GLG 101

Chapter 8

Geologic Time

 Early concepts and the age of the Earth

- Christian scholars: 6000 years old

- Buffon: gradual cooling, 75000 years

- depositional rates of various sediments

- salinity of oceans

- Kelvin: heat loss, 20-100 my

Relative dating methods

Nicolas Steno:

- principle of superposition: oldest rock on bottom

- principle of original horizontality: sediments are deposited horizontally

- principle of lateral continuity: sediments extend laterally

James Hutton:

- principle of cross-cutting relationships: younger features cuts older

William Smith:

- principle of faunal and floral succession: organisms occur in a predictable fashion

Unconformities

is a surface of non-deposition or erosion

- disconformity: surface of erosion or non-deposition between parallel rock layers

- angular unconformity: erosional surface on tilted or folded strata

- nonconformity: erosion surface cut into metamorphic or igneous rocks covered by sedimentary rocks

Correlation

demonstration of time equivalence of rock units

correlation criteria:

- similarity of rock type

- position in a sequence

- key beds (marker beds)

- guide fossils (index fossils)

"Absolute dating methods"

isotope: variable form (number of neutrons) of an element

- decay rate of unstable isotopes is used to determine age of rocks

three different types of radioactive decay:

- alpha decay: loss of 2 neutrons and 2 protons

- beta decay: loss of one fast electron

- electron capture: proton in nucleus captures one electron to become a neutron

half-life: of a radioactive isotope (parent) is the time it takes for half of the radioactive isotope to decay to a stable isotope (daughter)