Geologic Time and Earth History

Two Conceptions of Earth History: Catastrophism Assumption: Great Effects Require Great Causes Earth History Dominated by Violent Events Catastrophes Do Happen But Are Uncommon Uniformitarianism Earth History Dominated by Small-scale Events Typical of the Present. Such events as erosion and deposition

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 North America Formed Before The Glaciers Came Absolute - Know Dates Civil War 1861-1865 World War II 1939-1945 Glaciers Left N.A. About 11,000 Years Ago

Superposition: Mindoro Cut, Wisconsin

Remains of Ancient Plants And Animals, Evidence of Life 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

Natural or Sculpture?

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

CORRELATION The process of matching up rock layers from different locations.

Correlation

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

The Fundamental Rule of Absolute Ages The Earth is older than everything on or in it -Except its atoms -All ages are minimum ages

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.

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.