Measuring Geologic Time

Types of Rocks

• Igneous rock: any rock formed by cooling and crystallization of magma (e.g., granite) or by consolidation of debris ejected from volcanoes (e.g., ash)

• Sedimentary rock: any rock composed of sediment.  The sediment may be particles of various sizes formed by the breakdown of other rocks (e.g., sandstone or shale) or the remains of animals  (e.g., shelly limestone) or plants (e.g., coal), or chemical compounds that are deposited directly from solution (e.g., cave limestone).

• Metamorphic rock: any rock altered by high temperature or pressure and the chemical activity of fluids (e.g., slate, marble).

Examples of Sedimentary Rocks

Chert - a form of microcrystalline quartz

Anthracite coal

Siltstone

Reconstructing Geologic Time

• Relative dating: Determining the age of an event relative to other events.  Putting events in a chronological order.

• Absolute dating: Using geochemical and other methods to assign actual ages to specific events.

Construction of the Relative Time Scale of Rocks

• Stratigraphy: study of the composition, origin, geographic extent and age of layered rocks.

• Principle of Original Horizontality

• Principle of Original Lateral Continuity

• Principle of Superposition

• Principle of Uniformitarianism

• Principle of Cross-Cutting Relationships

• Principle of Inclusions

• Unconformity

Principle of Original Horizontality

• Sedimentary particles settle from fluids under the influence of gravity

• Thus, sediments must have been deposited in layers that were nearly horizontal and parallel to the surface on which they were accumulating

The Principle of Original Lateral Continuity

• At the time of deposition, soft sediments spread out laterally in all directions until they either thin out (due to a lack of material) or run into a barrier, such as the edge of a geologic depression.

The Principle of Superposition

• For any sequence of undisturbed strata, the oldest layers are on the bottom, and successively higher layers are successively younger

Dipping strata

Principle of Cross-Cutting Relationships

• Any structure that cuts across another feature must be younger than that feature.

Principle of Uniformitarianism

• The same processes and principles (e.g., erosion, transportation of materials, etc.) that govern change on the earth today, have operated in the past to produce similar changes

Principle of Inclusions

• Fragments with larger rock masses are older than the rock masses in which they are enclosed

Unconformity

• A surface that separates older from younger rock, with some amount of time missing between the older and younger units

Kinds of Unconformity

Angular unconformity – steeply inclined older strata have been beveled by erosion and covered by flat-lying younger layers Disconformity – parallel strata are separated by an erosional surfaceMay be caused by the withdrawal and subsequent advance of the sea

Nonconformity – surfaces where stratified rocks rest on older intrusive rocks

Question

• How do we recognize the same time period in different temporally-ordered stacks of rock that are sometimes separated by large distances?

Correlation

• The determination of the equivalence of bodies of rock in different localities

• Rock bodies may be equivalent in the lithology (composition, texture, color), in their age, in the fossils

Lithostratigraphic Correlation

• A formation may have changed somewhat in appearance between 2 localities, but if it always lies above or below a distinctive stratum of consistent appearance, then the correlation of the formation is confirmed

• Correlation based on similarity of rock type and position.

Biostratigraphic Correlation

• Correlation of strata based on biological similarity

• If life forms have varied through time (and fossils assemblages from different times are distinctive, and the relative ages of assemblages can be determined by superposition), then the occurrence of the same fossil assemblage in rocks from different regions indicates that the rocks formed at the same time, and a relative time-scale based on fossil assemblages can be constructed.

Magnetostratigraphic Correlation

• Correlation based on different orientation of materials in sediments due to changes in the Earth’s magnetic field over time

• Using these aforementioned methods, especially the biostratigraphy, scientists working over the last 200 years have developed a Relative Geologic Time Scale. 

Biostratigraphy and the Geologic Time Scale

• The German paleontologists Friedrich Quenstedt and Albert Oppel indicated that each increment of time in a stratigraphy could be characterized by a particular assemblage of fossil organisms, formally termed a biostratigraphic "zone"

• These zones could then be traced over large regions, and eventually globally.

• Groups of zones were used to establish larger intervals of stratigraphy, known as geologic "stages" and geologic "systems".

• The time corresponding to most of these intervals of rock became known as geologic "ages" and "periods", respectively. 

Construction of an Absolute Time Scale

Lord Kelvin's Challenge

• Kelvin believed that the earth was initially molten and had cooled to its present state

• He attempted to determine the rate of cooling: because we know the mean surface temperature of the Earth and we know the heat loss of the Earth, Kelvin reasoned that we can calculate how long it took this molten body to cool

• Kelvin determined the Earth could be as young as 10 to 25 million years old.

Atoms

• An atom is the smallest unit of matter that still retains the properties of an element• In the nucleus of an atom are neutrons and protons

• A neutron is a subatomic particle that is electrically neutral (no electric charge)• A proton is a subatomic particle with a single positive electric charge (+)

• The nucleus of an atom is surrounded by rapidly moving electrons • An electron is a subatomic particle with a single negative electric charge (-)

• The positive charge of a proton is equal to the negative charge of an electron• Electrons are very small subatomic particles - they have a mass that is 2000 times less than that of a proton or a neutron

Model of an atom of the

element Helium

Atomic Number and Mass Number

• Elements differ from one another in the number of subatomic particles in their atoms• All atoms of a particular element have the same unique number of protons; this is called the element's atomic number• An atom's mass number is the sum of the numbers of protons and neutrons in its nucleus

Isotopes and Radioactivity

• Some elements have variant forms called isotopes•Isotopoes are atoms of an element which have the same number of protons in the nucleus, the same atomic number, and the same chemical properties , but which have different atomic masses because they have different numbers of neutrons in their nuclei• A radioactive isotope is one in which the nucleus decays spontaneously, giving off particles and energy• Radioactive decay involves the spontaneous decay of unstable atomic nuclei (parent nuclei) until they reach a stable state, which we call daughter nuclei. 

Radioactive decay series of uranium-238 to lead-206

Alpha decay is a type of radioactive decay that yield alpha particles - positively charged ions of helium (An alpha particle contains two protons and two neutrons.)

Beta decay another type of radioactive decay that emits a beta particle - an electron discharged from the nucleus when a neutron splits into a proton and an electron)

A third kind of emission in radioactive decay is called gamma radiationIts consists of a form of invisible electromagnetic waves having even shorter wavelengths than x-rays

Radioactive Decay

• The rate of decay of radioactive isotopes is uniform and is not affected by changes in pressure, temperature, or the chemical environment• Each radioactive isotope has a particular mode of decay and a unique decay rate• As time passes, the quantity of the original parent nuclide diminishes, and the number of the newly formed, or daughter, atoms increases, thereby indicating how much time has elapsed since the clock began timekeeping•The determination of radioactive decay is accomplished using a mass spectrometer

Mass Spectrometry

Radioactive Decay and Half-Life

• The decline in the number of atoms is rapid at early stages but becomes progressively slower in the later stages• Because of this feature of radioactivity, it is convenient to consider the number of years needed for half of the original quantity of atoms to decay• This span of years is termed the half-life

• Some nuclides have very long half-lives, measured in billions or even trillions of years; others have extremely short half-lives, measured in tenths or hundredths of a second. • The decay rate and therefore the half-life are fixed characteristics of a nuclide; the half-life of a given nuclide is a constant.• At the end of the years constituting one half-life, 1/2 of the original quantity of radioactive element still has not undergone decay; after another half-life, 1/2 of what was left is halved, so that 1/4 of the original quantity remains; after a third half-life, only 1/8 would remain, and so on

Half-Life cont.

• Every radioactive element has its own half-life

Dating Methods

Carbon-14 Dating Method

Carbon 14 is being created continuously in the earth’s atmosphere 14C originates in the upper atmosphere when a neutron strikes an atom of 14N; as a result of the collision, the N atom emits a proton and becomes 14C• The newly formed 14C combines quickly with O2 to form CO2

• The 14C that is tied up in CO2 can be utilized by plants during photosynthesis; also, when plants are eaten by herbivores this 14C can become incorporated into animal (tissues) food chain as well

Eventually 14C decays back to 14N by the emission of a beta particle; A plant removing CO2 from the atmosphere should receive a share of 14C proportional to that in the atmosphere; a state of equilibrium But the age of some ancient piece of organic material is not determined by the ratio of parent (14C) to daughter (14N) nuclides The age is estimated from the ratio of 14C to all other carbon (namely 12C) in the sample• When an animals dies, there can be no further replacement of carbon from atmospheric CO2, and the amount of 14C already present in the once living organism begins to diminish in accordance with the rate of 14C decay

Carbon-14 Dating Method cont.

Summary of Radiometric Dating

• Radiometric dating provides numerical values for the age of an appropriate rock, usually expressed in millions of years. • Therefore, by dating a series of rocks in a vertical succession of strata previously recognized with basic geologic principles (recall Stratigraphic principles and relative time), it can provide a numerical calibration for what would otherwise be only an ordering of events -- i.e. relative dating obtained from biostratigraphy (fossils), superpositional relationships, or other techniques. • The integration of relative dating and radiometric dating has resulted in a series of increasingly precise "absolute" (i.e. numeric) geologic time scales