Transcript TIME ITS MEASUREMENT

TIME
ITS MEASUREMENT
THE TWO TYPES OF TIME
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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
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Relative—the geologic time column
A great deal can be (and has been)
done
Based on understanding how rocks are
formed and…
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Superposition
Cross-cutting relationships
Derived fragments
What’s up??
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Rocks and rock layers may be
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twisted,
tilted,
folded,
turned upside down
Features in rocks show ‘up’
Superposition
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Sedimentary (or in
some cases, lava)
rock
Need other evidence
of ‘up’
Younger on top
Cross-cutting relations
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Any rock
Feature which crosscuts is younger
Cross-cutting relations
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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
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An unconformity
Marks a time of loss
of record—also an
errosional surface
By two features, it is
younger
Derived fragments
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Sedimentary (can be
applied to some
igneous)
Also ‘law of inclusion’
Rock containing the
derived fragments as
inclusions is younger
Derived fragments
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These sedimentary
layers are upside
down!
Early Geologic Column--simple
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Quaternary*
Tertiary*
Secondary
Primary
Modern Geologic Time Column
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Modern column has
absolute dates as
well—ignore for the
moment
Geologic time column
ABSOLUTE AGE
DATES
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The problem in geology
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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
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The solution was not available until
approximately 1950; needed
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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
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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
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Presently usable on
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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 dating
an example—K40
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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 mineral
only a tiny part of lattice shown
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++++K+++++++K++K+++++++
K++++++K++++++++++K++++
++K++++++++++++K++++++++
K+++++K++++++++++++++K++
+++++++++++K+++++++++++
+++K++++K++++++K+K++++++
After one half-life; Ar:K40 = 1
or after 1.3 billion years
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++++A+++++++A++K+++++++
K++++++K++++++++++A++++
++K++++++++++++K++++++++
K+++++A++++++++++++++A++
+++++++++++A+++++++++++
+++K++++A++++++A+K++++++
After 2 half-lives; Ar:K40= 3
or after 2.6 billion years
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++++A+++++++A++K+++++++
K++++++A++++++++++A++++
++A++++++++++++A++++++++
K+++++A++++++++++++++A++
+++++++++++A+++++++++++
+++A++++A++++++A+K++++++
Other ratios
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A graph or a math formula can be
determined and is used for other ratios
of Ar to K-40 (including fractional ratios)
FOSSILS
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There are two aspects to fossils
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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
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Defined – remnant or evidence of a life
form, preserved in the geologic past
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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
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Probably Jurassic
reptile tracks
Note the hammer at
top-right for scale
1966, Hartford,
Connecticut (now a
park)
Fossil dinosaurs
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Top – Triceratops,
Cretaceous
Bottom – Stegosaurus,
Jurassic
Both reconstructed
and at the Amer.
Museum of Nat.
History
Fossil ‘bird’
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Probably one of the
best known of all
fossils
Archaeopteryx, a
toothed, earliest
bird, Jurassic,
Bavaria
Amer. Museum of
Nat. History
Fossil trilobites
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Trilobites,
Ordovician, ?
Fossil invertebrates
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In order – clam,
clam, clams, horn
(solitary) coral
Mid continent U. S.,
Devonian
‘trapping’ and preservation of
fossils
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#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 Park
Once 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
La Brea Tar Pit drawing
‘trapping’ and preservation of
fossils—generalized
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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
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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
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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
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Small, common, highly
varied in shape over
time, easily recoverable
Look again at the
Geologic time column
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All not to be
memorized
Major units and
~times