Earth’s History in Fossils
P.Lobosco
Fossils
 A fossil
if the remains or evidence of a living
thing. A fossil can be the bone of an organism or
the print of a shell. A fossil can be a tunnel lefts by
a worm. The most common fossils are bones,
shells, pollen grains and seeds.
 Most fossils are buried in sediments. Plants and
animals that live near water are preserved more
often. Most fossils are found in sedimentary rock.
Fossil Identification
 Fossils
are identified according to the process by
which it was formed.
 Petrification
 Molds and Casts
 Imprints
 Preservation of Entire Organism
 Trace Fossils
Petrification
 Petrification
means turning into stone. When the
dinosaurs died, the soft parts of their bodies
decayed quickly. Only the hard parts, the bones,
were left. Many of these bones were buried in
sediments of mud and wet sand. The water
dissolved minerals in the mud and as the water
flowed through pore in the bones, minerals were
deposited. The water evaporated and the bones
were turned to stone.
Molds and Casts
 Two
types of fossils are formed when an animal or
plant is buried in sediments that harden into rock.
If the soft parts of the organism decay and the hard
parts are dissolved by chemicals an empty space
will be left in the rock. The empty space, called a
mold, has the same shape as the organism.
Sometime the mold is filled in by minerals and
they harden to form a cast in the same shape as the
organism.
Imprints
 Sometimes
a fossil is formed before the sediments
harden into rock. Thin objects, such as leaves and
feathers, leave imprints or impressions in soft
sediments such as med. When the sediments
harden into rock, the imprints are preserved as
fossils.
Preservation of Entire Organism
 Freezing
prevents substances from decaying.
Some animals have been preserved by freezing.
Wooly mammoths have been found in large blocks
of ice. A furry rhinoceros was found in the loose
frozen soil in the arctic. When the resin form
evergreen trees hardens it forms amber. Insects
have been trapped and preserved in amber. Some
animals have been found in tar pits.
Trace Fossils
 Trace
fossils are fossils that reveal much about an
animal’s appearance without showing any part of
the animal. Trace fossils area the marks or
evidence of animal activities. Tracks, trails,
footprints, and burrows are trace fossils.
Interpreting Fossils
 Fossils
indicate that many different life forms have
existed at different times throughout the Earth’s
history. Some scientists believe that for every
organism that exists today, 100 have become
extinct. When fossils are arranged according to
age, they show that living things have evolved.
Fossils also indicate for the Earth’s surface has
evolved. Fossils give clues about past climate.
A History in Rocks and Fossils
 The
Law of Superposition states that in a series of
sedimentary rock layers, younger rocks normally
lie on top of older rocks. This idea was proposed
by James Hutton in the 18th century. The processes
of weathering, erosion and deposition have always
acted on the Earth’s surface. Scientists use this
law to determine whether a fossil or layer or rock
is older or younger than another fossil or layer of
rock.
Index Fossils
 The
law of superposition helps scientists put in
order the record of Earth’s past for one particular
location. Index fossils are fossils of organisms that
lived during only one short period of time.
Scientists assume that index fossils of the same
type of organism are all nearly the same age. So a
layer of rock with one type of index fossil in it is
close in age to another layer of rock with the same
type of index fossil in it.
Unconformities

Sedimentary rock layers and the fossils found within them
may be disturbed by powerful forces within the Earth.
The rock layers may be folded, bent or twisted.
Sometimes older layer are uplifted to the Earth’s surface.
At the surface, the exposed rocks are weathered and
eroded. Sediments are then deposited on top of the eroded
surface of the older rocks. The deposited sediments
harden to from new horizontal sedimentary layers. The
old eroded surface beneath the newer rock layer is called
an unconformity. There is a wide gap in the ages of the
rock layers above and below an unconformity.
Faults

A break or crack along which rocks move is called a fault.
The rock layers on one side of a fault may shift up or
down relative to the rock layers on the other side of the
fault. Because faults can occur only after rock layers are
formed, rock layers are always older than the faults they
contain. The relative age of a fault may be determined
form the age of the sedimentary rock. Scientists can
determine the forces that have changed the Earth’s surface
by examining the faults in rock layers.
Intrusions
 The
relative ages of igneous rock formations can
also be determined. Magma often forces its way
into layers of rocks. The magma hardens in the
rock layers and forms an intrusion. An intrusion is
younger than the sedimentary rock layers it passes
through.
Extrusions
 Sometimes
magma reaches the surface of the Earth
as lava and hardens. Igneous rock that forms on
the Earth’s surface is called an extrusion.
Extrusions are younger than the rock layers
beneath them.
Radioactive Dating

The discovery of radioactive elements in 1896 led to the
development of an accurate method of determining the
absolute age of rocks and fossils. An atom of radioactive
element has an unstable nucleus, or center, that breaks
down or decays. During radioactive decay, particles and
energy called radiation are released by the radioactive
element. As some radioactive elements decay, they form
decay elements. A decay element is stable. This
breakdown occurs at a constant rate.
Half-Life
 Scientists
measure the decay rate by a unit called a
half-life, the time it takes for half of the radioactive
element to decay. If certain radioactive elements
are present in a rock or fossil, scientists can find
the absolute age of the rock or fossil. Many
different radioactive elements are used to date
rocks and fossils. One radioactive element used to
date the remains of living things is carbon-14.
Nitrogen is the decay element of Carbon –14.
Other Methods of Absolute Dating
 There
are other methods of absolute dating.
 Varve
 Splitting
atoms
or fission of the nuclei of radioactive
Varve
 A varve
is sediment that shows a yearly cycle.
These sediments are often deposited in glacial
lakes. Each year a light-colored sandy layer is
deposited in the summer and a dark-colored layer
is deposited in the winter. Be counting the pairs of
layers it is possible to tell haw many years a a
glacier was in a particular place before it
disappeared.
Splitting of Radioactive Atoms
 The
splitting or fission of the nuclei of radioactive
atoms is another method of dating. The highenergy particles that are split off leave microscopic
tracks that can be counted. The older the rock, the
greater the number of tracks. This method can be
used to date moon rock samples and meteorite
samples as well as samples of Earth rocks.
The Age of the Earth
 Scientists
use radioactive dating to determine the
age of rocks. By finding the age of rocks, the can
estimate the age of the Earth. Scientists have
found some rocks in South Africa that are more
than 4 billion years old. Radioactive dating of
moon rocks brought back by the Apollo missions
show them to be 4 to 4.6 billion years old.