Earth in Time The Rock Record and Geologic Time GEOLOGY TODAY Barbara W. Murck Brian J. Skinner Chapter 3 Maroon Bells, Colorado N. Lindsley-Griffin, 1999.

Relative Age - Principles Original horizontality: Sediments are deposited on a horizontal surface Sedimentary rocks form in horizontal layers Sedimentary rocks not horizontal were disturbed after they formed Folded sedimentary strata, Crete N. Lindsley-Griffin, 1999

Relative Age - Principles Stratigraphic superposition: Each layer of sedimentary rocks is deposited over the previous layer Lower layers are always older than upper layers Sandstone, limestone, and shale strata, Grand Canyon N.P., AZ N. Lindsley-Griffin, 1999

Relative Age - Principles Cross-cutting relationships: A rock unit is always older than any feature which cuts across or disrupts it. Fractures cutting sandstone layers, Merseyside, UK N. Lindsley-Griffin, 1999

Relative Age - Principles Cross-cutting relationships: A rock unit is always older than any feature which cuts across or disrupts it. Fractures cutting sandstone layers, Merseyside, UK N. Lindsley-Griffin, 1999

Relative Age - Principles Correlation: Lateral continuity -- Strata can be traced from one location to another Physical similarity -- Same characteristics = same strata Similar sequence of strata in two sections = same sequence Coal and sandstone strata, Badlands N.P., SD N. Lindsley-Griffin, 1999

Relative Age - Principles Faunal succession: Each formation contains a unique fossil assemblage Assemblages succeed one another in orderly, predictable sequence Same everywhere in the world In general, simpler organisms precede more complex ones in the same group Dinosaur tracks N. Lindsley-Griffin, 1999

Relative Age - Principles Faunal succession used for correlation: Look for same fossils in same sequence of similar strata. Fig. 3.6, p. 63 N. Lindsley-Griffin, 1999

Absolute Age - Principles Radioactive Decay - Release of particles from nucleus ALPHA EMISSION: Two protons + two neutrons Mass - 4 Number - 2 BETA EMISSION: Neutron decays to a proton and gives off an electron Mass - no change Number + 1 BETA CAPTURE: Proton captures electron and becomes a neutron Mass - no change Number - 1 © Houghton Mifflin 1998; N. Lindsley-Griffin, 1999. All rights reserved

Absolute Age - Principles Half-life: Time needed for the number of parent atoms to be reduced by one-half At time zero, 100% P.A. After one half-life, 50% P.A. and 50% D.A. After two half-lives, 25% P.A. and 75% D.A. Figure 3.12, p. 70 N. Lindsley-Griffin, 1999

Carbon-14 Dating Neutrons in atmosphere change nitrogen-14 to carbon-14 C-14 incorporated into tissue of living organisms Ratio of C-14 to other C-isotopes remains constant in living tissue At death, C-14 not replenished, ratio of C-14 to other C-isotopes decreases Amount of C-14 remaining determines time since death of organism © Houghton Mifflin 1998. All rights reserved