HISTORICAL GEOLOGY LECTURE 4. THE GEOLOGIC TIME SCALE

Harry Williams, Historical Geology 1

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.

HISTORICAL GEOLOGYLECTURE 4. THE GEOLOGIC TIME SCALE.



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.


Early Paleozoic period names are based on outcrops in the west of Great Britain:

A. Cambrian

B. Ordovician

C. Silurian

D. Devonian


Mesozoic and Cenozoic period names are from locations in Europe and Russia.

Perm Basin in Russia


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 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.


Atomic number = number of protons – defines element.Atomic mass = number of protons+neutronsMass of proton=~1, neutron=~1, electron=~0

Radiometric Dating Methods.


Radiometric dating is based on radioactive decay i.e. the nucleus of certain elements (PARENT ELEMENT) are radioactive and spontaneously lose or gain atomic particles and in so doing change to a different element (DAUGHTER ELEMENT).



Example: Uranium 238 (U238) decays to Lead 206 (Pb206) via a large number of intermediate steps.


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.


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


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.


Instead, sedimentary rocks are often given minimum and maximum ages by radiometrically dating enclosing or intruding igneous rocks (including volcanic ash falls).


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.


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.


Production of C14 in the atmosphere by cosmic rays (high energy particles). The C14 spontaneously decays to N14 by Beta emission.


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.


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.


How can you possibly remember the period names in the correct order?

Answer = a mnemonic

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HISTORICAL GEOLOGY LECTURE 4. THE GEOLOGIC TIME SCALE