Geologic Time Geochronology – the study of the timing of geologic events and the ages of geologic...

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Geologic Time • Geochronology – the study of the timing of geologic events and the ages of geologic materials • Relative dating – the space/time relationship between two or more geologic events or units • Numerical dating – absolute time or measured dates through geochemical analysis

Transcript of Geologic Time Geochronology – the study of the timing of geologic events and the ages of geologic...

Page 1: Geologic Time Geochronology – the study of the timing of geologic events and the ages of geologic materials Relative dating – the space/time relationship.

Geologic Time

• Geochronology – the study of the timing of geologic events and the ages of geologic materials

• Relative dating – the space/time relationship between two or more geologic events or units

• Numerical dating – absolute time or measured dates through geochemical analysis

Page 2: Geologic Time Geochronology – the study of the timing of geologic events and the ages of geologic materials Relative dating – the space/time relationship.

Principles of Relative Age Dating• Uniformitarianism – James Hutton – The present is

the key to the past.• Horizontality/Superposition – Layers were originally

laid down horizontally with oldest at bottom/youngest at top

• Cross-cutting relationships – any intrusive formation must be younger than the rocks it intrudes or cuts.

• Inclusions – must be older material picked up during intrusion

• Faunal succession – there is a direct link between the procession of fossils and time (e.g. once a plant/animal becomes extinct it is never seen again)

Page 3: Geologic Time Geochronology – the study of the timing of geologic events and the ages of geologic materials Relative dating – the space/time relationship.

Unconformities

• Gaps in geologic record forming boundaries that separate rocks of different ages.

• Non-conformity – (pg. 251) boundary between unlayered intrusion and overlying sedimentary rocks

• Angular unconformity – (Fig. 9.6) nonconforming boundary between horizontal rocks that are rotated and overlying rocks that were deposited horizontally

• Disconformity – (pg. 251) boundary between parallel layers of sedimentary rocks

Page 4: Geologic Time Geochronology – the study of the timing of geologic events and the ages of geologic materials Relative dating – the space/time relationship.

Great Unconformity of the GC(angular unconformity)

Page 5: Geologic Time Geochronology – the study of the timing of geologic events and the ages of geologic materials Relative dating – the space/time relationship.

Great Unconformity of GC(nonconformity – Tapeats SS over crystalline basement)

Page 6: Geologic Time Geochronology – the study of the timing of geologic events and the ages of geologic materials Relative dating – the space/time relationship.

Great UnconformityBox Canyon Ouray, Colorado

Page 7: Geologic Time Geochronology – the study of the timing of geologic events and the ages of geologic materials Relative dating – the space/time relationship.

Disconformities in GC(gaps between sedimentary layers)

Page 8: Geologic Time Geochronology – the study of the timing of geologic events and the ages of geologic materials Relative dating – the space/time relationship.

Correlation

• Rocks that are of the equivalent age but are separated spatially (Fig.9.9) – Fossil evidence – biota of same age– Key beds – short duration event over a wide area

(Ft. Apache Lm. In Sedona)– Facies change within a unit

Page 9: Geologic Time Geochronology – the study of the timing of geologic events and the ages of geologic materials Relative dating – the space/time relationship.

Stratigraphic Sections

Page 10: Geologic Time Geochronology – the study of the timing of geologic events and the ages of geologic materials Relative dating – the space/time relationship.

Correlating Stratigraphic Sections

• Vertical measurement of rock unit thickness• Section is a hypothetical column of rock units

for a particular area• Multiple sections can be correlated by

drawing lines between similar or identical beds or units

• Facies change can be identified if type of rock between correlated units changes between measured sections

Page 11: Geologic Time Geochronology – the study of the timing of geologic events and the ages of geologic materials Relative dating – the space/time relationship.

Unit Correlation

Page 12: Geologic Time Geochronology – the study of the timing of geologic events and the ages of geologic materials Relative dating – the space/time relationship.

Stratigraphic Unconformities

Page 13: Geologic Time Geochronology – the study of the timing of geologic events and the ages of geologic materials Relative dating – the space/time relationship.

Numerical Dating Techniques• Isotope dating – rate of decay for radioactive isotopes – uses

known rates to measure the time elapsed since mineral formation– Half life – time it takes half the atoms of the parent isotope to decay into daughter

isotopes (Fig.9.15)– Typically more useful for igneous rocks– Most reliable in closed system– Limited by time

• Uranium-thorium-lead – 10million to 4.6 billion only for oldest rocks

• Potassium-argon – 100,000 to 4.6 billion very old and very young dates

• Rubidium-strontium – 10 million to 4.6 billion half-life only rocks > 10my

• Carbon 14 – good for < 100,000 yrs old

Page 14: Geologic Time Geochronology – the study of the timing of geologic events and the ages of geologic materials Relative dating – the space/time relationship.

Radioactive Parent Stable Daughter Half life

Potassium 40 Argon 40 1.25 billion yrs

Rubidium 87 Strontium 87 48.8 billion yrs

Thorium 232 Lead 208 14 billion years

Uranium 235 Lead 207 704 million years

Uranium 238 Lead 206 4.47 billion years

Carbon 14 Nitrogen 14 5730 years

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Parent/Daughter Ratio

Page 16: Geologic Time Geochronology – the study of the timing of geologic events and the ages of geologic materials Relative dating – the space/time relationship.

How does Carbon-14 dating work?

• Cosmic rays from the sun strike Nitrogen 1 4 atoms in the atmosphere and cause them to turn into radioactive Carbon 14, which combines with oxygen to form radioactive carbon dioxide.

• Living things are in equilibrium with the atmosphere, and the radioactive carbon dioxide is absorbed and used by plants. The radioactive carbon dioxide gets into the food chain and the carbon cycle.

• All living things contain a constant ratio of Carbon 14 to Carbon 12. (1 in a trillion).

• At death, Carbon 14 exchange ceases and any Carbon 14 in the tissues of the organism begins to decay to Nitrogen 14, and is not replenished by new C-14.

• The change in the Carbon 14 to Carbon 12 ratio is the basis for dating. • The half-life is so short (5730 years) that this method can only be

used on materials less than 70,000 years old. Archaeological dating uses this method.) Also useful for dating the Pleistocene Epoch (Ice Ages).

• Assumes that the rate of Carbon 14 production (and hence the amount of cosmic rays striking the Earth) has been constant (through the past 70,000 years).

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Geologic Time Scale(Fig. 9.17)

*Read Box 22.3

• Eons• Eras –

Cenozoic – Age of mammals, early primates, Ice Ages, man (3.4-3.8mya), spread of modern humans last 10,000yrs.

Mesozoic, age of dinosaurs, early flowering plants, formation of the Rocky Mountains

Paleozoic, - Mt. Building in N.A. and Eur., early land plants, coal forming swamps

Precambrian 4.6by Earth forms, ~1.5by Pangaea forms, .5by first multi-celled organisms

Page 18: Geologic Time Geochronology – the study of the timing of geologic events and the ages of geologic materials Relative dating – the space/time relationship.

Periods – Breakdown of Eras

•Paleozoic•Permian

•Pennsylvanian•Mississippian

•Devonian•Silurian

•Ordovician•Cambrian