chapter 8: the rock record - firelands elementary school 83.pdf · studying the shape of the...
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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
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