ES 105Earth Systems Science II

TextbookEarth Science,

11th ed.Edward J. Tarbuck & Frederick K. Lutgens

Geologic TimeChapter 11

American Geologic Exploration

• John Wesley Powell• Colorado River

Expedition—1869• Grand Canyon is

Earth’s “Book of Revelations”

Leaves of History

Historical notes

Catastrophism• Landscape developed by catastrophes • James Ussher, mid-1600s, concluded

Earth was only a few thousand years old

Historical notes

Modern geology James Hutton • Theory of the Earth• Published in the late 1700s • “…we find no vestige of a beginning—

no prospect of an end.”

Leaves of History

Modern geology

• Uniformitarianism• Fundamental principle of geology • "The present is the key to the past”

Modern geology

However, catastrophism has its place in our understanding.

• Tsunamis• Floods• Krakatau• Meteor impacts

Relative dating

Placing rocks and events in proper sequence of formation

Principles and rules of Relative Dating

• Law of superposition –• Principle of original horizontality –• Principle of cross-cutting relationships –

Law of Superposition

• Nicklaus Steno, 1669 • Oldest rocks are on the bottom

Superposition is well illustrated by the strata in the Grand Canyon

Principle of Original Horizontality

• Sediment is deposited horizontally• Not on slanted or vertical surfaces

Principle of Cross-cutting Relationships

• Younger feature cuts through an older feature – Something must exist first to be cut by

another thing• The ‘things’ cutting may be ‘things’, such

as igneous intrusions• Or they may be events, like fault breaks,

folding, or erosion periods

Cross-cutting relationships

Inclusions

• One rock contained within another • Rock containing the inclusions is

younger

How inclusions form

Formation of Inclusions

Now the fragments are included

Unconformities

• An unconformity is a break in the rock record

• Three types of unconformities

Three types of unconformities

• Angular unconformity –• Disconformity –• Nonconformity –

Angular Unconformity• tilted rocks are overlain by flat-lying

rocks

Formation of an

angular unconformity

Nonconformity

• Metamorphic or igneous rocks below the noncomformity

• Younger sedimentary rocks above the noncomformity

• Often has inclusions in the overlying bed

Formation of Inclusions

Now the fragments are included

Disconformity• Strata on either side are parallel• Represents a gap in geologic time• Most difficult of the three

unconformities to detect

Several unconformities are present in the Grand Canyon

Principles of Relative Dating

• Law of Superposition• Principle of Original Horizontality• Principle of Crosscutting Relationships• Inclusions• Unconformities