13 geologictime forstudents

Chapter 12

Geologic Time

Chapter 12

Outline• Geologic time: perspective & a bit of history

• Dating geologic materials-General: relative & absolute dating-Relative dating:

-7 Principals & their application to a geologic history-Fossil successions

• Gaps in the geologic record (unconformity)-3 types of unconformities-Stratigraphic correlation & the global geologic column

• Numerical (absolute) dating-Radioactive decay-Meaning of a radiometric date-Other numerical dating methods-Dating the geologic column, geologic time scale, & age of Earth

Chapter 12

Chapter 12

Geologic Time

Chapter 12

Geologic Time

• Understanding time permits assigning ages to…

Chapter 12

Geologic Time• Prior to late 1600s, geologic time was thought to =

historical time. • Archbishop James Ussher, Ireland, 1654.

Chapter 12

Geologic Time• Scientists began to find clues to an ancient Earth.

• Nicolaus Steno (1638–1686) – Danish physician.

fossilsharktooth

Chapter 12






Chapter 12

Chapter 12

Geologic Time2 ways to date geological materials:

1. Relative age –

2. Numerical (absolute) age –

Chapter 12

Relative vs. Absolute

Chapter 12

Relative Age

• Logical tools are useful for defining relative age.• Principles of:

1. Uniformitarianism

2. Superposition

3. Original horizontality

4. Original continuity

5. Cross-cutting relations

6. Inclusions

7. Baked contacts.

Chapter 12

Geologic Time1. Uniformitarianism – “The present is key to the past”.

•

Chapter 12

Defining Relative Age

2. Superposition.

Chapter 12

Relative Age

3 & 4. Horizontality and continuity.

Chapter 12

Relative Age5. Cross-cutting relations.

Chapter 12

Relative Age6. Inclusions – a rock fragment within another.

Chapter 12

Relative Age

7. Baked contacts.

Chapter 12

Relative Age• Determining relative ages empowers geologists to

unravel complicated geologic histories.

Chapter 12

Geologic History

• Deposition of horizontal strata below sea level in order 1-> 8 (old to young). *Horizontality & continuity*

Chapter 12

Geologic History

• Igneous intrusion of a sill. *baked contact*

Chapter 12

• Intrusion solidified into sill

• Tectonic compression

Geologic History

Chapter 12

• Compression results: • Folding (inference: layers had to exist to be folded).• Uplift (above sea level) & erosion.

• Intrusion of a pluton. *baked contact/cross-cutting*

Geologic History

Chapter 12

• Extension -> normal faulting.• Faulting cross-cuts pluton & rock layers.

Geologic History

Chapter 12

• Dike intrusion.• Dike cross-cuts everything (even normal fault).

Geologic History

Chapter 12

• Erosion to present landscape.• Removed volcano and cuts down the dike top.

Geologic History

Chapter 12

Geologic History• Relative ages help to unravel a complicated history.• Those rules permit one to decipher this diagram!

Chapter 12

Geologic History

• Test yourself at home:

Chapter 12

Fossil Succession

• Fossils (organism traces) can be preserved in sedimentary rocks.

Chapter 12

Fossil Succession• Species evolve, exist, and then go extinct.

Chapter 12

• Fossil range – first to last appearance.

• Permit correlation of strata.

Fossil Succession

Chapter 12






Chapter 12

Chapter 12

Unconformity

• An unconformity is• Causes:

Chapter 12

Unconformities3 Types:

1. Disconformity –

Chapter 12

DisconformitiesDisconformities

Chapter 12


2. Nonconformity –

Chapter 12

NonconformityNonconformity

Chapter 12


3. Angular unconformity – represents a big gap in time

Horizontal rocks deposited, then deformed

Then eroded

Then sediments horizontally deposited on erosion surface

Chapter 12

Types of Unconformity

This animation shows the stages in the development of three main types of unconformity in cross-section, and explains how an incomplete succession of strata provides a record of Earth history. View 1 shows a disconformity, View 2 shows a nonconformity and View 3 shows an angular unconformity. For more information, see Section 12.5 Unconformities: Gaps in the Record starting on p.423 and Figure 12.9 in your textbook.

Types of Unconformity

Chapter 12

Unconformities• Earth history is in

strata.• Missing strata =

missing history

Chapter 12

Stratigraphic Correlation

• In 1793, William “Strata” Smith noted strata could be matched across distances.

• Similar rock types in a similar order• Rock layers contained same distinctive fossils• He made the 1st geologic map of the UK

Chapter 12

Stratigraphic Correlation• Stratigraphic columns depict strata in a region.

• Drawn to portray relative thickness• Rock types depicted by fill patterns• Divided into formations (mapable rock units)• Formations separated by contacts

Chapter 12

Stratigraphic Correlation• National Parks of Arizona & Utah.

• Formations can be traced long distances • Overlap in rock type sequences• Overlapping rock columns are used to build a composite

Chapter 12

The Geologic Column• A composite global stratigraphic column exists.• Constructed from incomplete sections across the globe• It brackets almost all earth history

Chapter 12






Chapter 12

Chapter 12

Numerical (Absolute) Dating

• Based on radioactive decay of atoms in minerals.• Radioactive decay proceeds at a known fixed rate• Radioactive elements act as internal clocks• Numerical dating is called geochronology

Chapter 12

Radioactive DecayIsotopes Atoms with same # of protons, different # of neutrons

Have similar but different mass numbers

Some are Stable – never change

Some are Unstable (radioactive) –spontaneously change to something else (decay) at a fixed rate

Chapter 12

Radioactive Decay• Decay process has 2 main components:

• Parent – isotope that decays• Daughter – decay product isotope

• Decay process can: • Have 1 step (parent>daughter)• Have many steps (parent>daughter>etc)

• Decay product is unstable and hence also decays• Eventually proceeds to a stable endpoint

Chapter 12

Radioactive Decay Time• Half-life (t½) – time for ½ unstable parent to decay.

• t½ is unique for each isotope

• After one t½ -1/2 original parent remains

• After three t½ -1/8 original parent remains

• Parent disappears (nonlinear) daughter accumulates

Chapter 12

Radiometric Dating• Mineral age can be determined by:• Measuring parent/daughter isotope ratio• Calculating time

Chapter 12

Chapter 12

What Is a Radiometric Date?

• Time since a mineral began to retain all parent & daughter isotopes. • Requires cooling below “closure (blocking) temperature.”

• Daugther retained only below closure T• Daughter leaks out above closure T• Thus, if rock is reheated above closure T, the

radiometric clock can be reset to zero

Chapter 12

Other Numerical Ages

• Numerical ages are possible without isotopes. • Growth rings – annual layers from trees or shells • Rhythmic layering – annual layers in seds or ice

Chapter 12

Other Numerical Ages• Magnetostratigraphy –magnetic signatures in strata are

compared to global reference column

Chapter 12

Other Numerical Ages• Decay process can cause scars (tracks) in minerals.

• Decay by fission (explosion) produces scar (track)• Daughter isn’t another isotope, it’s a damage zone

Chapter 12

Dating the Geologic Column• Use geochronology to: • Date specific strata OR• Bracket those that cant be dated directly

Chapter 12

The Geologic Time Scale

Chapter 12

Age of the Earth

• Oldest rocks are 3.96 Ga.• Zircon minerals in some sandstones are 4.1-4.2 Ga.• Earth is ~4.57 Ga based on correlation with…

• Meteorites, moon rocks.

13 geologictime forstudents

Spiritual

Transcript of 13 geologictime forstudents