EARTH’S HISTORY EARTH FORMED ALONG WITH THE SOLAR SYSTEM 4.6 BILLION YEARS AGO (4,600 MYA) I GEOLOGIC TIME SCALE 1) A SUMMARY OF THE MAJOR EVENTS IN EARTH’S HISTORY EON ERA PERIOD EPOCH II RELATIVE TIME – PLACES EVENTS IN A SEQUENCE BUT DOES NOT IDENTIFY THEIR ACTUAL DATE OF OCCURRENCE 1) RULES FOR FINDING AGE A) LAW OF SUPERPOSITION- IN UNDISTURBED SEDIMENTARY ROCKS THE OLDEST ROCK LAYERS ARE AT THE BOTTOM AND THE YOUNGEST ARE AT THE TOP. B) LAW OF CROSS-CUTTING RELATIONSHIPS- AN IGNEOUS INTRUSION IS YOUNGER THEN THE ROCK IT HAS INTRUDED INTO. (LOOK AT THE WHISKERS!) C) LAW OF INCLUDED FRAGMENTS- IF FRAGMENTS OF ONE TYPE OF ROCK ARE FOUND IN ANOTHER ROCK LAYER THE ROCK FRAGMENTS MUST BE OLDER THAN THE ROCK LAYER IN WHICH THEY ARE FOUND D) UNCONFORMITY- A PLACE IN THE ROCK RECORD WHERE LAYERS OF ROCK ARE MISSING BECAUSE OF UPLIFT AND EROSION (IT APPEARS LIKE A SQUIGGLY LINE IN A CROSS-SECTION) III ROCK CORRELATION- MATCHING OF ROCK LAYERS THAT CAN BE SEEN AT THE EARTH’S SURFACE, OVER A LARGE AREA 1) OUTCROP- A ROCK LAYER(S) THAT IS EXPOSED AT THE EARTH’S SURFACE 2) INDEX FOSSIL A) EASILY IDENTIFIABLE B) WIDESPREAD OCCURRENCE C) SHORT-LIVED 3) KEY BEDS- THIN, WIDESPREAD LAYERS, USUALLY OF VOLCANIC ASH, THAT CAN BE USED TO CORRELATE AN EXACT POINT OF TIME IV ABSOLUTE TIME- IDENTIFIES THE ACTUAL DATES OF GEOLOGIC EVENTS 1) TREE RINGS- YEARS (SPRING/FALL) 2) VARVES- GLACIAL LAKE DEPOSITS IN YEARS (SUMMER/WINTER) 3) RADIOACTIVE DATING- USED TO DATE FAR BACK IN TIME. CERTAIN ROCKS CONTAIN RADIOACTIVE ELEMENTS A) HALF-LIFE – THE RATE AT WHICH A RADIOACTIVE ELEMENT DECAYS.