General Geology: Geologic time

Instructor: Prof. Dr. Boris Natalin

• Interpreting Earth history is the primary goal of geology

• Rocks contain information about their origin.• Rocks exist as individual material bodies (e.g.

layer or intrusion) occupying some space in the Earth.

• These bodies have contacts with each other which can be interpreted in terms of time – e.g. magmatic rock (batholith or dyke) intrudes sedimentary rocks.

• Geological event must be put into time perspective

Early estimate of geologic time• Herodotus (450 B.C) observed steady growth of the

Nile delta and conclude that the age of the Earth should be more that 20,000 years

• Dark ages and the Book of the Genesis – “Begat” method - Archbishop James Ussher of Ireland (1581-1665) declared that the Earth was created in the evening of October 22, 4004 BC.

• Comte de Buffon (cooling of iron bolls; age of the earth is 75,000 years)

• Salinity of the oceans (John Joly)Total age 90 Ma

James Hutton

1726-1797

“The results, therefore, of our enquiry is, that we find no vestige of a beginning – no prospects of an end”

Geologic time

• Absolute (numerical) dateThis date pinpoint the time in history when something took place

• Relative datingRocks are placed in their proper sequence of formation

Relative dating

Nicolaus Steno (1638-1686)

is the founder of relative dating

Nicolaus Steno

"The prodromus of Nicolaus Steno's

dissertation concerning a solid

body enclosed by a process of nature

within a solid"

Steno introduced three principals of spatial and temporal relationships of rocks1. Original horizontality 2. Original continuity3. Superposition

Original horizontality

Original horizontality

These folded rocks were originally horizontal

Law of superposition

Principle of cross-cutting relationships

Block diagram shows the succession of accumulation of layers, magmatic rocks, and deformations

Relative time of rock formation

Inclusions and relative dating

Relationships of sedimentary rocks

• ConformityThe relationships between adjacent sedimentary strata that have been deposited in orderly sequence with little or no evidence of time lapse; true stratigraphic continuity

• Unconformity-A break or gap in geologic record-The structural relationships between rocks that

are not in normal succession

Conformable relationships

Unconformable relationships

“The mind seemed to grow giddy by looking so far into abyss of time”

Hutton’s unconformity

Types of unconformities

• Angular unconformity• Disconformity (erosion of the underlying bed) • Paraconformity (time gap)• Nonconformity (crystalline rocks below the

unconformity)

Conformity and unconformity

Formation of an angular unconformity

- Accumulation

- Deformation

- Subsidence

- New accumulation

An angular unconformity represents an extended period during which deformation and erosion occurred

• Angular unconformityYounger sediments rest upon the eroded surface of tilted or folded rocks (An episode of deformation separates the rocks)

• DisconformityAn unconformity between beds that are parallel (A time gap exist between two rock groups)

• NonconformityAn unconformity between sedimentary rocks above and igneous or metamorphic rocks below (A magmatic or metamorphic episode separates two groups of rocks)

Record of relative time as determined by structural relation of rocks

Sill

Younger age!

Dike

Dike cuts the sill; its age is younger

Erosion of the previously formed rocks

Formation of younger rocks G-K

Lava flow

Formation of stream (erosion)

Relative dating and correlation

• Relative ages of rocks determined in individual outcrops must be correlated with each other.

• Correlation by physical criteria (type of rocks, succession of layers, thickness of beds, metamorphism, structures, etc.).

• Correlation by fossils (rocks containing similar fossils are synchronous).

Correlation by physical criteria

Methods• Walking along outcrop• Comparing the position

of beds• Comparing distinctive

minerals or rocks

Results• Succession of deposited

beds• Stratigraphic column

Comparing the position of beds

Comparing distinctive minerals or rocks

Stratigraphic succession

and stratigraphic columns

Fossils and correlation•William Smith

(1769-1839)•Principle of faunal

succession•Rocks containing similar

fossils are synchronous

Index fossilsThese fossils are wide spread geographically and are limited to a short span of geologic time

Graptolite

Ammonite

Relative age from assemblage of fossil

- Time intervals of fossils A, B, and C allows to divide geological history into 3 intervals

Fossils and correlation

• Age of Trilobite• Age of Fishes• Age of Coal Swamps• Age of Reptiles• Age of Mammals

Radiometric dating (absolute date)

• Earth is about 4.6 billon years old• Dinosaurs became extinct 66 million years ago

Atoms• Atom is composed of electrons, protons, and

neutrons• Atomic number is the number of protons in

nucleus• Atomic mass number is the number of protons

and neutrons• In the same element, a number of neutrons can

vary, and these variations or isotopes define the mass of element.

Radioactivity•Some isotopes are unstable

•The breaking apart, or decay, of a nucleus is called radioactivity •There are tree types of radioactive decay

Alpha emission (α) → two protons and two neutrons Beta emission (β) → (an electron or a positron) is emitted from an atomElectron capture → a proton-rich nuclide absorbs an inner atomic electron

Radioactive decay

• Parent isotopes (unstable isotope)

• Daughter isotopes• Radioactive decay

series

Radium

Radon

Polonium

Radioactivity and radiometric dating

• Rate of decay for many isotopes have been precisely measured and it do vary under the physical conditions that exist in Earth’s outer layers.

• Radioactive isotopes can be used for dating of rocks because content of parent and daughter elements can be measured.

• A radioactive mineral is captured during magma formation. If system is closed after the cooling the amount of appeared daughter element gives us a time elapsed.

Half of the radioactive parent element remains after one half-life

One quarter of the radioactive parent element remains after the second half-life

Change is exponential

Half-life as a rate measure

Radiometric dating

Choice of the method

1) Expected age and the half-life

2) Content of parent/daughter elements in rocks

Potassium-Argon dating

• Potassium-40 → Argon-40• The half life is 1.3 billion years• Isotopes are common in micas and feldspars

Potassium-Argon dating

• K39 (93% of total K), K40 (0.01167 of total K), and K41(7.9% of total K)

• K40 is radioactive • K40 decay by:

- electron capture (11% to argon-40 - beta emission 89% to calcium-40

• Ca40 is not useful

Potassium-Argon dating: errors

• System must be closed• Samples must be fresh• Cross check by other method must be applied

Radiocarbon dating

• Carbon-14 → Nitrogen-14• The half life is 5730 years• Isotopes are common organic material• The method dates events as far back as 75,000

years

Radiocarbon dating

Isotope of carbon is incorporated into carbon dioxide in atmosphere and then is absorbed by leaving material

Radiocarbon dating

• Carbon-14 is incorporated to carbon dioxide• Carbon dioxide is absorbed by living mater• As long as an organism is alive the content of

carbon-14 is stable• After the death of an organism the radioactive

decay of carbon-14 causes decrease of its content in organic tissue

The geologic time scale

• Relative dating of rocks have been used since Steno time but isotopic dating (absolute age) appeared only in 20thcentuary.

• The scale is mainly based on evolution of fossils

• Eon → Era → Period → Epoch → Stage

Precambrian → Paleozoic → Mesozoic → Cenozoic

542 Ma

Why relative dating is still important?• Radiometric (isotopic)

dating is mainly used for magmatic rocks.

• Sedimentary rocks can only rarely be dated by radiometric means

• Metamorphic rocks are affected by several deformational and metamorphic events

Radiometric dating is possible!