CHAPTER 12.1 Discovering Earth’s History Uniformitarianism = forces and processes seen today have been at work for a very long time. Relative Dating = tells the sequence of events through layers of sedimentary rock.

Law of Superposition = the layer on the bottom is oldest OLDER OLDEST Law of Horizontality = layers of sediment are deposited in a horizontal position.

Principle of Cross-Cutting Relationships = when a fault cuts through rock layers or when magma intrudes other rocks, we can assume that the fault or the magma are younger than the rocks they affected.

Inclusions = pieces of one rock unit that are contained in another Unconformity = time when deposition stopped, erosion removed rock, and then deposition resumed.

Angular unconformity = unconformity involving folding or tilting of rock during the time when deposition was halted. Disconformity= unconformity involving flat erosion during the time when deposition was halted.

Correlation of Strata = using multiple locations to piece together the history of a large region. Pg 341 Figure 8

Chapter 12.2 Fossils: Evidence of Past Life Fossil Formation: Unaltered Remains = remains that have not been changed over time. Ex. Teeth, bones, shells, frozen mammoth. Altered Remains = Remains that have been changed over time. Ex. Petrified wood, molds, casts, carbonized leaves and delicate animals.

Trace Fossils = indirect evidence left by prehistoric life. Examples: Tracks, burrows, dung, stomach contents.

Two Conditions Favoring Preservation: Rapid burial Possession of hard parts (shells, bones, etc.)

Principle of Fossil Succession = fossil organisms succeed one another in a definite and determinable order so that any time period can be recognized by its fossil content.

Index Fossil widespread geographically limited to a short geologic time span occur in large numbers

Using Fossils to Interpret Ancient Environments You might identify the ancient environment as… If you found… LAB: NC Fossil Hunt

Chapter 12.3 Dating with Radioactivity Radioactive Decay: Alpha Emission = element looses neutrons at a regular rate. Beta Emission = element looses beta particles at a regular rate. Electron Capture = an electron falls into then nucleus from the innermost orbit. Half-Life = the amount of time necessary for one half of the nuclei in a sample to decay to its stable isotope. Animated Modes of Radioactive Decay: http://www.wwnorton.com/college/Chemistry/gilbert/tutorials/ch2.htm Tutorial on Half-Life: http://www.colorado.edu/physics/2000/isotopes/lifetime.html

Radiometric Dating = procedure using rates of radioactive decay to determine the age of rocks and minerals. Example: As uranium decays it turns into lead. This decay from uranium into lead is done at a measurable rate. Carbon-14 Dating Carbon-14 is the radioactive isotope of Carbon. Normally Carbon-14 is lost into the atmosphere. We all have Carbon-14 in our bodies. As long as you are alive, you continue to replace the Carbon-14 and the balance between Carbon-14 and Carbon stays equal. When you die you immediately stop replacing the Carbon-14 and the balance changes. The difference in balance between Carbon-14 and Carbon tells you how hold the item is or how long it has been dead.

Carbon-14 Uranium-238 half-life of 5730 years up to 75,000 years ago used by anthropologists (study human history) used by geologists studying recent Earth history half-life of 4.5 billion years extremely old samples used by historical zoologists (study ancient fossils) used by geologists studying ancient Earth history

Chapter 12.4 The Geologic Time Scale 4.56 BYA EONS – ERAS – PERIODS – EPOCH – MYA Phanerozoic – Cenozoic – Quaternary – Holocene – 0.01 Problems with the Geologic Time Scale? The age of metamorphic rock may just indicate when it metamorphosed – not when it formed. Sedimentary rock is made of mixtures of older rocks that have weathered and settled there. Flash: Geologic Time Scale…30:00 Flash: Boneheads…(Jurassic Park?)