GEOLOGIC TIME

The Age of the Earth…

How old is it?

History of the World (Part I)

• Historical Geology tries to answer questions about the earth’s past:– How old is the earth?– How did great rock

layers in places like The Grand Canyon form?

– What are fossils, how do we find them, and where did they come from?

• Historical Geology also tries to unravel the mysteries of issues like: – How do geologists

piece together past geologic events in the correct order?

– How can we date or “time stamp” geologic events using radio-metric dating tools?

Volumes upon Volumes• Think of Earth’s past as

one giant, complicated history book that has already been written

• Geologists are trying to piece together all the volumes and chapters that have been written throughout history

• They do this by studying old rocks all around the world and finding out which chapters they belong in

• This is certainly not a very easy task

The strata of these rocks found within Canyonlands National Park Utah reveal the past by speaking to a geologist layer by layer

These layers are much like pages in a history textbook

The History Book of The Earth• Some information from previous

“pages” has been damaged or smudged

• Many pages and even chapters, especially from earlier volumes, are completely missing

• These missing details can make the task of piecing together earth’s history very difficult

• And yet, there is enough of the original story left behind and preserved that geologists have been able to piece together and decipher a relatively reliable story of the earth’s history

• This reconstructed history of the earth is called The Geologic Time Scale

The above panoramic of Monument Valley is a testament to the vast amounts of time that geologic history covers

Notice the horizontal strata supporting the towering monuments of sandstone

Is it Young or is it Old?

• The general accepted age of the earth has changed throughout recorded history– First estimates were that the earth was very young: In

the mid 1600s, Archbishop Ussher used Old Testament chronologies and calculated the formation of the earth to have occurred in 4004 B.C. (~6000 yrs old)

– In the 1700s, a French scientist calculated the age based upon the rate of cooling of the molten iron core (~80,000 yrs old)

– In the 1890s, an Irish scientist calculated the age based upon how long it would take the earth’s oceans to become as salty as they now are (~90,000,000 yrs old)

We’re getting Older!!!• Only 100 years ago, the science of chemistry was

completely revolutionized – Between 1898 & 1903 Marie Curie and her husband did

amazing new research which, for all intensive purposes, discovered the essence of radioactivity

• They discovered two new elements (Polonium & Radium)• They won the Nobel Prize in 1903

– Lord Rutherford shortly thereafter discovered half-life decay rates which would eventually lead to radiometric dating techniques of rocks, fossils, and meteorites (~4,600,000,000 yrs old)

– The current accepted age of the earth stands at a very well rounded 4.6 billion years

It’s a Freaking Disaster• How do the physical features

of the earth’s surface change over geologic time?

• Early theories in 1600s-1700s argued for an idea called catastrophism– A theory that earth’s features

have been formed by great disasters in very short time spans

– These major catastrophic events no longer occur

• Examples: worldwide floods, earthquakes, volcanoes, & landslides shaped the earth in only hundreds of years

“The Present is the Key to the Past”• Around 1790 James Hutton

formulated the idea of uniformitarianism– The geologic process that we

see going on today, have always held true throughout earth’s history

– These processes have shaped & changed the features of the earth over incredibly long time spans (millions of years)

• To understand ancient rocks, we must first understand present-day processes and their results

• Uniformitarianism is in stark contrast to catastrophism

Old Rocks…How Old?

• When it comes to finding the age of something geologically, there are two ways to do it:– Absolute date – this will give

you an actual definite age before the present date

• Years before present

– Relative date – this will only determine what sequence things happened in, not how old they are

• The order of formation

The rocks of Stonehenge stand as an ancient memorial to a civilization long gone

Nobody knows exactly when it was constructed

Relative Dating…3 Rules

• Around 1650, a Danish scientist named Nicolaus Steno recognized and formulated three simple, yet powerful, rules for the sequences of events for geologic formation– Law of Superposition– Principle of Original

Horizontality– Principle of Cross-Cutting

Relationships

Relative Dating

• Law of Superposition

In any sequence of undeformed sedimentary rocks, the oldest rock is always at the bottom and the youngest is at the top

• Therefore, each layer of rock represents an interval of time that is more recent than that of the underlying rocks

At Arizona’s Grand Canyon, it’s easy At Arizona’s Grand Canyon, it’s easy to imagine that the bottom layers were to imagine that the bottom layers were deposited before the upper layersdeposited before the upper layers

Relative Dating• Principle of Original

Horizontality – Sediment, when deposited, forms nearly horizontal layers because of gravity. Therefore, if we observe beds of sedimentary strata that are folded or tilted at a steep angle, we can assume that some deforming force took place after the sediment was deposited

All sedimentary layers are deposited All sedimentary layers are deposited flat due to gravity and settlingflat due to gravity and settling

When we see rock layers like these in When we see rock layers like these in Crete that have been drastically folded, Crete that have been drastically folded, we know that they must have been we know that they must have been moved into that position by tectonic moved into that position by tectonic forces forces afterafter they were deposited they were deposited

Relative Dating• Principle of Cross-Cutting

Relationships

Whenever a fault or an intrusive igneous rock cuts through existing sedimentary rocks, it must be younger than the structure it cuts

Intrusion – Igneous rock that cut Intrusion – Igneous rock that cut through sedimentary layers. Ex. through sedimentary layers. Ex. Dikes, Batholith, etc.Dikes, Batholith, etc.

Fault – break in the rock layersFault – break in the rock layers

Types of Unconformities• When we observe rock

layers that have been deposited without any kind of interruption, we call those layers conformable

• No location on earth has a complete and continuous set of rock strata without breaks

• Unconformities represent a long period during which deposition stopped, erosion removed previously formed rocks, and then deposition resumed

There are three distinct types of unconformities

Disconformity

Angular Unconformity

Nonconformity

Unconformities cont’d

• Angular unconformitiesA Deposition of the

original bottom sedimentary layers stops

B They are tiltedC They are erodedD Then finally they

are blanketed by new horizontal strata

Notice the 9 steps shown in the bottom diagram D

Winston-you’d better conform

• NonconformitesA Igneous intrusions

cut their way up into previously deposited rocks

B Erosion cuts down to the igneous rock

C New sedimentary layers blanket the eroded igneous rocks

Note: these can also form with sediments covering older metamorphic rocks that have been exposed

Relative Dating Practice• Layers 1 were

deposited• Layers 1 were tilted

(no other layers have been tilted)

• Angular Unconformity A

• Layers 2 were deposited

• Disconformity B • Layers 3 deposited• Igneous Intrusion B

(it cuts through layers 1,2,& 3 only)

• Disconformity C • Layers 4 were

deposited• Volcano A erupts

A

B

C

Practice – What is the sequence?

Explain what formed 1st, 2nd 3rd, etc…

And explain why you know it is in that order-- that is, explain what geologic rule determines the order for each layer

You find the sequence…1st ________

2nd ________

3rd ________

4th ________

5th ________

6th ________

7th ________

8th ________

9th ________

10th _______

11th _______

12th _______

13th _______

14th _______

15th _______

Layer 2

Layer 1

Layer 3

Layer 4

Layer 5

Layer 6

Layer 10Layer 9

Layer 8

Layer 7

History• Shale – An ocean covered the area and clay

was deposited forming shale (color)• Folding/Tilting– Stress causes the rock to

fold or tilt.• Fault – Stress causes the rock to break

forming a fault.• Intrusion/Dike – Igneous activity forms a rock

intrusion through layers.• Sandstone – Sand covers the area and forms

sandstone.

• Unconformity – Weathering and erosion caused an unconformity where.

• Limestone – An ocean covers the area and calcite deposits form limestone.

• Conglomerate – An ocean covers the area and deposits gravel which forms conglomerate.

More practice

To Review:• Earth’s history can be

thought of as a large, multi-volume history book

• Geologists try to piece together this history book by looking at rocks and correlating their ages

• The Geologic Time Scale is the geologic history of the earth in calendar form

• Early theories about the age of the earth were based upon short time spans and catastrophic changes

• James Hutton came up with idea of uniformitarianism

• Currently accepted models are based upon incredibly long time frames and slow repeating processes

• Geologic dating can be done by absolute dating or by relative dating

• Nicolaus Steno came up with the three laws to help with relative dating

• Unconformities represent breaks in the rock record