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29.2 RELATIVE TIME
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PAST MISCONCEPTIONS
Before the 1700’s, scientist thought the Earth was ~ 6000 y.o. Given geologic processes take much longer to take shape they were able to get a beter idea of how old Earth was.
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RELATIVE DATING
What is relative dating?
Relative Dating – The process of placing events in the sequence in which they occurred. Does not give exact dates, just ballpark figures.
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RELATIVE DATING
The Principle of Superposition
Remember the Law of Superposition? Explain. Youngest Top, Oldest
bottom Intrusions can be
youngest depending on their penetration.
Embedded fragments are older than the rock that surrounds it. (ex. A pebble in conglomerate is older than the conglomerate itself.)
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GAPS IN RELATIVE TIME
Unconformity Indicates
whether layers of rock are missing from strata (rock layers) sequence.
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GAPS IN RELATIVE TIME
Unconformity Angular
unconformity Strata is tilted
during uplift then young flat strata is then deposited over older tilted strata.
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GAPS IN RELATIVE TIME
Disconformity When layers are
eroded away and younger layers replace them causing gaps in the time sequence. (Optional writing) Ex.
Layers in a sequence is eroded then covered by a body of water. The sediments in the water settle to form new layers. Although there is a sequence, some of the “pages” are missing.
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GAPS IN RELATIVE TIME
Nonconformity When sediments are
deposited on top of uplifted, eroded igneous rock.
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ROCK LAYER CORRELATION
Can you define correlation?
Matching of rock layers from one area to another.
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ROCK LAYER CORRELATION
Walking the Outcrop An outcrop is a
part of the rock layer that can be seen at Earth’s surface. Compare visible
rock of one area to another
Can be difficult in areas of high vegetation and dense soil.
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ROCK LAYER CORRELATION
Matching Rock Characteristics
Simply compares rock characteristics such as color, appearance and composition
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ROCK LAYER CORRELATION
Using Index FossilsIndex fossils are organisms that died during a
certain time period. Fossils in one layer of sediment will be different
(age wise) than one in another layer of sediment.
Four characteristics of index fossils Unique (Easily Identifiable) Abundant Widespread Existed for only a brief period
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ROCK LAYER CORRELATION
Fossils as environmental indicators
Can indicate an areas past climate. (Optional) Ex. Coral
only forms in shallow, warm water b/t 30 N and 30 S. Coral discovered in an area that doesn’t normally have coral would indicate that an area would have been covered by warm, shallow water.
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ROCK LAYER CORRELATION
Matching Key Beds Key beds are like
index fossils Have to be unique Widespread Easily recognizable
Ex. Volcanic eruptions spread across great distance
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STRATIGRAPHIC MATCHING
This is simply matching multiple layers within a strata.