Chapter 4: A Trip Through Geologic Time

I. Fossils A fossil is the preserved remains or traces of an organism. The fossil record shows that life on Earth has changed over time (example: organisms used to be much simpler) Different types of fossils: Mold and cast Petrified fossils – when minerals replace the organism(and it turns into rock) Carbon film Trace fossils – provide evidence of ancient organisms’ activities Preserved remains - tar, amber, or ice Species of organisms that are no longer living are extinct. Fossils usually occur in sedimentary rock. Only the hard parts of organisms can become fossilized because soft parts decay too quickly.

II. The Relative Age of Rocks Relative age of a rock is its age compared to the ages of other rocks. Law of Superposition - in horizontal layers of sedimentary rock, each layer is older than the layer above it and younger than the layer below it (this helps geologists determine relative ages) Finding relative age: Igneous intrusions – always younger than the rock layers around and beneath it Igneous extrusions – always younger than the rock layers below it Faults –always younger than the rock layers it cuts through Index fossils – fossils of organisms that were only alive for a short time period but lived in many places Geologists use index fossils for finding the relative ages of rocks

III. Radioactive Dating Radioactive decay happens when the atoms of an unstable element break down into atoms of a new element A half-life is how long it takes for half of a radioactive element’s atoms to decay into a different element. Geologists use radioactive dating to determine the absolute ages of rocks. Radioactive dating works best with igneous rocks.

IV. The Geologic Time Scale The geologic time scale is Earth’s timeline – it is a record of life forms and geologic events in Earth’s history. Geologists use it because the time span of Earth’s past is so great, and it splits Earth’s long history into smaller units. The geologic time scale is split into divisions called eras: Paleozoic Era Mesozoic Era Cenozoic Era Eras are split into periods (just like years are split into months). Example: The Mesozoic Era is subdivided into three periods: the Triassic, the Jurassic, and the Cretaceous.

V. Early Earth Early life The planet forms (Precambrian Time) – scientists believe that Earth formed approximately 4.6 billion years ago Earth’s atmosphere - mostly carbon dioxide from volcanic eruptions Earth’s oceans - when Earth cooled down and water vapor turned to liquid water Earth’s continents - rock formed from the magma that cooled down Early life Very simple, single-celled organisms Similar to bacteria

VI. Eras of Earth’s History Paleozoic Era (544 million-245 million years ago) Cambrian Period – “explosion” of life forms called the Cambrian Explosion Ordovician Period – first vertebrates Silurian Period – insects and spiders Devonian Period – Age of Fishes, but the first amphibians show up here Carboniferous Period – Appalachian Mountains begin to form Permian Period – mass extinction, perhaps because of a climate change after continental drift Mesozoic Era (245-66 million years ago) Triassic Period – Age of Reptiles & Pangaea still mostly together Jurassic Period – dinosaurs rule the Earth; Pangaea breaks apart Cretaceous Period – mass extinction and end of dinosaurs, perhaps because of a giant object hitting Earth Cenozoic Era (66 million years ago to today) Tertiary Period – Age of Mammals Quaternary Period – humans!

Chapter Review Review Questions Test Format Pg. 149-150, #1-25 M.C. and T/F Short Answer (diagrams) “Rock Layers” “Radioactive Decay of Strontium-90”

**Be able to determine the relative ages of the rock layers and explain how you know.**

**Be able to read the graph and find the half-life of strontium-90.**