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THE TWO TYPES OF TIME
Relative time—two events; known is their relation to each other but not the time between
Absolute time—two events; known (in some time units) is the time between them
EXAMPLES
EARLY USE OF TIME IN GEOLOGY
Relative—the geologic time column A great deal can be (and has been)
done Based on understanding how rocks
are formed and… Superposition Cross-cutting relationships Derived fragments
What’s up??
Rocks and rock layers may be twisted, tilted, folded, turned upside down
Features in rocks show ‘up’
Cross-cutting relations An intrusive rock
(an igneous dike) Also (by some)
called the ‘law of intrusion’—any rock may be intruded
The intrusion is younger
Cross-cutting relations An unconformity Marks a time of
loss of record—also an errosional surface
By two features, it is younger
Derived fragments Sedimentary (can
be applied to some igneous)
Also ‘law of inclusion’
Rock containing the derived fragments as inclusions is younger
ABSOLUTE AGEDATES
The problem in geology Need a clock that operates over looong
times That is accurate even over looong
times One that keeps a record of the
passage of time And is a part of the rocks and may be
preserved
Absolute time The solution was not available until
approximately 1950; needed An understanding of isotopes and
radioactivity Accurate ways of measuring the
ratios of isotopes present in a sample An accurate determination of half-
lives and decay processes
Absolute time These became
available following the research into atomic energy during and after WWII; and the availability of that information
Now, isotopic determinations, using a mass spectrograph, are routine
Radioactive age dating Presently usable on
igneous and metamorphic rocks (give date of solidification and of metamorphism)
Carbon bearing materials that were once living and are less than about 60,000 years old (gives date of death)
There are specific procedures and problems for each set of isotopes and type of rock
Radioactive age datingan example—K40
Decay – K40 + e- Ar40; ½-life = 1.3 by Magma – K common, Ar is rare; K fits in
many minerals, Ar (a noble gas) doesn’t Let K represent a K40 atom, A represent
an Ar40 atom (daughter) derived from a K40, and ‘+’ represent a K39 atom
As far as a mineral is concerned, all isotopes of K are chemically the same; and Ar is not a fit, but it is physically trapped in the crystal lattice as a decay product (daughter atom)
Crystallization of a K mineralonly a tiny part of lattice shown
++++K+++++++K++K+++++++ K++++++K++++++++++K++++ ++K++++++++++++K++++++++ K+++++K++++++++++++++K++ +++++++++++K+++++++++++ +++K++++K++++++K+K++++++
After one half-life; Ar:K40 = 1or after 1.3 billion years ++++A+++++++A++K+++++++ K++++++K++++++++++A++++ ++K++++++++++++K++++++++ K+++++A++++++++++++++A++ +++++++++++A+++++++++++ +++K++++A++++++A+K++++++
After 2 half-lives; Ar:K40= 3or after 2.6 billion years ++++A+++++++A++K+++++++ K++++++A++++++++++A++++ ++A++++++++++++A++++++++ K+++++A++++++++++++++A++ +++++++++++A+++++++++++ +++A++++A++++++A+K++++++
Other ratios
A graph or a math formula can be determined and is used for other ratios of Ar to K-40 (including fractional ratios)
FOSSILS There are two aspects to fossils
As remnants of life forms and how they are formed, preserved, and interpreted
As a way of doing another type of relative age dating
Fossil = remnant of life form
Defined – remnant or evidence of a life form, preserved in the geologic past Remnants are usually hard parts—bone,
teeth, shell, scales, claws, seeds (rare), pollen; these don’t rot or are not eaten (or are passed undigested)
Evidence—tracks, footprints, trails, imprints, casts, carbon outlines, etc.
Geologic past—if it smells, it belongs to biology
Fossil tracks Probably Jurassic
reptile tracks Note the hammer
at top-right for scale
1966, Hartford, Connecticut (now a park)
Fossil dinosaurs Top – Triceratops,
Cretaceous Bottom –
Stegosaurus, Jurassic
Both reconstructed and at the Amer. Museum of Nat. History
Fossil ‘bird’ Probably one of
the best known of all fossils
Archaeopteryx, a toothed, earliest bird, Jurassic, Bavaria
Amer. Museum of Nat. History
Fossil invertebrates In order – clam,
clam, clams, horn (solitary) coral
Mid continent U. S., Devonian
‘trapping’ and preservation of fossils #1-quick burial #2-hard parts By far the most common—marine
creatures—widespread seas with abundant life and burial by sediments
Rarest—hominids, jelly fish, forest birds—land creatures are rarely trapped and buried and jelly fish have no hard parts
‘trapping’ and preservation of fossils—Rancho La Brea—
Hancock ParkOnce in a while things work exactly right—in L.A., pits containing oil seeps were commonly water holes for the land animals for about the last 50,000 years; many stepped or got pushed into the sticky tar and trapped—the tar is also an excellent preservative, preserving seeds, skin, feathers, hide, fur, small and large animal bones
‘trapping’ and preservation of fossils—generalized In the seas past and present—
moderately common—maybe 1 in 10,000
On the land—maybe 1 in 10 million Alpine forests and deserts—maybe
1 in 100 million Then preserving for a looong time,
finding and recognizing
Fossils for relative dating After many of the major sedimentary
rock units were dated relatively, it was discovered that many forms of life in the seas succeeded one another in an consistent manner
This came to be a commonly used and useful way to do relative dating referred to as ‘using faunal succession’; there are probably more than 5000 references detailing examples of faunal succession
Fossil foraminifera These are drawings of
one of the more important fossils used in relative age dating
Actual size 0.1 – 1 mm Widely used in the
petroleum industry Small, common, highly
varied in shape over time, easily recoverable