TIMEITS MEASUREMENT

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’

Superposition Sedimentary (or

in some cases, lava) rock

Need other evidence of ‘up’

Younger on top

Cross-cutting relations Any rock Feature which

cross-cuts is younger

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

Derived fragments These

sedimentary layers are upside down!

Early Geologic Column--simple

Quaternary* Tertiary* Secondary Primary

Modern Geologic Time Column

Modern column has absolute dates as well—ignore for the moment

Geologic time column

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 trilobites Trilobites,

Ordovician, ?

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

La Brea Tar Pit drawing

‘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

Look again at the Geologic time column

All not to be memorized

Major units and ~times