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 World War II 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 Beringers Fossils

Beringers 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 QuaternaryLatin, fourth1822 TertiaryLatin, third1760 CretaceousLatin creta, chalk1822 JurassicJura Mountains, Switzerland1795 TriassicLatin, three-fold1834 PermianPerm, Russia1841 CarboniferousCarbon-bearing1822 DevonianDevonshire, England1840 SilurianSilures, a pre-Roman tribe1835 OrdovicianOrdovices, a pre-Roman tribe1879 CambrianLatin Cambria, Wales1835

Absolute Ages: Early Attempts The Bible Add up Dates in Bible Get an Age of B.C. For Earth John Lightfoot and Bishop Ussher 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 Yr. Primordial K-Ar (K-40) 1.25 B.Y. Rb-Sr (Rb-87) 48.8 by U 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: 150,000,000 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