Unit 5: History of Earth
Ch. 4 A Trip Through Geologic Time
Objectives:
1. Describe the different types of fossils and explain how they formed.
2. Explain how the fossil record is used to describe organisms and environments of the past.
3. Use the law of superposition, rock changes, and index fossils to explain how scientists determine the
relative ages of rocks within a layer.
4. Explain how and why scientists use the geological time scale.
5. Describe the structure of the geological time scale and how the time periods are defined.
6. Describe what took place during early Earth, Precambrian time, and the 3 eras in terms of geological events
and development of life.
Vocabulary:
Fossil
Mold
Cast
Petrified fossil
Carbon film
Trace fossil
Paleontologist
Evolution
Relative age
Absolute age
Law of Superposition
Unconformity
Index fossil
Ammonite
Stromatolite
Geologic time scale
Era
Precambrian
Paleozoic
Mesozoic
Cenozoic
Period
Comet
Invertebrate
Brachiopod
Trilobite
Vertebrate
Amphibian
Reptile
Mass extinction
Mammal
Ch. 4.1 Fossils
Read p. 110-116 and make notes by answering the following questions.
Briefly, describe how a fish could be preserved as a fossil.
- A fish dies and sinks to the bottom of a lake
- Before it could decay, layers of sediments (sand) cover it
- Minerals from the sand get inside the fish and replace the bones
- Pressure changes the sediment into rock, over millions of years
Why are fossils so important?
- They provide evidence of how life has changed over time, how Earth’s surface has changed, and what past
environments and climates were like
Why are most fossils found in sedimentary rock?
- Sediments bury organisms when they die, which eventually hardens into sedimentary rock
- Sedimentary rock is also the most common rock on the surface of the Earth, where organisms live
What part of an organism is preserved in fossils and why?
- The hard parts (bones, teeth, scales, shells, seeds, woody stems) because the soft parts are quickly eaten by
other organisms or are decayed by bacteria
Describe the five main types of fossils, including how they formed, and describe an example of each.
- Molds and Casts
o Mold = hollow area in rock that takes on the shape of the organism after the organism has decayed
o Cast = solid copy of the shape of the organism, the mold is filled in with minerals that harden in this
shape (compliments the mold) *a cast is just a rock in the shape of an organism*
 Example: Ice cube = cast; ice cube container = mold
 Most common type of fossil found
- Petrified Fossils
o Minerals replace all or part of an organism – the organism decays and the minerals are left behind
o Trees can become petrified
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Carbon Films
o A very thin coating of carbon on rock (like a stamp)
o All living things contain carbon and once the organism decays, the carbon makes a print on the rock
o Insects, plants, and even fish can leave behind carbon films
Trace Fossils
o Not a fossil of an organism, but evidence of the activities of that organism
o Footprints, nest, burrow, feces
Preserved Remains
o Animals become trapped in tar, which prevented bacteria from decaying the organism
o Amber, tree sap, traps insects and they get covered in it. The amber eventually hardens.
o Frozen remains – animals are preserved through freezing in cold climates
Explain how scientists use the fossil record.
- The fossil record provides evidence about the history of life and past environments on Earth. The fossil
record also shows that different groups of organisms have changed over time.
- We learn what organisms were like in the past, how they were related to each other, what environments
were like and how they changed over time.
Ch. 4.2 The Relative Age of Rocks
Read p. 117-121 and make notes by answering the following questions.
What are the similarities and differences between relative age and absolute age? Use an example in your
explanation.
- Similarities – tells you which sample is older and which is younger
- Differences – relative age only tells you if something is older or younger, relative to a comparison. For
example, Ms. L is older than her students, but younger than Mr. Sibley. Absolute age tells you the number
of years it has been since the sample formed (its age in number form). Ms. L is 37 years old.
Explain the law of superposition. Can it be used to determine relative or absolute age and why?
- Rock layers are older on the bottom and younger on the top of a sample. If you have a stack of magazines,
the bottom magazine was placed on the table first, and the top magazine was placed there last. The law of
superposition can be used to determine relative age because you can tell which layer is older and which
layer is younger; you cannot give an exact age to each layer.
Describe the four ways in which scientists can determine the relative age of a rock.
- Extrusion – the extrusion (lava or igneous rock) is always younger than the rocks below it
- Intrusion – the intrusion (magma or igneous rock) is always younger than the rock layers around and
beneath it.
- Fault – the fault is always younger than the rock it cuts through.
- Gaps – unconformities in rock layers means that new rock layers meet a much older rock surface beneath
them (the newest rock is much older than the rock below it because some middle layers are missing)
What characteristics must a fossil have to be classified as an index fossil?
- Widely distributed (everywhere) and represent a type of organism that existed only briefly
Give examples of index fossils.
- Ammonites and trilobites
Ch. 4.4 The Geologic Time Scale
Read p. 127-129 and make notes by answering the following questions.
Using a clock, model the history of the Earth by putting in a few of the major events that have taken place.
12 AM – Earth forms
7 AM – Single celled organisms appear
7 AM – 9 PM – soft-bodies organisms (jellyfish and worms) develop
After 9 PM – Larger, more complex organisms develop in the oceans
10 PM – Reptiles and insects appear
Before 11 PM – Dinosaurs appear
11:30 PM – Dinosaurs disappear
11:59:59 PM – Humans appear
How was the geologic time scale created?
- From studying rock layers and index fossils worldwide, they could group similarities together
- This information allowed them to place Earth’s rocks in order by relative age
- Later, absolute age was used to make more divisions in the geologic time scale
Describe eras and periods.
Eras – the largest division in the time scale – 3 Eras: Paleozoic, Mesozoic, Cenozoic (before this time was the
Precambrian)
Periods – subdivisions of Eras, names from places around the world where geologist first described the rocks and
fossils of that period
*A major change in the fossil record is what ends one era, and a new one begins. The divisions of the geologic
time scale depend on events in the history of life on Earth.*
Ch. 4.6 Eras of Earth’s History
Read p. 134-145 and make notes by answering the following questions.
Describe the lifeforms found towards the end of Precambrian Time.
- Single-celled organisms and things resembling jellyfish (soft-bodied). Feathery plant-like animals attached
to the ocean floor.
What is the Cambrian Explosion?
- A relatively short time in which a large number of organisms evolved.
Describe the events that took place during the Paleozoic Era.
- Organisms developed hard parts – shells and skeletons
- All animals lived in the sea
- Invertebrates dominated (jellyfish, worms, sponges, brachiopods, trilobites)
- Invertebrates and fish arose – jawless fish were the first vertebrate
- Plants developed on land
- Fish then dominated
- Amphibians, who live in the water and on the land during their life cycle, were able to live on the land and
they then dominated
- Reptiles, who live completely on the land, then developed
- Winged insects and trees developed on land
- Pangaea formed
What happened at the end of the Paleozoic Era?
- A mass extinction took place for plants and animals, both on land and in the oceans
Describe the events that took place during the Mesozoic Era.
- Reptiles dominated
- Dinosaurs and mammals appeared
- Birds appeared
- Flowering plants evolved
What happened at the end of the Mesozoic Era?
- A mass extinction took place, wiping out all the dinosaurs
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It was most likely caused by a comet or asteroid smashing into Earth. The impact caused dust to go into the
atmosphere, blocking out the sun, preventing plants from growing properly, and lowering temperatures.
Volcanic activity could have also contributed to this extinction.
Describe the events that took place during the Cenozoic Era.
- Mammals now flourished once the dinosaurs disappeared
- The climate warmed, then cooled again, causing a series of ice ages, which eventually melted
- Plants and animals thrived in all environments, on land and in the oceans
- Human evolved