Transcript No Slide Title

HISTORICAL GEOLOGY
LECTURE 4. THE GEOLOGIC TIME SCALE.
Introduction: The standard geologic time scale was not
developed in an organized and systematic manner. Instead,
the scale took shape over centuries as a number of geologists
discovered and named the various intervals, on the basis of
time-rock units identified by superposition, fossils and
correlation (absolute ages from radiometric dating have only
recently been added). Often the units were named after the
local area or the type of rocks present.
Harry Williams, Historical Geology
1
Harry Williams, Historical Geology
2
Divisions:
Eons: Archean – (4000-2500 million years ago). No rocks remain
from ~4600-4000 million years ago.
Proterozoic (2500-542 million years ago) modern
tectonics and sedimentation started in this eon.
(Archean & Proterozoic together = “Precambrian”)
Phanerozoic (evident life) fossils become abundant.
Eras: Based on major changes in fossils.
Paleozoic (542-251 m.y.);
Mesozoic (251-65 m.y.);
Cenozoic (65m.y. - present day).
Periods: Again, based on fossil changes. Named after local features.
Harry Williams, Historical Geology
3
Early Paleozoic period
names are based on outcrops
in the west of Great Britain:
A. Cambrian
B. Ordovician
C. Silurian
D. Devonian
Harry Williams, Historical Geology
4
Mesozoic and
Cenozoic period
names are from
locations in Europe
and Russia.
Perm Basin
in Russia
Harry Williams, Historical Geology
5
1. Cambrian; after cambria (latin for Wales) 542-488 mybp.
2. Ordovician; after ordovices, early celtic tribe, 488-447 mybp.
3. Silurian; after silures, English/Welsh tribe, 447-416 mybp.
4. Devonian; after Devonshire, 416-359 mybp.
5. Mississippian; from the Mississippi River basin, 359-318 mybp.
6. Pennsylvanian; rock outcrops in Pennsylvania, 318-299 mybp.
(last two only used in N. America, elsewhere = Carboniferous,
based on coal beds in northern England).
7. Permian; from Perm, a province of Russia, 299-251 mybp.
8. Triassic; rocks in Germany, 3 divisions, 251-200 mybp.
9. Jurassic; the Jura mountains in Switzerland, 200-145 mybp.
10. Cretaceous; from 'creta' for chalk, 145-65 mybp.
11. Paleogene (65 – 23 mybp); beginning of “Age of Mammals”.
12. Neogene (23-2.6 mybp); “Neo” = newer part of Cenozoic, pre-ice
ages.
13. Quaternary; suggested by a French geologist, 1.8-0 mybp. Time of
ice ages.
(Absolute ages were not available when these divisions were first
Harry Williams, Historical Geology
6
made).
Epochs:
Pleistocene – time distinguished by many glaciations or “ice ages”.
1.8 million – 10,000 years ago.
Holocene – after the last ice age. 10,000 years ago to present day.
Harry Williams, Historical Geology
7
Radiometric Dating Methods.
Based on radioactive decay i.e. the nucleus of certain elements
(PARENT ELEMENT) spontaneously lose or gain atomic
particles and in so doing change to a different element
(DAUGHTER ELEMENT).
Atomic number = number of protons – defines element.
Atomic mass = number of protons+neutrons
Mass of proton=1, neutron=1, electron=0
Harry Williams, Historical Geology
8
Harry Williams, Historical Geology
9
Example: Uranium 238 (U238)
decays to Lead 206 (Pb206)
via a large number of
intermediate steps.
Harry Williams, Historical Geology
10
When dealing with a large number of atoms, studies have shown how
long it will take FOR HALF OF THEM TO DECAY (change from
parent to daughter material) - this is the HALF LIFE.
Harry Williams, Historical Geology
11
The ratio of PARENT ELEMENT to DAUGHTER ELEMENT
thus follows a predictable pattern. Therefore, radiometric dating
methods are based on finding the radioactive element in a
mineral from which none of the daughter element has escaped
and calculating the ratio between them to find the age of the
mineral.
Lead 206
Harry Williams, Historical Geology
12
These radiometric dating methods (except C14) are best suited to
IGNEOUS rocks, since the radioactive elements and their
daughter elements are "locked" in the mineral crystals when the
rock solidifies from magma. These methods are not well-suited to
metamorphic rocks since heating of the original rock may release
the daughter elements, "resetting" the radiometric clock to the
time of the last episode of heating.
Using these methods for sedimentary rocks will give the
age of the rock and mineral particles that make up the rock, not
the time of deposition of the sediment.
Harry Williams, Historical Geology
13
Instead, sedimentary rocks are
often given minimum and
maximum ages by
radiometrically dating
enclosing or intruding igneous
rocks (including volcanic ash
falls).
Harry Williams, Historical Geology
14
Or, a rock of
unknown age can be
correlated (e.g. by
fossils) to a rock of
known age.
As a result of these
methods, thousands
of sedimentary
rocks around the
world have been
dated and absolute
ages have been
added to the
geologic time scale.
Harry Williams, Historical Geology
15
Radiocarbon Dating
Radiocarbon (C14) has a half-life of 5730 years and is
usually used for dating ORGANIC MATERIAL up to about
100,000 years old - useful for archaeology and very recent
geologic events, such as the last ice age (ended 10,000 years
ago). The organic material can be any kind e.g. bones buried in
a swamp; pieces of tree buried in a river floodplain; shells
buried in ocean floor muds.....etc.
Harry Williams, Historical Geology
16
Production of C14 in the atmosphere by cosmic rays (high energy particles).
The C14 spontaneously decays to N14 by Beta emission.
Harry Williams, Historical Geology
17
C14 is naturally produced in
the atmosphere by cosmic
rays at a fairly steady rate (2
atoms/second/square cm).
Since C14 is absorbed by
plants (the bottom of the
food chain), it makes its
way into all living things.
Harry Williams, Historical Geology
18
REGULAR CARBON (non-radioactive carbon 12 - the most
common carbon) is also absorbed by all living things, such that:
RATIO OF C14/C12 CONSTANT IN ALL LIVING
ORGANISMS
Once the organism dies the C14/C12 ratio decreases, since the
organism is no longer absorbing new C14. If C14/C12 ratio = 50%
of modern ratio, material is 1 half-life, or 5730 years, old; if ratio =
25%, material is 2 half-lifes, or 11,460 years old and so on.
Harry Williams, Historical Geology
19
How can you possibly
remember the period
names in the correct
order?
Answer = a mnemonic
Harry Williams, Historical Geology
20