Chapter 12.1 Principle of Uniformitarianism “Current geological processes are the same as those at work in the past” James Hutton (1726-1797) Father of modern geology Men of Rock – Deep Time (Stop at 39 min) http://www.youtube.com/watch?v=bakeXEdlXXc Hutton’s Uncomformity (BBC 3:48) http://www.youtube.com/watch?v=lMfPSdrrjZI Sir Charles Lyell (1797-1875) Author “Principles of Geology” Foremost geologist of his day James Hutton- The Discovery of deep time 6:17 http://www.youtube.com/watch?v=2ybBPqwWF4Q

Determining Relative Age (What is older, or younger... not how old chronologically) 1. Law of Superposition – layers of sedimentary rock are older than the layers above it, and younger than the layers below it.

2. Unconformities Movements of the earth lift up rock layers. Layers are exposed to erosion. Sediments are deposited, forming new rock layers Stratified rock rests upon unstratified rock Stratified rock rests upon tilted stratified rock Stratified rock rests upon stratified rock, with a large time gap

3. Crosscutting Relationships – a fault or an intrusion is always younger than the rock layers it cuts through A – oldest; deepest layer B – next oldest, layer above C – next oldest, layer above D – dike cuts through existing layers E – crack filled through existing layers; youngest Identify the oldest to the youngest layers  Turn & Talk

12.2 Fossils: Evidence of Past Life Fossils – remains or traces of animals or plants from a previous geologic time Paleontology – the study of fossils Fossils provide clues to past geologic events, climates, and evolution of species. Fossils are generally found in sedimentary rock. Igneous rock forms from magma; remains burn up Metamorphic rock forms from heat and pressure, destroying any fossils inside

Kinds of Fossils Preservation of Organisms - Usually only the hard parts of organisms fossilize; in rare cases, the whole organism is preserved... a. Mummification - drying c. Tar Beds – large variety b. Amber – tree sap d. Freezing

Otzi, a 5300 frozen human in Russia. He had many tattoos. He had just eaten dinner. He had 2 wounds, one fatal… was he attacked? Freezing keeps an animals skin and muscles intact.

2. Petrification – minerals in ground water replace the original organic material; slow process, often resulting in perfect replica Florissant fossil beds National Monument

3. Traces of Organisms – no part of the original tissue survives Coprolites Trace fossils – example: footprints, burrows Gastroliths Imprints Molds Casts

Interpreting the Fossil Record Index Fossils – fossils found exclusively in rock layers of a particular geologic age. Trilobites (245-550 mya) & Ammonites (65-200 mya) are good index fossils

12.3 Dating with Radioactivity What is radioactivity? Determining absolute age More accurate than reconstruction of geologic events.

Radioactive Decay More accurate method for finding the absolute age of rocks Unstable isotopes lose particles and energy at a constant rate. This rate of change is measureable and is not affected by external factors U-238  emits 2+ & 2N... mass now 234 element now 90 (Thorium) Review from Chapter 8: The atomic number = # protons. The atomic mass = the sum of protons + neutrons U-238 has 92 protons & 146 neutrons (mass = 238) This cycle continues until a stable form (Pb-206) is reached. SEE TABLE p. 239

Radioactive Decay (con’t) Half-Life – time it takes for half the mass of a given element to decay into its daughter elements Uranium-238 has a very long half life (4.5 billion years) useful for rocks 10 million to 4.5 billion years old Carbon-14 has a very short half life (5730 years) useful for rocks between 100 to 30,000 years only used for organic materials (once living)

Determining Absolute Age (How old?... 100,000 yrs old? A million years old?) Erosion and Deposition Erosion rates are accurate, only w/in the last 10-20 thousand years. Over long periods of time, erosion rates can vary greatly. The edge of Niagara Falls is eroding at 1.3 meters/year. Estimate the falls formed 9,900 years ago. Only useful w/in ~10-20 thousand years. Less dependable for older features. (i.e. Grand Canyon) because erosion rates vary greatly. ~30 cm of sedimentary rock is deposited every 1000 yrs. But, floods can deposit many meters of sediment in a single day

Varve Count May be inconsistent. Why? Like counting tree rings, Varve counts are based on annual sedimentary deposits Layers (varves) of alternating light and dark colored bands Light bands – course particles Dark bands – fine particles Varves usually formed in glacial lakes. During the summer, snow and ice melt rapidly; water can carry large amounts of sediment into a lake. Most course particles settle quickly to form layer on the bottom of the lake. With the coming of winter, the surface of the lake begins to freeze. Fine cay particles, still suspended in the water settle slowly to form a thin layer on top. The winter and summer layers form one varve. May be inconsistent. Why?