Transcript the key to the past

the key to the past
relative time vs. absolute time
relative time
order of events or objects from first (oldest) to last (youngest)
she is older than he is; she was born first and he was born last
absolute time
age of events or objects expressed numerically
she is twenty-one and he is nineteen
relative age dating concepts
original horizontality
all sediment originally deposited in water formed in horizontal layers
sediments will settle
to bottom
and blanket
the sea floor
relative age dating concepts
superposition
youngest
within a sequence of undisturbed
rocks, the oldest rocks are at the
bottom and youngest at the top
….young upward…
oldest
relative age dating concepts
cross-cutting
relationships
a disrupted pattern is older than
the cause of the disruption
e.g. an intrusion is younger
than the rocks it intrudes
relative age dating concepts
inclusions
fragments of other rocks contained
in a body of rock
must be older than the
host rock
e.g.
1) xenoliths in granite are older
than granite and
2) pieces of rock in
conglomerate are older
than conglomerate
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relative age dating concepts
unconformities
• A break in the geologic record, a gap in time
• Created when rock is eroded or when sediment is not deposited
• If rock was consistently deposited, and never eroded, we would be
able to see all rocks from all time periods (all 4.6 billion years)
relative age dating concepts
different types of unconformities
angular unconformity
• found between horizontal layers of sedimentary rock and layers
that have been tilted
• contacts between formations
may represent significant
amounts of time
angular unconformity
from: http://www.uakron.edu/envstudies/parks/rmgcan2.html
angular unconformity
angular unconformity
relative age dating concepts
different types of unconformities
disconformity
• contact separates sedimentary layers that are parallel
• missing time is difficult to recognize (may need other
information)
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relative age dating concepts
different types of unconformities
nonconformity
• horizontal sedimentary layers lie on top eroded igneous or
metamorphic rock
• large gap in geologic record
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what events occur?
angular unconformity
nonconformity
what events occur?