Fossils
• A fossil is the remains or evidence of a living thing.
• To scientists, fossils create a picture of the earth’s past.
• Fossils are generally incomplete
• Most fossils are preserved in sedimentary rocks.
• For fossils to form, the organism must be buried quickly.
• Plants and animals that lived in or near
water were preserved more often than other organisms.
• Fossils are almost never found in
igneous or metamorphic rocks because the heat or pressure involved in their
• There are many different kinds of fossils.
• They are classified according to how they form.
• 1. Petrification- a process which forms
• 2. Molds and Casts- two types of fossils formed when an organism is buried in sediments that turn to stone.
• A mold is the empty space left behind when the soft parts of an organism are dissolved.
• The mold has the same shape as the
organism.
• 3. Imprints- fossils that form when thin objects leave impressions in soft
sediments that later harden into rock.
• 4. Preservation of Entire Organisms- whole organisms are preserved in
• Freezing- prevents the organism from decaying. Occurs in arctic regions
where it is very cold.
• Amber- resin, or sap, from evergreen trees trap insects and then harden. Insects trapped in amber are usually perfectly preserved.
• Tar Pits- large pools of tar contain
fossils of entire organisms that were trapped in the sticky substance when they came to drink the water that
• 5. Trace Fossils- are the marks or evidence of animal activities.
• Tracks, trails, footprints, and burrows are all trace fossils.
• They reveal much about an animal
Interpreting Fossils
• Fossils tell scientists that many
different life forms have existed at different times throughout earth’s history.
• When fossils are arranged according to
• Fossils also tell how the earth’s surface has changed over time.
• Ex: If fossils of sea organisms are
found in rocks high above sea level, the land was probably once covered by
ocean.
• Fossils give clues to the earth’s past
climate.
• Fossils tell scientists about the
appearance and activities of extinct animals.
• Ex: Footprints can tell how big or heavy an organism was or how fast it
moved.
• Fossil teeth can tell what kind of food
History in Rocks
• Using evidence from rocks and fossils, scientists can determine the order of
events that occurred in the past and the approximate time at which events
happened.
Law of Superposition
• states that in a series of undisturbed
sedimentary rock layers, younger rocks normally lie on top of older rocks.
• Scientists use the law of superposition
Index Fossils
• fossils of organisms that lived during only one short period of time.
• One layer of rock with an index fossil in it is the same age as another layer of
Unconformities
• formed when sedimentary rock layers are
uplifted, eroded, and new sediments are laid on top of it.
• The old eroded surface beneath the newer
rock is called an unconformity
• There is a wide gap in the ages of rock layers
above and below the unconformity.
• By studying unconformities, scientists can
Faults, Intrusions, and
Extrusions
• Faults, or breaks in rock layers, can only occur after rocks are laid down. So faults are younger than the rock layers they cut through.
• Intrusions are igneous rocks that cut
• Extrusions are formed when igneous rock layers harden at the surface.
• Extrusions are younger than the rock layers beneath them.
• All of the above are ways scientists can
determine relative age of rocks.
• Relative age tells how old rocks are
• Throughout history, scientists have tried to develop ways to determine absolute age of rocks.
• Absolute age is the actual number of years old rocks are.
• One way to determine absolute age of rocks is radioactive dating.
• Radioactive dating is based upon the knowledge that radioactive elements are unstable and that they
• The breakdown of a particular element occurs at a constant rate, called the
half-life.
• The half-life of a radioactive element is the amount of time it takes for half of the radioactive element to decay into decay element.
• By measuring the ratio of decay
element to radioactive element in a
• Many different radioactive elements are used to date rocks and fossils.
• Another way to determine absolute age is through the use or varves, or
sediments that show yearly cycles.
Age of Earth
• Scientists have used radioactive dating to determine the age of the earth.
• The oldest rocks on earth are about 4 billion years old.
• Moon rocks brought back to earth have been dated between 4 and 4.6 billion years old.
• Since scientists believe that the earth and
