Ch. 8 The Rock Record
8-1 Determining Relative Age
Key Ideas
State the Principle of Uniformitarianism
Explain how the Law of Superposition can be used to determine the relative ages
of rocks.
Compare three types of unconformities.
Apply the Law of Crosscutting Relationships to determine the relative ages of
rocks.
Academic Vocabulary—sequence (the order in which things come or events happen)
I.
Uniformitarianism
Geologists estimate that the age of the Earth is 4.6 billion years old.
_______________—proposed by James Hutton says that geologic processes
that occurred in the past can be explained by current geologic processes.
A. Earth’s Age
Before Hutton’s research many people thought the Earth was about ______
years old. Hutton reasoned that _______ of years must be needed for forces to
create the rock structures observed on Earth’s crust.
II.
Relative Age
Relative Age _________________________________________________ but
_______________________________. Scientists commonly study Sedimentary
rock layers.
III.
Law of Superposition
__________________ is the principle that a sedimentary rock layer is older than
the layers above it & younger than the layers below it if the layers are not
disturbed.
IV.
Principle of Original Horizonality
Principle of Original Horizonality states that sedimentary rocks left ___________
will remain in _________ layers. So sedimentary rock layers that are not in
horizontal layers have been _______________________ movements.
A. ________________
The arrangement of layers in which _____________ particles are located
in the bottom layers.
B. ______________
When sand is deposited sandy sediment forms beds at an _____ to the
bedding plane.
C. ______________
Small waves that form on the surface of sand due to the action of _____ or
_____.
V.
Unconformities
____________ is a break in the geologic record. This shows that deposition either
stopped for a period of time or erosion removed rock before deposition resumed.
Three types:
_____________—stratified rock rests upon unstratified rock
Angular unconformity—boundary between a set of __________ & a set of
horizontal layers.
____________—The boundary between horizontal layers of old
sedimentary rock & younger, overlying layers that are deposited on an
eroded surface
A. Crosscutting Relationships
Law of Crosscutting Relationships states that a fault or ______
_______ is always ________than the rock layer it cuts through
1. _____—a break or crack in Earth’s crust along which rocks
shift
their positions.
2. _______—a mass of igneous rock that forms when magma is
injected into rock and then cools and solidifies.
8-2 Determining the Absolute Age
Key Ideas
Summarize the limitations of using the rates of erosion and deposition to
determine the absolute age of rock formations.
Describe the formation of varves.
Explain how the process of radioactive decay can be used to determine the
absolute ages of rocks.
Academic Vocabulary—estimate (to calculate approximately)
I.
Absolute Dating Methods
Absolute Age is the __________________, often stated in years before the
present, as established by an absolute-dating process, such as radiometric dating.
A. Rates of ________
One way to estimate absolute age is to study rates of erosion. Only
practical for geologic features formed in the past 10,000 to 20, 000 yrs.
B. Rates of Deposition
Another estimation by calculating the rate of ________________.
Approximately 30 cm of sediments are deposited every 1000 yrs.
C. ______ Count
A varve is a pair of sedimentary layers (one course, one fine, representing
one year) that is deposited in an _____ cycle, commonly in glacial lakes,
and that can be used to determine absolute age. (similar to counting _____
on a tree trunk).
II.
Radiometric Dating
________ are atoms of the same element that have a different number of
neutrons.
__________ isotopes have nuclei that emit particles and energy at a
constant rate.
_________________—the method of using radioactive decay to measure
absolute age.
Parent Isotope is the _______ material.
________Isotope is what the parent isotope changes into over time.
__________—when parent isotopes give off 2 protons & 2 neutrons; the
daughter isotope is an element whose atomic number is 2 less than the
parent isotope. It can be stopped by skin or a few sheets of paper.
_________-- when a parent isotope has a neutron break up into1 proton &
1 electron; the daughter isotope is an element whose atomic number is 1
more than the parent isotope. It can be stopped by a 2x4 or a sheet of
Aluminum foil.
____________—produces a ray with no mass. It can be stopped by several
feet of concrete, lead or both.
A. Half-Life
Half-life is the time required for ____ of a sample of a radioactive isotope
to break down by radioactive decay to form a _____________. The greater
the percentage of daughter material in a sample the older the rock is.
B. Radioactive Isotopes
The amount of time that has passed since a rock formed determines which
radioactive isotope will give a __________________________.
238
U is most useful for samples 10 million yrs. Or older
40
K is most useful for samples 50,000 & 4.6 billion yrs. old.
II.
Carbon Dating
Radiocarbon dating or ________ dating is measuring the amount of
radioactive Carbon from once living material (wood, bones, shells etc.)
Plants absorb __ during photosynthesis. Some of this is ____. When animals
eat these plants the 14C becomes part of their tissues. When animals die
they no longer are ________ C.
14
C is most useful for samples less than 70,000 yrs. old.
8-3 The Fossil Record
Key Ideas
Describe four ways in which entire organisms can be preserved as fossils
List five examples of fossilized traces of organisms
Describe how index fossils can ve used to determine the age of rocks.
Academic Vocabulary—evidence (information showing whether an idea is true or valid).
_______—the trace or remains of an organism that lived long ago, most commonly
preserved in sedimentary rock. The formation of I. Rock & M. Rock generally destroys
organic structure.
Paleontology—the scientific _______ fossils.
I.
Interpreting the Fossil Record
The fossil record provides information about the _____________ of Earth.
Revealing how organisms and environments have changed through Earth’s
past.
II.
Fossilization
___________________ usually act upon dead plant & animals so to
become a fossil this material must be protected from decay or buried
quickly. Wood, bones, shells & teeth generally can become fossils.
Sometimes entire organisms can be preserves while other times only
evidence that life once existed can be seen.
_____________—special preparations of remains; in very dry places.
Amber—Hardened tree sap trapped organisms.
Tar Seeps—Animals became trapped and could not escape.
_______—Organisms buried in frozen soil or ice do not decay.
Petrification—Minerals can replace original organic material.
Carbon Films—gases escape from tissues & can leave Carbon on surfaces.
____________—empty spaces (molds) fill with sediment (cast).
Coprolites—Fossilized dung can reveal the feeding habits of animals.
Gastroliths—stones used to grind food in a dinosaur’s _______.
III.
Types of Fossils
Trace fossils are fossilized marks that form in sedimentary rock by the
movement of an animal on or within soft sediment. Ex: ________,
_______________.
IV.
Index Fossils
Index fossil is a fossil that is used to establish the age of rock layers
because it is __________________________ & existed for only a short
span of geologic time. Ex: ammonites & trilobites.
V.
Index Fossils and Absolute Age
Scientists can use Index fossils to _________________ of specific rock
layers.