CO- The Geologic time scale and geologic history.

LO- describe in writing the eras of the Geologic Time scale

The Geologic Time ScaleMeasured in millions or

billions years (instead of hours and days)

Represents 4.6 billion years of Earth history

It is divided into time periods based on the rock and fossil record.

The Geologic Time Scale is divided into eons, eras, periods, and epochs based on one or more of the following:

1) major changes in earth’s surface 2) Major changes in earth’s climate.3) major changes in the life forms on

earth.

Dividing up Geologic Time

The Geologic Time Scale:• Is divided into Eons–Which are broken down into Eras•Which are broken down into Periods–Which are broken down into Epochs

Four major eras in the geologic time scale.

Precambrian Time – 88% of geologic time

Paleozoic- Ancient LifeMesozoic- Middle LifeCenozoic –Recent Life

Precambrian Eon – 88% of geologic timeFrom 4500 to 540 million years agoLife on earth begins – Single celled organisms lived

pre- “ multicellular life”

Oldest layers of rockBacteria and Cyanobacteria (Blue-Green algae)Evolution of first multicellular begins at the end

Paleozoic Era- Ancient life 540-248 million years ago.Evolution of multi-cellular organismsInvertebrates fish and amphibians Evolution of land plants land Ended with the

“Permian Mass Extinction”-90% of all life went extinct.

Mesozoic Era -Middle life248-65 million years agomore complex organismsAge of reptilesdinosaursReptiles, birds, first mammals Ended with mass extinction – “asteroid

impact”

Cenozoic Era – recent life 65 million years ago to presentAge of mammalsHumans take over the world

CO-Determining earth’s age and history.LO-Describe in writing how we determine the age of the earth

How Old Is The Earth?Scientists estimate that the earth is about 4.567 billion years old.

The dating of the earth is based on evidence found in layers of rock in the earths crust and from meteorites.

The events in Earth’s geologic history has been determined through a combination of fossil evidence and absolute and relative dating techniques.

Relative V.S Absolute Time

• Relative age - Relative means that we can determine if something is younger than or older than something else. Relative time does not tell how old something is, all we know is the sequence of events.

• For example: the sandstone in this area is older than the limestone.

• Absolute age- Absolute age means that we can more or less precisely assign a number (in years, minutes, seconds, or some other units of time) to the amount of time that has passed. Thus we can say how old something is.

• For example: The sandstone is 300 million years old.

Relative vs. Absolute Dating

Absolute Dating is determining an exact age of something. This is done primarily with a technique known as radiometric dating.Radiometric dating – Radioactivity discovered in 1896.– By 1905 the first time radioactive dating attempted.

• By measuring the amount of a known radioactive element left in a rock, and knowing the rate of decay of that element, we can tell how old it is

RELATIVE DATING

Relative Dating – is the science determining the relative order of rock layers and past events.

- By using a series of principles (laws) the relative age of rock layers and order geologic events can be determined.- can only determine the

sequential order in which a series of events occurred, not when they occur

The Earth’s Crust is made up of layers of rock called Strata

The Law of Uniformitarianism

The current geologic forces and processes are the same processes that were at work in the past.These processes include:

-erosion and deposition- volcanism- plate tectonics

The Law of SuperpositionIn an undisturbed sequence of rocks that is deposited in layers: • the youngest layer is on top and the

oldest on bottom• each layer is younger than the one

beneath it and older than the one above it.

For example- The Grand canyon

The Law of Superposition

The Law of Superposition

Law of Original HorizontalityThe Law of Original Horizontality:Most sedimentary rock layers, when originally formed, were laid down horizontally.

If rock layers are not horizontal, we know that some geologic process changed them and that this process occurred after the layers were originally laid down. Processes include:folding and tilting.

The Law of Original Horizontality

The Law of Cross-Cutting RelationshipsThe Law of Cross-Cutting Relationships states that a fault or body of rock is younger than any other body of rock that it cuts through.

Things that cut through rock layers are:

1) Faults2) Igneous Intrusions

List these in order from oldest to youngest.