Fossils Remains, imprints, or traces of once living organisms. Two things are required for a fossil to develop: A quick burial and hard parts. Fossils can tell a geologist when, where, and how an organism lived. A general type of fossil is an Index fossil – Fossil of species that existed for a short period of time and were widespread. Not all rocks contain fossils. Some rocks melt and reform, destroying the fossils. Plus, fossils need a quick burial and hard parts.

Types of Fossils Cast – When sediments fill in a cavity (hole) made when an object decayed. (The shell copies we made!) Carbonaceous film – thin layer of carbon atoms and molecules. Mold – Cavity in rock made when an organism decayed (our Play- Doh!) Permineralized remains – skeleton (calcium) replaced by minerals Trace fossils – Clues organisms left behind, such as animal tracks or fossilized poop. Trace fossils can show how an organism ate, or if it lived in groups, or what type of habitat it lived in, and how it ate.

Cast Mold Permineralized

Relative Ages The relative age of rock layers only tells us if a rock layer is older or younger than rock layers next to it. It does not give an absolute age (like 75 million years old). Relative dating is putting the order of events or age of rocks in order by examining the position of rocks in a layer. PRINCIPLE OF SUPERPOSITION says older rocks are on the bottom and rocks become younger as you move upward in the undisturbed layers.

Unconformities and Faults If a fault breaks through a rock layer, it is younger than that rock layer. If it does not, it is older than that rock layer. If similar rock layers are found far apart but match up, they are said to be CORRELATING ROCK LAYERS. Unconformities are gaps in the rock records made when agents of erosion remove existing rock layers (makes surface of rock layer look jagged, bumpy, or wavy.) UNIFORMITARIANSIM = The idea that Earth’s processes occurring today are similar to those that occurred in the past.

Principle of Superposition D is the youngest layer. A is the oldest layer. Intrusion B is younger than A, older than D. Intrusion E is younger than D. F (a fault) is younger than E. Where is the unconformity? Between tan and gray

Absolute Age of Rocks Uses atoms in rocks and other objects to determine their ages. Absolute Dating gives an actual number for the age of something, like 34,000 years old, not just if it is older or younger than something.

Radiometric dating is a method where a geologist can calculate the absolute age of the rock by knowing the half-life of an isotope. Half-life – Time it takes for half an isotope’s atoms to decay. ONLY radiometric isotopes have a half-life. So if you know how much isotope was there when you started, and how much there is now, you know how old the fossil or rock is. Carbon-14 is used to date the fossils of living things, but is only good to about 75,000 years. Radiometric Dating