Geologic Time. Geologic time scale How geologists explain Earth’s rock record Neptunism: all rocks were precipitated from an ocean (1780’s) –Werner (p.

Geologic Time

Geologic time scale

How geologists explain Earth’s rock record

• Neptunism: all rocks were precipitated from an ocean (1780’s)– Werner (p. 6)– In line with biblical flood

• Appeared to agree with rock layering– Problems:

• Where did the water go?• What about volcanoes?

• Too simple!

How geologists explain Earth’s rock record

• Catastrophism (1800’s)-Ch.1

• All of Earth’s history resulted from sudden, widespread catastrophes

• 6 major catastrophes

• Last catastrophe was the flood

• Problem: need more than 6 events

How geologists explain Earth’s rock record

• Uniformitarianism (1800’s)-in Ch. 1– James Hutton

– Charles Lyell

• Present is the key to the past • Processes occurring today have occurred throughout

geologic history AT THE SAME RATES• Can’t explain all rocks found on earth

– Rocks formed in conditions that no longer exist

– Conditions still exist, but at great depth

– Conditions exist, but not observable over human lifetime

Relative dating

• Establishing relative sequence of events– Based on uniformitarianism– Doesn’t put a specific age on event

• This is what your homework is!

Applying Uniformitarianism to rocks• Horizontal sedimentary rocks

• Which rock units are older?

Principle of superposition• With horizontal sed rocks, the oldest are on

the bottom

Near vertical Sedimentary Rocks• Were these sed rocks

deposited like this?

Priciple of lateral continuity• A water-laid layer of

sediment, at the time it was formed, must continue laterally in all directions until it thins out as the result of non-deposition or until it abuts against the edge of the original basin of deposition.

Principle of original horizontality• Sediments are deposited

nearly horizontal. If they aren’t today, they must have been moved or deformed

Faulted Rocks• Which is younger, rocks or fault?

Cross-cutting relationships• Which is younger, dark rock or

lighter rock? (both are igneous)

Principle of cross-cutting relationships

• An igneous intrusion or fault must be younger than the rocks it intrudes/displaces

Principle of inclusions• Fragment of rock

contained in another unit of rock is older than rock that surrounds fragment

Principle of Inclusions

Which rock is oldest. How do you know?

Principal of Superposition

Original Horizontality

Principle of Original Horizontality

Principal of Cross-cutting relationships

Rock ID Quiz Wed. 30 JanLook at your rock and answer the following

1.Describe your rock. Include the texture, color and composition of your rock. (4pts)

2.Is your rock igneous, sedimentary or metamorphic? (3pts)

3.What is the name of your rock? (3pts)

Bonus (2pts):

What is a mineral (you must have all characteristics to get the points)?

Stratified sedimentary rocks

• Stratigraphy: study of layered rocks

Bedding planes in sed rocks

• Layers separated by surfaces called bedding planes

Relative dating and stratigraphy

• Which is younger?

Disturbed (deformed) sed rocks

• Which is younger? Which way is up?

Relative dating and stratigraphy

• Which way is up?

Principles of relative dating

• Assumes constant deposition of rocks

• What if not constant?

• What if rocks removed from record?

• Unconformity: break in geologic time

Unconformities

• Missing time in the rock record

• Caused by…what?

• 3 kinds– Disconformity– Angular unconformity– Nonconformity

• Relative dating and stratigraphy

Erosion and unconformities

• How do unconformities occur?

• Erosion

• Occurs when rocks are above sea level…why is this??

Disconformity• Erosional surface

is in between parallel beds

Disconformity

Angular unconformity

• Tilted beds overlain by younger, horizontal beds

Angular unconformity

• Titled beds overlain by younger horizontal beds

Nonconformity

o Bedded rocks rest on an eroded surface of crystalline rocks (what are those types of rocks?)

Nonconformity

Types of unconformities

Relative dating/unconformities

Sedimentary environments

• How are different types of rock deposited at the SAME time?

• Think of a modern day ocean environment…

Sedimentary environments

Sedimentary environments

• Sea level rising, shoreline moves inland transgression

• Sea level falling, shoreline moves off land regression

• What can cause changes in sea level?

Sea level rising: transgression

Transgression

How to recognize a transgression

How to recognize a transgression

Sea level dropping: regression

Marine regression

How to recognize a marine regression

Applying stratigraphy and relative dating

Early attempts to date the Earth

• St. Augustine of Hippo ( A.D. 354-430)

• Established idea of A.D. and B.C.

Archbishop Ussher in 1600’s

• Counted geneologies in the bible back to Adam and Eve

• Earth formed: October 22, 4004 B.C.

Buffon in 1700’s

• Assumed Earth started molten and cooled to today

• He measure the cooling rate of melted balls of iron

• Earth ~ 75,000 years old

• Problems:– Assumes no new source of heat

Other techniques to estimate age:

• Deposition rates of sediments

• Ages range from less than 1 million to more than 2 billion

• Problems: – Assumes constant rates of deposition– Doesn’t take erosion into account

Other techniques to estimate age:

• Salt content in oceans

• Age of earth ~ 90 million years

• Problems:– Assumes oceans start as fresh water– Assumes all salt comes from rivers– Assumes no salt is removed from oceans

Lord Kelvin’s age of Earth (1866)

• Returned to cooling Earth calculations– Earth was hot when first formed, cooling ever

since

• Earth looses heat through time

• Age: 20 million to 40 million years old

• Problem: – Too young to account for uniformitarian

interpretation

• Discovery of radioactivity defeated this idea

Where does this leave us?

• Earth is old with slow processes

• Earth is younger than 400 million with fast processes

• Discovery of radioactivity changes things

• Radioactivity– Decay of elements through time– Major heat source for Earth

Modern uniformitarianism

• Less literal uniformitarianism

• Processes are the same, rates change

• Uniformitarianism and catastrophism combined

• Occasional catastrophic events do happen

• Earth is 4.6 billion years old

Help with relative dating exercise

• Geologic time/relative dating

• Picture with solution