Geological TimeThe Present is the Key to the Past

Interpreting Geologic History

• The Earth was thought to have formed about 4.6 billion years ago.

• Geologists try to put the pieces of the Geologic past back together using several methods.

Interpreting Geologic History

• Uniformitarianism• Original

Horizontality• Law of

Superposition• Intrusions and

Extrusions• Folds and Faults• Unconformities

Interpreting Geologic History

• Uniformitarianism – The processes we see today are similar to those that have been taking place throughout geologic time.

“The present is the key to the past.”• Original Horizontality – Rock is

always older than the process that changed it. Ex. Tilting, folding, faulting. Layers were always flat first.

Law of Superposition

• Rocks found at the bottom of an undisturbed exposure are usually the oldest.

D, K, A, G

Intrusions and Extrusions

• Melted Rock can often rise and cut through sedimentary layers.

• If the magma solidifies in the earth it will form an igneous intrusion surrounded by contact metamorphism.

• If the magma reaches the surface (Lava) It can form an igneous extrusion.

Intrusions and Extrusions

Intrusion

Intrusions and Extrusions

R, Y, C, Z, J

An Intrusion is probably going to be Granite or Gabbro

Intrusions and Extrusions

Extrusion

Intrusions and Extrusions

R, Y, C, Z, J

An Extrusion is probably going to be Basalt or Rhyolite

Intrusions and Extrusions

R, Y, C, Z, J, M

A Buried Extrusion Is Still An Extrusion

Intrusions and Extrusions

Faults and Folds

• Faults and Folds are a result of the Earth’s shifting tectonic plates.

• Folds are Bends in the rock layers.• Folds are the EXCEPTION to the law

of superposition.• Faults are cracks where layers are

offset from each other.• REMEMBER: rocks must be older then

the process that alters them.

Folds

Lets add some heat and pressure

Folds

Lets Convert the colors to Symbols

Folds

Lets add some more heat and pressure

Folds

If a Fold is so extreme that it bends back over itself it is called an Overturn

Lets take a closer look

Folds

Young

old

Young

Folds

Folds

Faults

R, Y, C, Z, J, M, K

Faults

Faults

Unconformities

• An Unconformity is a buried erosional surface.

• They happen when an area of bedrock is weathered and eroded and then buried again by another layer of sedimentary rock.

• This can happen to any kind of rock; sedimentary, igneous, metamorphic, folded, tilted or faulted.

Unconformities

Erosional Agent

Unconformities

Unconformity

Remember!! An unconformity is not an unconformity until it is buried!!

R, Y, C, Z, J, H, P

Let’s Try Some Relative Age Dating

____ Young____________________________________________________ Old

Geologic History

• The Geologic Timeline is divided up into different units of time.

• The largest unit is the Eon, often made up of billions of years.

• Each Eon is divided up into Eras.• Each Era is made up of Periods.• Each Period is made up of Epochs.

Geologic History

• In your ESRT you have a chart that has a timeline of geologic history.

• This chart primarily focuses on New York State.

• It is very important that we understand how to read this chart.

• It includes information about rock type, life-forms, fossils, geologic events and continental layouts throughout geologic history.

Geologic History

Geologic History

Let’s try a sample question

During what Period did the earliest birds appear?

Geologic History

Jurassic

Radioactive Decay

Radioactive Decay

• Radioactive Decay refers to the breakdown of unstable elements into stable elements.

Unstable element

Stable Element

Radioactive Decay

• The rate at which these unstable (radioactive) elements decay is predictable and constant.

• This rate can enable us to perform Absolute Dating.

Absolute Dating

• Where Relative Age Dating can only give us the age of material as it relates to other material… (which came first, last, before etc…)

• Absolute Dating can give us a measurable value for the age of material.

• This is because of that predictable decay rate called Half-Life.

Half-Life

• Half-Life is defined as the amount of time it takes for ½ of an unstable (radioactive) element to decay into a stable (daughter/decay) element.

• Only half of the radioactive material will decay per each Half-Life.

• This means that there can never be 0% radioactive material remaining or 100% decay material created.

Half-Life

Radioactive

Stable

Here we start with 100% Radioactive Material

Half-Life

Radioactive

Stable

After 1 half-life 50% of the radioactive material has decayed into stable material

Let’s see what happens after the second Half-Life

50% 50%

Half-Life

Radioactive

Stable

After 2 Half-Life’s the remaining radioactive material has now decayed half way. Only 25% of

the original is remaining.

25%

75%

…and a third

Half-Life

Radioactive

Stable

After a 3rd Half-Life there is only 12.5% of the original radioactive material remaining.

12.5%

87.5%

Half-Life

The chart for any Radioactive Material will look like this.