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Geologists estimate that the earth is
about 4.6 billion years old!!!
4,600,000,000 years
How are we able to estimate something
like this?
RELATIVE TIME
VS.
ABSOLUTE TIME
PLACES EVENTS IN A
IDENTIFIES THE
SEQUENCE BUT DOES
ACTUAL DATES OF
NOT IDENTIFY THEIR
GEOLOGIC EVENTS
ACTUAL DATE OF
OCCURRENCE
EXAMPLE
A LIST IN
CHRONOLOGICAL
ORDER OF WHAT YOU
HAVE DONE TODAY UP
UNTIL THIS TIME
THE EXACT TIMES AT
WHICH YOU DID
THESE THINGS
LAW OF SUPERPOSITIONIN UNDISTURBED
SEDIMENTARY ROCKS THE
OLDEST ROCK LAYERS
ARE AT THE BOTTOM AND
THE YOUNGEST ARE AT
THE TOP.
Unconformities
An unconformity is a contact between two
rock units in which the upper unit is
usually much younger than the lower unit.
Unconformities are typically buried
erosional surfaces that can represent a
break in the geologic record of hundreds of
millions of years or more.
Non-conformity: sedimentary rock sits on top of metamorphic;
gap of time that erosion took place
Angular Unconformity: sedimentary rock deposited in layers;
results from faulting and folding typically
Disconformity: sedimentary rocks exposed, eroded, and then
deposition resumes
LAW OF CROSS-CUTTING
RELATIONSHIPS-
AN IGNEOUS INTRUSION IS YOUNGER
THAN THE ROCK IT HAS INTRUDED
INTO.
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
FAULTED AND FOLDED LAYERS –
LAYERS OF ROCK THAT HAVE BEEN
FAULTED OR FOLDED MUST HAVE
BEEN PRESENT BEFORE THE
ACTIONS OF FAULTING OR FOLDING
TOOK PLACE
FOLDING
FAULTING
PLACE THE FOLLOWING EVENTS IN ORDER
STARTING WITH THE OLDEST, FROM A TO M.
L
M
K
I
H
G
J
E
D
C
F
B
A
MATCHING OF ROCK
LAYERS THAT CAN BE SEEN
AT THE EARTH’S SURFACE,
OVER A LARGE AREA
AN OUTCROP IS EXPOSED
ROCK LAYERS AT THE
EARTH’S SURFACE
A KEY BED IS A THIN, WIDESPREAD LAYER, USUALLY OF
VOLCANIC ASH, THAT CAN BE USED TO CORRELATE AN
EXACT POINT OF TIME
MEASURING ABSOLUTE TIME
RATES OF EROSION AND DEPOSITION
IF SCIENTISTS CAN MEASURE THE RATE AT WHICH A
STREAM ERODES ITS BED, THEY CAN ESTIMATE THE
APPROXIMATE AGE OF THE STREAM.
EXAMPLE:
NIAGARA FALLS THE
EDGE IS ERODING AT AN
AVERAGE RATE OF 1.3
METERS PER YEAR.
SCIENCTISTS CAN THEN
ESTIMATE THAT IT
LIKELY FORMED 9,900 YEARS AGO
MEASURING ABSOLUTE TIME
TREE RINGS
EACH RING REPRESENTS A SINGLE
YEAR (SPRING/FALL) THE WIDTH OF THE
RING DEPENDS UPON THE
TEMPERATURE AND RAINFALL
VARVES
GLACIAL LAKE DEPOSITS. A THICK
LIGHT COLORED LAYER IN THE
SUMMER AND A THIN DARK LAYER
IN THE WINTER
RADIOACTIVE DATING
USED TO DATE FAR BACK IN TIME. CERTAIN ROCKS CONTAIN
RADIOACTIVE ISOTOPES
RADIOACTIVE ISOTOPES ARE ATOMS OF
ELEMENTS THAT GIVE OFF RADIATION FROM
THEIR NUCLEI
RADIOACTIVE DECAY IS THE PROCESS BY
WHICH A RADIOACTIVE ISOTOPE CHANGES
INTO A NEW STABLE ELEMENT
THE RATE AT WHICH A RADIOACTIVE ELEMENT DECAYS. IT
IS THE TIME IT TAKES FOR HALF OF THE ATOMS OF THE
RADIOACTIVE ELEMENT TO DECAY TO A STABLE END
PRODUCT
AT THE END OF EACH HALF-LIFE, HALF OF THE
RADIOACTIVE MATERIAL REMAINS
PARENT ISOTOPE = THE RADIOACTIVE ISOTOPE THAT BEGINS
DAUGHTER ISOTOPE = THE STABLE ISOTOPE THAT HAS BEEN CHANGED
RADIOCARBON DATING
USES THE RADIOACTIVE ISOTOPE CARBON-14
FOUND IN ALL LIVING THINGS.
BECAUSE CARBON-14 IS CONTINUALLY ABSORBED
BY FOOD AND WATER IT STAYS CONSTANT IN LIVING
THINGS. WHEN THE LIVING THING DIES THE
PERCENTAGE OF CARBON-14 DECREASES AT THE
RATE OF ITS HALF-LIFE.
CAN BE USED TO DATE BACK ABOUT 100,000 YEARS
URANIUM LEAD METHOD IS USEFUL TO DATE ROCKS
OLDER THAN 10 MILLION YEARS. CAN BE USED ONLY ON
IGNEOUS ROCKS THAT CONTAIN THE RIGHT KIND OF
URANIUM
RUBIDIUM-STRONTIUM METHOD CAN ALSO BE USED
TO DATE OLDER ROCKS BECAUSE OF ITS LONG HALF-LIFE. IT
IS ALSO VERY COMMONLY FOUND IN IGNEOUS ROCKS.
POTASSIUM-ARGON METHOD IS VERY USEFUL SINCE
POTASSIUM-40 CAN BE FOUND IN METAMORPHIC,
SEDIMENTARY, AND IGNEOUS ROCKS. IT CAN DATE OLDER
ROCKS BUT MAY ALSO DATE ROCKS AS YOUNG AS 50,000
YEARS
End
IT IS ESTIMATED THAT THE EARTH FORMED
ALONG WITH THE SOLAR SYSTEM 4.6 BILLION
YEARS AGO (4,600 MYA)
GEOLOGIC TIME SCALE
A SUMMARY OF THE MAJOR EVENTS IN EARTH’S HISTORY
EON – largest segment of geologic time
ERA
PERIOD
EPOCH – smallest segment of geologic time
Precambrian time is all of the geologic
time before the Paleozoic Era.
Precambrian rocks lack fossils because most of the
organisms were microscopic bacteria and algae
Stromatolites are algal reefs that make up a large part of the
fossils from the Precambrian.
HALF OF THE WORLDS METALLIC MINERALS ARE
FOUND IN PRECAMBRIAN ROCKS
It is very difficult to interpret the rock record from the
Precambrian for 3 reasons
1.) THE RECORD COVERS AN INCREDIBLY LARGE
PERIOD OF TIME
2.) Many Precambrian rocks are severely deformed and
folded
3.) MOSTLY LACKING IN INDEX FOSSILS WHICH
MAKES ROCK CORRELATION DIFFICULT
THE PALEOZOIC ERA MARKS THE BEGINNING OF AN
ABUNDANT FOSSIL RECORD.
MANY ANIMALS HAD HARD
SHELLS AND SKELETONS
NORTH AMERICA WAS OVER THE EQUATOR AND
OUR CLIMATE WAS WARM WITH FEW SEASONAL
CHANGES
CAMBRIAN PERIOD
TRILOBITES AND BRACHIOPODS ARE THE MOST
COMMON FOSSIL
TRILOBITES ARE FOUND THROUGHOUT THE
PALEOZOIC ERA BUT NEVER MORE ABUNDANT AND
WIDESPREAD THEN THE CAMBRIAN (INDEX FOSSIL)
THE BURGESS SHALE IS A ROCK FORMATION FOUND
IN THE ROCKY MOUNTAINS OF CANADA. 120
DIFFERENT KINDS OF ANIMALS FROM THE MIDDLE
OF THE CAMBRIAN PERIOD CAN BE FOUND THERE.
ORDOVICIAN
ALL LIFE STILL IN THE
OCEANS
THE END OF THE PERIOD IS MARKED BY THE TACONIC
OROGENY WHEN A PIECE OF AFRICA BROKE OFF AND
COLLIDED WITH N. AMERICA. THIS PRODUCED THE
TACONIC MOUNTAINS
SILURIAN
THE EURYPTERIDS APPEARED DURING THIS
PERIOD. THEY ARE THE NYS FOSSIL. THEY WERE
CALLED SEA SCORPIONS AND ARE THOUGHT TO
BE DISTANT COUSINS OF THE TRILOBITE.
LAND ANIMALS ALSO APPEARED. DISTANT
COUSINS OF SPIDERS, MILLIPEDES, AND
SCORPIONS
CLIMATE BECAME VERY DRY IN N. AMERICA
AND SHALLOW SEAS THAT COVERED THE
CONTINENT EVAPORATED. (ROCK SALT BEDS IN
WESTERN NY)
DEVONIAN
PERIOD
“AGE OF THE FISHES”
FISH WERE JAWLESS AND
WERE COVERED WITH
HEAVY PLATES.
POOR SWIMMERS THAT
REACHED UP TO 9 METERS
IN LENGTH
MISSISSIPPIAN AND PENNSYLVANIAN
PERIOD
REPTILES EVOLVED AS THE NEW LAND
ANIMAL
CLIMATE WAS WARM AND RAINY AND THE LAND
WAS COVERED BY MANY SWAMPS. AS TREES AND
FERNS FELL INTO THE SWAMP THEY SLOWLY
CHANGED OVER TIME INTO COAL.
A FOSSIL IS ANY EVIDENCE OF EARLIER LIFE
PRESERVED IN THE ROCK
ORIGINAL REMAINS (RARE) – THE ACTUAL
UNCHANGED REMAINS OF THE PLANT OR
ANIMAL ARE PRESERVED.
REPLACED REMAINS – THE SOFT PARTS OF THE
ORIGINAL ANIMAL HAVE DISAPPEARED AND THE
HARD PARTS HAVE BEEN REPLACED BY MINERAL
MATERIAL. (PETRIFIED WOOD)
MOLDS AND CASTS – FOSSIL SHELLS OR BONES ARE
DISSOLVED COMPLETELY OUT OF THE ROCK LEAVING
A HOLLOW DEPRESSION IN THE ROCK. NEW MINERAL
MATERIAL FILLS THE MOLD IT FORMS A CAST OF THE
ORIGINAL FOSSIL.
TRACE FOSSILS – EVIDENCE OF
LIFE OTHER THEN REMAINS,
WHICH INCLUDES ANY
IMPRESSIONS LEFT IN THE ROCK.
(TRAILS, FOOTPRINTS, TRACKS,
BURROWS)
INDEX FOSSIL
EASILY IDENTIFIABLE
SHORT-LIVED
WIDESPREAD OCCURRENCE