Geologic Time and Earth History Two Conceptions of

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 Ages Relative - 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 Occur Almost 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

Correlation

The Geologic Time Scale Quaternary 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 Silurian Devonshire, England Silures, a pre-Roman tribe 1840 1835 Ordovician Ordovices, a pre-Roman tribe 1879 Cambrian Latin Cambria, “Wales” 1835

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 19 th 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 Rates Cutting of Grand Canyon • 2 km/3 m. y. = 1 cm/15 yr Uplift 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 Rates Movement 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 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