THE HISTORY OF

THE EARTH

EARTH SCIENCE

CHAPTER 21

FOSSILS

Paleontology is the study of life that existed in prehistoric times

A fossil is any evidence of earlier life preserved in rock.

Shells

Bones

Petrified trees

Footprints

Impressions made by leaves

Burrows made by worms

FORMATION OF FOSSILS

Fossils may form in several ways:

From original remains

From replaced remains

Being preserved in molds and casts

Being preserved as trace fossils

Being preserved in carbonaceous film

ORIGINAL REMAINS

In rare cases, organisms may be preserved in

their entirety.

Frozen wooly mammoths in Siberia and Alaska

Prehistoric insects captured in resin

Usually, soft body parts decay and only the

hard parts are preserved.

Bones

teeth

REPLACED REMAINS

The soft parts of plants and animals have decayed

and the hard parts have been replaced by minerals

Circulating groundwater removes the original

organic material and replaces it with minerals such

as calcite, silica, and pyrite.

This results in an exact copy of the original plant or

animal.

Ex. Petrified wood

MOLDS AND CASTS

Organisms can be buried in mud or other

sediments. As the sediments become rock,

the hard body parts become fossils.

If the fossil dissolves out of the rock, a hollow

depression in the rock called a mold results.

Minerals may then seep into the mold and fill

it, forming a cast, or copy, of the original

fossil.

TRACE FOSSILS

Indirect evidence of life that is left behind

without being actual remains of the organism

or plant is called a trace fossil.

Trails, footprints, bite marks, burrows

CARBONACEOUS FILMS

Decomposing organisms may be exposed to

intense heat and/or pressure.

Their tissues will undergo chemical reactions

that sometimes leave behind a very thin film

of carbon that resembles a silhouette of the

organism. This is called a carbonaceous

film.

RELATIVE TIME

Most often, scientists have no way of knowing the exact dates of geological events on Earth.

Therefore, they often use a process called relative dating.

Relative dating does not assign exact dates to events but places events in a sequence, thus saying when they occurred in relation to other events.

THREE RULES OF

RELATIVE DATING

The Principle of Superposition

In an undisturbed sequence of sedimentary strata, the oldest rock layer will be at the bottom and the youngest layer will be on the top.

The Principle of Cross-Cutting Relationships

An igneous intrusion is always younger than the rock it has intruded or cut across.

Embedded Fragments

Rocks that are embedded in another rock must be older than the rocks in which they are embedded.

Example #1

Example #2

Example #3

1. Oldest layer

2. Youngest layer

3. Oldest organism

4. Place the following actions in the correct order of formation. (1-oldest, 5-

youngest)

-Granite dike intrusion, -folding of gray shale, -deposition of tan sandstone, -erosion

of tan sandstone, -deposition of red sandstone

Grand Canyon

Cross Section

GAPS IN RELATIVE TIME:

UNCONFORMITIES

An unconformity indicates where layers of rock are missing in the strata sequence—means evidence is missing

Angular unconformity

Younger, flat strata are deposited on top of older strata…older strata could have been tilted during uplift

Disconformity

All layers are horizontal, but some are missing due to erosion before the deposition of new layers

Nonconformity

Occurs when sedimentary layers are deposited on top of igneous or metamorphic rock

ROCK LAYER CORRELATION Correlation is the matching of rock layers from one area to

those in another area

Several methods are used to correlate rock layers: Walking the outcrop—outcrop is the part of the rock layer that can be

seen at Earth’s surface

Matching rock characteristics—see if the rocks have the same appearance, color, and composition

Using index fossils--(next slide)

Fossils as Environmental indicators—the presence of certain fossils can indicate climate and/or sea level changes

Matching key beds—a single rock layer that is unique, easily recognizable, and widespread

Stratigraphic matching—when the sequence of three of more layers or “strata” is repeated in two different areas

INDEX FOSSILS Index fossils are the remains of animals that lived and died

within a particular time segment of Earth’s history.

Remains are buried and animals become extinct, so those rock layers can become distinctly tied to that time period.

Index fossils have four characteristics:

The fossils are easily recognizable.

The fossils are abundant.

The fossils are widespread in occurrence.

The fossils occur only in a few rock layers because the organisms only existed for a brief time period

ABSOLUTE TIME Absolute time identifies the actual dates of events.

Historical methods for measuring absolute time: Estimating rates of erosion and sedimentation

Not very accurate because rates are not constant

Counting tree rings Still used today

One ring roughly equals one year

Have dated trees back to 2000 BC

Counting varves A varve is any sediment that is deposited on a yearly cylce

Used most accurately in glacial lakes

Have dated sediments back 15 000 years

RADIOACTIVITY

Recall that isotopes are atoms of the same element

with different numbers of neutrons.

Many elements have radioactive isotopes.

Radioactive isotopes are those that exhibit

radioactive decay by emitting or capturing tiny

particles in order to try and achieve stability.

Whenever, particles are emitted or captured, it can

change the atomic mass and even the atomic number

(identity) of the atom.

THREE TYPES OF

RADIOACTIVE DECAY

Alpha decay Two protons and neutrons are emitted

So, atomic number of original isotope decreases by two and atomic mass decreases by four

Beta decay A neutron splits into a proton (+) and an electron(-). The electron,

called a beta particle, is emitted.

The atomic number of the original isotope increases by one and the atomic mass does not change.

Electron capture A proton captures an electron and becomes a neutron.

The atomic number of the original isotope decreases by one and the atomic mass does not change.

CONTINUAL DECAY

The original element in the decay process is

called the parent isotope.

After each decay process, a daughter isotope

is formed.

If the daughter isotope is also radioactive,

decay continues until a stable, nonradioactive

isotope is formed as the daughter.

HALF-LIFE

Radioactive isotopes decay at a constant rate. This rate is not affected by changes in temperature or pressure, or by the passage of time

The amount of time it takes for exactly one-half of the radioactive atoms in a sample to decay to a stable product is called the half-life.

After two half-lives, 25% of the original sample will remain. ( ½ x ½ = ¼).

After three half-lives, only 12.5 % of the original sample will remain. ( ½ x ½ x ½ = 1/8

RADIOMETRIC DATING

Scientists measure the amounts of a parent

and a daughter isotope within a rock or

mineral and use the ratio to find the age of the

rock

Carbon-14 is used to date organic material.

Uranium-lead, Rubidium-strontium, and

potassium-argon are used to date rocks and

minerals.

RADIOCARBON DATING

Radioactive carbon-14 and stable carbon-12 both exist in the tissues of living organisms.

After death, the carbon-14 begins to decay. Scientists use the ratio of carbon-12 to carbon-14 to determine how long ago the plant or animal died.

Two serious limitations:

Can only be used to date things that were once alive

Carbon-14’s half-live is 5730 years (short amount of time), so it can only be accurately used to date items that are 70 000 years old or younger.

URANIUM-LEAD DATING

Radioactive Uranium-238 decays to form

stable lead-206.

Uranium-238 is 4.5 billion years, so it is

possible to date the oldest rocks of Earth’s

crust using this isotope.

Is naturally found in zircon, which is in some

igneous rocks—very rarely found in

sedimentary or metamorphic rocks

RUBIDIUM-STRONTIUM DATING

Rubidium-87 decays to form strontium-87.

The half-life of Rubidium-87 is 47 billion

years (over 10x the age of Earth)

Is the best method for dating the oldest of

Earth’s rocks

Is found in common minerals like feldspars

and micas.

POTASSIUM-ARGON DATING

Potassium-40 decays to form Argon-40.

The half-life of potassium-40 is 1.3 billion

years.

Are found in micas, feldspars, and

amphiboles.

Can be used to date sedimentary and

metamorphic rocks as well as igneous ones.

Geologic Time

Dinosaurs

Mesozoic Era

Title: Geologic Timeline

• The Earth is about 4.6 byo based on the absolute dating of rock samples. The Earth’s history is broken down into Eons, eras, periods and epochs. Each time frame does not last an even amount of time and the division of time is based on geologic and biologic events or evolutions.

Precambrian Eon Today, Cenozoic Era

Geologic Time Defined by changes in species or major geologic events. Time

frames are NOT uniform (NOT evenly divided!)

Earth is 4.6

BYO

Fossil evidence

shows life at 3.5

BYA

Life evolved

between 3.9-3.5

BYA

Precambrian ERA 4,600-544mya

• 87% of the Earth’s History

• Seas formed

• Mountains grew

• Oxygen built up in the atmosphere – Cyanobacteria

• Life : bacteria, jellyfish, algae (Prokaryotes and Eukaryotes)

• No life on land yet, only in seas!

Precambrian Life

Stromatalites!

Ediacara

Paleozoic ERA 544-248 mya

• Age of Fishes

• Appalachian Mountains form

• Warm, shallow seas cover the majority of North America

Paleozoic Era

• Periods

• 1. Cambrian Period – “Explosion” of life

– Trilobites (skeletons, hard parts evolved)

• 2. Ordovician Period – corals, sponges, first fish

– Mass extinction- 57% of marine species (glaciation)

• 3. Silurian Period – first land plants

Paleozoic ERA • Periods • 4. Devonian

– age of fishes (armor-plated fish and amphibians) lungfish, coelacanths

– Ferns (seeds!) – mass extinction! 50%

of marine species- (glaciation)

• 5. Mississippian – Crinoids

(Invertebrates) – Many amphibians

Paleozoic Era

• 6. Pennsylvanian Period – Insects (cockroach,

centipede, dragonfly)

– first reptiles

– Coal deposits

• 7. Permian – abundant reptiles

– Mass extinction- 95% of land and marine species (regression)

Mesozoic Era 225-65my

• Age of Reptiles

• Mass extinctions at the beginning and end of this era

• Mississippi River System forms

• Rocky Mountains

• Atlantic Ocean

Mesozoic Era 225-65my • Periods • 1.Triassic

– Pangaea breaks up

• 2. Jurassic – First birds

• 3. Cretaceous – first flowering plants

• Life Forms of Mesozoic Era – Dinosaurs, First Mammals,

and Modern Fish

• Mass extinction – Meteorite, volcanism – Wipes out dinos

Cenozoic ERA 65mya- today

• Age of Mammals

• Himalayan Mountains

• Alps

• Grand Canyon

• Great Lakes

• Chesapeake Bay

Cenozoic Era

• Periods

• 1. Tertiary – Mastodons, Mammoths,

Primates

– Early humans • Homonids (walk upright)

• Neanderthals

– extinction (45% of species)

• 2. Quaternary – Civilization begins10,000 years

ago with end of most recent ice age and invention of agriculture