Rock Layers & Fossils March 2018

Learning Target I can explain the relationship between dating rock layers and the age of fossils.

About Fossils Found in sedimentary rocks Sedimentary rock typically is layered Layers derived from different periods of history

About the Methods Scientists combine several well-tested techniques to find out the ages of fossils. Relative Age Superposition Index Fossils Absolute Age 

Relative Age Think about a layer cake, you know that the layer at the bottom was the first one the baker put on the plate, and the upper ones were added later. In the same way, geologists figure out the relative ages of fossils and sedimentary rock layers; rock layers, and the fossils they contain, toward the bottom of a stack of sediments are older than those found higher in the stack

Superposition law of superposition. a basic scientific law of telling Earth-time It states that in any undisturbed sequence of rocks deposited in layers, the youngest layer is on top and the oldest on bottom, each layer being younger than the one beneath it and older than the one above it.

Index Fossils Also known as guide fossils or indicator fossils Used to define and identify geologic periods Index fossils must have a short vertical range, wide geographic distribution and rapid evolutionary trends. Think of them as geologic post-its

Absolute Age Geologists find absolute ages by measuring the amount of certain radioactive elements in the rock. As time passes, the "parent" radioactive elements change at a regular rate into non-radioactive "daughter" elements. A.K.A. Radiometric Dating You may have heard of Carbon-14