1. Earth’s History

2. Earth’s History

3. Earth’s History
Earth probably formed from an accumulation of rock, dust, and gases drawn together by its own gravity about 4.6 billion years ago.
The rocks of Earth’s crust preserves clues that help us unravel the mystery of our changing planet, its environments, and the development of terrestrial life.

4. Uniformitarianism
Geologists think that the forces that they observe today are similar to processes that occurred throughout Earth’s history.
“The present is the key to the past.”

5. Law of Superposition
The rocks at the bottom of an undisturbed exposure are usually the oldest.
There are occasional exceptions to this law.

6. Original Horizontality
A rock is always older than the process that changed it.
Sediments are usually deposited in layers.
When we see sedimentary layers, we usually assume that these layers were deposited level, and that they were tilted after they had turned into sedimentary rock.

7. Igneous Extrusions
An Extrusion occurs when molten rock flows onto Earth’s surface, where it crystallizes to form igneous rock.
An extrusion is younger than the rock below it, but older than the rock that will form on top.
The rock below the extrusion will show a zone of contact metamorphism where the hot lava baked it.

8. Igneous Intrusions
An Intrusion is an internal process where magma squeezes into or between layers of pre-existing rock.
The hot molten rock changes the surrounding rock immediately above and below and next to it by contact metamorphism.

9. Folds and Faults
Folds are bends in rock layers produced by movements of Earth’s crust, generally related to Earth’s tectonic plates.
Faults are breaks in the rock where movement has occurred often associated with earthquakes.
Offset layers are indications of faulting.
Folds and Faults occur after the rock has formed.

10. Fossils The preserved remains or traces of living things.
Can reveal a great deal about past life forms and environments.
Can also provide clues about the past geological events or relative ages of rock layers.
Trace Fossils do not contain the remains of the organisms that produced them.
Include the impressions of shells, dinosaur footprints, oddly shaped formations from sediments filling in animal burrows and petrified drippings.
Trace fossils reveal much about an organisms behavior and relationship to its living and nonliving environment

11. Correlation of Rocks
Geologists try to match similar rock strata in different locations to see if they formed at the same time or under similar conditions.
Color, texture, composition.
Compare index fossils in the strata.

12. Correlation of Rocks Example
Which layers are the same?
Of the rock layers E and F, which is the oldest?
What is the correct sequence of rock layer from oldest to youngest?
An unconformity (buried erosional surface) is represented by the interface between which two layers?
What type of rock is layer A?

13. Geologic Time Scale
Based on rock formations that contain characteristic fossil groups and on changes in the kinds of organisms that inhabited Earth.
The scale is divided into eras, periods, and epochs.

14. Evolution of Life
Geologists believe that life forms existed in the Precambrian Period.
They had no hard parts , therefore few left fossils.
Precambrian fossils are very rare.
More complex organisms developed as time went on.
Some disappeared (went extinct) from the fossil record .
Within each species there are variations in size, shape and other traits.

15. Evolution of Life
The Evolution of Life (Charles Darwin) states that individuals that have traits that better adapt them to their environment will survive longer and have more offspring to pass on these desirable traits.
This process of Evolution of Life, leads to the extinction of some species and formation of new ones.

16. Evolution of Life
Paleontologists (geologists who study fossils) have found remains of a large variety of plants and animals that lived in many different environments.
Some still exist, but most have become extinct.
Most organisms decompose or are consumed by other organisms after they die, only a very small percentage leave any fossil remains.
Because of this, many forms of life will never be known.

17. Life and The Atmosphere
Microscopic organisms that developed about 2.2 billion years ago changed the mixture of gasses in our atmosphere.

18. Life and The Atmosphere
About 3.8 billion years ago, the atmosphere probably consisted of a mixture of carbon monoxide, carbon dioxide, hydrogen, nitrogen, ammonia, and methane.
The atmosphere today is 78% nitrogen and 21% oxygen.

19. Past Geologic Events
No single location shows a complete record of the geologic past.
If an area was above sea level for a while, it is likely that sediments were not deposited and older rocks have been destroyed by erosion.
When a new rock layer is layered on a surface left by erosion, it forms a buried erosion surface (unconformity).

20. Radioactive Dating
Measurements of natural radioactivity in the rocks have allowed the geologic time scale to become an absolute time scale.
One that gives the absolute age (numerical age) of an object (measured in years).
Chemical elements often have several forms (isotopes) that differ in the number of neutrons in their atomic nuclei.

21. Radioactive Dating
If the nucleus of an isotope has more or fewer than the normal number of neutrons, the isotope may be radioactive.
A radioactive isotope will break down naturally into a lighter element called a decay product.
In the process, it gives off radioactivity.
A sample of a radioactive element contains millions of atoms, from which we can predict a rate of decay.

22. Half-Life
The rate of decay of a radioactive element is measured by its half-life.
Different radioactive elements have different half-lives.
A half-life is the time required for half of an element’s atom in a sample to change to the decay product.
At the end of one half-life, a sample contains equal amounts of the radioactive element and its decay product.

23. Half-Life
In each succeeding half-life, half of the remaining atoms decay.
As the element decays, fewer radioactive atoms remain in the sample, and more decay product accumulates.
The higher the ratio of decay product to radioactive element, the older the sample.

24. Decay-Product Ratio
The ratio between the mass of a radioactive element and its decay product in a sample.
After we determine this ratio, we can calculate how many half-lives have gone by since the sample was formed and then determine its age.

25. Selecting the Best Radioactive Element for Dating a Sample
The sample to be dated must contain a measurable quantity of a radioactive element and its decay product.
A sample containing the remains of living organisms is likely to contain radioactive carbon-14.
The sample’s age must also be considered.
Carbon-14 can only date samples no older than about 50,000 years.
Uranium-238 can measure samples of the oldest rocks on our planet.