Earth’s History Geologic time scale-a timeline that organizes the events in Earth’s history Like a calendar extending from Earth’s formation to the present Divided into eons, eras, periods, and epochs Earth’s History www.fas.org •Eon-the largest group; divided by the Cambrian boundary (about 550 million years ago) when the variety of life forms explodes; billions of years long •Era: mass extinctions mark the boundaries between the eras; hundreds of millions years long •Period: blocks of time when a unique rock series was laid down; tens of millions of years long •Epoch: divisions of the most recent periods; several million years long •Imprints or remains of organisms that were once alive •Typically form in sedimentary rock, however: •May form in rocks •Amber •Petrification •Asphalt •Frozen Fossils •Trace fossils-any naturally preserved evidence of animal activity •Mold-a cavity in rock where a plant or animal was buried •Cast-created when sediment fills in a mold and becomes rock History of Change •The fossil record reveals a history of environmental change. Why? •Studying the relationship between older and younger fossils •Fossils of organisms that lived during a relatively short, well-defined geologic time span •Found in rock layers throughout the world Ammonites 230-208 MYA louisvillefossils.blogspot.com/2010/01/remark. Trilobite ~400 MYA brianlean.wordpress.com/2008/08/ Layers of Rocks 1. Layer A is the oldest due to the law of superposition-oldest layers are set down first 2. Radiometric dating because the rock layers are inorganic 3. The geologist did not include dates because they included the age of the extrusion and intrusion. They also used index fossils 4. < 250 million years old but > 100 million years old because it is between the extrusion and intrusion 5. No because the first vertebrates did not appear until the Ordovician Period (505 MYA) 6. A major event in Earth’s history was the evolution of dinosaurs and flowering plants 7. Layer F was deposited during the Cenozoic Era because it contains a whale fossil.