STUDY GUIDE
Chapter 7: A Trip Through Geologic Time
Lesson 1
* Most fossils form when sediment hardens into rock, preserving the shapes of organisms. Fossils
are the preserved remains of traces of organisms that lived in the past.
* Fossils include molds, casts, petrified fossils, carbon films, trace fossils and preserved remains.
Know what each of these are and examples of each.
* Fossils provide evidence about Earth’s history. They have allowed scientists to describe the
past environments and the history of life.
Lesson 2
* Understand the difference between relative and absolute age and when/how each is used to
describe the layers of rocks
* In horizontal sedimentary rock layers, the oldest layer is generally at the bottom. Each layer is
younger that the layers below it. (Law of Superposition)
Lesson 3
* Know that igneous rocks can give clues to the relative ages of rock layers. Extrusions are
always younger than the rocks below it. Intrusions are always younger than the rock layers
around and beneath them. (Law of Cross Cutting)
* A fault it always younger than the rock it cuts through. (Law of Cross Cutting)
* Index fossils are widely distributed and represent an organism that existed for a geologically
short period of time. They are useful because they tell the relative age of rock layers in which
they occur.
* Gaps in the geologic record and folding can change the position in which rock layers
appear. An unconformity shows where rock layers have been lost due to erosion. Folding
sometimes occurs when forces in the Earth fold layers and can even occur so much that the
layers are turned over completely.
* Be able to read a model and identify the relative ages of layers and some events that have
taken place over time. (Refer to practice activities and 1uiz)
Lesson 4
* During radioactive decay, the atoms of one element break down to form atoms of another
element.
* Radioactive dating is used to determine the absolute ages of rocks and works best with
igneous rocks.
* In radioactive dating, scientists compare the amount of radioactive element in the rock with
the amount of the stable element into which the radioactive element decays. Know how to
determine the half life of an element and how half lives/years it will take to occur when given
data. Know how to calculate how many atoms may be remain/exist after a certain number of
half lives. (Refer to the practice worksheets and quiz)
* Isotopes of Potassium and Uranium are commonly used to radioactive date rocks, while
isotopes of Carbon are commonly used to date things that once lived.
* Because the time span of Earth’s past is so great, geologists use the geologic time scale to
show Earth’s history.
* Know the 4 main divisions (Precambrian, Paleozoic, Mesozoic and Cenozoic) and the periods
that make up each. Know general characteristics of events that happened during these times
and which era and period we live in today.
Lesson 5
* Earth’s atmosphere, oceans and continents began to form during the first several million years
of Precambrian Time.
* Scientists think that the Earth began as a ball of dust, rock and ice in space. Gravity pulled
this mass together.
* Scientists believe that the Earth and the moon are approximately 4.6 billion years old
Lesson 6
* During the Cambrian Explosion that began the Paleozoic Era, a great number of different
organisms evolved. Most were invertebrates that lived in the sea.
* Reptiles spread widely during the Mesozoic Era (the Age of Reptiles)
* During the Cenozoic Era, mammals evolved to live in many different environments.
* During the Devonian Period, animals that could live on land began to evolve. These animals
were called amphibians and spent part of their time in the water.
* When the environment changed quickly and no individual members of a species were able
to adapt to the new environment, then the species became extinct.
* Know the difference between vertebrates and invertebrates and examples of each.
Use your vocabulary packet, Review & Reinforce worksheets, Enrich worksheets, lab activities,
quizzes and textbooks to help you prepare!
These are mostly vocabulary terms and examples.
You need to know these so you can APPLY the concepts to questions on the assessment!