This is only an outline of the lecture. You will need to go to class
       to fill in the outline, although much of the relevant information is
       also in the text.

Examples of relative time:
 
 
 
 
 
 
 

Examples of absolute time
 
 

Principle of Stratigraphy: #1 --Original Horizontality
 
 
 
 
 
 

Principle #2 --Superposition
 
 
 
 
 
 

Principle #3 --Crosscutting Relationships
 
 
 
 
 
 

 Principle #4 --Inclusion
 
 
 
 
 
 

Principle #5 --Faunal Succession
 
 

Disconformity
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Nonconformity
 
 
 
 

 Angular Unconformity
 
 

Maps and Cross-Sections

You have seen geologic maps and simple cross-sections in lab.

Careful measurements of the orientation and inclination of beds
 
 
 
 
 

This allows ages of layers of rock to be compared
 
 
 
 
 
 

Fossils
 
 
 
 
 
 
 
 

Key Beds

Use of key beds allows correlation over a large area.

A layer of volcanic ash
 
 
 
 
 
 

The ash layer is the same age everywhere it is found.
 
 

The Geologic Time Scale began to be developed in the 19th Century,
 
 
 

We now are able to say how old rock strata and fossils are in years.
 
 
 
 
 

The Time Scale is divided into Eons, which are each divided into Eras.
 

The Eras are divided into Periods, and the Periods into Epochs.  The Epochs are divided into Ages.
 

The Eons are:
 
 
 
 
 

 The Phanerozoic Eon
 
 
 
 
 

Dinosaurs lived during the Mesozoic Era.
 

Mammals have dominated life on Earth during the Cenozoic Era.
 
 

These dates are given with a + or - estimate, such as 38.1 million + 0.6 million years.  The "error" of 0.6 million is the precision of the date.
 
 

What We Date:  Usually igneous rocks, because crystallization of magma "sets the clock."
 
 

Some Isotopes of some elements are "unstable," and decay (change) into isotopes of other elements.
 
 

Remember that an isotope consists of those atoms of an element that all have the same number of neutrons.
 
 

Parent Isotope:  the unstable isotope that is decaying.
 
 

Daughter Isotope:  the isotope formed by decay.
 
 

 Rate of Decay:  Decay of radioactive isotopes occurs at a constant rate.
 

This observation gave people the idea of using the ratios of parent to daughter to find how old the rock is.
 

Setting the Clock:  The newly formed igneous rock has only the parent isotope and none of the daughter, because of magmatic processes that removed whatever daughter had formed earlier.
 

(For example, in some cases, the daughter isotope is an element that follows the H2O as it bubbles out of the magma.)
 

So, a long time after the rock crystallized, we can use the ratios of the parent isotope to the daughter isotope, to determine when the rock crystallized.
 

What isotopes do geologists use?   A variety, including isotopes of rubidium & strontium; potassium & argon; and uranium & lead.
 

People working on young sediment (< 50,000 years) use radiocarbon dating.
 
 

All types involve gain or loss of particles from the nucleus.
 
 

 
Alpha decay:  Nucleus of the unstable atom loses 2 neutrons and 2 protons (alpha particle).
 
 

Beta decay:  A neutron decays into a proton and an electron.  The protons stays.  The electron (beta particle) is kicked out.
 
 

Electron capture:  A nucleus absorbs one of the atom's electrons.  It combines with a proton to make a neutron.

All of these processes change the atom from one element to another.
 
 

"Constant rate of decay" means that  a certain fraction of the radioactive isotope will decay in a specific length of time.

Half life:  This is how we express the decay rate.  We say that 1/2 will decay in one half-life.
 
 

Then half of the rest will decay in the next half-life.  And so on.
 

This is exponential decay, not linear decay.

Therefore, when you graph time against the fraction remaining, you get a curve, rather than a straight line.
 
 

We use radiometric ages of igneous rocks, plus the principles of stratigraphy, to bracket the ages of sedimentary rocks.
 

We determine that sedimentary rock layers crosscut or intruded by igneous rocks are older than the igneous rocks.
 

If we have several plutons or dikes of different ages, we can determine the youngest possible and/or oldest possible ages of at least some of our rock layers.
 
 
 
 

Download the Outline of "The Rock Record and Geologic Time" by clicking here.