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.