Geologic Time and Earth History

Geologic Time and

Earth History

Two Conceptions of Earth History:

Catastrophism• Assumption: Great Effects Require Great Causes • Earth History Dominated by Violent Events

Uniformitarianism• Assumption: We Can Use Cause And Effect to

Determine Causes of Past Events • Finding: Earth History Dominated by Small-scale

Events Typical of the Present. • Catastrophes Do Happen But Are Uncommon

Uniformitarianism Continuity of Cause and Effect

• Apply Cause and Effect to Future - Prediction

• Apply Cause and Effect to Present - Technology

• Apply Cause and Effect to Past – Uniformitarianism

The Present is the Key to the Past

Ripple Marks, Bay Beach

Fossil Ripple Marks, Baraboo Range

Modern Mud Cracks

Fossil Mud Cracks, Virginia

Two Kinds of AgesRelative - Know Order of Events But Not

Dates• Civil War Happened Before W.W.II • Bedrock in Wisconsin Formed Before The

Glaciers Came Absolute - Know Dates• Civil War 1861-1865 • World War II 1939-1945 • Glaciers Left Wisconsin About 11,000 Years

Ago

Superposition:Mindoro Cut,

Wisconsin

Geologic Map

Fossils

Remains of Ancient Plants And Animals, Evidence of Life

Commonly Preserved:

Hard Parts of Organisms:

• Bones

• Shells

• Hard Parts of Insects

• Woody Material

Rarely Preserved

Soft or Easily Decayed Parts of Organisms:

• Internal Organs

• Skin

• Hair

• Feathers

Types of Fossils

• Original Material

• Casts & Molds

• Replacement (Petrified Wood)

• Carbonized Films (Leaves)

• Footprints, Tracks, Etc. – “Trace Fossils” – Our only preserved record of

behavior of fossil organisms

Dinosaur Tracks, Texas

Rubbing Rock?

Wisconsin

Rubbing Rock?

California

Pseudofossils

Look Like Fossils But Aren't

• Dendrites

• Concretions

Pseudofossils

Natural or Sculpture?

Johannes Beringer’s “Fossils”

Beringer’s Book

Where Fossils OccurAlmost Exclusively in Sedimentary Rocks• Heat of Melting or Metamorphism Would Destroy

Almost Every Type of Fossil • Rare Exceptions:

– Some Fossils in Low-grade Metamorphic Rocks

– Trees Buried by Lava Flow

To Be Preserved, Organisms Have to Be:• Buried Rapidly After Death • Preserved From Decay

Fossil Tree in Lava Flow, Hawaii

Good Index Fossils

• Abundant

• Widely-distributed (Global Preferred)

• Short-lived or Rapidly Changing

CorrelationCorrelation

The Geologic Time ScaleQuaternary Latin, “fourth” 1822

Tertiary Latin, “third” 1760

Cretaceous Latin creta, “chalk” 1822

Jurassic Jura Mountains, Switzerland 1795

Triassic Latin, “three-fold” 1834

Permian Perm, Russia 1841

Carboniferous Carbon-bearing 1822

Devonian Devonshire, England 1840

Silurian Silures, a pre-Roman tribe 1835

Ordovician Ordovices, a pre-Roman tribe 1879

Cambrian Latin Cambria, “Wales” 1835

Absolute Ages: Early Attempts

The Bible

• Add up Dates in Bible

• Get an Age of 4000-6000 B.C. For Earth

• John Lightfoot and Bishop Ussher - 4004 B.C. (1584)

• Too Short

Absolute Ages: Early Attempts

Salt in Ocean• If we know rate salt is added, and how much salt

is in ocean, can find age of oceans.

Sediment Thickness• Add up thickest sediments for each period,

estimate rate.

Both methods gave age of about 100 million years• Problem: Rates Variable

Age of The Sun• If sun gets its heat from burning or other chemical

reactions, could only last 10,000 years or so. • Best 19th century guess: sun was slowly

contracting. • Problem: only 30 million years ago, sun would

have extended out to earth's orbit! • Geologists wanted more time, but you can't fight

the laws of physics... • Sun actually gets its energy from nuclear reactions

and can keep going for billions of years • The Geologists were right after all. Go Team.

The Fundamental Rule of Absolute Ages

The Earth is older than everything on or in it

-Except its atoms

-All ages are minimum ages

Radiometric Dating: Half-Life

Present Radiometric Dating Methods

Cosmogenic

• C-14 5700 Yr.

Primordial

• K-Ar (K-40) 1.25 B.Y.

• Rb-Sr (Rb-87) 48.8 by

• U-235 704 M.Y.

The Geologic

Time Scale

Some Geologic RatesCutting of Grand Canyon• 2 km/3 m.y. = 1 cm/15 yrUplift of Alps• 5 km/10 m.y. = 1 cm/20 yr.Opening of Atlantic• 5000 km/180 m.y. = 2.8 cm/yr.Uplift of White Mtns. (N.H.) Granites• 8 km/150 m.y. = 1 cm/190 yr.

Some Geologic RatesMovement of San Andreas Fault

• 5 cm/yr = 7 m/140 yr.

Growth of Mt. St. Helens

• 3 km/30,000 yr = 10 cm/yr.

Deposition of Niagara Dolomite

• 100 m/ 1 m.y.? = 1 cm/100 yr.

1 Second = 1 Year• 35 minutes to birth of Christ

• 1 hour+ to pyramids

• 3 hours to retreat of glaciers from Wisconsin

• 12 days = 1 million years

• 2 years to extinction of dinosaurs

• 14 years to age of Niagara Escarpment

• 31 years = 1 billion years

Were The Dinosaurs Failures?Dinosaurs: 150,000,000 years

Recorded History: 5000 years

• For every year of recorded history, the dinosaurs had 30,000 years

• For every day of recorded history, the dinosaurs had 82 years

• For every minute of recorded history, the dinosaurs had three weeks