Chapter 8: THE ROCK RECORD
I. DETERMINING RELATIVE AGE
- geologists estimate that the Earth is 4.6 billion years old
- James Hutton
- 18th century Scottish physician and farmer
- wrote about agriculture, weather, climate physics and philosophy
- drew conclusions based on geological events
A. Uniformitarianism
1. Hutton theorized that the same forces that changed the landscape presently changed the landscape in the past
2. studying the present, could learn about the past
3. Uniformitarianism
a. current geologic processes are the same processes that were at work in the past
1a. volcanoes
2a. erosion
b. one of the basic foundations of the science of geology
c. processes of the past and present are the same, the rate of the processes may vary over time
4. Earth’s Age
a. before Hutton, people thought the Earth was only 6000 yrs old
b. Hutton observed that forces the operated slowly
c. reasoned that millions of years must be needed to create the complicated rock structures of Earth’s crust
B. Relative Age
1. strata
a. layers of rock
b. show the sequence of events that took place in the past
c. relative age
1c. indicates that one layer is older or younger than another layer
2c. does not indicate age of rock in years
2. various types of rock form layers
a. igneous rock form layers when successive lava flows stack on top of each other
b. metamorphic rock, marble, can show layers
c. sedimentary rock is commonly studied by scientists
C. Law of Superposition
1. sedimentary rock forms when new sediments are deposited on old sediments and solidify
2. rock layers are called beds
3. the boundary between beds is called a bedding plane
4. Law of Superposition
a. used to determine the relative age of a layer of sedimentary rock
b. the law that a sedimentary rock layer is older than the layers above it and younger than the layers below it if the layers are not disturbed
c. Fig. 3
D. Principle of Original Horizontality
1. sedimentary rocks left undisturbed will remain in horizontal layers
2. layers that are not horizontal have been tilted or deformed by crustal movements that happened after the layers formed
3. tectonic forces
a. forces can push older layers on top of younger ones
b. law of superposition cannot be easily applied
4. clues to apply the Law of Superposition
a. graded bedding
1a. largest particles of sediment are deposited in the bottom of the layer
2a. graded bedding – arrangement of layers in which coarse and heavy particles are located in the bottom of the layer
3a. if larger particles are located at the top of the layer, the layer may have been overturned
b. cross beds
1b. cross beds – sandy sediment forms curved beds at an angle to the bedding plane
2b. tops of layers commonly erode before new layers are deposited
3b. sediment appear to be curved at the bottom
c. ripple marks
1c. ripple marks – small waves that form on the surface of sand because of the action of wind and/or water
2c. ripple marks may be preserved in sandstone
3c. crests of ripple marks point upward
E. Unconformities
1. unconformity
a. a break in the geologic record created when rock layers are eroded or when sediment is not deposited for a long period
of time
b. three types (table 1)
1b. nonconformity – stratified rock rests upon unstratified rock
2b. angular unconformity – boundary between a set of tilted layers and set of horizontal layers
3b. disconformity – boundary between horizontal layers of old sedimentary rock and younger, overlying layers that
are deposited on an eroded surface
2. all rocks beneath an unconformity are older than the rocks above the unconformity
3. Crosscutting Relationships
a. fault – a break or crack in Earth’s crust along which rocks shift their position
b. intrusion – a mass of igneous rock that forms when magma is injected into rock and then cools and solidifies
c. Law of Crosscutting Relationships – the principle that a fault of body of rock (intrusion) is younger than any other
body of rock that it cuts through (Fig. 5)
II. DETERMINING ABSOLUTE AGE
- absolute age – 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
A. Absolute Dating Methods
1. Rates of Erosion
a. rate at which a stream erodes its bed
b. measuring rate of erosion can lead to an estimated age of the stream
c. practical only for features formed within the past 10 000 to 20 000 years ago (i.e. Niagara Falls)
d. method is less reliable for features formed over a long period of time (i.e. Grand Canyon)
2. Rate of Deposition
a. in general, about 30 cm of sedimentary rock are deposited over a 1000 year period
b. this method is not always accurate, it merely provides an estimate
1b. floods can deposit many meters of sediment in just one day
2b. rate of deposition may change over time
3. varve count
a. tree’s age can be estimated by counting growth rings
b. varve – a banded layer of sand and silt that is deposited annually in a lake, especially near ice sheets or glaciers can
be used to determine absolute age
1b. generally from in glacial lakes
2b. consists of light-colored band of coarse particles and a dark band of fine particles
aa. coarse particles deposited in summer
bb. fine particles settle during winter
c. one varve represents one year
B. Radiometric Dating
1. rocks generally contain small amounts of radioactive material
a. isotope – atoms of the same element that have a different atomic mass
b. radioactive isotope – isotopes whose nuclei emits particles and energy at a constant rate regardless of surrounding conditions
2. scientists use the natural breakdown of isotopes to measure absolute age
3. radiometric dating – a method of determining the absolute age of an object by comparing the relative percentages of a radioactive isotope and a stable
isotope
a. parent isotope – the original radioactive isotope that is going through radioactive decay
b. daughter isotope – the stable isotope that results from the breakdown of the parent isotope
4. scientists use the decay rate and compare the percentages of the parent and daughter isotope to determine absolute age
5. Half-Life
a. radioactive decay happens at a relatively constant rate that is not changed by temperature, pressure or other environmental conditions
b. half-life – the time required for half of a sample of a radioactive isotope to break down by radioactive decay to form a daughter isotope
c. the greater the amount of the daughter isotope, the more half-lives and therefore the older the rock
6. Radioactive Isotopes
a. accuracy of radiometric dating depends on two items
1a. if too little time has gone by, there may not be enough of the daughter isotope
2a. if too much time has gone by, there may not be enough of the parent isotope
b. radiometric dating uses a ratio of the parent and daughter isotopes
c. uranium-238 is most useful for dating features that are more than 100 million years old
d. potassium-40 is used to date rocks that are between 50 000 and 4.6 billion years old (Table 1 page 195)
C. Carbon Dating
1. younger rock layers may be dated indirectly by dating organic material found within the rock
2. only accurate for layers of rock that are less than 70 000 years
3. carbon-14 dating (radiocarbon dating) – the use of carbon-14 found in once living organisms and comparing it to carbon-12
a. half-life of carbon-14 is 5730 years
b. scientists first determine the ratio of C-14 to C-12
c. compare that ratio to the ratio of C-14 to C-12 in living organisms
III. THE FOSSIL RECORD
- fossils – the trace or remains of an organism that lived long ago, most commonly preserved in sedimentary rock
- fossils are an important source of information for finding the relative and absolute age of rocks
- provide clues to the past
- geological events
- climate
- evolution of living things
- paleontology – the scientific study of fossils
- almost all fossils are discovered in sedimentary rock
A. Interpreting the Fossil Record
1. provide important clues to the environmental changes that occurred in Earth’s past
2. scientists use this information to learn about how environmental changes have affected living organisms
B. Fossilization
1. only dead organisms that are buried quickly or protected from decay can become fossils
2. generally, only the hard parts of organisms become fossils
a. wood
b. bone
c. shells
d. teeth
3. in some instances, only a replica of the original organism remains (Table 1 page 198)
C. Trace Fossils
1. fossilized evidence of past animal movement
2. examples
a. tracks
b. footprints
c. borings
d. burrows
3. an important clue to the animal’s appearance and activities
D. Index Fossils
1. a fossil that is used to establish the age of rock layers
2. four requirements
a. present in rocks scattered over a large region
b. features that clearly distinguish it from other fossils
c. organisms must have lived for a short period of time
d. fossils must occur in fairly large numbers within the rock layers
E. Index Fossils and Absolute Age
1. index fossils are used to determine absolute age
2. rock layers containing index fossils can be dated accurately
3. index fossils can be used to date rock layers in separate areas