Dating Methods

10.11.07 / 10.12.07

Essential Question

• How can scientists determine the ages of geologic material?

What are strata?

• Layers of rock

Strata

Strata

Folded strata

What is an angular unconformity?

• When rock deposited in horizontal layers is folded or tilted, then eroded, and then covered by more horizontal layering

Angular unconformity

Angular unconformity

What is the law of cross-cutting relationships?

• The fault or igneous intrusion is always younger than the rock layer it cuts through.

Cross-cutting relationship

What are nonconformities?

• Unstratified igneous or metamorphic rock is eroded and then covered by layers of sediment.

Nonconformity

Nonconformity

What are disconformities?

• The boundary between younger sedimentary rock layers deposited on an eroded layer of older sedimentary rock

disconformity

What are faults?

• A break or crack in Earth’s crust along which rocks shift their positions

Fault

Earth history in 24 hours

scale:

1 sec = 52,000 years

1 min = 3,125, 000 yrs

1 hour = 187,500,000 yrs

What is correlation?

• Matching geologic materials from different regions to ages

Correlating tree rings

Rates of erosion and deposition

• Measure the recent rates

• Estimate age of geologic feature– Becomes less reliable as age increases– Most reliable within last 10,000-20,000 years

Niagara Falls

Varve count

• Similar to counting tree rings

• Count annual layers of sediment

• Common in glacial lake beds

• Distinguished by layers of dark fine particals and light heavy particles

Radiometric dating

• A method of determining the absolute age of an object by comparing the relative percentages of a radioactive (parent) isotope and a stable (daughter) isotope.

• Atoms of the same element with different numbers of neutrons are called isotopes.

Radioactivity and radiometric dating

Radiometric dating • Half-life – the time for one-half of the

radioactive nuclei to decay • Requires a closed system• Cross-checks are used for accuracy • Complex procedure • Yields numerical dates

The radioactive decay curve

Dating sedimentary strata using radiometric dating

Radioactivity and radiometric dating

Carbon-14 dating • Half-life of only 5730 years • Used to date very recent events • Carbon-14 produced in upper atmosphere

• Incorporated into carbon dioxide • Absorbed by living matter

• Useful tool for anthropologists, archeologists, historians, and geologists who study very recent Earth history