Relative dating superposition Clues in Igneous rock Index Fossils Determining whether an object or event is older or younger than other objects or events Younger rocks lie above older rocks Extrusive rock is always younger Intrusive rock is always younger Help geologists match rock layers Tells the relative age of the rocks

The time it takes for ½ of a radioactive sample to decay Absolute Dating Radioactive decay Half-life Establishing the age of an object, so as a fossil or rock layer by determining the number of years it has existed The atoms of one element break down to form atoms of another element Occurs at a steady pace Helps to date rocks accurately Only happens with certain elements The time it takes for ½ of a radioactive sample to decay

Types of radiometric dating Determining the absolute age of a sample based on the amount of decay left in an object Potassium-Argon Half-life of 1.3 billion years – great for rocks Carbon-14 Radioactive carbon All plants & animals have some Half-life of 5730 years Not good for rocks or things older than 50k

Dividing Geologic Time Geologic column Dividing Geologic Time A record of the geologic events as shown in the fossil records Oldest to youngest Helps when you have areas that are not complete or have faults Uses rocks from all over the world Precambrian 88% of Earth’s history Few fossils Eras Time between precambrian & present Triassic, Jurassic, Cretaceous Periods Smaller chunks of time in each era Names come from places where rocks/fossils were first described.

Preserved remains or traces of living things Occur in many forms Fossils Fossils in Rocks Molds & Casts Petrification Preserved remains or traces of living things Occur in many forms Most recognizable are preserved organisms The hard parts of dead organisms buried quickly in sediment and preserved in rock Hard parts of organisms most likely to be fossils because they decompose slower Mold= hollow area in sediment in the shape of an organism or part of an organism Cast = solid copy of the shape of an organism Minerals completely replace an organism’s tissues Petrified wood

Thin coating of carbon on rock Carbon Films Trace Fossils Preserved Remains Thin coating of carbon on rock Preserves delicate parts of leaves & insects Evidence of ancient organisms Footprints & tracks Burrows (homes) Coprolites (fossilized poo!) Processes that preserve the whole specimen Animals sink into tar pits and are preserved Amber traps insects/plants Ice freezes fossils as perfect replicas of what they were in life Mummification dries/preserves tissue