GEOLOGIC TIME

Dr. Wan Zuhairi Wan YaacobProgram Geologi UKM

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GEOLOGIC TIME

Based on rock basic principles & fossil succession that were

discovered hundreds years ago

Relative geologic time scale

Radiometric dates(natural rates of decay of

radioactive elements)

Supplementary data (age) for geologic time scale

Geologic time scale

Relative dating Absolute dating

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Geologic time

• Relative age ( Usia relatif ) :– Placing geologic events in a sequential order, i.e.

Know the sequence of events.– Something is younger than or older than something

else.– Does not tell how old something is– For example: the sandstone in this area is older than

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the limestone.

• Absolute age ( Usia mutlak ) : – Spesific dates for rock units/events expressed in

years before the present – Thus, we can say how old something is.– For example: The sandstone is 300 million years old.

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Absolute age- Radiometric dating

Relative age-Rock principles- fossil

Geologic Time ScaleA dual scale; a relative scale based on rock and fossil sequences, with radiometric dates expressed as years before the present

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Fundamental principles of relative dating

• Nicolas Steno (1638-1686)– Superposition – Original horizontality– Lateral continuity

• James Hutton– Cross cutting relationship– Inclusions

• William Smith (1769-1839)– Fossil succession

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Principles of StratigraphyNicolas Steno (1638-1686)

1) Principle of superposition. – The oldest stratum at the bottom and the youngest

layer at the top

2) Principle of original horizontality. – A sequence of sedimentary rock layers that is

steeply inclined from horizontal must have been tilted after deposition and lithification

3) Principle of original lateral continuity. – Sediment extends laterally in all directions until it

thins and pinches out or terminates against the edge of the depositional basin

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Principle of superpositionThe oldest layer is at the bottom and the youngest layer is at the top

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Steeply inclined sedimentary rocks - Principle of original horizontality.

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James Hutton• Principle of cross cutting relationship

(Prinsip potong memotong… Don’t use this if you drive home this raya!!!)

– An igneous intrusion or fault must be younger than the rocks it intrudes or displaces

• Principle of inclusions ; “Xenolith”(prinsip kandung mengandung)– Inclusions or fragments of one rock contained

within a layer of another, are older than the rock layer itself

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Principle of superposition vs Principle of cross cutting relationship

Principle of superposition Principle of cross cutting relationship

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Principle of inclusions(prinsip kandung mengandung)

Baked zone

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1. Principle of Superposition2. Principle of Inclusions3. Principle of Cross cutting

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UNCONFORMITIES(ketakselarasan)

• Surface of nondeposition or erosion that separates younger strata from older rocks

• Represents a break in our record of geologic time (time break)

• Conformable = layers of rocks have been deposited without interruption (no break !).

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KETAKSELANJARAN V KETAKSELARASAN (discontinuities v unconformities)

• Ketakselanjaran – semua jenis struktursekunder pada batuan

• Ketakselarasan – sempadanhakisan/tiada pegendapan sedimenberlaku

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Three types of unconformities

(i) Disconformity (ketakselarasan selari)• Parallel erosion surface

(ii) Angular unconformity (ketakselarasanbersudut)

• Tilted or folded erosion surface

(iii) Nonconformity (Tak Selaras)• Erosion surface cut into metamorphic or igneous

rocks, covered by sedimentary rocks

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Disconformity

A period of nondeposition / erosion- Significant amount of geologic times

Sharp bedding planeDepositional break of minutes, hours, years (inconsequential)

Older strata

Younger strata

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Hiatus (rumpang) = any interval of geologic time not represented by strata (missing time !)

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Angular Disconformity

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Angular Unconformity

Angular Unconformity

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Nonconformity

Igneous or metamorphic rock

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Angular unconformity

Nonconformity

Siccar Point Scotland First described by James Hutton more than 200 years ago

Disconformity

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Questions: • D and L; • C and M; • E and N;• B and A;• I and Q;• A and M• Surface A• Surface B• The youngest?• The oldest?

Surface B ?Surface A ?

1. D is older than L – Principle of Superposition2. C is older than M – Principle of Cross cutting

relationships* C is cut by K which is in turn cut by M.

3. E is older than N – Principle of Cross cuttingrelationships

* E is cut by I, which is in turn cut by the erosion surface beneath N.

4. B is older than A – Principle of Inclusions5. I is older than Q – Principle of Cross cutting

relationships* I is cut by the erosion surface beneath N, and Q cuts N

6. A is older than M – Cross cutting relationships

* A is cut by C, which is cut by K, which is cut by M.dwzwy2006-2007

Principle of Faunal (Fossil) Succession

• Fossils (for sale !!?):– Remains of organisms that lived for certain

length of time.– indicator for time & depositional environment

• Fossil organisms succeed (come next after) one another through time in a definite order.

• Index fossils:-– (1) Easily identified– (2) Geographically widespread– (3) Existed for a short interval of geologic time

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Trilobite Isotelus

InoceramusLingula

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Fossils

Floral fauna

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FOSSILS (micro/macro)Microfossil macrofossil

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Fossil correlation

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Fossils correlation

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ABSOLUTE DATING METHOD

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Sophisticated analytical equipment capable of counting or measuring the quantities of different isotopes of an element

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Chemical elements

atoms

Number of protons = atomic numberProtons + neutrons = atomic mass number

Isotopes = variable forms of the same element (various number of neutrons)

Stable isotopes

Unstable isotopes(radioactive)

DECAY

Stable forms

Decay rate (or half life) gives absolute

age of rocks

Radioactive decay = the process whereby an unstable atomic nucleus is spontaneously transformed into an atomic nucleus of a different element

Half life = the time for one-half of the atoms of the original unstable parent element to decay to atoms of a new, more stable daughter element.

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A mineral crystallizing in a cooling magma will contain only a radioactive parent atoms but no stable parent atoms

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Metamorphism tends to blur the result

Metamorphic rocks are difficult to date accurately, because heat and intense pressure affect the

parent-daughter ratio.

Leakage:(1) If daughter atoms have leaked out ----

calculated age will be too young(2) If parent atoms have been removed ---

age will be too great !

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One half life = 1 m.yParent/daughter ratio = 1:15 (4 half lives)Age = 4 x 1m.y. = 4 m.y.

Parent element(unstable)

Daughter element(stable)

Radiometric Dating

Several conditions must be satisfied in the selection of radioactive isotopes:(1) Must be abundant enough in sample(2) Half life on parent must be appropriate

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Parent

Daughter

Three isotopes of potassium:

(1) 39K* non-radioactive (stable)

(2) 40K* Radioactive with a half life of 1.3 billion years**40K decays to 40Ar and

40Ca, only K-Ar branch is used in dating

(3) 41K - non-radioactive (stable)

Potassium-Argon Dating (1)

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K is an element that goes into many minerals,eg: feldspars and biotite.

Ar, which is a noble gas, does not go into minerals when they first crystallize from a magma because Ar does not bond with any other atom.When a K-bearing mineral crystallizes from a magma it will contain K, but will not contain Ar.With passage of time, the 40K decays to 40Ar, but the 40Ar is now trapped in the crystal structure where the 40K once was.Thus, by measuring the amount of 40K and 40Arnow present in the mineral, we can determine how many half lives have passed since the igneous rock crystallized, and thus know the absolute age of the rock.

Potassium-Argon Dating (2) R.I.Y

Absolute ages of sedimentary rocks can be determined by dating associated igneous rocks

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The Oligocene rocks are younger than the 30 m.y old lava flow and older than the 20 m.y. old lava flow.

The Paleocene rocks are older than both the 36 m.y. old dike and the 57 m.y. old dike (thus the Paleocene is older than 57 m.y.

The Eocene rocks are younger than the 57 m.y. old dike and older than the 36 m.y. old dike that cuts through them.

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The Geologic Time Scale

Geologic Time ScaleA dual scale; a relative scale based on rock and fossil sequences, with radiometric dates expressed as years before the present

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Precambrian

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Sempadan MASA(time boundary)

Even dinosaurs cannot pass through !!

Eat me! Eat me! Booo..!!!

THE END

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FINAL EXAM(total questions = 33)

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percentage (%)

#1 #2 #3 #4Topics

Final exam

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Quiz : next Friday !!!

• 13th October 2006

• Time: 11.15-12.00pm (45 minutes)

• Venue: HERE !!

• Multiple choice questions (OMR!!)

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• SELASA 18.10.2006 JAM 2pm

• Tempat: Makmal Tahun 1

• Kumpulan A : 2.00-2.45pm

• Kumpulan B : 3.00-3.45pm

• Lihat notice board/web untuk maklumatlanjut

Peperiksaan amali hablur

Selamat berpuasa dan selamat hari raya. Maaf zahir dan zahir…

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