MR. SURRETTE
VAN NUYS HIGH SCHOOL
CHAPTER 4: EARTH’S HISTORY
CLASS NOTES
RELATIVE DATING
The lower levels of sedimentary rocks and lava flows are formed before top layers. Along these lines,
geoscientists use five basic principles to discern the relative ages of rocks.
ORIGINAL HORIZONTALITY
Layers of sediment are deposited evenly, with each new layer depositing on top of older sediment.
SUPERPOSITION
In undeformed sequences of sedimentary rocks, upper layers are younger than the lower levels.
SUPERPOSITION AT THE GRAND CANYON
CROSS-CUTTING
An igneous intrusion or fault that cuts through preexisting rock is younger than the rock it cuts across.
For example, in the following diagram, A is the youngest rock, B is the next youngest, and C is the
oldest.
CROSS-CUTTING
INTEGRATED SCIENCE
PAGE
1
MR. SURRETTE
VAN NUYS HIGH SCHOOL
INCLUSIONS
Inclusions are pieces of one rock type contained within another. Any inclusion is older than the rock in
which it is found.
FAUNAL SUCCESSION
The evolution of life on Earth is recorded in the rock record in the form of fossils.
FOSSILS
Fossils are the preserved evidence of ancient life. Some fossils are called body fossils. These are the
preserved parts of plants and animals. Most fossils, however, are trace fossils. Trace fossils are
remains of the activities of plants and animals like tracks, trails, or scratch marks.
INDEX FOSSILS
Index fossils are fossils of widespread organisms that lived for only a short interval of geologic time.
Index fossils are used to divide the fossil record into chapters. For example, dinosaurs created index
fossils for the Mesozoic era.
UNCONFORMITIES
Most rock layers are deposited without interruption. However, weathering and erosion, crustal uplifts,
and other geologic processes can interrupt the normal sequence. This breaks, or gaps, the rock record.
These gaps are called unconformities.
GRAND CANYON UNCONFORMITIES
RADIOMETRIC DATING
Although relative dating indicates which rocks are older or younger, it does not determine age. The age
of rocks can be estimated by radiometric dating, which measures the ratio of isotopes to their decay
products. Isotopes are atoms of the same element that contain different numbers of neutrons.
COMMON RADIOMETRIC ISOTOPES
Radiometric dating with uranium-238 is useful for very old rocks. In fact, scientists have used
uranium-238 to estimate that the oldest rocks on Earth are 3.8 billion years old. Carbon-14 is used to
date organic matter (bone, wood, charcoal, shells, etc.). Because of its short half-life (5730 years),
carbon-14 only dates events within the last 50,000 years.
INTEGRATED SCIENCE
PAGE
2
MR. SURRETTE
VAN NUYS HIGH SCHOOL
RELATIVE AND RADIOMETRIC DATING
Scientists learn the sequence of geologic events by using both relative and radiometric dating.
Although radiometric dating provides the age of minerals and organic matter, it does not give the age of
rocks. Scientists must then use the principles of relative dating.
GEOLOGIC TIME SCALE
By convention the geologic time scale is divided into four eras: the Precambrian (time of hidden life),
the Paleozoic (time of ancient life), the Mesozoic (time of middle life), and the Cenozoic (time of recent
life).
GEOLOGIC TIME SCALE
[Please see Handout]
MASS EXTINCTIONS
Extinction is the total disappearance of a species. According to the fossil record, large numbers of
species became extinct during short intervals of geologic time. The extinction of one species often has a
domino effect: if one species vanishes, so do many others.
PRECAMBRIAN
Precambrian time ranges from 4.6 billion years ago, when the Earth formed, to 544 million years ago.
The Precambrian makes up 85% of Earth’s history.
PRECAMBRIAN
The first evidence of single-celled organisms (green algae) occurs in rocks 1.5 billion years old.
Multicellular plants and animals emerged at the end of the Precambrian 700 million years ago.
PALEOZOIC ERA
The Paleozoic era began 544 million years ago and lasted 300 million years. During this time, sea levels
rose and fell several times, allowing shallow seas to cover the continents. Marine life flourished. The
Paleozoic is divided into six major time periods.
CAMBRIAN PERIOD
Almost all major groups of marine organisms came into existence during this time. A highlight of the
Cambrian period was the evolution of organisms that could create shells. A variety of marine
organisms emerged, including the trilobite, the armored “cockroach” of the Cambrian sea.
TRILOBITE
INTEGRATED SCIENCE
PAGE
3
MR. SURRETTE
VAN NUYS HIGH SCHOOL
ORDOVICIAN PERIOD
The Ordovician Period marks the appearance of vertebrates. In particular, the appearance of a jawless
eel called the agnath. Many extinctions mark the end of the Ordovician, especially shallow-water
marine groups.
SILURIAN PERIOD
More land emerged during the Silurian period. Much of North America was at or above sea level. This
allowed the development of terrestrial plants and other forms of terrestrial life.
DIVONIAN PERIOD
The Divonian period is known as the “age of fishes.” More animal life moved to land and the first
amphibians appeared.
CARBONIFEROUS PERIOD
Warm, moist climatic conditions contributed to lush vegetation and dense swampy forests during the
Carboniferous period. The remains of these swampy forests created many of today’s coal fields. Giant
insects and the first reptiles flourished.
PERMIAN PERIOD
The evolution of reptiles continued during the Permian period. A massive animal extinction marks the
end of the Permian. Marine invertebrates were especially affected.
THE MESOZOIC ERA
The Mesozoic era began 245 million years ago (245 MYA). It is known as the “age of reptiles.”
MESOZOIC PERIODS
Within the Mesozoic era are three time periods: the Triassic, the Jurassic, and the Cretaceous. The
highlight of the Mesozoic era was the rise of dinosaurs. Flowering plants arose during the
Cretaceous period. The end of the Cretaceous, 66 million years ago, was another time of great
extinctions.
THE CENOZOIC ERA
The era we live in, the Cenozoic, began 66 million years ago. It is known as the “age of mammals.” It
is composed of the Tertiary and the Quaternary time periods.
THE CENOZOIC ERA
Flying bats, large land mammals, and marine animals like whales and dolphins began to occupy niches
left vacant by the extinct Mesozoic reptiles. In other words, these animals participated in adaptive
radiation. The Quaternary period also marks the appearance of humans.
INTEGRATED SCIENCE
PAGE
4