Geology 2, Historical Geology Your Name___________________

Lesson 3 - Geologic Time

Readings:

Earth Through Time (Levin):

p. 15-18, 20-24

p. 29-45: Geologic Time (Chapter 3)

The Rocks Don’t Lie (Montgomery): Chapter 3, P. 93-96 of Chapter 6

Explain the Difference between Relative Geologic Time and Numeric Geologic Time

Relative Geologic Time

Describe the following relative geologic time principles:

Superposition

Original Horizontality

Cross-cutting relationships

Describe what occurs to produce the following features:

An Unconformity

An Inclusion

Numeric Geologic Time

Explain what occurs during radioactive decay using the words: parent and daughter elements, and half-

life. Describe what occurs during a “half life”

Draw a graph showing how parent material changes with time. This graph should show the

exponential decay of the parent material.

Use your graph to show the % of parent that would remain after 4 half lifes. Do this by showing where

you find the number of half lifes on the graph and labeling the % (provide the number) that exists after

4 half lifes.

List at least two common parent-daughter pairs used to date rocks. List the half life of each pair

The geologic time scale has been determined bit-by-bit over the years through relative dating,

correlation, examination of fossils, and radiometric dating. Boundaries on the time scale are drawn

where important changes occur in the fossil record, such as extinction events. The time scale is divided

into a number of types of units of differing size. From the largest units to the smaller units, they are:

Eons.

Eras.

Periods.

Epochs.

Draw a Geologic Time scale showing Eon, Eras, and Periods.

Write the times where each interval begins and ends (in millions of years) next to each division. This URL has a practice page for remembering the time scale. http://tinyurl.com/G2TimeScale

Mnemonic to memorize the Periods of the Paleozoic: Cambrian, Ordovician, Silurian, Devonian,

Mississippian, Pennsylvanian, Permian,: Can Old Silly Dumb Men Play Poker or

Cold Oysters Seldom Develop Many Precious Pearls

Mnemonic to memorize the Periods of the Mesozoic and Cenozoic: Triassic, Jurassic, Cretaceous,

Paleogene, Neogene, Quaternary: Three Jokers Carry Pale New Quarters

Use relative geologic time principles to order the events that produced the outcrop shown below.

Indicate the relative time principle you used to determine each step. Feature C is an igneous intrusion,

D is a fault, and the boulders shown in F are made of material from layer B.

Identify the feature that represents an Unconformity.

Assume Radioisotopic Dating has been used to date Layer G (an igneous sill) as 23 million years old,

and the igneous intrusion C as 5 million years old.

What is the age of the most recent motion on the fault?

Do you consider the age you determined for the motion of the fault as a relative or numeric age?

Explain.

Refer to the figure below. This is a cross-section of the Grand Canyon. The symbols used are:

Wavy lines – metamorphic rock; small Vs – igneous rock; dots – sandstone; dashes – shale; bricks –

limestone.

a. Assume Uranium-235 (parent) and Lead-207 (daughter) have been found in the Zoroaster Granite, and the

Uranium/Lead are in a closed system.

The Uranium (parent) represents 12.5% of the system and the lead (daughter) represents 87.5% of the

system. How many half-lifes have occurred? ___________________

Each half-life is 704 million years, How old is the Granite? ______________________

b. Refer to the Figure below. What is the youngest possible age of the Tapeats Sandstone if it is part of the

Cambrian System?

c. List the geologic history using a numbered sequence from oldest to youngest. Include periods of tilting,

erosion, and intrusion. The “Grand Canyon Supergroup” is a series of sedimentary rock layers that can be

considered as a single unit in your history. The wavy lines indicate unconformities.

d. Write the approximate numeric age next to the time units (e.g., early Paleozoic)

e. Unconformities are especially useful in reconstructing Earth history. An unconformity is a surface

that corresponds with a gap in sedimentation, either nondeposition or erosion. Rocks above an

unconformity are younger than those below it.

Three main types of unconformities are recognized:

(1) angular unconformity, in which beds above and below the surface are not parallel;

(2) nonconformity, in which sedimentary layersoverly crystalline rocks (either igneous or

metamorphic); and

(3) disconformity, in which beds above and below the surface are parallel, but the surface itself is

irregular, exhibiting evidence of erosional relief. In geologic block diagrams and cross-sections,

unconformities are usually drawn as a wavy line.

Identify the type of uniformity that exists at each of the following boundaries within the Grand Canyon Section

a. Tapeats Sandstone – Vishnu Schist boundary

b. Tapeats Sandstone – Grand Canyon Supergroup boundary

c. Redwall Limestone-Supai boundary

Investigate the Grand Canyon samples in the lab and use the table below to:

1. Determine the rock types of the Vishnu and Zoroaster.

2. Are the other samples consistent with the symbols shown in the diagram? Explain any

differences.

3. How can you determine that the Muav sample is a limestone?

4. What is the basis for classifying the Supai as a shale?

Simplified Chart of Lithologic (rock type) map symbols

The following questions are related to the description of Siccar Point (Scotland) in Chapter Six of the

The Rocks Don’t Lie

List the geologic events (mountain building, erosion, or deposition) that James Hutton proposed to

explain formation of the Siccar Point outcrop: Number each event and list the rock type formed or eroded

List the approximate time (in millions of years) for each numbered event you have listed.