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) date Relative dating This date pinpoint the time in history when something took place Relative dating Rocks are placed in their proper sequence of formation

is the founder of 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 rocks 1. Original horizontality 2. Original continuity 3. Superposition

Original horizontality

Original horizontality These folded rocks were originally horizontal

Law of superposition

Principle of cross-cutting relationships Relative time of rock formation Block diagram shows the succession of accumulation of layers, magmatic rocks, and deformations

Inclusions and relative dating

Relationships of sedimentary rocks Conformity The 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 Hutton’s unconformity “The mind seemed to grow giddy by looking so far into abyss of time”

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 unconformity Disconformity Nonconformity Younger sediments rest upon the eroded surface of tilted or folded rocks (An episode of deformation separates the rocks) Disconformity An unconformity between beds that are parallel (A time gap exist between two rock groups) Nonconformity An 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 cuts the sill; its age is younger

Erosion of the previously formed rocks

Lava flow Formation of younger rocks G-K

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 fossils Graptolite These 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 atom Electron capture → a proton-rich nuclide absorbs an inner atomic electron alpha particle is the same as a helium-4 nucleus, which has mass number 4

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-life as a rate measure 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

Radiometric dating Choice of the method Expected age and the half-life Content of parent/daughter elements in rocks

Potassium-Argon dating 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!