CATASTROPHISM Archbishop - James Usher (1664) Earth only about 6,000 years old Based on genealogies and history recorded in Bible Creation date: 4004 BC Earth’s Landscape developed by major catastrophes Worldwide disasters triggered by unknown causes No longer operating today

Scientific Efforts & attempts to date the Earth Georges Louis de Buffon (mid 1700’s) Assumed Earth was originally molten Calculated age to be about 75,000 yrs. Based on present temperature and a rapid rate of cooling

Scientific Efforts & attempts to date the Earth Rate of Sedimentation Calculated age based on deposition rate of various sediments and the thickness of sediments to be from 3 million to 1.5 billion yrs. Problem: deposition rate is not consistent complete record of sedimentation does not exist

Scientific Efforts (cont.) Ocean Salinity – John Joly (1800’s) Calculated Earth’s age at 90 million yrs. from current salinity of ocean Assumed water was originally pure Salt was derived from erosion Problem: erosion rates are not constant and loss and recycling of salt not considered

PRINCIPLE OF UNIFORMITARIANISM The Physical, chemical, and biological laws (processes) that operate today have also operated in the geologic past. James Hutton (mid -1700’s) “The Present is the Key to the Past” A Fundamental Principle of Geology Sir Charles Lyell (mid-1800’s) Published “Principles of Geology” Popularized and showed more convincingly, Hutton’s ideas

Lord Kelvin (1866) - Most influential physicist of his time Made calculations indicating the Earth’s age to be between 20 million yrs. and 100 million yrs. Assumed on the originally molten Earth idea and conventional heat sources for the Earth and Sun Discovery of radioactivity in late 1800’s proved his calculations to be invalid

Millions of years of Earth History Exposed 1 st observed by John Wesley Powell Led expedition through the canyon Realized that the evidence for an ancient Earth was concealed in the rock.

3 Geologic Principles are followed:  LAW (PRINCIPLE) OF SUPERPOSITION  PRINCIPLE OF ORIGINAL HORIZONTALITY  Principle of CROSS-CUTTING RELATIONSHIPS The Sequence of Formation  Placing rocks in proper order  Which ones formed first, second, third, and so on  Cannot tell exact age, only order of events

 LAW OF SUPERPOSITION  PRINCIPLE OF ORIGINAL HORIZONTALITY PRINCIPLE OF LATERAL CONTINUITY  LAW OF CROSS-CUTTING RELATIONSHIPS PRINCIPLE OF INCLUSIONS PRINCIPLE OF FOSSIL (FAUNAL) SUCCESSION

LAW OF SUPERPOSITION PRINCIPLE OF ORIGINAL HORIZONTALITY Principle of CROSS-CUTTING RELATIONSHIPS

In an undeformed sequence of sedimentary rocks y Each bed is younger than the one beneath it Nicolaus Steno ( ) Also applies to volcanic rocks/surface-deposits Lava flows and beds of volcanic ash

Layers of sediment are generally deposited in a horizontal position from water under the influence of gravity Form Rock layers that are flat have not been disturbed

Original Horizontality (d) Lateral Continuity (e)

Deposition of sediment occurs during:  Submergence – below Sea-level  Sediment layers are deposited and sedimentary rock formed Erosion and deformation of rock occur during:  Emergence – above Sea-level  Rock layers uplifted above sea-level and exposed to deformation and erosion.

An intrusion or fault that cuts through another rock is younger than the rock it cuts. Sills vs. buried lava flow? Look for heat effects

Time 1Time 2

Figure 8.4

A F E C B D

PRINCIPLE OF INCLUSIONS UNCONFORMITIES CORRELATION OF ROCK LAYERS (AT DIFFERENT LOCATIONS) PRINCIPLE OF FOSSIL SUCCESSION

An inclusion is a piece of rock that is enclosed within another rock Inclusions are older than the rock that contains them Rocks that contain inclusions are younger Inclusions (Older) (Younger)

Which layer is younger?

A break in the rock record Period in which deposition stops Folding and/or tilting of strata can occur erosion removes some of the existing rock Deposition resumes

ANGULAR UNCONFORMITY DISCONFORMITY NONCONFORMITY

Tilted or folded sedimentary rocks are overlain by flat-lying rock strata Folding or tilting occurs during pause in deposition Horizontal Sedimentary rock Tilted/Folded Sedimentary rock Angular Unconformity

Break in the rock record caused by erosion or non-deposition of sediment Horizontal sedimentary rock layers are separated by an uneven surface Sedimentary Strata on either side is parallel Fossils must be used to determine the length of break in deposition Sedimentary rock Disconformity

Sea Level Deposition of rocks 1,2,3,4,5 & 6 Uplift & erosion 5 & 6 Submergence & Deposition 8 & 9 Disconformity

An unconformity between two very different rock types. Eroded Boundary between an eroded igneous intrusion or metamorphic rock and overlying sedimentary rock. Older Igneous & Metamorphic rock strata found beneath sedimentary rock strata Inclusions may be useful in identifying Igneous or Metamorphic rock Sedimentary rock Nonconformity

Unconformities in the Grand Canyon: This cross-section through the Grand Canyon illustrates the three basic types of unconformities

GRAND CANYON A record of a huge expanse in time, and multitude of sedimentary environments… But: The record of time is not continuous because of geological processes that happened after…

MATCHING ROCKS OF SIMILAR AGE IN DIFFERENT REGIONS Geologists use CORRELATION Reliance upon FOSSIL EVIDENCE Which rock layers are found in both Zion N.P and Grand Canyon N.P.? (Kaibab Ls & Moenkopi Fm) Which rock layers are found in both Bryce Canyon N.P. and Zion N.P.? (Navajo Ss & Carmel Fm)