# ESS 8.4 Earth’s History.

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ESS 8.4 Earth’s History

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 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

Dating Techniques Either relative or absolute
Time is: a measured period during which an event, process, or condition exists or continues.

One way in which geologists look at time: Relative Age

Relative Age of a rock… …is its age compared to the ages of other rocks. Example: My niece, Amber, is older than my other niece, Jordan. What happened first? What happened next?

Absolute Age of a rock… …is the number of years since the rock formed.
Example: My niece, Amber, is 3 years old and my niece, Jordan, is 3 months old.

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 rocks are on the bottom and the youngest rocks are on the top.

Principle of Superposition
Q#1 Which rocks are older? Which rocks are younger? Younger Rocks Older Rocks

LAW OF CROSS-CUTTING -- AN IGNEOUS INTRUSION IS YOUNGER THAN THE ROCK IT HAS INTRUDED INTO.

Principle of Cross-Cutting
You can't cut something until that something exists, therefore the cut has to be younger. So faults are younger than the rock it cuts through. Volcanic vents cutting across sedimentary rock layers - Which is youngest?

Principle of Cross-Cutting
Q#2 What is younger? The rocks or the fault line?

Principle of Inclusions
The inclusion is always younger than the rock layers around and beneath it.

Principle of Inclusions
Q#3 Which is older? The rock layer or the rocks included in the layer?

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

Folding and Faulting Folding Faulting

UNCONFORMITY- A PLACE IN THE ROCK RECORD WHERE LAYERS OF ROCK ARE MISSING BECAUSE OF UPLIFT AND EROSION. THE RESULT CAN BE A LARGE AGE DIFFERENCE BETWEEN THE ROCKS ABOVE AND THOSE BELOW THE EROSIONAL SURFACE (IT APPEARS LIKE A SQUIGGLY LINE IN A CROSS-SECTION)

Where is the youngest rock?
How do you know?

What rock layer is the oldest?
How do you know?

Which is younger? The fault or the rock layers?
How do you know?

PLACE THE FOLLOWING EVENTS IN ORDER STARTING WITH THE OLDEST.
B I G K M F C A D H J J H C A F D K G I M B E L

Using Fossils to Date Rocks
Give relative age to a layer of rock Match layers of rock at other locations Use index fossils to match rock layers Index fossils are distributed in different areas & only occur in one area

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

MEASURING ABSOLUTE TIME
TREE RINGS EACH RING REPRESENTS A SINGLE YEAR (SPRING/FALL) THE WIDTH OF THE RING DEPENDS UPON THE TEMPERATURE AND RAINFALL

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

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