Chapter 3 “The Rock and Fossil Record”
Section 1: Earth’s Story and Those Who First Listened
The Principle of Uniformitarianism
For 30 years a Scottish farmer and scientist named John 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.
Uniformitarianism Versus Catastrophism
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.
A Victory for Uniformitarianism
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.
Modern Geology – A Happy Medium
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.
Paleontology – The Study of Past Life
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.
Section 2: Relative Dating: Which Came First?
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.
The Principle of Superposition
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
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.
Disturbed Rock Layers
4 Ways Rock Layers become disturbed:
1.
2.
3.
4.
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.
Gaps in the Record – Unconformities
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
Types of Unconformities
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
Rock –Layer Puzzles
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
Section 3: 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.
Radioactive Decay
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.
The Process of Dating Rocks:




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
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.
Types of Radioactive Dating
Scientists use different radiometric dating techniques based on the estimated age of the object. The older the object is
the greater half-life of an isotope you will need.
1.
Potassium-Argon Method
 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
2. Uranium-Lead Method
 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 (lead-206) 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
3. Rubidium-Strontium 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
4. Carbon-14 Method
 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
Section 4: Looking at Fossils
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.
7 Types of Fossils:
1.
2.
3.
4.
5.
6.
Fossils in Rocks
 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
Fossils in Amber
 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
 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
Fossils in Asphalt
 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
Frozen Fossils
 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.
Trace Fossils
 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
7. Molds and Casts
 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
Using Fossils to Interpret the Past
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.
History of Environmental Change
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.
History of Changing Organisms
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
Using Fossils to Date Rocks
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.
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.
Section 5: Time Marches On
Geologic Time
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.
The Geologic Time Scale
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 second-largest 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.
The Paleozoic Era – Old Life
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.
The Mesozoic Era – The Age of Reptiles
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.
The Cenozoic Era – The Age of Mammals
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.