A. Isotopes – variety of an element based on the number of atoms (are
unstable)
B. Radioactive Decay – change in an isotope from unstable to stable
C. Decay product – this is a stable isotope
D. Half-Life – the amount of time it takes one half of the material to
decay(change) from unstable to stable, this amount of time NEVER CHANGES,
nothing affects it, not heat, pressure, location
Ancient Life & Evolution:
 Most life forms on Earth are now extinct, everything that has ever
lived has eventually become extinct
 Comparisons between strata allow interpretations of past
environments
 Chances of fossilization are small, more types of life have left no
evidence of their existence