Ch 12outline

Historical Geology

Chapter 12 Geologic Time

Determining the Age of the Earth

_____________ – Greek (570-470B.C.) realized the fossils were the ancient remains of life on Earth. 

• _____________ – Greek, (450 B.C.) dug into the Nile River bank and counted the layers. He determined the Earth to be may thousands of years old.

• _____________– 1779, compared the Earth to a ball of cooling iron. He determined the Earth’s age to be 75,000 years.

• Several scientists tried to use the saltiness of the ocean.

• They measured how much salt was being added by rivers and concluded the age to be 90 million years.

(Salt precipitates out of ocean water when it becomes supersaturated.)

• _____________(Lord Kelvin) 1897, used heat conduction combined with actual measurements of the rate of heat flow out of the Earth’s surface.

• He concluded the Earth to be no more than 20-40 million years old.

• His calculations upset the biological science community of the time.

• Most thought the Earth to be much older.

• _____________ wrote that he would die unhappy if Thompson’s calculation were correct.

• Radioactivity proved that Darwin was correct.

Discovering Earth’s History

• _____________, 1869, led an expedition down the Colorado River through the Grand Canyon.

• Rocks record geological events and changing life forms of the past.  

• How is this history revealed? _____________

• Geological events are almost meaningless unless they are put into a time perspective.  

• The Earth is much older than anyone had previously thought and that its surface and interior have been changed.

A Brief History of Geology

• The primary goal of geologists is to interpret Earth’s history.

• In the mid-1600s, _____________ constructed a chronology or time line of both human and Earth history.

• He determined the Earth to be 5,000 years old.

• He believed the Earth to be created in 4004 B.C.

• In the 1700s, _____________, published his Theory of the Earth.

• He set forth the _____________. • Uniformitarianism means that the

forces and processes that we observe today have been at work for a very long time.

• Scientists know that these processes have not always as observable as they were in the past.

EX: large meteorites have hit the Earth even though no one saw them happen.

• This leads to the belief that the Earth is very old.

Relative Dating: Key Principles

• Relative dating tells us the sequence in which events occurred, not how long ago they occurred.

• Nicolaus Steno…… _____________ : states that in un-deformed sequence of sedimentary rocks, each bed is older than the one above it and younger than the one below it.

• Principle of _____________ - means that layers of sediment are generally deposited in horizontal position.

EX: _____________

• Principle of _____________– when a fault cuts through or when magma intrudes other rocks and crystallizes, we can assume the fault and the intrusion are both younger than the rocks affected.

• Law of _____________ :_____________ – pieces of one rock

contained in another. Ex: _____________ rocks are formed in

streambeds.

• _____________ – represents a long period during which deposition stopped, and erosion removed previously formed rocks, and then deposition resumed.

• _____________– indicates that during the pause in deposition, a period of deformation (folding and/or tilting) and erosion occurred.

• Two sedimentary rock layers that are separated by an erosion surface are called a _____________.

• _____________ – means the erosion surface separates older metamorphic or igneous intrusions from younger sedimentary rocks

• _____________ of Rock Layers – matching rocks of a similar age in different regions.

• _____________ help with correlation.

Distance is 12 miles between these two rock units

Distance is 17 miles between these two rock units

Fossils: Evidence of Past Life

• Fossils contain traces of prehistoric life.

• They are important components of sediment and _____________ rocks

Fossil Formation

The type of fossil that is formed is determined by the condition under which an organism died and how it was buried.

1. _____________– fully preserved mammoth, insects preserved in amber.

• 2. _____________– _____________wood and bones.

• Mineral rich water soaks into the small pores and cavities of the original organism.

• The minerals later crystallize.

• 3. _____________– common in shelled organisms.

• Similarly carbonation occurs when pressure squeezes out liquids and gaseous components of an organism leaving behind a thin residue of carbon

Fossil Types

Indirect Evidence

• 4. _____________– animal tracks, foot prints, burrows or holes, worm tubs, coprolites or petrified animal dung, gastroliths or gizzard stones

• Conditions Favoring Preservation:1. _____________– slows decay2. _____________–

Fossils and Correlation

• William Smith in the 18th century determined that fossils weren’t randomly distributed.

• Each layer contained distinct fossils that may not occur in the layers above and below it.

• The Principle of _____________– states that fossil organisms succeeded one another in a definite and determined order.

• Therefore, any time period can be recognized by its fossil content can recognize any time period.

• Geologists have identified an order of fossils; an Age of Trilobites, an Age of Fishers, an Age of Coal Swamps, an Age of Reptiles, an Age of Mammals.

• Once fossils were recognized as tie indicators, they became more useful in correlating rocks of similar age.

• Index fossils – are…..• 1. _____________• 2. _____________• 3. _____________• Ex. _____________

Interpreting Environments

• Fossils can also be used to interpret and describe ancient environments.

• Ex: Determining where ocean water and shorelines once were, climate, water temperature, corals indicate warm shallow oceans

Dating with Radioactivity

• Earth is about _____________ billion years old.

• Basic atomic structure

• An atom’s _____________ is the number of protons and neutrons in an atom’s nucleus.

• The number of neutrons can vary, and these variants, or _____________, have different mass numbers.

Radioactivity

• Radioactivity is when atomic nuclei are unstable and spontaneously break apart, or _____________.

• What is an isotope? _____________ An unstable or radioactive isotope of an

element is called the _____________.• The isotopes that result from the decay of

the parent are called the _____________ products.

• When the unstable nuclei begin to break down, radioactive decay begins and continues until a stable or non-radioactive isotope is formed.

• Example – _____________decays until lead 206 (Pb-206) is formed.

• This process has 13 intermediate steps before the stable Pb-206 is reached in the 14th

Half - life

• _____________ is the common way of expressing the rate of radioactive decay.

• _____________ is the amount of time necessary for ½ of the nuclei in a sample to decay to its stable isotope.  

• The half - life of U-238 is _____________ billion years.

Radiometric Dating

• is the process by which the age of rocks and minerals can be determined by using certain isotopes.

• The rates of decay have been precisely measured and _____________ under the physical conditions that exist in the Earth’s outer layers.

• Each radioactive isotope has been decaying at a constant rate since the formation of the rocks in which it occurs.

• Example – When uranium is incorporated into a mineral that crystallizes from magma, lead isn’t present from previous decay. The radiometric “clock” starts at this point.

• As the uranium decays, atoms of the daughter product are formed, and measurable amounts of lead eventually accumulate.

• The five radioactive isotopes in this table exist in nature and have been useful in determining ages of ancient rocks.

• An accurate radiometric date can be obtained only if the mineral remained in a closed system during the entire period since its formation.

• Example: potassium – argon method stems from the fact that argon is a gas and may lead from the sample making the measurement inaccurate.

• Cross checking or using more than one radiometric method is used to insure accuracy.

Dating with Carbon – 14

• To date recent events, _____________ is used in a method called radiocarbon dating.

• _____________ is continuously produced in the upper atmosphere and is quickly incorporated in carbon dioxide. As a result, all organisms contain a small amount of carbon – 14.

• While the organism lives, carbon – 14 is continually replaced.

• The ratio of carbon – 14 to carbon – 12 remains constant. 

• When an organism dies, the amount of carbon – 14 gradually decreases as it decays.

• By comparing the ratio of carbon – 14 to carbon – 12 in a sample, radiocarbon dates can be determined.

• Because the half-life of carbon – 14 is only _____________ years, it can be used to date recent geologic events up to about _____________ years ago.

Importance of Radiometric Dating

• Radiometric dating has supported the ideas of James Hutton and Charles Darwin.

• These dating methods have proved that there has been enough time for the processes we observe to have accomplished tremendous tasks.

Other Dating Methods Used

1. _____________ from _____________ trees (3,000 years)

2. _____________ – which are sediment layers found in glacial lakes (15,000 years)

Bristlecone Pine

Varve

The Geologic Time Scale

The geologic time scale divides Earth’s 4.56 billion year history into specific units of time.

Structure of the Time Scale

Time is divided into units, which include:

• _____________ – longest unit of time• _____________• _____________• _____________ – shortest unit of

time.

• Phanerozoic eon began 450 million years ago. It means “visible life.”

There are three eras with in the Phanerozoic.

1 – _____________ – meaning ancient life

2 – _____________ – middle life3 – _____________ – recent life

• Any unit of time is separated by a major change in life forms.

• Each era is subdivided into periods, which are separated by somewhat less significant changes in life forms.

• The Cenozoic is further divided into epochs.