Geologic time the present is the key to the past

• Relative Time – determination of the age of a rock or geologic event by comparing with other rocks/geologic events Can you infer who is older?... In undisturbed sedimentary rock or la

 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 horiz ontal layers

parallel to Earth’s surface and will remain that

way unless disturbed

Original Horizontality

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

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

Law of Cross-Cutting

Relationships

Any feature that cuts across a rock or layer of

sediment is y ounger 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-cutting

Example 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 Fragments

Sediments 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

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

Angular unconformit y

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

- alternate forms of an element

- some are

release of high-energy particles from

R a d i oa c ti v e D e c a y

radioactive isotopes give off radioactive

particles until they become stable isotopes (new elements)

Example:

Uranium - 238 Radioactive Isotope

Radioactive Decay

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

Since the Half - Life of Every

We can use it to find:

Uranium 238 Lead 206

Half-Lives of Special Isotopes

4.5 x 10 9 years (4,500,000,000 years)

14 5.7 x 10 3 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

DECAY TIME (YEARS)

Carbon 14 half - life = 5,700 years sample size = 800 g Complete the

following table Half Life Radioactive C Remaining 14 (g) Number of Years

1 2

DECAY TIME (YEARS)

Absolute Time vs Relative Time

- less expensive

- easier to do

Absolute Dating & Geologic

History

QuickTime™ and a Sorenson Video 3 decompressor are needed to see this picture.

s

-the remains of plants and animals

that lived in the past

- mainly formed in sedimentary rock

Fossils Form in Several

Ways

Form 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

Fossils form in Several

n

- 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

Key Bed

- single rock layer that is

recognizable, widespread, and exists for a short time

- matching rock layers from one

area to another

This can be done in several ways:

1 “Walking the outcrop”

2 Matching similar rock features

3 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)

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

Page 8 & 9 of the Reference Tables.

Each black vertical bar represent the existence of a group of

organisms.

Extinction

DECAY TIME (YEARS)

Isotopes - different forms of the same element

- normal # of protons

- more than normal # of neutrons

Normal Hydrogen -1 proton

- 0 neutrons

- 1 electron

Radioactive Isotope of Hydrogen

1 proton

2 neutrons

1 electron Radioactive Hydrogen

H-3