Earth History

Geologists estimate that the earth is about 4.6 billion years old!!! How are we able to estimate something like this?

READING THE ROCK RECORD RELATIVE TIME VS. ABSOLUTE TIME PLACES EVENTS IN A SEQUENCE BUT DOES NOT IDENTIFY THEIR ACTUAL DATE OF OCCURRENCE IDENTIFIES THE ACTUAL DATES OF GEOLOGIC EVENTS 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

FINDING THE AGE OF ROCKS WITH RELATIVE TIME LAW OF SUPERPOSITION- IN 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 C D F B A

ROCK CORRELATION 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

HALF-LIFE 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 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

PALEOZOIC ERA 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.

FOSSILS 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