Physical Geology

Chapter 6

The Rock Record

Uniformitarianism Is a theory that rejects the idea that

catastrophic forces were responsible for the current

conditions on the Earth. The theory suggested instead, that continuing

uniformity of existing processes were responsible for the present

and past conditions of this planet.

• In the mid-seventeenth century, biblical scholar and Archbishop James Ussher determined that the earth had been created in the year 4004 BCE. Just over a century later James Hutton, known as the father of geology, suggested that the earth was much older and that processes occurring in the present were the same processes that had operated in the past, and would be the processes that operate in the future. This concept became known as uniformitarianism and can be summarized by the phrase "the present is the key to the past."

• Sedimentary rocks are formed particle by particle and bed by bed, and the layers are piled one on top of the other. Thus, in any sequence of layered rocks, a given bed must be older than any bed on top of it. This Law of Superposition is fundamental to the interpretation of Earth history, because at any one location it indicates the relative ages of rock layers and the fossils in them.

Very simply put…………..

• Law of Original Horizontality states that most sediments, when originally formed, were laid down horizontally. However, many layered rocks are no longer horizontal. Because of the Law of Original Horizontality, we know that sedimentary rocks that are not horizontal either were formed in special ways or, more often, were moved from their horizontal position by later events, such as tilting during episodes of mountain building.

If not horizontal, look for clues to original position of layers and then apply the law of

superposition

Clues

• Graded beds – beds of decreasing sized sediment particles

• Cross beds – curved shaped beds at an angle to the bedding plane

• Ripple marks – peaks of ripples oriented upward

Unconformities

• disconformity

• Nonconformity

• Angular unconformity

• Breaks in the geologic record

Crosscutting relationships• A fault or igneous intrusion is always younger

than the rock layers it cuts through• If through an unconformity, it is younger than all

above and below the unconformity

How Old is the Earth?

1654: Archbishop J ames Ussher of I reland studied the timing of biblical events described in the Old Testament, and concluded that the the world was created on - October 23, 4004 B.C.E. at 9:00 AM.

Thus, the age of the earth, based on Ussher's results, would be: 4004 + 2003 = 6,007 years old.

Many calculations of this type have been done by other biblical scholars, and in general, the Earth was f ound to be about 6,000 years old based on these types of calculations.

Current Estimate of the Age of the Earth

Scientific methods of estimating the age of the earth are based on analysis of evidence f ound in rocks and on radiometric dating .

Based on these methods, scientists have concluded that the earth is much more than 6,000 years old.

Recent estimates of the age of the earth:

4.6 billion years old

(4.6 billion = 4,600,000,000)

Age of the Earth

Geologic Time

Geologic time is measured in millions of years.

How do we know about such long periods of time?

Geochronology: The science of determining the ages of rocks.

Relative Geologic Ages

1669 - Nicholas Steno: Studied layers of rock called strata.

Steno described how the position of strata could yield relative ages of rock units (i.e., older versus younger). Steno applied the f ollowing simple rule that has come to be called the principle of superposition:

I n any sequence of "undisturbed" strata of sedimentary rocks, the oldest layer is on the bottom, and successively higher rocks are successively younger.

Sedimentary rocks: Rocks f ormed f rom weathered products of pre-existing rocks (sediments) that have been transported, deposited, and lithified (converted into rock).

Geologic Time Scale

During the 16, 17 and 1800's geologists developed a geologic time scale based on relative ages.

Absolute Age of the Earth

During the 16, 17 and 1800's geologists developed a geologic time scale based on relative ages.

But: What about absolute ages? What is the absolute age of the earth?

Defi ne: Absolute age of the earth = amount of time since the earth solidifi ed.

At some point in the earth's early history, the entire planet was a mass of molten rock material. When did the earth cool and solidify into its present state?

Fossils and Evolution

Fossils = remains of ancient lif e.

For example, molds of ancient sea shells.

Mid-1800's: Charles Lyell - Used evolution to estimate absolute ages of rocks.

Fossils and Evolution

Mid-1800's: Charles Lyell - Used evolution to estimate absolute ages of rocks.

To estimate ages of rocks: Need to fi nd a natural process that leaves a tangible record in the rocks.

Then: if you can estimate the amount of time associated with that process, the age of the rock can be estimated.

Evolution is such a process.

Geology and Geophysics

Geology is the science that deals with the history of the earth as recorded in rocks.

Unif ormitarianism

The Principle of Uniformitarianism = The physical, chemical and biological laws that operate today have also operated in the geologic past.

Lyell compared the amount of evolution exhibited by marine f ossils f ound in rocks of diff erent relative ages with estimates of rate of evolution. Using estimates of rate of evolution and observations of f ossil f orms -- estimate age of rocks.

How long a period of time would be required f or the evolution observed in the rock strata to occur?

Fossils and Evolution

How long a period of time would be required f or the evolution observed in the rock strata to occur?

Lyell calculated: Certain rocks were about 80 million years old. But, ages of older rocks were more difficult to estimate.

Problems:

Missing parts of f ossil record. Rates of evolution diffi cult to estimate f or older lif e f orms.

General conclusion based on this type of approach to estimating age of earth:

Earth is at least 10's of millions of years old.

Fossils and evolution

Rates of deposition of sediments

Cooling of the Earth f rom a molten state

Radiometric Dating

Methods f or Estimating the Age of the Earth

Fossils and Evolution

How long a period of time would be required f or the evolution observed in the rock strata to occur?

General conclusion based on this type of approach to estimating age of earth:

Earth is at least 10's of millions of years old.

Fossils and evolution

Rates of deposition of sediments

Cooling of the Earth f rom a molten state

Radiometric Dating

Methods f or Estimating the Age of the Earth

Rates of Deposition of Sediments

Estimate the amount of time required to deposit a given thickness of strata.

A = Age of sequence of sedimentary rocks. T = Thickness of strata. R = Rate of sediment accumulation.

A = T/R

Example:

(100 f t)/ (0.0002 f t/ yr) = 500,000 yr

Rates of Deposition of Sediments

TT = total thickness of sedimentary strata that has been deposited during earth history.

Age of Earth = TT/R

Rates of Deposition of Sediments

Because of poor estimates of rates of deposition, this method gave a very wide range of results.

But, this method generally suggests that the earth isat least 10's of millions of years old, and (based on both f ossil evidence and rates of deposition of sediments):

Many geologists and biologists in the 1800's concluded that the earth was several hundreds of millions of years old.

Fossils and evolution

Rates of deposition of sediments

Cooling of the Earth f rom a molten state

Radiometric Dating

Methods f or Estimating the Age of the Earth

Based on f ossils and evolution, and on rates of deposition of sediments:

By the late 1800's many geologists and biologists thought that the earth was several hundreds of millions of years old.

Cooling of the Earth f rom a Molten State

Late 1800's: A physicist named Lord Kelvin, used physics and mathematics to challenge geologists and biologists who, by then estimated the age of the earth to be at least 100's of millions of years old.

Kelvin estimated: Earth was about 24 to 40 million years old.

Many geologists and biologists of the time argued that the earth was much older.

Cooling of the Earth f rom a Molten State

Kelvin's Analysis:

1. Assume that the Earth cooled f rom a molten state.

2. Assume that cooling f ollowed ordinary laws of heat conduction and radiation (unif ormitarianism).

3. Apply laws of heat conduction and radiation, and calculate the number of years it would take for the earth to cool f rom a hot molten mass to it's present state.

Kelvin's Result: Earth is 24 to 40 million years old.

Late 1800’s

Lord Kelvin: 10's of millions of years old Cooling of earth f rom molten state Physics and mathematics "Quantitative" arguments

Geologists and Biologists:

100's of millions of years old Fossils, evolution, and rates of

deposition of sediments I ntuition "Qualitative" arguments

“The real purpose of the scientific method is to make sure that nature hasn’t misled you into thinking that you know something that you don’t actually know.”

Robert M. Pirsig, Zen and the Art of Motorcycle Maintenance: An Inquiry into Values

Radioactivity

Discovery of radioactivity in the 1890's showed that the geologists and biologists were right:

Radioactivity as an internal heat source. Radiometric dating.

So, earth is much older than Kelvin thought…

Fossils and evolution

Rates of deposition of sediments

Cooling of the Earth f rom a molten state

Radiometric Dating

Methods f or Estimating the Age of the Earth

Radioactivity and

Radiometric Dating

• Radioactive elements have half-lives

• # of half lives may be calculated

• Years since formation of rock or strata may then be calculated

Absolute Dating Methods

The Fossil Record

• Fossil – remains of organisms

• Index fossil – occur only in and are identified with specific strata

• Paleontology – study of fossils

• Sedimentary rock – “home” of fossils

How about…….?

Or……?

Or……?

Dinosaur tracks

burrows

Worm tubes

Index Fossilslived for a short

geologic time but over a large area

so are indicative of a particular era in

geologic time

Index Fossil example – Indicative of the Silurian and

Ordovician periods