Relative Dating Determining relative ages of rocks or strata compared to another rock or strata Can say which layer is older and younger but cant say absolute age For example Rock Layer A is older than rock Layer B ID: 552186
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Relative Dating of StrataSlide2
Relative Dating
Determining relative ages of rocks or strata compared to another rock or strata.
Can say which layer is older and younger but can’t say absolute age.
For example, Rock Layer A is older than rock Layer B.
Cannot say that Rock layer A is 32 million years old and rock layer B is 30 million years old. Slide3
Law of Superposition
E
ach
bed in a sequence of sedimentary rocks (or layered volcanic rocks) is younger than the bed below it and older than the bed above it.
Based on two assumptions:
1.) Principle of Original Horizontality
2.) The beds of strata are not overturnedSlide4Slide5
That which cuts through is younger than the
Object that is cut
dike cuts through
granite is cut
Cross Cutting Relationships
What happened first in the picture on
t
he right? Second? Third?Slide6
Principle of Faunal Succession
States that fossils aren’t randomly placed in strata, but rather follow a succession where they succeed each other in a definite and determinable order.
Organisms live for a certain amount of time and then go extinct, so we can use them to tell us age of rocks.
Index Fossils:
A good index fossil is is widespread but short-lived.
Certain Trilobites are good index fossils. Slide7
Fossil Correlation
Fossils can be used to correlate strata hundreds of miles apart.
If an index fossil is found in two different rock layers, we know they must be the same age.
We can then correlate or connect them together. Slide8
Relative Ages of Lava Flows and SillsSlide9
Principle of Inclusions
•
Inclusions (one rock type contained in another rock type) are
older than
the rock they are embedded in. That is, the younger
rock contains the inclusionsSlide10
Principle of Inclusions
Example:
Storm beds can include rip-up
clasts
.
Which is younger in each case?Slide11
Unconformities
Gaps in the geologic record, called unconformities, are common where deposition stopped and erosion removed the previously deposited material.
Drawn as wavy lines. Slide12Slide13
3 Types of Unconformities
Angular Unconformity
Disconformity
NonconformitySlide14
Angular Unconformity
Where tilted beds have been eroded and horizontal beds have been deposited on top.
Gap in time between tilting of beds and deposition of horizontal beds. Slide15
Disconformity
Erosion surface (gap in time) between two horizontal beds of sedimentary rocks.
For example:
Limestone is deposited
Limestone is eroded
Sandstone deposited on top of Limestone. Slide16
Nonconformity
U
nconformities (gap in time) that
separate igneous or metamorphic rocks from overlying sedimentary rocks.
They
usually indicate that a long period of erosion occurred prior to deposition of the
sediments.
Example: Sandstone overlying granite.