W12L1-GeologicTimeDay1.pdf

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GEOLOGIC TIME DAY 1RELATIVE AGE DATINGMARSHAK CHP 12

Fall 2014 W12L1


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Geologic Time

! TPS: How old is Earth? How do we know how old it is?

Numeric

Age

Dating


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Geologic Time

! TPS: How did this form? Which layers formed first?Which formed later?

Relative

Age

Dating


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Relative vs. Numerical

! There are two ways of

dating geological materials

! Relative ages assign

an order to events

! Numerical ages assign

exact dates to events


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Geologic Time

! In your everyday life

!

What do you consider old?

! How do you measure time?

! What are other ways to measure time? In other

words how do we see time pass?


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Geologic Time

! James Hutton (172697), the father of modern geology

!

First to articulate Principle of Uniformitarianism

" Same natural laws and processes that operate in the universe now

have always operated in the universe in the past and applyeverywhere


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Geologic Time

! James Hutton - The present is the key to the past" Processes seen today are the same as those of the past

" Geologic change is slow; large changes require a long time

"

Therefore, there must have been a long time before humans


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Physical Principles

! Sir Charles Lyell wrote Principles of Geology in 183033! Laid out a set of principles for deciphering Earth history

!

Used to establish relative ages of Earth materials! The Principle of Uniformitarianism"

Processes observed today were the same in the past

" Mudcracks in old sediments formed like mudcracks today


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Physical Principles

! The principle of uniformitarianism

!

Processes observed today were the same in the past

" We can observe lava flowing and cooling into solid rock

"

Ancient lava flows are the products of volcanic eruptions


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Physical Principles

! The principle of original horizontality

!

Sediments settle out of a fluid by gravity

! This causes sediments to accumulate horizontally

! Sediment accumulation is not favored on a slope

!

Hence, tilted sedimentary rocks must be deformed


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Physical Principles

! The principle of superposition!

In an undeformed sequence of layered rocks:" Each bed is older than the one above, and

"

Younger than the one below! Younger strata are on top; older strata below


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Physical Principles

! The principle of lateral continuity

! Strata often form laterally extensive horizontal sheets

! Subsequent erosion dissects once continuous layers

! Flat-lying rock layers are unlikely to have been disturbed.


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Physical Principles

! The principle of cross-cutting relations

!

Younger features truncate (cut across) older features

! Faults, dikes, erosion, etc., must be younger than the material

that is faulted, intruded, or eroded! A volcano cannot intrude rocks that arent there

Similar situation#creates baked contacts


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Physical Principles

! Principle of inclusionsa rock fragment within another

!

Weathering rubble must have come from older rock

! Fragments (xenoliths) and igneous intrusion are older

! Inclusions are always older than the enclosing material


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Physical Principles

! Physical principles allow us to sort out relative age -even in complex situations


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Physical Principles

! What happened 1stin this region?

A) Granite pluton intrusion

B) Layers 1-7 folded

C) Layers 1-7 deposited

D) Faulting

E) Sill intrusion


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Physical Principles

! What happened 2ndin this region?

A) Granite pluton intrusion

B) Layers 1-7 folded

C) Layers 1-7 deposited

D) Faulting

E) Sill intrusion


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Physical Principles

! When did the Granite pluton intrude this region?

A) 3rd

B) 4th

C) 5th

D) 6th

E) 7th


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Geologic History

! A sequence of horizontal strataaccumulates!

Superposition (1 oldest)

! An igneous sill intrudes

! Inclusions of 4 and 5 in sill

confirm it is younger

! Folding, uplift, and erosion take place.! Folding occurs before intrusion


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Geologic History

! A granitic pluton intrudes the folded sediments! Granite cuts folded layers

! Heat alters intrusive contact

! Xenoliths fall into magma

! A fault cuts the granite and folded sediments

!

A basalt dike cuts across the block! The dike cools and is eroded


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The Principle of Fossil Succession

! Fossil rangethe first andlast appearance!

Each fossil has a unique range

! Range overlap narrows time

! Index fossils are diagnostic of

a particular geologic time

! Fossils correlate strata! Locally

! Regionally

! Globally


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! Fossils are often preserved in sedimentary rocks

! Fossils are time markers useful for relative age-dating

!

Fossils speak of past depositional environments

! Specific fossils are only found within a limited time range

Fig. 12.6


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! Species evolve, exist for a time, and then disappear

! The first appearance, range, and extinction used for dating

! Fossils succeed one another in a known order

! A time period is recognized by its fossil content


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Unconformities

! An unconformity is a time gap in the rock record, from:!

Nondeposition

!

Erosion

! There are three types of unconformity

! Angular unconformity! Nonconformity

! Disconformity

Fig. 12.8b


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Angular Unconformity

! An angular unconformity represents a huge gulf in time!

In the situation featured below:" Horizontal marine sediments deformed by orogenesis

" Mountains eroded completely away

" Renewed marine invasion

" New sediments deposited


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! Huttons Unconformity, Siccar Point, Scotland

! A common destination for geologists" Vertical beds of Ordovician sandstone

" Overlain by gently dipping Devonian redbeds

"

Missing time: 50 million years

Fig. 12.8a


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Unconformities

! Nonconformityigneous/metamorphic rocks capped by

sedimentary rocks

! Crystalline igneous/metamorphic rocks were exposed by erosion

! Sediment was deposited on this eroded surface

Granite

Sandstone


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Unconformities

! Disconformityparallel strata bounding nondeposition!

Due to an interruption in sedimentation" Pause in deposition

" Sea level falls, then rises

" Erosion

! Sometimes hard to see

Fig. 12.9c,d


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Which rock layer is older

than 600 million years?

A) Tapeats Sandstone

B)

Redwall Limestone

C)

Dox Siltstone

D) Zoraster Granite

Handout Part I


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Handout Part I

What type of unconformity

is #1?

A) Disconformity

B)


C)

Non-conformity1

2


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Handout Part I

What type of unconformity

is #2?

A) Disconformity

B)


C)

Non-conformity

2

1

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W12L1-GeologicTimeDay1.pdf