Geologists use what two references for time

Relative time and Absolute time

Relative Time

subdivision of the earth's history in a specific order based upon relative age relationships. These subdivisions are given names, most of which can be recognized globally, usually on the basis of fossils.

Absolute Time

Numerical ages in 'millions of years'. These are most commonly obtained via radiometric dating methods performed on appropriate rock types.

Principle of Uniformitarianism

James Hutton, The Present is the key to the past.

Six Fundamental Principles for Geology.

Superposition, Original Horizontality, Lateral continuity, Cross-cutting relationships, Inclusions, and Fossil Succession.

Superposition

Oldest units of rock on the bottom, the higher up, the younger the rock.

Original horizontality

Sedimentary rocks deposited as flat layers, any other attitude implies a deformation event after deposition.

Lateral continuity

Strata extends laterally in all directions until it thins, pinches out, or reaches the edge of the depositional basin.

Cross-cutting relationships

a feature or unit that cuts another is younger

Inclusions

fragments of one rock contained within a layer of another are older than the layer containing them

Fossil succession

fossil assemblages succeed one another through time in a regular and predictable order

William Smith 1795

Spent 24 years mapping and tracing strata in Great Britain: Surveyor and engineer. Predicted the kinds and thicknesses of rock that had to be excavated in future engineering projects.

Fossils groups and their prominent times

Age of Trilobites Cambrian Age of Fishes Devonian Age of Coal Forests Carboniferous Age of Reptiles Mesozoic Age of Mammals Cenozoic Age of Humans Quaternary

Fossils represent?

Organic remains, these organisms lived, evolved, and died in the past. Therefore only the rocks formed during the same age.

Contact between rocks

Conformable - reflecting more or less continued deposition OR Unconformable - there is a time break, causing a surface of erosion or non deposition.

Three types of Unconformities

disconformity - an erosional surface between parallel beds angular unconformity - an erosional surface on top of tilted strata, over which younger rocks have been deposited nonconformity - an erosional surface on top of igneous or metamorphic rock

Correlation

the process by which stratigraphic units in two or more separated areas are demonstrated to be laterally similar in character or mutually correspondent in stratigraphic position, as based on geologic age, lithologic characteristics, fossil content, or any other property

Marker bed

highly distinctive and easily correlated units, such as coal beds, volcanic ash or lava flow layers, extended for a long distance and occurred in short period of time.

Absolute-Dating Methods

Radioactive isotopes spontaneously decay to more stable isotopes, releasing particles and energy in the process The constant rate of decay gave geologists a ‘clock’ to accurately date rocks, as well as a mechanism to explain Earth’s high residual heat

Atoms, Elements and Isotopes

The nucleus of an atom is composed of protons and neutrons, with electrons encircling it number of protons define the elements atomic number Not all atoms of the same element have the same numbers of neutrons and are called isotopes same amount of protons but variable number of neutrons Some of these isotopes are radioactive and the decay rate of isotopes is measured to determine the absolute age of a rock

Radioactive Decay

The process whereby an unstable atomic nucleus is spontaneously transformed into an atomic nucleus of a different element

Radioactive Decay 3 types

Alpha Decay - 2 protons and 2 neutrons are emitted from the nucleus - Loss of two atomic numbers and four atomic mass numbers. Beta Decay - a fast-moving electron is emitted from a neutron in the nucleus, changing that neutron to a proton and consequently increasing the atomic number by one, with no resultant atomic mass number change. Electron Capture - a proton captures an electron and thereby converts to a neutron, resulting in a loss of one atomic number and no change in the atomic mass number

Half-lives

A half-life is the time it takes for one-half of the unstable parent element to decay to atoms of a more stable daughter element Half-lives for given elements are constant and have a wide range (less than a second to 49 billion years) Decay is geometric, not linear.

Radiometric Dating

When magma crystallizes, only the parent radioactive isotopes will fit within the structure of certain minerals because they are the right size and can fit into crystal structure, whereas the stable daughter atoms can’t fit due to their different size By measuring the parent-daughter ratio and knowing the half-life of the parent the age of the sample can be calculated

Sources of Uncertainty

The most accurate radiometric dates are obtained from igneous rocks. The time being measured is the time of crystallization of the mineral containing the radioactive atoms, not the time of formation of the radioactive atoms. Sedimentary rocks can’t be reliably dated, one would be measuring the age of a particular mineral rather than the time it was deposited as a sedimentary particle.

Fission Track Dating

Crystal structure was damaged during decay, such damage leaves tracks that are visible after etching with hydrofluoric acid The number of tracks corresponds to the age of the sample: greater number of tracks = older sample Most useful between 40,000 and 1.5 m.y.a.

Radiocarbon Dating Method

Carbon 14 dating is based on the ratio of C12 to C14 and is used to date once-living organisms

Tree-Ring Dating Methods

Counting the growth-rings in the lower part of the stem. Each ring represents one year’s growth. The pattern of wide and narrow rings can be compared among trees to establish the exact year in which the rings were formed.

Lithostratigraphic Unit

is a defined stratum or body of strata (sed. & extrusive igneous, metasedimentary, or metavolcanic) which is distinguished and delimited on the basis of lithic characteristics and stratigraphic position. Generally conforms to the law of superposition. Layered or tabular in form. Supergroup, Group, Formation, Member, Bed

Biostratigraphic Unit

is a body of rock defined or characterized by its fossil content. Biozone, Subzone

Quatermary Period has two divisions

Holocene (present) and Pleistocene

Mesozoic Period has three divisions

Cretaceous, Jurassic, and Triassic

Time between Mesozoic and Cenozoic

66 million year