Geologic Time, Fossils and the

Rock Record

Unit 2

The Geologic Time Scale

• Record of geologic events and major changes in life forms (mass extinction or new life)

• By studying the characteristics of rocks and the fossils, geologists can interpret the environments the rocks were deposited in, reconstruct Earth’s history, and possibly predict events or conditions in the future.

• What kind of events on earth could precipitate such events?

Geologic Time Scale

Geologists have divided the history of Earth into time units based on fossils found within rocks.

These time units are part of the geologic time scale, which is a record of Earth’s history from it’s origin (4.6 bya).

The development of the geologic time scale is continually refined.

The scale allows scientists, world-wide, to correlate geologic events, environmental changes, and the development of life-forms preserved within the rock record.

Geologic Time

The oldest division is at the bottom of the scale

As you move up the scale, each division is younger

The scale is divided into smaller units

◦ Eons◦ Era◦ Periods – measured in

tens of millions to hundreds of millions of years

◦ Epochs – smallest, measured in millions to tens of millions of years

Reading the Scale

The greatest expanse of time – the longest unit of time

Measured in billions of years

The most recent eon, the Phanerozoic (visible life) began 540 MYA.

Precambrian Time makes up approximately 90% of geologic time and is divided into two eons, the Archean and the Proterozoic

Eons

Next longest span of time, marked by significant changes in life

Measured in hundreds of millions to billions of years

Defined by the differences in life-forms found in rocks

Names are based on the relative ages of the life forms found◦ Paleo – “old”◦ Meso – “middle”◦ Ceno – “Recent”◦ Zoic – “Life”

Eras

Eras are broken into even smaller divisions, called periods, which are ultimately defined by the life forms that are abundant or have became extinct during the time in which specific rocks were deposited▫ Paleozoic Era has 7 Periods▫ Mesozoic Era has 3 Periods.▫ Cenozoic Era has 3 Periods

Measured in tens of millions to hundreds of millions of years.

Some named for region in which the rocks were first observed (Mississippian Period).Periods

Periods are broken into even smaller divisions, called epochs

Measured in millions to tens of millions of years

Different groups of organisms are used to distinguish between the various epochs

The current era, Cenozoic, is divided into 3 periods and 7 epochs

The Cenozoic is the only era to include epochs

Epochs

Studying Earth’s History

Geologic Laws and Principles

States that processes occurring today have been occurring since the Earth formed. Only the rate, intensity, and scale with which they occur have changed

“The present is the key to the past”

To say that geological processes in the past were the same as those occurring today is not to suggest that they always had the same relative importance or that they operated at precisely the same rate (Ex. The Colorado River cutting through the Grand Canyon)

If uniformitarianism is actually the guiding principle of how Earth works… where does the concept of Catastrophism fit in?

The Principle of Uniformitarianism

Explains the differences in fossil forms encountered in successive stratigraphic levels as being the product of repeated cataclysmic occurrences and repeated new creations

In the case of catastrophism, as applied to geology or paleontology (the study of fossils), there can be little doubt that it has seemed as defunct as any theory could be in the eyes of the scientific community. In recent decades however, catastrophism has made a real contribution to geology and the evolutionary theory

Right or wrong, it has generally been thought that the catastrophists of the 19th Century and earlier believed that God was directly involved in determining the history of the Earth. So, for example, American palaeontologist, Steven Stanley, claimed in 1987 that catastrophism was the outmoded belief that sudden, violent and widespread events caused by supernatural forces formed most of the rocks visible at the earth's surface

Thinking in regard to erosion, what processes have occured that would support the idea of catastrophism? What processes would contradict catastrophism?

The Doctrine of Catastrophism

Original Horizontality

Superposition Cross-Cutting Relationships

States that sedimentary rocks are deposited in horizontal or nearly horizontal layers

We assume that the oldest rocks are at the bottom and each layer above is younger than the one below as sedimentary particles settle from fluids through the force of gravity

States that in an undisturbed rock sequence, the oldest rocks are at the bottom and each successive layer is younger than the layer beneath it

Think of this like a clothes hamper… if on Monday, the hamper was empty and you placed clothing in it each day, the clothing you would wear on Monday would be at the bottom and Friday’s clothing would be on top.

States that an intrusion or a fault is younger than the rock it cuts across

Additional Geologic Laws and Principles

Relative Age of Rocks

Relative-Age dating places the ages of rocks and the events that formed them in order, but without exact

dates, through comparisons of rock layers to one another.

Relative age can be determined where an overlying rock layer contains particles of rock material from the layer beneath it.

These particles are called inclusions

The presence of inclusions indicates that the rocks in the lower layers are older than those in the top layers as they would’ve had to be present to be “included” in the new layer

Inclusions

It is difficult to find an undisturbed rock layer due to the ever-changing processes of the Earth

As result, we must look at unconformities that have resulted, which is when a buried erosional surface results in a gap in the rock record

Other Means to Determine Relative Age

Disconformities

When horizontal sedimentary rocks overlie horizontal sedimentary rocks

Types of Unconformities

When sedimentary rocks overlie nonsedimentary rocks, suggesting an uplifting. The contact point between the nonsedimentary and sedimentary rock is called a nonconformity.

Nonconformities

When horizontal sedimentary rocks are uplifted and tilted, exposing them to weathering and erosion

When deposition resumes, horizontal layers are laid down on top of the erosional surface

The layers beneath remain in tact, but at an angle

Angular Unconformity

Correlation is the matching of outcrops of one geographic region to another.

◦ Can be used for exploration of distinctive fossils◦ Can be used to exploration of unique rock or mineral features◦ Can be used in exploration of oil or other valuable minerals

Why Correlate Rock Strata?

Absolute-Age Dating of RocksAbsolute-age dating enables scientists to determine the actual age of a rock, fossil, or other object. Scientists have a method for dating very old objects by using the decay rate of radioactive isotopes.

Through radiometric dating, scientists attempt to determine the ratio of parent nuclei to daughter nuclei within a given sample of a rock or fossil.

As the number of parent nuclei decreases, the number of daughter nuclei increases.

As it takes a long time for the entire isotope to decay, geologists use the length of time it takes for one-half of the original amount to decay. This is called the half-life

Use of Radioactive Isotopes

Tree Rings Seasonal Climatic Changes

Distinctive Sediment Layers

The age of a tree can be determined by counting the number of annual tree rings in a cross section of the tree.

The width of tree rings are directly related to the climatic conditions during growth periods (spring and winter rings equal one year)

Dendrochronology is the science of comparing annual growth rings to date events and changes in past environments

Varves are bands of alternating light and dark colored sediments of sand, clay, and silt. In the summer months, deposits are light colored and thick… in the winter they are darker and thinner

Varves are similar to tree rings as they show evidence of cyclic events

Varves from different lakes can be compared to determine the ages of glacial lake sediments

When layers are formed by instantaneous or short-lived events, geologists may be able to determine the time of the event through radiometric dating.

This layer is called a key bed, and can correlate rock layers across large areas

Other Ways to Determine Age

Remains of Organisms in the Rock Record

Fossils are the evidence or remains of once-living plants or animals.

• The rock record provides evidence of geologic events and life forms of the past

• Processes observed on Earth in the present also acted in the past

• The Earth is very old and has changed over geologic time

The Rock Record

Fossils with original preservation are the soft and hard parts of plant and animal remains that have not undergone any kind of change since the organism’s death.

This is extremely uncommon because preservation would require extraordinary circumstances (freezing, drying out, oxygen-free environments)

Fossils

Altered Hard Parts

When all the organic material has been removed and the hard parts of a plant or animal have been changed by mineral replacement or by recrystallization

When pore spaces are filled with mineral substances, it is referred to as permineralization.

Molds and Casts Indirect Evidence of Past Life

Mold – hollowed-out impression of the shell that was left in sedimentary rock

Cast – When the cavity of the hollowed-out impression becomes filled with minerals or sediments

Referred to as “trace fossils”

Examples: worm trails, burrows, footprints

Provide information about how an organism lived, how it moved, and how it obtained food

Remains of plants or animals that can be used by geologists to correlate rock layers over large geographic areas or to date a particular rock layer

Found exclusively in sedimentary rocks

Easily recognized & abundant

Widely distributed geographically

Must have lived during a short period of time

Provide clues to the environments in which the rocks formed

Index Fossils

Volcanic Time Markers and CorrelationViolent volcanic eruption can deposit a thin layer of volcanic ash over the entire Earth.

This layer is very distinguishable and is similar to index fossils (cover entire earth and represent a very short period of time)

http://www.youtube.com/watch?v=QW9DVoa-BJM

We correlate fossils as we correlate rocks. So, rocks with the same fossil are considered to be the same age

Why Study Fossils?

Allows scientists to interpret and describe Earth’s history

Provides evidence of the past existence of a wide variety of life-forms

Provides evidence that a population has undergone changes in their environment, which is referred to as evolution

Shows evidence of ancient environmental conditions

Helps scientists find patterns and cycles that can be used to predict future phenomena