Chapter 3
Our World in One Minute!!`
http://www.youtube.com/watch?v=ZSt9tm3RoUU

For 30 years a Scottish farmer
and scientist named James
Hutton studied rock formations
in Scotland and England.

He was trying to answer
questions like, “how do
mountains form,” “how is new
rock created,” and “how old is the
Earth”

His observations led to the
foundation of modern geology –
the study of the Earth, the
materials of which it is made, the
structure of those materials, and
the processes acting upon them

In 1788, Hutton collected his notes and wrote
Theory of the Earth.
 In this book he stated that the key to understanding
Earth’s history was all around us.
 What he meant was that the processes that we
observe today, such as erosion and deposition, remain
uniform, or do not change, over time.

Uniformitarianism is the idea that the same
geologic processes shaping Earth today have
been at work throughout Earth’s history.

Catastrophism is the principle that states that all
geologic change occurs suddenly.

Supporters thought that Earth’s features, such as
mountains, canyons, and seas, formed during
rare, sudden events called catastrophes

In Hutton’s time most people thought the world
was only a few thousand years old.

A few thousand years was not enough time for
the gradual geologic processes that Hutton
described to have shaped our planet.

Despite Hutton’s work,
Catastrophism remained
geology’s guiding principle for
decades.

From 1830 – 1833 a British
Geologist named Charles Lyell
published three volumes,
collectively titled Principles of
Geology, in which he
reintroduced Uniformitarianism.
 Armed with Hutton’s notes and new
evidence of his own, Lyell saw no
reason to doubt that major geologic
change happened at the same rate
in the past as it happens in the
present – gradually.

During the late 20th century, scientists
such as Stephen J. Gould challenged
Lyell’s Uniformitarianism. They
believed that catastrophies do, at
times, play an important role in shaping
Earth’s history.

Today, scientists realize that neither
Uniformitarianism nor catastrophism
accounts for all geologic change
throughout Earth’s history

Although most geologic
change is gradual and
uniform, catastrophies
that cause geologic
change have occurred
during Earth’s history
 Huge craters have been
found where asteroids and
comets are thought to have
struck Earth in the past
 One being the asteroid that
could have potentially killed
off the dinosaurs.

The study of past life is
called paleontology.

Scientists who study this
are called paleontologists.

The data these scientists
use are called fossils.

Fossils are the remains of
organisms preserved by
geologic processes.
Edwin Colbert was a 20thcentury vertebrate
paleontologist who made
important contributions to the
study of dinosaurs

Geologists try to determine the order in which
events have happened during Earth’s history.
 They rely on rocks and fossils to do so.

Determining whether an object or event is older
or younger than other objects or events is called
relative dating.

Layers of sedimentary rock provide a sequence
of history. As you move to the bottom, the layers
are older.

Superposition is the principle that states that
younger rocks lie above older rocks in
undisturbed sequences.

Some rock layers are disturbed by forces within
the Earth; these forces can push other rocks into
a sequence, tilt or fold rock layers, and break
sequences into movable parts.

When rocks sequences are disturbed, or out of
order, geologists can use the geologic column as
a tool to help.

The geologic column is an ideal sequence of rock
layers that contains all the known fossils and
rock formations on Earth, arranged from oldest
to youngest.

Geologists use this column to identify the layers
in puzzling rock sequences.
4 Ways Rock Layers become disturbed:

Fault: a fault is a break in the Earth’s crust along which blocks of
the crust slide relative to one another

Intrusion: An intrusion is molten rock from the Earth’s interior
that squeezes into existing rock and cools

Folding: Folding occurs when rock layers bend and buckle from
Earth’s internal forces

Tilting: Tilting occurs when internal forces in the Earth slant
rock layers.

Sometimes layers of rock are missing altogether,
creating a gap in the geologic record.

An unconformity is a surface that represents a
missing part of the geologic column.

Unconformities also represent missing time –
time that was not recorded in layers of rock.

Unconformities are created by:
 Nondeposition – the stoppage of deposition when a
supply of sediment is cut off
 Erosion
1.
Disconformities
 The most common type
 Are found where part of a sequence of
parallel rock layers is missing
 Can form when a sequence of rock
layers is uplifted. Younger layers at the
top of the material is deposited
elsewhere. At some future time,
deposition resumes, and sediment
buries the old erosion surface. The
disconformity that results shows where
erosion has taken place and rock layers
are missing
 Represents thousands to millions of
years of missing time
2.
Nonconformities
 Are found where
horizontal sedimentary
rock layers lie on top of
an eroded surface of
older intrusive igneous or
metamorphic rock
 Represent millions of
years of missing time
3.
Angular Unconformities
 Are found between
horizontal layers of
sedimentary rock and
layers of rock that have
been tilted or folded.
 The tilted or folded layers
were eroded before
horizontal layers formed
above them.
 Represent millions of years
of missing time

Geologists often find
rock-layer sequences that
have been affected by
more than one of the
events and features
mentioned in this section

Determining the order of
events that led to such a
sequence is like piecing
together a jigsaw puzzle
Absolute Dating: A Measure of Time

The process of establishing the age of an object
by determining the number of years it has
existed is called absolute dating.

To determine absolute ages of fossils and rocks,
scientists analyze isotopes of radioactive elements.

Atoms of the same element that have the same
number of protons but have different numbers of
neutrons are called isotopes.

Most isotopes are stable, but some are unstable and
referred to as radioactive.

Radioactive isotopes tend to break down into
stable isotopes of the same or other elements in
a process called radioactive decay.

Radioactive decay occurs at a steady rate so it
can be used to calculate an objects true age.

An unstable radioactive isotope (the parent isotope) of one
element breaks down into a stable isotope (daughter isotope)

The decay of the parent isotope into the daughter isotope can
occur in one step or a series of steps, but in either case the rate
of decay is constant.

Therefore, to date a rock, scientists compare the amount of
parent materials with the amount of daughter material.

The more daughter material there is, the older the rock is.

Radiometric dating is determining the absolute
age of a sample, based on the ratio of parent
material to daughter material

A half-life is the time that it takes one-half of a
radioactive sample to decay.

Example: Let’s say a rock contains an isotope
with a half-life of 10,000 years. So for this rock,
in 10,000 years half of the parent material will
have decayed and become daughter material. If
you analyze the sample and it has equal amounts
of parent and daughter material, then that rock
is about 10,000 years old.

After every half-life, the amount of parent material
decreases by one-half.

Example: With a 10,000 year half life, after 10,000 years
you would still have ½ of the parent material. After
20,000 years, you would have ¼ of the parent material
(half of ½ is ¼). After 30,ooo years you would have 1/8
of the parent material (half of ¼ is 1/8), etc.




Imagine traveling back
through time before
Columbus arrived in
America.
You are standing on what
will be the bluffs of the
Mississippi River.
You see dozens of people,
Native Americans, building
large mounds
They are building burial
mounds, now an
archaeological site called
Effigy Mounds National
Monument

People lived at Effigy from 2,500 to 600 years
ago.

How do we know these dates????

Scientists use different radiometric dating
techniques based on the estimated age of the
object. The older the object is the greater halflife of an isotope you will need.

Isotope used is Potassium-40

Half-life of 1.3 billion years

Decays to argon and calcium, measuring
argon as the daughter material

Used to date rocks older than 100,000 years

Uranium-238 decays in a series of steps to Lead-206

Half life is 4.5 billion years

The older the rock is the more daughter material (lead206) there will be in the material

Can be used for rocks more than 10 million years old

Younger rocks do not contain enough daughter material to
be accurately measured by this method

Rubidium-87 is an isotope that forms the
stable daughter isotope strontium-87

Half life is 49 billion years

Used to date rocks older than 10 million years

Carbon is found in three forms: Carbon-12, Carbon-13 and the radioactive form
Carbon-14

These carbons combine with oxygen to form the gas carbon dioxide, which is
taken in by plants to perform photosynthesis

As long as a plant is alive, new carbon dioxide with a constant carbon-14 to
carbon-12 ratio is continually taken in

Animals then eat the plants and contain the same ratio of carbon isotopes

When a plant or animal dies, no new carbon is taken in.

The amount of carbon-14 begins to decrease as the organism decays, and the
ratio to carbon-12 to carbon-14 decreases.

This decrease can be measured in a lab

Because the half-life of carbon-14 is only 5,730 years, this dating method is used
mainly for dating things that lived within the last 50,000 years

The remains or physical evidence of an organism
preserved by geologic processes is called a fossil.

Fossils are most often preserved in sedimentary
rock, but other materials can also preserve
evidence of past life.

Usually when an organism dies, it immediately begins to
decay or is eaten by another organism

Sometimes though organisms are quickly buried by
sediment when they die

The sediment slows down decay. Hard parts like shells and
bones are more resistant to decay than soft tissues

When sediments become rock, the hard parts of animals
are much more commonly preserved than are soft tissues

Hardened tree sap is called Amber

Sometimes organisms, usually types of insects,
are trapped in tree sap which hardens over time.

Organisms are fully preserved inside that tree
sap, DNA and all

Petrifaction is a process in which minerals
replace an organisms tissues

Permineralization is a process in which the pore
space in an organisms hard tissue (bone or wood)
is filled up with mineral

Replacement is a process in which the organisms
tissues are completely replaces by minerals

There are places on Earth where asphalt wells up at the
Earth’s surface in thick, sticky pools (are also called tar
pits)

The La Brea asphalt deposits in Los Angeles are at least
38,000 years old

These pools have trapped and preserved many kinds of
organisms from the past 38,000 years

From these fossils scientists have learned about the past
environment of southern California
“Early excavations at the La Brea
tar pits of central Los Angeles
during the period 1913–1915
unearthed roughly a million bones
from nearly a hundred sites. Photo
courtesy of the George C. Page
Museum. Source:
Fossils discovered in the tar
pits of La Brea
http://www.youtube.com/watch?v=G7FK59
waeo0
6:09
 Cold temperatures slow decay
 Many types of frozen fossils are preserved from
the last ice age
 In October 1999, scientists recovered a complete
20,000 year old woolly mammoth frozen in the
Siberian tundra – woolly mammoths became
extinct approximately 10,000 years ago.
http://player.discoveryeducation.com/index.cfm?guidAssetId=FDCCD32E-D697-408B-8EA328F781244276&blnFromSearch=1&productcode=US#

Any naturally preserved evidence of animal activity is called a trace
fossil

Examples are tracks, burrows, coprolite

Footprints that fill with sediment and become preserved in rock
reveal a lot about the animal that made them, including how big it
was, how fast it moved. Parallel track ways showing dinosaurs
moving in the same direction have led scientist to hypothesize that
dinosaurs moved in herds

Burrows are shelters made by animals such as clams, that burry in
the sediment. Burrows are preserved when they are filled in with
sediment and buried quickly
Cross-section of Mammoth
footprints
Footprints from
the Triassic Period
Burrows

A cavity in rock where a
plant or animal was buried
is called a mold

A cast is an object created
when sediment fills a mold
and becomes a rock

A cast shows what the
outside of the organism
looked like
This photograph shows two molds
from an ammonite. The image on the
left is the internal mold of the
ammonite, which formed when
sediment filled the ammonite’s shell,
which later dissolved away. The image
on the right is the external mold of the
ammonite, which preserves the
external features of the shell.
United Streaming: The Formation of Fossils 3:16
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d834f-99b5-44d2-be32-80a6846af348

The fossil record offers only a rough sketch of the
history of life on Earth. There are huge gaps in the
fossil record.

The fossil record contains more information about
organisms with hard parts, and organisms that lived
in locations that favor the production of fossils.

The record is incomplete because most animals
have never become fossils, and those that have,
have yet to be discovered.

The fossil record reveals a
history of environmental
change.

Marine fossils help scientists
reconstruct ancient coastlines
and the deepening and
shallowing of ancient seas.

Using the fossils of plants and
land animals, scientists can
reconstruct past climates.
This scientist has found marine fossils
on mountaintops in the Yoho National
Park in Canada. The fossil of Marrella,
shown above, tells the scientist that
these rocks were pushed up from below
sea level millions of years ago.

By studying the relationships between fossils,
scientists can interpret how life has changed over
time.

Because the fossil record is so incomplete, they
look for similarities between fossils, or between
fossilized organisms and their closest living
relatives, and try to fill in the blanks of the fossil
record
Fossil Record of Horses
http://www.livescience.com/13670-25-amazing-ancient-beastsdinosaurs-reptiles.html

To be considered an index
fossil, a fossil must be found in
rock layers throughout the
world.

One example of an index fossil
is ammonites called Tropites.
Tropites was a marine mollusk
similar to a modern squid. It
lived in a coiled shell. Lived
between 230 and 208 million
years ago and is an index fossil
for that period of time.
Tropites is a genus of coiled
ammonites. Tropites existed for
only about 20 million years,
which makes this genus a good
index fossil

Fossils of Trilobites called
Phacops are another
example of an index fossil.
Trilobites are extinct, and
their closest living relative is
the horseshoe crab.

By dating rock,
paleontologists determined
that Phacops lived
approximately 400 million
years ago. So when they
find one in rock layers
anywhere on Earth, they
assume that these rock
layers are also about 400
million years old.
Paleontologists assume that any rock
layer containing a fossil of the trilobite
Phacops is about 400 million years old

Using Radioactive Dating to date the oldest rocks on
record, scientists have estimated the Earth to be 4.6
billion years old.

One of the best places in North America to see the
Earth’s history recorded in rock layers is in Grand
Canyon National Park. The Colorado River has cut the
canyon nearly 2 km deep in some places. Over the
course of 6 million years, the river has eroded countless
layers of rock. These layers represent almost half, or
nearly 2 billion years, of Earth’s history.
Bones of dinosaurs that lived
about 150 million years ago are
exposed in the quarry wall at
Dinosaur National Monument in
Utah.
Well-preserved plant and animal fossils
are common in the Green River
formation. Clockwise from the upper
right are a fossil leaf, a dragonfly, a fish,
and a turtle.

The geologic time scale
is a scale that divides
Earth’s 4.6 billion year
history into distinct
intervals.

The largest divisions of
geologic time are eons.

There are four eons:
1. The Hadean eon
2. The archean eon
3. The Proterozoic eon
4. The Phanerozoic Eon

The Phanerozoic eon is
divided into three eras,
which are the secondlargest divisions of geologic
time

The three eras are further
divided into periods, which
are the third-largest
division of geologic time

Periods are divided into
epochs, which are the
fourth-largest divisions of
geologic time

The boundaries between geologic time intervals
represent shorter intervals in which visible changes
took place on Earth.

At certain times during Earth’s history, the number
of species has increased or decreased dramatically.

Extinction is the death of every member of a
species.

Gradual events such as global climate change and
changes in ocean currents, can cause mass
extinctions; a combination of these events can also
do the same.

Lasted from about 542 million to 251 million years
ago. It is the first era well represented by fossils.

Marine life flourished at the beginning of the
Paleozoic. The oceans became home to a diversity
of life.

However, there were few land organisms.

By the middle of the Paleozoic, all modern
groups of land plants had appeared.

By the end of the era, amphibians and reptiles
lived on the land, and insects were abundant.

The Paleozoic came to an end with the largest
mass extinction in Earth’s history. Some
scientists believe that ocean changes were a
likely cause of this extinction, which killed nearly
90% of all marine species.
Jungles were present during the Paleozoic era, but there were no birds
singing in the trees and no monkeys swinging from the branches. Birds
and mammals didn’t evolve until much later

Began about 251 million years ago.

Is known as the Age of Reptiles, because reptiles,
such as the dinosaurs, inhabited the land.

Small mammals appeared about the same time
as the dinosaurs, and birds appeared late in the
Mesozoic Era.

Many scientists think that birds evolved directly
from a type of dinosaur

At the end of the Mesozoic, about 15% to 20% of
all species on Earth, including the dinosaurs,
became extinct. Global climate change may
have been the cause.
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
Began about 65.5 million years ago and continues to the
present.

During the Mesozoic, mammals had to compete with dinosaurs
and other animals for food and habitat. After the mass
extinction, mammals flourished.

Unique traits, such as regulating body temperature internally,
and bearing young that develop inside the mother, may have
helped mammals survive the environmental changes that
probably caused the extinction of the dinosaurs.
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