Transcript Geologic History - Perry Local Schools

Who “dun” it?
Relative Time:
 Using layers of sequenced rock to find out
which is older and younger
 Ages of events are placed in order of
occurrence.
 No exact date is identified.
 Miss Miller is older than her students.
 Edison is an older building than Pfeiffer.
 WWI occurred before WWII.
 List one more example of relative age:
___________________________________
Finding age with relative time:
1. The Law of Superposition –
In a sequence of undisturbed
sedimentary rocks, the oldest rocks will
be at the bottom.
The law of cross-cutting relationships:
Anything that cuts across layers of
rock is younger than the rocks that it
has intruded into. This applies to
faults and igneous intrusions.
• Magma that rises thru to
the surface and hardens
into rock is younger than
the rock it is cutting
through.
• This is usually igneous rock,
or faults– breaks in rocks.
The Law of Folds or Tilts:
Tilts in rocks are younger than the
rocks themselves.
Original Horizontality:
Rocks are usually
deposited flat and
level.
Original horizontality
Other Guidelines for determining a sequence:
 Sedimentary rocks are usually formed
under water.
 Weathering and erosion usually
happen above water (on dry land).
 Contact metamorphism shows that the
rock that was changed was there first
when the intrusion happened.
Formation of an
Unconformity
Unconformities
 This is a rough/wavy surface that
represents “missing” time, separating
young rock from old rock.
Layers are formed according to superposition.
Something happens to uplift the area
• folding
• faulting, etc.
Erosion wears away the uppermost layers.
Area submerges and deposition begins again.
Here’s the
unconformity!
The law of included fragments :
The fragments that make up a rock
are older than the entire sample.
Absolute Time:
 Identifies the exact date of an event.
 65 million years ago
 1995
 two years older
Absolute Time
 Tree rings- every year a tree grows a new ring.
When the tree dies, the rings can be counted
and events such as forest fires can be dated.
(Evidence of the same fire may be seen as a
layer of ash in the rocks to correlate a rock
layer with the tree rings).
Tree rings can be used to date events up to
3000 years ago.
 Radioactive Dating Radioactive elements
decay (fall apart) at steady rates. One "halflife" of time has passed when exactly half of
the element remains. Half lives differ from
element to element. When they fall apart they
form a different material. A ratio between the
original material (parent material) and the
decay product (daughter material) can be
used to determine how many half-lives the
material has undergone.
Sequence 1: Uplift & Erosion
1.
2.
3.
4.
5.
Limestone deposited
Sandstone deposited
Shale Deposited
Uplift
Erosion
Sequence 2: Faulting
Limestone
deposited
2. Sandstone
deposited
3. Shale deposited
4. Faulting
1.
Sequence 3: Folding
Limestone deposited
2. Sandstone deposited
3. Shale deposited
4. Folding
1.
Sequence 4: Igneous Intrusion
Limestone deposited
2. Sandstone deposited
3. Shale deposited
4. Igneous Intrusion
Note: contact
metamorphism is the
same event as the
intrusion.
1.