Uniformitarianism

Laws of nature DON’T change with time

Past events explained & estimated by modern processes

“The Present is the Key to the Past”

Uniformitarianism

sedimentary rocks form in horizontal layers parallel to Earth’s surface and will remain that way unless disturbed

Original HorizontalityOriginal Horizontality

CLAY

SILT

CLAY

SAND

SILT

Horizontal Layers of the Grand Canyon

• Relative Time – determination of the age of a rock or geologic event by comparing with other rocks/geologic events

He is

Can you infer who is older?

3 Ways to Determine Relative Time………?

Coming Right Up!!

In undisturbed sedimentary rock or lava flows:

Law of Superposition

Top = youngest layer

In undisturbed sedimentary rock or lava flows

Bottom = oldest layer

Law of Cross-Cutting Relationships

Any feature that cuts across a rock or layer of sediment is younger than the rock or layer it cuts

Example of Cross-Cutting

Intrusion - igneous rock that forms when magma squeezes between existing rock and hardens

Intrusion - igneous rock that forms when magma squeezes between existing rock and hardens

Examples of Cross-Cutting Example of Cross-cutting

Extrusion - igneous rock that forms when lava flows on Earth’s surface and hardens

Example of Cross-cuttingExample of Cross-cutting

Inclusion - pieces of older rock trapped within younger rock

Examples of Cross-Cutting

Faults, joints, tilts, and veins also follow the law of cross--cutting relationships

Law of Included FragmentsSediments are older than the cement and rock they are part of

The pebble is older than the conglomerate rock it is found in.

However, there are exceptions to these laws…….

Exceptions to Law of Superposition

Rock layers can be overturned, older layers pushed on top of younger layers

THEREFORE, geologists use these clues to find the original position of rock layers…

Exceptions to Law of SuperpositionGraded Bedding - coarse, heavy particles - bottom layer

Exceptions to Law of SuperpositionRipple-Marks - tops of ripple marks point

Exceptions to Law of SuperpositionCross-Beds - curved at bottom layer, cut off at the top

Unconformity

Angular unconformity

Gaps or missing layers in the rock record due to erosion

Formation of an Unconformity

Formation of an Unconformity

Formation of an Unconformity

Formation of an Unconformity

Formation of an Unconformity

Picture of Unconformity

• Absolute Time – finding an exact date for rocks or geologic events in YBP (years before present)

Born 2004 or 1 YBP

Born 1922 or 83 YBP

How do geologists measure

Absolute Time?

1. Radioactive Decay

2. Biological Clocks - Tree rings, coral growth rings, glacial lake layers of sediment

Before you measure ABSOLUTE TIME you must understand the following terms…..

ISOTOPES

- alternate forms of an element

- some are RADIOACTIVE

Radioactive isotope- an isotope that undergoes radioactive

decay

- used to determine absolute age

Examples:

Uranium 238 and Carbon 14

Radioactivityrelease of high-energy particles from

unstable atoms, maybe cancer-causing

Radioactive Decayradioactive isotopes give off radioactive particles until they become stable isotopes (new elements)

Example:

Uranium - 238 Radioactive Isotope

Radioactive Decay

Lead-206 Stable Isotope

Amount of time it takes 1/2 the atoms in a given sample to go through radioactive decay is called it’s

Half - Life

Let’s look at the half - life of Uranium 238

???

U238 - Radioactive

Pb206 - Stable decay product

1 : 0 3/4 1/2 : 1/2

1/4

7/8

1/8

15/16

1/16

31/32

1/32

Since the Half - Life of EverySince the Half - Life of Every Isotope is ….Isotope is ….

1. Unique1. Unique2. Always the same time2. Always the same time3. Unaffected by:3. Unaffected by:

Mass, Volume, Mass, Volume, Temperature & PressureTemperature & Pressure

We can use it to find:We can use it to find:

TTHHE E

AABBSSOOLLUUTTEE

AAGGEE

OOFF

RROOCCKKSS!!!!!!

Uranium 238Uranium 238 Lead 206 Lead 206

Half-Lives of Half-Lives of Special IsotopesSpecial Isotopes

4.5 x 104.5 x 109 9 yearsyears

(4,500,000,000 years)(4,500,000,000 years)

Carbon 14Carbon 14 Nitrogen Nitrogen 14 14 5.7 x 105.7 x 103 3 yearsyears

(5,700 years)(5,700 years)

Unknown Radioactive Isotope half - life = 3000 years sample size = 200 g Let’s complete the table below.

Half Life Remaining Unknown Radioactive Isotope (g)

Number of Years

0 200 0

1

2

3

4

5

25

50

12.5

6.25

6000

9000

12,000

15,000

100 3000

DECAY TIME (YEARS)

MA

SS

OF

UN

KO

WN

(G)

3000

6000

9000

12000

15000

NOW LET’S GRAPH THE HALF-LIFE OF THE UNKNOWN

0

1 Half - L

ife

2 H - L 3

4

200

100

0

0 Half - L

ives

Carbon14 half - life = 5,700 years sample size = 800 g Complete the following table.

Half Life Remaining Radioactive C14 (g)

Number of Years

0 800 0

1

2

3

4

5

100

200

50

25

11,400

17,100

22,800

28,500

400 5,700

DECAY TIME (YEARS)

MA

SS

OF

C14 (G

)

5,700

11,400

17,100

22,800

28,500

NOW LET’S GRAPH THE HALF-LIFE OF CARBON 14

00 H

alf - Lives

1 Half - L

ife 2

3

4

800

400

Absolute Time vs. Relative Time

Most geologists use relative time because it’s

- less expensive

- easier to do

Absolute Dating & Geologic History

QuickTime™ and aSorenson Video 3 decompressorare needed to see this picture.

Fossils

-the remains of plants and animals that lived in the past

- mainly formed in sedimentary rock

Fossils Form in Several WaysForm in several ways

1. Original remains - unchanged remains of a plant or animal

Ex. Dinosaur bones/teeth

2. Replaced remains - soft parts of original animal replaced by minerals

Ex. Petrified wood

Fossils Form in Several Ways

3. Mold - fossilized shell or bone dissolves and leaves a hollow depression in a rock

Ex. Ferns, leaves, or fish

4. Cast - new mineral material fills a mold

Ex. Shellfish

Fossils form in Several Ways

5. Trace fossils - fossil evidence of animal movement

Ex. Trails, footprints, burrows & borings

Evolution- process of change that produces

new life forms over time

- fossils provide evidence of evolution

Natural Selection-theory of evolution

-best adapted organisms will survive in large numbers and pass on these adaptations to their offspring

Index Fossils

- special fossils that give the relative age of the rocks that contain them

Index fossils must:1. Be recognizable2. Widespread3. Exist for a short time

Key Bed- single rock layer that is recognizable, widespread, and exists for a short time

Correlation- matching rock layers from one area to another

This can be done in several ways:1. “Walking the outcrop”2. Matching similar rock features3. Index fossils

Correlating or matching index fossils from three outcrops.

Correlate the 3 rock sequences to make a single rock column for the geologic history of the region. (oldest on bottom)

Oldest

Youngest

No place on the earth has all the rocks formed through time. So geologists use correlation to

make a single rock column that shows the entire history of the earth.

• Correlation means “to match”• Geologists match all of the following:

– rock types– rock sequences– ash layers from volcanic eruptions– index fossils

Page 8 & 9 of the Reference Tables.

Each black vertical bar represent the existence of a group of organisms.

Extinction

First appearance

DECAY TIME (YEARS)

MA

SS

OF

UN

KN

OW

N (G

)

DECAY TIME (YEARS)

MA

SS

OF

C14 (G

)

Isotopes - different forms of the same element - normal # of protons - more than normal # of neutrons

Normal Hydrogen

-1 proton

- 0 neutrons

- 1 electronIsotope of Hydrogen

- 1 proton

- 1 neutron

- 1 electron

Radioactive Isotope of Hydrogen

1 proton

2 neutrons

1 electron

Radioactive Hydrogen H-3