Clues to Earth’s Past
Chapter 10
Background Knowledge
Atoms are made of a positive nucleus surrounded by
negative electrons
2. Some kinds of organisms that once lived on Earth have
completely disappeared, although they were something
like others that are alive today.
3. Fossils can be compared to one another and to living
organisms according to their similarities and differences.
Some organisms that lived long ago are similar to
existing organisms, but some are quite different.
1.
Section 1: Fossils
Key Points to Remember:
Fossils include carbon films, molds, casts, and trace
fossils
Dead Organisms are more likely to become fossils if they
have hard parts and are buried quickly after they die
Paleontologists use clues from fossils to learn about
ancient life and the environments ancient organisms
lived in.
Vocabulary to know:
Fossil, catastrophism, uniformitarianism, carbon film,
mold, cast, trace fossil, paleontologist
Fossils pg 326
Question
Answer
Are the insects in the
amber alive?
No, they died millions of
years ago
What are some other ways
in which organisms can be
preserved as fossils?
Leaf imprints, petrified
wood, dinosaur bones, or
shells; these are all ways in
which organisms can be
preserved
Evidence of the Distant Past
Rocks provide clues to Earth’s past. Some of the most
obvious clue that are present in rocks are fossils, the
preserved remains or evidence of ancient living
organisms.
Some early scientists thought that fossils formed from
the remains of organisms killed in disasters
Catastrophism is the idea that conditions and
organisms on Earth change because of quick, violent
events.
Continued…
James Hutton proposed that changes on Earth take
place very slowly.
He proposed the principle of uniformitarianism.
According to this principle, the same geologic process
that occur today occurred in the past
This principle says that most changes on Earth’s surface
occur in slow, continuous cycles
In addition to the events explained by
uniformitarianism, sometimes catastrophic events
quickly change Earth’s surface
Formation of Fossils
An organism is more likely to become a fossil if it
contains hard parts such as teeth or bones
Fossils are more likely to form if an organism is buried
quickly after it dies
Tiny fossils are called microfossils
Types of Preservation
If the actual organism becomes a fossil, it was
completely enclosed over a long period of time to keep
it away from air and bacteria
Some fossils are called carbon films, which form when
pressure drives off the gases and liquids from an
organism’s tissues, leaving only the carbon behind.
Fossils also form when minerals in groundwater replace
the tissues of dead organisms
Types continued
Some fossils are molds, which is the impression in a
rock left by an ancient organism
When a mold fills with sediment or mineral deposits,
it forms a fossil called a cast
Trace fossils show evidence of the activity of ancient
organisms, such as tracks or nests
Ancient Environments
Paleontologist are scientists who study fossils
Paleontologists compare fossils of ancient organisms
with organism that live today.
They use uniformitarianism to learn about the
environments of ancient organisms
If a fossil of an ancient organism is similar to an
organism that lives today, these two organisms might
have lived in a similar environment
Fossils show that Earth’s climate has changed many
times in the past
Ancient Environments
Fossils of plants such as ferns indicate that much of
Earth was warm and humid 100 million years ago
Fossils of some species such as the woolly mammoth
help scientists learn about times that the climate of
Earth was cool
END OF SECTION
Answer questions on page 334 #2, 4-8
Complete worksheet 14/16
Section 2: Relative-Age Dating
Key Points to Remember
Relative age dating is the age of rocks and geologic
features compared with other rocks and features near by.
The relative age of rock layers can be determined using
geologic principles, such as the principle of
superposition and the principle of inclusion
Vocabulary: relative age, superposition, inclusion,
unconformity, correlation, index fossil
Relative-Ages of Rocks
Relative age is the age of rocks and geologic features
compared to other rocks and features nearby
The relative age of lower layers of rock is greater than the
relative age of rocks above them. (meaning the rocks on the
bottom are older than those on the top)
The principle of superposition states that in undisturbed
rock layers, the oldest rocks are on the bottom.
According to the principle of original horizontality, most
rock-forming material was deposited in horizontal layers
Relative ages of Rocks
The principle of lateral continuity states that
sediments are deposited in large, continuous sheets in
all lateral directions.
A piece of older rock that becomes part of a new rock
is called an inclusion. The rock containing pieces of
other rocks must be younger than the pieces.
Sometimes, rock formations break, or fracture
When rocks move along a fracture, the fracture is called
a fault
According to the principle of cross-cutting
relationships, when a geologic feature cuts across
another feature, the feature it cuts across is older.
Unconformities
An unconformity is the surface between older, eroded rock
and the younger rock on top of it
Young sedimentary layers that form on top of older,
horizontal sedimentary layers that have eroded create a
disconformity
Young sedimentary layers that form on top of older, tilted or
folder sedimentary layers that have eroded create an angular
unconformity.
Young sedimentary layers that form on top of older
metamorphic or igneous layers that have eroded create a
nonconformity.
Images on page 340
Correlation
Matching rocks and fossils from different locations is called
correlation. It helps scientists determine the geologic history
of an area.
Fossils are often used to learn the relative ages of rock layers
that are far apart from each other.
If two formations contain similar fossils, the formations are
probably about the same age
Fossils that are useful for determining relative age are from
species that were abundant and existed for a short period of
time in many different areas on Earth.
Such fossils are known as index fossils.
End of Section : Answer 1 & 2, 4, 7 & 8 from book
Complete worksheet 26, 31, 34
Section 3: Absolute-Age
Dating
Key Points to Remember
Absolute age is the age in years of a rock or object
The radioactive decay of unstable isotopes occurs at a
constant rate, measured as half-life.
To date a rock or object, scientists measure the ratios of
its parent and daughter isotopes
Vocabulary: Absolute age, isotope, radioactive decay,
half-life
Absolute Ages of Rocks
The numerical age, in years, of any object is its absolute
age
Radioactivity can be used to help determine the
absolute age of rocks
Radioactivity is the release of energy from unstable
atoms
Atoms
An atom is the smallest part of an element that has the
properties of that element.
An atom contains protons (positive charge) and
neutrons (neutral charge) in the nucleus.
Atoms also have electrons (negative charge), which
surround the nucleus.
All atoms of the specific element contain the same
number of protons
Nuclei of atoms of the same element can contain
different numbers of neutrons.
Atoms of the same element that contain different
number of neutrons called isotopes.
Atoms
An isotope that does not change under normal
conditions is said to be stable
Isotopes that are not stable are radioactive isotopes
During radioactive decay, an unstable element naturally
changes into an element that is stable
The element that undergoes radioactive decay is called
the parent isotope.
The element formed by radioactive decay is called the
daughter isotope.
Atoms
Different radioactive isotopes decay at different rates,
but the decay rate is CONSTANT for each specific
isotope
For each radioactive isotope, the half-life is the time
necessary for half of the parent isotopes to decay into
daughter isotopes
After two half-lives, one-fourth of the original parent
isotopes remain
Radiometric Ages
Radiocarbon dating uses isotopes of carbon-14 to
determine the age of once-living organisms.
As long as an organism lives, the amount of carbon-14 it
contains is constant
Radiocarbon dating involves comparing the amount of
carbon-14 to the amount of carbon-12 in an organism
that has died
The half-life of carbon-14 is 5,730 years
About 50,000 years after an organism has died, the
carbon-14 has decayed so much that it cannot be
measured.
Radiometric Ages
Because carbon-14 is useful for only dating materials that are
organic, it cannot be used to date rocks
Radioactive isotopes are most likely to be trapped in igneous
rocks, which means they can be dated by comparing numbers
of parent and daughter isotopes
Radioactive isotopes are not very useful in dating sedimentary
rocks, because they are composed of grains of older rock.
To date old rocks, scientists use radioactive isotopes with long
half-lives
The age of the oldest rocks on Earth put Earth’s age at more
than 4 billion years.
END OF SECTION: Answer 1,2, 4 - 7, 9 from book
Complete worksheet pg 51/56