Chapter 8: The Rock Record

Vocabulary 1. Uniformitarianism

2. Relative age

3. Strata

4. Beds

5. Law of superposition

6. Unconformity

7. Law of crosscutting relationships

8. Principle of original horizontality

9. Graded bedding

10. Cross-beds

11. Ripple marks

12. Fault

13. Intrusion

14. Absolute age

15. Varves

16. Radiometric dating

17. Radioactive isotopes

18. Parent isotope

19. Daughter isotope

20. Half-life

21. Carbon-14

22. Fossils

23. Paleontology

24. Trace fossil

25. Amber

26. Petrification

27. Molds

28. Coprolite

29. Gastroliths

30. Index fossil

I. Determining Relative Age

A. Uniformitarianism

1. Principle created by James Hutton, a

Scottish farmer in the 1700s

a. Observed geologic processes occurring on

his farm & concluded that the same forces

that changed his farm must have changed

Earth in the past

2. Definition: current geologic processes

(volcanism, erosion, earthquakes, etc.)

are the same processes that changed

Earth’s surface in the past

a. Only thing that changes is the rate at which

processes occur

b. Brought up questions about Earth’s age

B. Relative Age

1. Definition: the age of an object in relation to the ages of other objects;

how old something is in comparison to something else; NOT a number

2. Looking at rock layers & determining the order in which they were

deposited can give us

information about

Earth’s past

3. One way to learn about

Earth’s past is to

determine the order in

which rock layers &

other rock structures

formed

i. Strata (layers of rock)

can show the order in

which events occurred

4. There are a few principles that can be

used to determine the order in which

rock layers formed.

C. Law of Superposition

1. Definition: a sedimentary rock layer is

older than the rocks above it and

younger than the layers below it if rock

layers are not disturbed

2. Sedimentary rocks are formed when

new sediments (sand, silt, mud, etc.)

are deposited on top of preexisting

layers of sediment

3. Sediments accumulate & harden into

layers called beds (boundaries between

beds are bedding planes)

D. Principle of Original Horizontality

1. Sedimentary rocks left undisturbed will form & remain in

horizontal layers (gravity!)

2. Sedimentary rocks that are not horizontal were most likely

affected by tectonic activity/crustal movement after the

layers were deposited

a. Sometimes tectonic forces push older layers on top of younger ones or

overturn a group of rock layers – there are ways to determine if this

happened & what the original layer positions were

3. Graded Bedding

a. Usually, the largest particles of sediment (biggest rocks) are located at

the bottom, and smaller particles are on top of those

b. Graded bedding is the arrangement of layers in which coarse and heavy

particles are in the bottom layers

c. If larger particles are located in the top layers, the layers may have been

overturned by tectonic forces

4. Cross-beds

a. Scientists can also look at shape of the bedding planes.

b. When sand is deposited, sandy sediment forms beds at an angle to the

bedding plane, called cross-beds

Studying the shape of the cross-beds can help determine the original

position of the layers

5. Ripple Marks

a. Definition: small waves that form on surface of sand due to wind or

water; when sand solidifies into sandstone, marks can be preserved

b. Usually, the crests (tops) of ripple marks point upwards; if not, the

rock layers have been disturbed

E. Unconformities

1. Definition: a time gap in the

geologic record, most likely due to

rock layers being eroded or a time

period in which sediment is not

being deposited

2. Three types:

a. Nonconformity: horizontal layers on top

of unstratified rock

b. Angular unconformity: horizontal layers

on top of tilted layers

c. Disconformity: horizontal layers

deposited on top of older horizontal

layers with an uneven surface

II. Determining Absolute Age

A. Definition: the numeric age of an

object or event, often stated in years

before the present, as established by

an absolute-dating process, such as

radiometric dating

1. Scientists use a variety of ways to

determine absolute age of a rock

formation. Some methods involve

geologic processes that can be

observed and measured over time;

others involve the chemical

composition of certain materials in

rock

B. Absolute Dating Methods

1. Rates of Erosion: only practical for geologic features

that formed within the past 10,000 to 20,000 years

2. Rates of Deposition: by using data collected over an

extended period of time, geologists can estimate the

average rates of deposition for common sedimentary

rocks.

1. Not always accurate because not all sediment is deposited

at an average rate – this usually only provides an estimate of

absolute age

3. Varve Count: some sedimentary deposits show definite annual layers, called varves.

a. Definition: a pair of sedimentary layers (one coarse, one fine) that is deposited in an annual cycle

b. The varves can be counted much like tree rings to determine the age of the sedimentary deposit

c. Thick layer is generally light-colored & sandy, deposited in summer

d. Thin layer is generally darker & clay-like, deposited in winter

4. Radiometric Dating

a. Definition: 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

b. Rocks generally contain small amounts of radioactive material

that can act as natural clocks

c. Atoms of the same element that have different numbers of

neutrons are called isotopes

d. Radioactive isotopes have nuclei that emit particles and energy

at a constant rate regardless of surrounding conditions; this

rate & the breakdown of the atoms can be used to determine

absolute age

e. Half-Life

i. Definition: time required for half of a sample of a radioactive

isotope to break down by radioactive decay to form a daughter

(stable) isotope

ii. The time required for half of any amount of a specific radioactive

isotope to decay is always the same & can be determined for any

isotope.

iii. By comparing the amounts of parent and daughter isotopes in a

rock sample, scientists can determine the age of the sample.

iv. The greater the percentage of daughter isotopes present in the

sample, the older the rock is.

f. Useful Radioactive Isotopes i. Uranium-238 (238U) is an isotope of uranium that has an

extremely long half-life; most useful for dating geologic samples that are more than 10 million years old (half-life: 704 million years)

ii. Potassium-40 (40K) has a half-life of 1.25 billion years; used to date rocks that are between 50,000 and 4.6 billion years old (half-life: 1.25 billion years)

iii. Rubidium-87 has a half-life of about 49 billion years; used to verify the age of rocks previously dated by using 40K (48.1 billion years)

iv. Long half-lives are useful for dating older rock; shorter half-lives are useful for dating younger rock

g. Carbon Dating

i. Younger rock layers may be dated indirectly by dating

organic material found within the rock.

ii. Organic remains, such as wood, bones, and shells that are

less than 70,000 years old can be determined by using a

method known as carbon-14 dating, or radiocarbon dating.

iii. All living organisms have both the carbon-12 and carbon-

14 isotope.

iv. To find the age of a sample of organic material, scientists

first determine the ratio of 14C to 12C.

v. Then they compare that ratio with the ratio of 14C to 12C

known to exist in a living organism.

vi. While organisms are alive, the ratio of 14C to 12C remains

relatively constant. Once a plant or animal dies, the ratio

begins to change – the amount of 14C decreases steadily.

vii. Scientists can determine the age from the difference

between the ratios of 14C to 12C in the dead organism.

III. The Fossil Record

A. Fossils: trace or remains of

an organism that lived long

ago, commonly preserved

in sedimentary rock

1. Paleontology is the

scientific study of fossils

2. Can provide info on relative

& absolute age of rocks,

clues to past geologic

events & climates, and

evolution

B. Fossilization Processes

1. Fossils can only be formed from dead organisms that are

buried quickly or protected from decay; only hard parts are

fossilized (bones, teeth, shells, wood, etc.)

2. Mummification

a. Found in dry locations – place where decay-causing bacteria can’t

survive

b. Some ancient civilizations mummified their dead by extracting the

body’s internal organs and then wrapping the body in prepared strips of

cloth

3. Amber: hardened tree sap

a. Insects become trapped in the

sticky sap and are preserved

when the sap hardens

b. Delicate features are usually

preserved (legs, antennae)

4. Tar Seeps : thick petroleum on

Earth’s surface covered by

water

a. Animals that come to drink the

water can become trapped in the

sticky tar; the remains of the

trapped animals are covered by

the tar and preserved.

5. Freezing

a. Frozen soil & ice can protect and preserve organisms because most

decay-causing bacteria cannot survive freezing temperatures

6. Petrification

a. Occurs when minerals (usually as a

part of groundwater) seep through

layers of sediment & replace the

organic content of a dead organism.

b. Once water evaporates,

the minerals, now in a near-perfect

replica of the organism, are all that

remains

c. Common petrifying minerals: silica,

calcite, pyrite

C. Types of Fossils

1. Trace fossil: fossilized mark that formed in sedimentary rock

by the movement of an animal

a. May or may not contain parts of the original organism

b. Can provide info about prehistoric life and the animal’s appearance &

activities

2. Carbon films: carbonized residues of leaves, stems, flowers, &

fish made in soft mud or clay

a. When original organic material partially decays, it leaves behind a

carbon-rich film that displays the surface features of the organism

3. Molds and Casts

a. When a shell is buried in sediment, it will

eventually decay & leave an empty space called

a mold

b. When sand or mud fills a mold and hardens, a

cast forms: replica of the original organism

4. Coprolites: fossilized dung or waste

materials from ancient animals

a. Can be cut into thin sections & microscopically

observed to gain information on the feeding

habits of ancient animals, such as dinosaurs

b. Animals that come to drink the water can

become trapped in the sticky tar; the remains of

the trapped animals are covered by the tar and

preserved.

5. Gastroliths: stones from digestive systems of dinosaurs

a. Some dinosaurs had stones in their digestive systems to help grind their

food, which often survive as fossils

b. Have a smooth, polished surfaces & are often found near dinosaur

remains

6. Index Fossils: used to establish age of rock layers because it is

unique, common, & only existed for a short period of

geologic time

a. Can be used to determine relative & absolute ages of rock layers