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Reading the Rock Record
Ch 23.1 in text
Also 21.6

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Rock Layers

Sediments and lava flows form distinct layers as they are deposited
A sedimentary layer that can be distinguished from the layer above and below is called a stratum (pl. strata)

http://www.joshushund.com/terragen_01-06/wall_of_strata.jpg

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What Information do Strata Provide to a Geologist?

To answer this question we need to look at some geologic principles. Text has six.

#1 Sedimentary deposits or lava flows generally form horizontal layers.

Principle of Original Horizontality

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What Information do Strata Provide to a Geologist?

#2 Unless disturbed by an unusual process, younger layers will be on top of older layers.

Principle of Superposition

youngest

oldest

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Layers 1-4 are positioned

oldest to youngest

(superposition)

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http://www.winona.edu/geology/MRW/mrwimages/Earth%20History/cross_section.jpg

Sediment deposition can be interrupted

Intrusion

Fault 6 is youngest

because it cuts

through 5

Formation 5 is called

an intrusion (typically

Of igneous rock) and

is younger

than 1-4 since it

cuts through them

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What Information do Strata Provide to a Geologist?

#3 When an intrusion of igneous rock or fault cuts across other rocks of any type the intrusion is younger than the layers that are cut across.

Principle of Cross-Cutting Relationships

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#4 When a feature contains pieces of other layers it has

to be younger than the layers it contains.

Principle of Inclusion

Notice that the

intrusion has

incorporated pieces

of other layers.

Intrusion

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Example of inclusion

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A sedimentary rock that contains (includes) pebbles. The pebbles

have to be older than the rock.

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What information do strata provide to a geologist?

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Sediments are deposited over a large area and are continuous.

Other processes like a fault or erosion can separate them later

#5 Principal of Lateral Continuity

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http://www.gc.maricopa.edu/appliedscience/gjc-nsf/images/grndcynxsect.cv.jpg

Whats the youngest?

What kind of rock is it?

Grand canyon has examples of all 5

Where are examples of inclusion and intrusion?

Superposition:

The Bright Angel

shale is younger

than the Tapeats

Sandstone but older

than the Muav

Limestone.

The Vishnu Schist

is the oldest

formation (made of

metamorphic rocks.

Wavy lines to remind

You that they come from

High pressure (squeezing)).

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One last principle: Fossils

#6 Fossils can be used to determine the approximate age of a stratum since specific living organisms lived over identified time periods. If a particular fossil is present in a layer, the layer must be the same age as other layers containing the same fossil, no matter where the layers occur.

Principle of Faunal Succession

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The 6 principles of Strata

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Original Horizontality

Superposition

Cross Cutting Relationships

Inclusion

Lateral Continuity

Faunal (fossils) Succession

General theme: the processes that have shaped our earth are the same over billions of years even though the surface looks very different

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Relative vs. Absolute Dating

Example:

You have a 20 year old younger sister

Who’s absolutely dated, who’s relatively dated?

Answer:

Your sister is absolutely dated- specific number

You are relatively dated- all we know is that you are

older than 20. You could be many years older.

Principles of strata only tell us relative dating

Relative dating: order only

Absolute dating: gives a real number for the age

Relative vs. Absolute Ages

For absolute ages, we do not use strata

 Radioactive elements are unstable and convert into other elements

Rate of conversion is known. We look at the fraction of conversion of radioactive elements in a rock sample.

Relative age: older or younger (no numbers) 

just the ordering

Absolute age: gives the actual number (65 million years ago)

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After 1

half-life has

passed, half

the atoms

have changed

or emitted

energy.

After a 2nd

half-life has

passed half of

the remaining atoms

have also changed

Radiometric Dating

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How we know how old the Earth is

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Certain types of lead only come from uranium.

If we see equal amounts of this lead and uranium, then it means

half the original uranium has decayed. So the rock is

one half life old.

Uranium has a half life of 4.5 billion years. Oldest mineral crystals

contain almost equal amounts of uranium and lead. Dated

to 4.4 billion years old.

Moon rocks are also 4.4 billion years old.

Some meteorites (chunks of rock from space) are 4.5

billion years old. This is when the planets must have

started forming.

If the rock had more lead and less uranium, it means it would maybe

have had more time for conversion  even older (not found)

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Why Aren’t All Strata Straight?

Geologic processes generate pressures that cause layers to become deformed.
Gaps from uplift
Folding (2 types) and Faulting (3 types)

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Gaps in strata: unconformities

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Angular. Older rocks got

tilted upwards at an angle.

(probably from converging

plates at some point)

Then erosion flattened the

surface.

Then the Tapeats sandstone

was deposited on top

Figure 23.5 in text is a simplified version of this

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anticline

syncline

This can push younger rock below older rock

FOLDS

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How to remember

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Anticline: Arch

Syncline: Sag

Sideling Hill on I68- Western MD

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Folds and converging plates

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Converging Plates

The Sideling Hill folds are from the formation of Pangea

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Why Aren’t All Strata Straight?

Geologic processes generate pressures that cause layers to become deformed.
The second type of deformation is faulting.

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Faults

Faults occur when sections of Earth’s surface shift position with respect to another
One side of the fault is designated as a footwall block and the other a hanging wall block

Illustrations on p. 576 of

Text are good

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How to remember hanging and foot walls

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The little guy’s feet are

on the footwall and his

lantern is hanging from

the hanging wall.

From your text- Figure 21.32

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Normal Faults

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Hanging wall moves down

NORMAL FAULT

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Normal faults occur when plates are pulled apart: diverging

lava

Blocks slide downwards as plates separate

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Reverse Faults

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Hanging wall moves upward

REVERSE FAULT

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Faults and Plate Tectonics

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How do you get this?

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Hanging wall pushed up

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Pushing up means converging plates

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Slip or Transform Faults

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Transform plates in an orchard

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The most famous slip-fault!

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Diverging plates

Normal faults

Converging plates

Reverse faults

Plates sliding past

each other. Transform

Faults (San Andreas)

Faults and Plate Tectonics: Summary

Picture of San Andreas Fault

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Near San Francisco

In the California desert E of Los Angeles

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Can you date the features from oldest to youngest?

Which are probably igneous rock? What letter

marks an angular unconformity?

http://www.winona.edu/geology/MRW/mrwimages/Earth%20History/cross_section.jpg

Ans: M, R, S: igneous

I: unconformity

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Vocabulary/Concepts Summary

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6 principles of strata: All assume that earth’s processes

are unchanged

Things that can distort and cause gaps

a. Unconformities (including igneous intrusions)

b. Deformations: faults and Folds

2 types of folds

3 types of faults

Difference between relative and absolute dating

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Some practice

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#44. If a sedimentary rock contains inclusions of metamorphic rock,

Which rock is older? Defend your answer.

#48. In a sequence of sedimentary rock layers, the youngest rock

Is found at the bottom and the oldest rock is at the top?

What does this mean?

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Practice/answers

44. Metamorphic rock is older. Inclusions are older

(don’t mix up- intrusions are younger (came late to cut through pre-existing rock. But inclusions are older- layers formed after and included older rock pieces)

48. This must’ve been due to some fold because normally

superposition means youngest at the top.

#44. If a sedimentary rock contains inclusions of metamorphic rock,

Which rock is older? Defend your answer.

#48. In a sequence of sedimentary rock layers, the youngest rock

Is found at the bottom and the oldest rock is at the top?

What does this mean?

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More practice

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#52 Suppose you see a sequence of sedimentary rock layers covered

By a basalt flow. A fault displaces the bedding (i.e. the layers) of the

Sedimentary rock, but does not cut into the basalt.

Relate the fault to the ages of the two rock types in the formation.

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More practice/answers

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#52 Suppose you see a sequence of sedimentary rock layers covered

By a basalt flow. A fault displaces the bedding (i.e. the layers) of the

Sedimentary rock, but does not cut into the basalt.

Relate the fault to the ages of the two rock types in the formation.

Sedimentary rock is the oldest.

Fault is next- came after to cut through the pre-existing rock.

Basalt is the youngest. It came after the fault, otherwise the fault

would’ve split it also

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