Unit 3

Chapter 8

The Rock Record

Section 1 Determining Relative Age

The age of the Earth is approximately 4.6 billion years old. This idea came from James Hutton, an 18th century Scottish Physician and Farmer. He is the “Father of Modern Geology”. He came up with his ideas from observing changes taking place on his farm. He also stated “The Present is the Key to the Past”.

Uniformitarianism –

1. Geologic processes now operating were also active in the past.

a. Volcanoesb. Erosionc. Depositiond. Earthquakes

2. Present day geologic features were formed by these processes over long periods of time.

Before Hutton’s time people (science) thought all of the world’s features were created by catastrophic events.

Earths AgeBefore Hutton most people thought the age of the Earth

was about 6000 years old. They also thought that all geologic features were created the same way and over a long period of time. He concluded that the Earth had to be older for the changes that he found, to occur.

Relative AgeRelative dating is using rock layers to place events in the

sequence in which they occurred. This does not identify the actual date, but a date close to when the event occurred.

LAW OF SUPERPOSITION

The oldest rocks in a rock layer will be on the bottom, the newest rocks are on the top. Sedimentary rocks forms in horizontal layers. If the rock layers are overturned (flipped over) this is not true.

Principals of Original Horizontality

Sedimentary rocks generally form in horizontal layers. The principal states that left undisturbed, the layers will remain horizontal. If the layers are not horizontal, some tectonic force (action) had to occur. Scientist’s would have to retrace what happen first in order to apply the Law of Super Position.

Graded beddingLarge particles usually on the bottom are smaller are on top.

Cross-bedsWhen sand is deposited, it usually forms beds at an angle because the

particles slide down.

Ripple MarksSmall waves formed on the surface of sand that later becomes a rock. Can be

used to show which layer was on top.

UnconformityIs a gap in the geologic rock record sequence; it is a layer of rock

that is missing in the strata sequence. Shows that either the deposition stopped or part of the layer was removed by erosion.Ex. Erosion of rock layers

NonconformityWhen sedimentary layers are deposited on igneous and

metamorphic rock layers that have been uplifted and parts eroded away.

Angular UnconformityRocks that have tilted during uplift, or folded, then eroded. When

erosion stops, deposition once again occurs.

DisconformitiesLayers that have been uplifted (not tilted still horizontal) and then

eroded away. Then new layers are deposited. This makes gaps in the layering.

Cross Cutting Relationships

They are igneous layers that are intruded into sedimentary rocks must be younger than the rocks they flowed through.

Section 2 Determining Absolute Age

Absolute Age (time)

The actual age is the actual time an event happened like when school gets out at 2:45pm.

Absolute Dating Methods

If we know the absolute time between two separate events we can determine the relative time of all the events that occurred between them. Also we can determine how long it took for the processes to form the Earth’s features.

Rates of ErosionAn estimate of absolute age based on known erosional rates of geologic

features. This can only be used for features that were formed within the past 10-20 thousand years. It can be used for Niagara Falls, but not the Grand Canyon.

Rates of DepositionThis can be used to estimate absolute age based on the deposition rates of common rocks such as limestone, shale and sandstone. Generally the rate is about 30cm in 1000years. If flooding occurs, this formula can not be used.

Varve CountingThis is like counting the rings on a tree. Certain layers of sediment occur annually and can be counted. In areas with glacial lakes, fine sediments are deposited in the winter where larger more course sediments are deposited in the summers. Geologists look for the color variation and count the layers.

Radiometric Dating

Radioactive decay is the spontaneous emission of energy from an atom.

It measures the amount of a parent and daughter isotope within a rock or mineral. By using a known decay rate they can determine the age of something.

Each time a particle is emitted or captured from a parent isotope, the isotopes atomic number changes and becomes a different element (daughter isotope). Radiation is released until a stable isotope is formed.

Half-life

At the time a rock is formed it starts to decay at a constant rate. Half life is the rate at which a radioactive element decays. It is the time it takes for half of the radioactive atoms in a sample to decay to a stable product.

The ratio of the amount of radioactive element left in the rock to the amount of stable product can be used to determine the absolute age of the rock.

Radioactive Isotopesa. Potassium-Argon Dating - Potassium 40 decays to Argon 40 - It has a half life of 1.3 billion years

- Potassium is very common, found in feldspar, micas and amphiboles.

- It is common in all rock families - It can date rocks a young as 50,000 years and as old as 4.6

billion.

b. Rubidium-Strontium Dating - Rubidium 87 decays to Strontium 87 - Its half life is 49 billion years - Best for dating extremely old rocks from 10 million to 4.6 billion - It is common in feldspar & micas (most all igneous rocks)

c. Uranium-Lead Dating: - Uranium 238 decays to Lead 206

- Its half life is 4.5 billion years - It occurs only in trace amounts in Zircon (igneous)

d. Radio Carbon Dating: It measures the ratio of Carbon 14 to Nitrogen14.

N14 Normal Nitrogen while C14 Radioactive Carbon

Limitations:

It can only be used on organic objects

Its half life is 5730 so it can only be used for objects 70,000 years old or younger.

It rarely in Sedimentary or Metamorphic Rocks

It does not give reliable results for rocks younger than

10 million years old

Section 3 Fossil Records

Fossils

Evidence of early life preserved in Rock

Paleontology

The study of prehistoric times

Paleontologist

The scientist that studies the evidence

Interpreting the Fossil Records

By looking at fossils we can see how an organism lived and how they have either changed or gone extinct. This process allows us to glimpse their past and how Earth has changed by climate, land mass, temp etc.

Fossilization

1. Mummification –Taking internal organs out and replacing it with a substance usually herbs, then wrapping them in cloth strips prepared with a solution to preserve a body.

2. Original Remains a. Organism that is preserved in its entiretyb. Frozen animals and humans (wooly mammoth)c. Bone and teethd. Any organism trapped in amber

3. Tar SeepsTar from underground comes to the surface and traps an animal in it. La Brea Tar Pits, Cal.

4. FreezingThis occurs when an animal or organism is frozen quickly, bacteria did not decay the soft tissue. These objects are frozen whole. The wooly mammoth is an example.

5. Petrifaction It is when soft organic material is decayed away to form a mold and the minerals such as calcite, silica and pyrite are deposited to form the original shape.

Types of Fossils

Carbonaceous filmsIt is when carbon compounds making up the tissue undergo a chemical change and

form a thin film on the object.

2. Molds and Castsa. Molds are a hollow depression in the rock that shows the original shape and surface

of the fossil.b. Casts form as hard parts (and soft sediments) sediment hardens.

3. CoprolitesFossilized dung or waste material

4. GastrolithUse to help dinosaurs digest. They have a distinct shape, smooth and round.

Trace fossilsThey are preserved indirect evidence of prehistoric life. Includes impressions left in

rocks like footprints, tracks, burrows and bite marks.

Index Fossils

Using Index FossilsThey are the remains of animals that lived for a

short period of time and then became extinct. Therefore they would be found only in one layer.

There are 4 characteristics:Easily recognizableAbundantWidespread in occurrenceExisted for only a brief period of time

Index Fossils and Absolute Age

Since they are only around for a short period scientists can use them to absolute age an area. This can be done in separate locations to prove they have something in common or even to find oil and gas reserves.